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Residency of adult Atlantic cod (Gadus morhua) in the western Gulf of Maine
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Fisheries Research 91 (2008) 123–132
Residency of adult Atlantic cod (Gadus morhua) in the western Gulf of Maine W. Huntting Howell a,∗ , Michael Morin a,1 , Nathan Rennels b , David Goethel c a
Department of Zoology, University of New Hampshire, Spaulding Life Sciences Building, Durham, NH 03824, USA b UNH Coastal Marine Laboratory, P.O. Box 474, New Castle, NH 03854, USA c F/V Ellen Diane, 23 Ridgeview Terrace, Hampton, NH 03842, USA Received 4 April 2007; received in revised form 12 November 2007; accepted 13 November 2007
Abstract A mark and recapture study of cod in the western Gulf of Maine was conducted to study the seasonal movements of fish, particularly as they related to areas closed to commercial fishing. A total of 27,772 cod were tagged, and 1334 (4.8%) were recaptured with sufficiently detailed recapture location and date to be included in the study. Results indicated that the group is resident to the area and sedentary. Although there were a small percentage of fish (2.5%) that traveled long distances (>100 km), most were recaptured near their release location. There was no linear relationship between fish length and linear distances traveled, and linear distance traveled was only weakly related to days-at-large. For all groups of cod tagged and released in particular areas and months, mean distances traveled were small (<65 km), rates of travel were slow (<0.2 km/day), and rates of group dispersion were <50 km2 /day. There were no recognizable spatial or temporal patterns in the mean angles of travel for groups released in various month/area combinations, and the angular deviations associated with the mean angles were quite large. Although there seems to be little pattern in the movement of cod in our study area, temporal and spatial changes in abundance indicate that movements are occurring. The general pattern was a concentration of large cod into one small, inshore area (Area 133) in both the spring and winter, and dispersion from this area in the ensuing months. Monthly percentages of ripe cod in Area 133 provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July. Thus it is likely that the observed spring and winter concentrations of fish in Area 133 were associated with spawning. Fish in the spring group were the largest encountered in the study. The study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Although there is some exchange of fish between most of the closed areas, the timing of the closures appears to protect the largest aggregations of cod. © 2007 Elsevier B.V. All rights reserved. Keywords: Atlantic cod; Gadus morhua; Movement; Natal homing; Spawning
1. Introduction The Atlantic cod (Gadus morhua) is widely distributed on both sides of the North Atlantic, and supports important commercial and recreational fisheries throughout its range. The National Marine Fisheries Service (NMFS) manages Atlantic cod found in U.S. waters as two stocks: Georges Bank and the Gulf of Maine (Mayo and Col, 2006). Because of the broad distribution and historical importance of the species, the
∗ 1
Corresponding author. Tel.: +1 603 862 2109; fax: +1 603 862 3784. E-mail address: [email protected] (W.H. Howell). Current address: 19 St. Louis Avenue, Gloucester, MA 01930, USA.
0165-7836/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2007.11.021
movements and migratory behaviors of a large number of cod populations, including those in the western North Atlantic, have been studied (see review by Robichaud and Rose, 2004). There is a good deal of variation in movement patterns between populations, ranging from sedentary to those that disperse over large areas. These differences have important management implications for two reasons. First, the stock (management unit) may be composed of subpopulations, or spawning components, between which there may be some exchange caused by movements (Smedbol and Stephensen, 2001). Understanding the degree of this exchange is essential in conservation efforts. Second, the management of cod in some regions is tied to their migratory behavior and their resultant temporal and spatial distribution. An example of this is found in the western Gulf of Maine,
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where there are time/area closures, in which commercial fishing is restricted during certain months of the year, and a permanently closed area, the western Gulf of Maine Area Closure (WGMAC). The effectiveness of permanent or rolling closures as a management tool depends on many things, including size and location. Of particular importance are the abundance of fish within them, the degree of fish movement in and out of the areas, and whether the areas are used for spawning (Appeldoorn, 1997; Zeller and Russ, 1998; Kaiser, 2005; Gibb et al., 2007). Mark and recapture techniques have proven effective in studying the temporal and spatial distribution patterns and movements of fish, and they have been used with great effectiveness in studying the movements of fish in and out of closed areas (Appeldoorn, 1997; Lawson and Rose, 2000; Martell et al., 2000; Hunter et al., 2006). The objective of this research was to study the movements of cod in the western Gulf of Maine, particularly in and around the closed areas, using mark and recapture techniques. Catchper-unit-effort data, calculated from tows made to collect fish for tagging, were used to determine if spatial and temporal changes in abundance were associated with observed movements. Although the movements of cod on Stellwagen Bank, to the southeast of our study area have been studied (Groger et al., 2007; Lindholm et al., 2007), to our knowledge the temporal and spatial distribution of cod within these area closures is unknown, so it is hoped that results will provide fisheries scientists and managers with information that will contribute towards an improved understanding of cod movements in the western Gulf of Maine, and the effectiveness of permanent and rolling closures.
2. Methods and materials The study areas included the Western Gulf of Maine Area Closure (WGMAC) and the adjacent time/area closure blocks 124, 132, 133, and 139 (Fig. 1). Both time/area closures and the permanent closure were established in 1998 (New England Fishery Management Council, 1998), with the purpose of reducing fishing mortality rates on Gulf of Maine cod by eliminating fishing in times/areas where cod are aggregated. The WGMAC is located off the coasts of northern Massachusetts, New Hampshire and southern Maine. It is approximately 32 km wide (east–west) and 97 km long (north–south), and covers a large portion of Jeffreys Ledge. The six adjacent time/area (rolling) closures (Areas 124, 125, 132, 133, 139 and 140) are each about 48 km on a side, with some (124, 133 and 139) overlapping the WGMAC (Fig. 1). In these, fishing is restricted during certain times of the year when cod biomass is especially high. The timings of the western Gulf of Maine rolling closures have been changed several times since their implementation; with the intent to further reduce fishing mortality rates. During the course of this study, Areas 124 and 125 were closed in April and May, and again in October and November. Areas 132 and 133 were closed from April through June, and Areas 139 and 140 were closed in May and June. The study was conducted in cooperation with five members of the New England commercial fishing industry. They were selected because of their interest in the project, and because all had extensive fishing experience in the study areas. The National Marine Fisheries Service issued a scientific sampling permit to each vessel that enabled sampling in
Fig. 1. Areas where cod were tagged in the western Gulf of Maine. Numbered blocks are 48 km × 48 km, and are closed to commercial fishing in selected months. The western Gulf of Maine area closure (WGMAC) is 32 km × 97 km, and is closed to fishing year-round.
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Table 1 Summary of sampling effort, number of cod tagged, and number of cod recaptured in each of the study areas Area
No. of sampling days No. of cod tagged No. cod recaptured Percent recaptured Effort adjusted no. recaptures Percent change
124
132
133
139
WGMAC
Totals
16 3,796 197 5.19 47 −75.9
37 5,962 223 3.74 75 −66.6
45 14,186 834 5.88 177 −78.7
35 1,846 23 1.25 23 0.0
21 1,982 57 2.88 25 −56.5
154 27,772 1,334 4.8 347
‘Effort adjusted’ numbers of recaptures are based on the percentage of recaptures in each area, and percent change reflects the difference between the actual number recaptured and the effort adjusted number of recaptures.
the closed areas. Fish were collected by commercial vessels using otter trawls (15.2 cm codend mesh) in short duration tows (15–30 min). Fish were collected as shallow as possible to minimize decompression of the swimbladder. Towing depths ranged from 30 to 270 m, but more than 90% of the tows were less than 180 m in depth. Upon completion of a tow, fish to be tagged were placed into onboard tanks supplied with flowing seawater. The primary species tagged was Atlantic cod (Gadus morhua), but haddock (Melanogrammus aeglefinus), pollock (Pollachius virens), American plaice (Hippoglossoides platessoides), yellowtail flounder (Limanda ferruginea), winter flounder (Pseudopleuronectes americanus), witch flounder (Glyptocephalus cynoglossus), goosefish (Lophius americanus) and wolffish (Anarhichas lupus) were also tagged as time permitted. Fish were measured to the nearest centimeter, and a 7 cm FloyTM T-bar tag was inserted into the musculature at the base of the first dorsal fin. The tag was either bright orange or yellow in color and included an identification number and telephone number to call when the fish was recaptured. Gender and reproductive condition were noted when obvious, i.e. if the fish extruded milt or eggs as it was handled. Once tagged, the fish was placed headfirst back into the water. The time it took to tag a fish and return it to the ocean was typically less than 30 s. The date, Loran C coordinates, start and stop time, and depth of the tow were recorded. A poster that contained a brief description of the project, and information on how to report a tag return, was distributed at commercial fishing cooperatives in Maine, New Hampshire, and Massachusetts. Fishermen involved in the project also encouraged other fishermen to return tag information. When a tag return was reported, the tag number, date, and position of capture were recorded. Only those recaptures with complete date and positional information were used. All release and recapture data were compiled into Microsoft ExcelTM worksheets for analyses. We used POSAID2TM software to convert all Loran C positions into latitude and longitude coordinates. For each recapture, straight-line distance traveled (km) and vector angle, measured as degrees from true north, were obtained by plotting the release and recapture position into a Maptech® digital Chartkit 2001 software program. This information was then used to calculate, for groups of fish, north–south and east–west velocities (km/day), a mean-vector angle (degrees relative to true north), angular deviation (degrees), and dispersion rates
(km2 /day), using the equations given in Saila and Flowers (1968). Rayleigh’s test (Zar, 1999) was used to examine the distribution of recapture locations of groups of interest (e.g. fish released in a particular place and time). If the z-statistic was insignificant (P > 0.05), the recapture locations were uniformly distributed around the release location, and movement was considered random. Alternatively, if the z-statistic was significant, the distribution of recapture locations was not uniform around the release location, and movement away from the release location was considered to be directed. Because fishing effort data on the small spatial scale reported in this study were not available, we used the percentage of recaptures in each area as a proxy for fishing effort, and adjusted the number of recaptures in each area accordingly into an ‘effort adjusted number of recaptures’ (Table 1). This adjustment was done by scaling the percentages to a common number (the lowest percentage of 1.25 found in area 139), and multiplying the actual number of recaptures by this scaled ratio. This adjustment procedure resulted in reductions of the number of recaptures ranging from 0% in the area with the lowest ‘effort’ (Area 139) to 78.7% in the area (133) with the highest ‘effort’ (Table 1). To determine spatial changes in the abundance of Atlantic cod in the western Gulf of Maine over time, catch-per-unit-effort (CPUE, kg/h) was calculated for each tow made to collect fish for tagging. Since fish were not weighed, weight was estimated from the following length-weight relationship (O’Brien and Munroe, 2001): loge weight (kg, live) = −11.7231 + 3.0521 loge length (cm) Because catch-per-unit-effort data are ratios and are asymmetric around the arithmetic mean, geometric means were calculated for each data set. Geometric means are more suitable for calculating confidence intervals for lognormal data (McConnaughey and Conquest, 1993). Mean lengths were calculated to examine changes in fish length over time within an area. Because in most cases the gender of the fish was not determined, mean lengths represent a combination of males and females. Differences in mean length between areas within month were analyzed using analysis of variance tests, followed by a Tukey’s test if significant differences (P < 0.05) were found.
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Fig. 2. Frequency distribution of number of recaptures vs. days at large.
3. Results A total of 154 days, between February 2001 and June 2003, were spent collecting and tagging. The distribution of days among areas and the number of cod tagged and recaptured are given in Table 1. Months when tagging occurred included February through December 2001, January through October 2002, and April through June 2003. Towing locations and months in the WGMAC were limited because a large portion of the area is unsuitable for trawling, and because of the seasonal abundance of lobster gear. A total of 27,772 cod were tagged in the 870 tows made. Of these, 1334 (4.8%) were recaptured prior to 31 December 2004, and reported with sufficiently detailed recapture location and date. It is these cod that are included in this study. Days at large ranged from 0 (recaptured the same day) to 1343. Most (59.7%) were recaptured within 120 days of being tagged and released (Fig. 2). The maximum linear distance between release and recapture was 362 km, but most (73.8%) were recaptured less than 30 km from their tagging location. There was no linear relationship (P = 0.48) between fish length and linear displacement (Fig. 3a), but displacement and days-atlarge was linearly related (P < 0.01). The relationship, however, was not strong, and days-at-large explained only 5.6% of the variation in linear displacement (Fig. 3b). Mean total lengths of tagged cod, for all combinations of sampling month and tagging area, are given in Table 2. Virtually all fish were adults, and there was some variation in mean size over time within all areas. Mean values ranged from 43.6 cm (Area 124 in June) to 80.8 cm (Area 133 in March). The simplest way to consider the movement of fish is to determine if they were recaptured within the same general location where they were tagged, or if they were recaptured in some other location. Although it was not possible to calculate effort adjusted recapture percentages for fish tagged in one area and recaptured in another, Table 3 shows the unadjusted percentage of cod released in each area that were recaptured in the same or different areas. In 3 of the 5 tagging areas (124, 132 and 133), the majority of tagged fish were recaptured in the area where
they were released. A small percentage (16.8%) of fish tagged in Area 124 moved into Area 132 to the north, but few if any moved into Areas 139 and the WGMAC. Although some fish tagged in Areas 132 and 133 were recaptured in each of the other areas, the greatest exchange was between these two areas, in an east-west direction. In areas 139 and the WGMAC, an equal or greater percentage of fish were recaptured outside the tagging area. Almost equal percentages of the fish tagged in Area 139 were recaptured in the tagging area (25.8%), in Area 133 to
Fig. 3. (a) Relationship between total length (cm) and linear displacement (km); (b) days at large and linear displacement by Atlantic cod tagged in the western Gulf of Maine.
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Table 2 Mean total length (cm) and standard error of cod tagged in the indicated area and months Month
Area 124
132
Mean January February March April May June July August September October November December
S.E.
133
Mean 66.0b
59.1b
0.7
56.1b 57.0b 43.6d 65.6a 48.2c 65.1a
0.3 0.3 1.0 1.1 0.9 0.9
56.5b
0.5
46.0d 70.2a 65.8ab 60.1c 58.6c 65.2b 56.6cd 52.3d 66.5ab 66.9ab
139
S.E.
Mean
S.E.
0.9 0.6 1.5 0.5 0.3 0.5 0.5 0.8 1.4 0.4 0.5
71.0b
0.4 0.7 1.1 0.4 0.2 0.6 1.4 3.6 1.6 2.9 0.6 0.5
70.0bc 80.8a 79.2a 79.3a 79.3a 76.6a 51.2d 68.1bc 72.0abc 66.5bc 66.5c
WGMAC
Mean
S.E.
56.0ab 68.8ab 68.7ab 77.8a 62.8ab 46.9c
8.7 3.1 0.6 0.7 1.7 2.9
59.2bc
4.2
70.4a
3.2
Mean
S.E.
67.3a 69.4a 65.8a 58.7b 50.0c 59.9b 60.2abc
0.7 0.6 1.1 0.9 1.4 1.3 4.8
Means within columns with the different letter are not significantly different (P > 0.05).
the southwest (25.8%), and in Area 132 to the south (29.0%). Data from the WGMAC were quite different than other areas. A relatively small percentage (2.3%) of fish were recaptured in the tagging area, presumably because most of the recaptures in the study came from commercial harvesters, and this area is closed to commercial fishing year-round. Some fish tagged in the WGMAC were recaptured in each of the other areas. Exchanges were greatest with adjacent areas to west (Area 132), southwest (Area 124), and northwest (Area 139). Taken together, the data in Table 3 suggest that although some exchange occurs between most of the areas, the movement of cod in the southwestern Gulf of Maine is quite limited. Movement statistics for groups of cod tagged and released in selected month/area combination, all years combined, and irrespective of recapture location, are provided in Table 4. Combinations from which we had <10 recaptures were not analyzed. Mean distances moved between release and recapture for the various groups ranged from 7.2 km (Area 132, August) to 65 km (WGMAC, April). Both N–S and E–W velocities for the groups were low, ranging from 0.0011 to 0.1956 km/day north–south, and from 0.0032 to 0.1509 km/day east–west. Dispersion coefficients (km2 /day) for the groups also varied by month and area of tagging, ranging from 1 (Area 132 released in August) to >49 km2 /day (Area 139 released in May). There were no spatial or temporal patterns in the mean angles of travel, and the angular deviations associated with the mean angles were quite large. Table 3 Percent recaptures in various areas associated with each tagging area Tagging area
124 132 133 139 WGMAC
Recapture area 124
132
133
139
WGMAC
“Out”
65.8 2.8 6.8 6.4 16.3
16.8 70.8 18.8 29 27.9
0.6 18.8 59.8 25.8 4.6
0 1.6 4.3 25.8 11.6
0 0.8 3 6.4 2.3
16.8 5.2 7.2 6.4 37.2
“Out” refers to recaptured made outside the general study area.
Only 10 of the 20 month/area tagging combinations had a significant Raleigh Z value (Table 4), and the results were almost certainly influenced by the distribution of commercial fishing effort. For example, directed movements away from the release location were most common among fish tagged in Area 133. Five of the seven groups showed directional movement, and in all cases it was towards the southeast quadrant. It is likely that these results may have been due, in part, to the lack of fishing to the west of the area, which includes inshore state waters where commercial fishing is prohibited. The data reported in Table 4 were calculated from those groups of fish where sufficient numbers were recaptured, independent of recapture date. To interpret movement patterns in the 30 days following tag and release, returns from specific months and areas of release were grouped (Table 5). There were six tagging month/area combinations from which enough fish (>10) were recaptured to conduct quantitative analyses of their movement during the 30 days after release. Mean distances traveled by the six groups ranged from 7.5 to 19.5 km over the 30 days, and directional movement was evident in only two of the six groups; those released in May and June in Area 133. In both cases the mean angle of travel was towards the southeast. One group of cod, tagged in Area 133 in May, provided enough recaptures to examine their position, relative to their release location, at 30 days intervals (Table 5). These analyses were done in an attempt to identify temporal changes in direction, distance, and dispersion. In each of the five 30 days intervals examined, the fish were located to the southeast of their release location. Distance from their release location increased with time, but even after 150 days, this particular group of fish was <65 km from their release location. Only 34 of the 1334 (2.5%) recaptured fish moved >100 km from their release location. Some of these long distance movers came from each of the five tagging areas, and in each of the areas they were recaptured in all directions from their point of release. Of the 34, three were recaptured off the west coast of Nova Scotia, 3 on or near Georges Bank, 7 along the northern Maine or New Brunswick, Canada coasts, 6 from the central
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Table 4 Summary of movement statistics of all cod released in the indicated month and area, regardless of recapture location Tagging month
Tagging area
Mean distance Mean angle of (km) travel (◦ )
Angular deviation (◦ )
North–south velocity (km/day)
East–west velocity Dispersion coefficient (km/day) (km2 /day)
Raleigh Z value
n
April May January April May June July August October January February April May June November December May June April May
124 124 132 132 132 132 132 132 132 133 133 133 133 133 133 133 139 139 WGMAC WGMAC
29.6 23 13.8 17.9 10.6 12 46.4 7.2 21.5 24.9 25.5 26.1 25.7 17 63.2 45.8 64.6 41.8 65 53.7
72.8 68.9 66.8 76.7 74.2 59.6 75.1 61.2 57.5 50.6 67.9 79.2 57.2 62.5 38.4 45.4 48.3 51.7 50.9 69.1
0.0437 S 0.0401 N 0.0132 S 0.0011 S 0.0129 S 0.0330 S 0.0794 S 0.0015 S 0.0390 S 0.0403 S 0.0036 S 0.0077 S 0.0424 S 0.0333 S 0.1648 S 0.1624 S 0.1956 S 0.1209 S 0.0900 N 0.0253 N
0.0585 E 0.0476 E 0.0035 W 0.0090 E 0.0097 W 0.0083 W 0.1182 E 0.0290 W 0.0174 W 0.1509 E 0.0203 E 0.0367 E 0.0838 E 0.0460 E 0.0860 E 0.0869 E 0.0476 W 0.0399 W 0.0707 W 0.0901 E
2.35 3.45* 1.22 0.37 2.13 12.97*** 0.4 2.2 5 8.16*** 0.88 0.3 131.43*** 6.21** 6.02*** 5.65** 6.65*** 4.21** 8.81*** 0.96
64 45 12 34 83 57 20 12 10 22 10 149 521 38 10 12 16 12 24 13
186.5 21.8 283.3 223.5 255.1 176.6 248.3 220.6 229.3 103.9 126.1 53.9 120.1 133.6 160 148.6 196.3 196.2 320.9 62.9
34.5 17.1 3.2 4.1 3.6 4.0 20.8 1.0 7.2 16.0 16.6 18.3 28.2 19.7 14.2 19.1 49.3 40.0 44.7 23.0
Tabulated results are a combination of all years combined. n is the number of recaptures from those released in the indicated month/area. Significant Raleigh Z values are indicated (* P < 0.05; ** P < 0.01; *** P < 0.001).
Gulf of Maine, and the remainder from locations east or south of Cape Cod, Massachusetts. Although there seems to be little pattern in the directional movement of cod in our study area, the temporal and spatial changes in abundance indicate that some movements are occurring. Catch-per-unit-effort (kg/h) values for the various month/area combinations are given in Fig. 4. Because of the small number of tows, and the inherent patchy distribution of schooling species such as cod, mean CPUE values were quite variable. Seasonal coverage was incomplete in all areas except 133, so we cannot be certain of seasonal patterns in CPUE in the other areas. There are, however, sufficient data to speculate on the seasonality of CPUE in all areas (Fig. 4). Mean values ranged from as low as 2.3kg/h (Area 139 in March), to more than 1400 kg/h (Area 133 in May). In general, the lowest values were
seen in Areas 139 and the WGMAC, and the highest values were seen in Areas 132 and 133. In Area 132, which was sampled in all months except December, there appeared to be two seasonal peaks in abundance. The first, and highest, was seen in October and November, and this was followed by a dramatic decline from January through March. CPUE then increased again in April, fluctuated through August, and declined to nearly its lowest level of the year in September. A bimodal pattern was also seen in Area 133, which was sampled in every month. Here, the highest mean CPUE values were found in April, and especially May, and these fish had the largest mean lengths of any of the groups we caught (Table 2). Mean CPUE in Area 133 then declined in June, and fluctuated at relatively low levels through October. Mean CPUE then rose again in November, to reach a second, but smaller annual peak in December and
Table 5 Data for cod tagged and released in the indicated month and area, and recaptured within a relatively short time Tagging month
Tagging area
Recaptured within (day)
Mean angle of travel
Mean distance Angular (km) deviation
North–south velocity
East–west velocity
Dispersion coefficient
Raleigh Z Value
n
April April May May May June May May May May May
124 133 124 132 133 133 133 133 133 133 133
0–30 0–30 0–30 0–30 0–30 0–30 0–30 31–60 61–90 91–120 121–150
179.8 312.3 37.7 279.7 128.5 155.4 128.5 118.8 119 124.8 116.5
12.1 13.1 19.5 7.5 10.1 11.6 10.1 31.2 39.3 40.8 64.3
0.0851 S 0.2145 N 1.1832 N 0.1055 S 0.3884 S 0.6140 S 0.3884 S 0.1928 S 0.2467 S 0.1549 S 0.0963 S
0.4954 E 0.1506 W 0.4324 W 0.2802 W 0.3366 E 0.4299 E 0.3366 E 0.4365 E 0.4011 E 0.2532 E 0.3788 E
21.0 4.4 0.0 3.3 16.9 13.5 16.9 3.6 1.3 0.8 1.8
2.4 0.3 0.2 2.1 131.4*** 6.2** 131.4*** 1.8 0.9 0.6 0.2
22 35 23 22 252 16 252 48 26 23 11
77.9 76.1 71.3 78.4 74.9 72.9 74.9 78.6 79.3 79.6 80.3
Mean angle is relative to due north, velocities are km/h, dispersion coefficients are km2 /day, and n is the number of recaptures. ** P < 0.01; *** P < 0.001.
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Fig. 4. Mean catch-per-unit-effort (kg/h) in the indicated months and areas. Vertical bars represent 2SE. Shaded bars represent months that the area was closed to commercial fishing.
January. It is clear from the CPUE data that there are temporal and spatial changes in cod abundance within our study area. There is no clear correspondence however, between the directional movements of the fish, as indicated by mark-recapture, and their changing abundance. The monthly percentages of ripe cod in Area 133, which is well known as an important cod spawning ground (Bigelow and Schroeder, 2002; Ames, 2004), are shown in Fig. 5. Results provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July (Fig. 5). The presence of two cod spawning groups utilizing this area has long been suggested by fishermen, and it has been recently been shown that the two groups are genetically distinct (Wirgin et al., 2007). Morin (unpublished data) also found that the winter group had a significantly shorter mean length than the spring group. This study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Seven hundred and thirty-three fish, tagged during the spawning months of April and May in Area 133 were
recaptured. A plot of mean distance from the tagging location against days at large is shown in Fig. 6. Although there is some variation, mean distances from the spawning ground tended to increase during the first 6–8 months, to reach approximately 70 km. Mean distance then decreased over the following 4–6
Fig. 5. Percentage of ripe cod in Area 133 in the indicated sampling month.
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Fig. 6. Distance from release location vs. days-at-large for those cod released in Area 133 in April and May.
months, such that the mean distance from the spawning ground where the fish were tagged was only about 20 km one year after tagging. A similar pattern was shown in cod at large for more than a year (Fig. 6). This movement away from, and then back towards, the spawning location is interpreted as natal homing behavior. A similar cyclic pattern may be indicated in Fig. 3b, which includes all cod, independent of tagging location and date. It should be noted, however, that the pattern seen in Fig. 3b is influenced by the large number of fish, in the more appropriate subset, depicted in Fig. 6. 4. Discussion The movements of scores of cod populations around the world have been examined. Several studies, including two from Stellwagen Bank in the southwestern Gulf of Maine (Groger et al., 2007; Lindholm et al., 2007), have found that groups of cod in different localities are composed of a mixture of both resident and migratory fish (Neat et al., 2006; Wright et al., 2006; Svedang et al., 2007). The fact that a small portion of the fish tagged in this study moved long distances, suggests that this is probably true in our study area as well. After reviewing many of these studies, Robichaud and Rose (2004) proposed four migratory behavioral categories based on the degree of site fidelity and homing. “Sedentary” populations are found year-round in a relatively small geographical area. “Accurate homers” display seasonal movements and home to a relatively small area, and “inaccurate homers” display seasonal movements and home to a much broader area. The fourth category (“dispersers”) includes populations that move and spawn in a haphazard pattern over large geographical areas. Results of this study indicate that the group of cod in the southwestern Gulf of Maine can be categorized as “sedentary resident”. In most areas, the cod were recaptured within the same area they were released (Table 3), and most (73.8%) traveled less than 30 km. Further, only 3% of the recaptures were from distances >100 km from the point of release. It should be noted, however, that these finding might have been influenced by the short time at large for many of the fish. Another indication of the sedentary nature of these fish was the low average rate of linear displacement (∼0.2 km/day).
This is lower than the range of rates (2–66 km/day) reported by Groger et al. (2007) for cod on Stellwagen Bank in the western Gulf of Maine. Their rates, however, were calculated from swimming speed rather than observed displacement, while our rates were calculated from linear displacements of groups of fish. Our low average rate of displacement may have been due to two factors. One was the inclusion of fish that were at large for a limited number of days. The other was that many of our fish were tagged on the spawning grounds, and it has been shown that schools of spawning cod remain relatively immobile, exhibiting a daily displacement of only 1–2 km (Rose, 1993). Although it could be argued that time at large, which was relatively short for many tagged fish, influenced these findings, we found only a very weak positive relationship between linear displacement and time at large (Fig. 3b). Similar local movements have been found in many groups of cod studied, and are often associated with reproductive cycles, feeding, site fidelity, and/or seasonally changing water temperatures (Wise, 1963; Campana and Simpson, 1984; Rose, 1993; Taggart et al., 1998; Hunt et al., 1999; Robichaud and Rose, 2001, 2002, 2004; Neat et al., 2006; Wright et al., 2006; Groger et al., 2007; Lindholm et al., 2007). The limited movements of most of the fish in this study, the small percentage that moved greater distances, and their probable homing, indicates that this group of cod probably has limited spatial and temporal overlap with other groups of cod in the western Gulf of Maine, and thus constitutes a subpopulation. Although this subpopulation appears to remain in the general area year-round, there are local movements that result in small-scale seasonal changes in distribution. The CPUE data indicates that the fish become concentrated in particular areas and times, and then disperse. Thus it is likely that cod movements in the western Gulf of Maine, at least at certain times of the year, are directed. Based on seasonal changes in abundance, the area undergoing the greatest seasonal change is 133, with a strong movement into the area in both the spring (April/May) and winter (December/January), followed by movement out of the area in the succeeding months. Because these movements are associated with spawning (Fig. 5), and because some cod populations return to the same spawning areas each year (Rose, 1993; Robichaud and Rose, 2001, 2002; Svedang et al., 2007), it seems likely that pre-spawning movement into the area is directed. Post-spawning movements out of the area may be either dispersive or directed. Distance from the spawning area increases over the following 6–8 months (Table 5, Fig. 6), to a mean distance of 70–90 km. In the following 4–6 months mean distance decreases, which we interpret as a homing migration associated with reproduction. Similar homing behavior, combined with site fidelity, has led to a mosaic of small, genetically distinct cod populations in Norway (Fevoldon and Pogson, 1997), Newfoundland (Ruzzante et al., 1998, 1999; Beacham et al., 2002), Labrador (Morris and Green, 2002), and Iceland (Jonsdottir et al., 1999). Results of this study, as well as those of Wirgin et al. (2007), suggest that a similar mosaic may exist in the western North Atlantic. The time/area closures referred to in this study were established to reduce the fishing mortality of Gulf of Maine cod by
W.H. Howell et al. / Fisheries Research 91 (2008) 123–132
protecting dense aggregation of fish. At present, Area 124 is closed to commercial fishing in April and May, and again in October and November. Although our study lacks data for October, CPUE was relatively high in April, May and November (Fig. 4), and thus these months of closure appear to be appropriate. Areas 132 and 133 are presently closed in April, May and June. In Area 133, CPUE was relatively high in April and May, but not in June (Fig. 4), which suggests that the June closure may not be contributing as much to the desired conservation effects as the other two months. We note, however, that this is an important spawning area for cod in the spring, so the June closure is almost certainly protecting some spawning fish. CPUE in Area 132 (Fig. 4) was relatively low during the months of closure (April, May and June). In fact, mean CPUE values were 2–4 times higher in October and November. Thus there is an apparent disconnect between months in which the grounds were closed and months of highest abundance. Protecting the cod in this area in April, May and June, may however, be important because this area may serve as a pre-spawning staging area (Morin, unpublished data). Area 139 is closed in May and June. These two months correspond with the months of highest CPUE (Fig. 4), and thus the temporal closure appears appropriate. It is worth noting, however, that mean CPUE values in this area (<175 kg/h) were low relative to other areas, suggesting that the conservation value of this temporal closure is less than those of other areas. The WGMAC is closed to commercial groundfish fishing year-round. We were unable to sample the WGMAC in all months, and thus we do not have a clear idea of seasonal changes in abundance. In the months we did sample (April through October), both CPUE and mean length decreased steadily as the year proceeded. Very few ripe cod were caught in the area in the months we sampled, leading us to conclude that the WGMAC is not an important cod spawning area in the southwestern Gulf of Maine; particularly relative to the very large aggregations of spawning fish found in Area 133 in the spring. The size of closed areas relative to the movements of the species they are designed to protect is also important (Appeldoorn, 1997; Ingram and Patterson, 2001). The time/area closures in this study are each about 2300 km2 , and the WGMAC is about 3100 km2 . Because the dispersion coefficients (0–21 km2 /day) and mean distances traveled (7.5–19.5 km) in the 30 days following release (Table 5) were both small relative to closed area size, it is likely that the time/area and permanent closures are large enough to protect the aggregations of cod. 5. Conclusion In summary, the group of cod in the southwestern Gulf of Maine can be classified as sedentary and resident, staying within the general area year around. There are however, local seasonal movements, almost certainly associated with spawning, that result in dense aggregations of fish in certain areas and times. Time/area closures currently in place appear to be appropriate, in that they are closed to fishing in those months when cod abundance is highest.
131
Acknowledgements We would like to thank the dedicated fishermen who helped us capture and tag the cod, including Frank Mirachi, Vincent Balzano, Jim Ford, and Carl Bouchard, and all the commercial and recreational fishermen who generously returned tag information. The study would not have been possible without the help of a number of students who helped with the fieldwork and data entry, including John Weideman, Scott Elzey, Jeff Kneebone, and Mike Ball. We are also grateful to the Northeast Consortium who funded the research. References Ames, E.P., 2004. Atlantic cod stock structure in the Gulf of Maine. Fisheries 29, 10–28. Appeldoorn, R., 1997. Dispersal rates of commercially important coral reef fishes: what do tagging studies tell us about potential emigration from marine fisheries reserves. Proc. Gulf Caribb. Fish. Inst. 49, 54–58. Beacham, T.D., Brattey, J., Miller, K.M., Le, K.D., Withler, R.E., 2002. Multiple stock structure of Atlantic cod (Gadus morhua) off Newfoundland and Labrador determined from genetic variation. ICES J. Mar. Sci. 59, 650–665. Bigelow, H.B., Schroeder, W.C., 2002. In: Collette, B.B., Klein-MacPhee, G. (Eds.), Fishes of the Gulf of Maine, Third edition. Smithsonian Institution Press. Campana, S.E., Simpson, J.E., 1984. The 4× Cod Fishery: A Biological Update. CAFSAC Research Document 84/43. 40 pp. Fevoldon, S.E., Pogson, G.H., 1997. Genetic divergence at the synaptophysin (SypI) locus among Norwegian coastal and northeast Arctic populations of Atlantic cod. J. Fish Biol. 41, 895–908. Gibb, F.M., Gibb, I.M., Wright, P.J., 2007. Isolation of Atlantic cod (Gadus morhua) nursery areas. Mar. Biol. 151, 1185–1194. Groger, J.P., Rountree, R.A., Thygesen, U.H., Jones, D., Martins, D., Xu, Q., Rothchild, B.J., 2007. Geolocation of Atlantic cod (Gadus morhua) movements in the Gulf of Maine using tidal information. Fish. Oceanogr. 16, 317–335. Hunt, J.J., Stobo, W.T., Almeida, F., 1999. Movement of Atlantic cod, Gadus morhua, tagged in the Gulf of Maine area. Fish. Bull. U.S. 97, 842–860. Hunter, E., Berry, F., Buckley, A.A., Stewart, C., Metcalfe, J.D., 2006. Seasonal migration of thornback rays and implications for closure management. J. Appl. Ecol. 43, 710–720. Ingram, G.W., Patterson, W.F., 2001. Movement patterns of red snapper (Lutjanus campechanus), greater amberjack (Seriola dumerili), and gray triggerfish (Balistes capriscus) in the Gulf of Mexico and the utility of marine reserves as management tools. Proc. Gulf Caribb. Fish. Inst. 52, 686– 699. Jonsdottir, O.D.B., Imsland, A.K., Danielsdottir, A.K., Thorsteinsson, V., Naevdal, G., 1999. Genetic differentiation among Atlantic cod in south and southwest Icelandic waters: synaptophysin (SypI) and haemoglobin (HbI) variation. J. Fish Biol. 54, 1259–1274. Kaiser, M.J., 2005. Are marine protected areas a red herring or fisheries panacea? Can. J. Fish. Aquat. Sci. 62, 1194–1199. Lawson, G.L., Rose, G.A., 2000. Seasonal distribution and movements of coastal cod (Gadus morhua L.) in Placentia Bay, Newfoundland. Fish. Res. 49, 61–75. Lindholm, J., Auster, P.J., Knight, A., 2007. Site fidelity and movement of adult cod Gadus morhua at deep boulder reefs in the western Gulf of Maine, USA. Mar. Ecol. Prog. Ser. 342, 239–247. Martell, S.J.D., Walters, C.J., Wallace, S.J., 2000. The use of marine protected areas for conservation of lingcod (Ophiodon elongatus). Bull. Mar. Sci. 66, 729–743. Mayo, R.K., Col, L.A., 2006. The 2005 Assessment of the Gulf of Maine Atlantic Cod Stock. Northeast Fisheries Science Center Reference Document 02-06. McConnaughey, R.A., Conquest, L.L., 1993. Trawl survey estimation using a comparative approach based on lognormal theory. Fish. Bull. U.S. 91, 107–118.
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Morris, C.J., Green, J.M., 2002. Biological characteristics of a resident population of Atlantic cod (Gadus morhua L.) in southern Labrador. ICES J. Mar. Sci. 59, 666–678. Neat, C.F., Wright, P.J., Zuur, A.F., Gibb, I.M., Tulett, D., Righton, D.A., Turner, R.J., 2006. Residency and depth movements of a coastal group of Atlantic cod (Gadus morhua L.). Mar. Biol. 148, 643–654. New England Fishery Management Council, 1998. Framework Adjustment 25 to the Northeast Multispecies Fishery Management Plan. http://www.nefmc.org/nemulti/frame/FW25 Finaldoc.pdf. O’Brien, L., Munroe, N.J., 2001. Assessment of the Georges Bank Atlantic cod stock for 2001. Northeast Fisheries Science Center. Ref. Doc. 0110:126. Robichaud, D., Rose, G.A., 2001. Multiyear homing of Atlantic cod to a spawning ground. Can. J. Fish. Aquat. Sci. 58, 2325–2329. Robichaud, D., Rose, G.A., 2002. The return of cod transplanted from a spawning ground in southern Newfoundland. ICES J. Mar. Sci. 59, 1285– 1293. Robichaud, D., Rose, G.A., 2004. Migratory behavior and range in Atlantic cod: Inference from a century of tagging. Fish Fish. 5, 185– 214. Rose, G.A., 1993. Cod spawning on a migration highway in the northwest Atlantic. Nature 366, 458–461. Ruzzante, D.E., Taggart, C.T., Cook, D., 1998. A nuclear DNA basis for shelf- and bank-scale population structure in northwest Atlantic cod (Gadus morhua): Labrador to Georges Bank. Mol. Ecol. 7, 1663– 1680. Ruzzante, D.E., Taggart, C.T., Cook, D., 1999. A review of the evidence for genetic structure of cod (Gadus morhua) populations in the NW Atlantic
and population affinities of larval cod of Newfoundland and the Gulf of St. Lawrence. Fish. Res. 43, 79–97. Saila, S.B., Flowers, J.M., 1968. Movements and behavior of berried female lobsters displaced from offshore areas to Narragansett Bay, Rhode Island. J. Cons. Perm. Int. Explor. Mer. 31 (3), 342–351. Smedbol, R.K., Stephensen, R.L., 2001. The importance of managing withinspecies diversity in cod and herring fisheries of the northwestern Atlantic. J. Fish Biol. 59, 109–128. Svedang, H., Righton, D., Jonsson, P., 2007. Migratory behavior of Atlantic cod Gadus morhua: natal homing is the prime stock-separating mechanism. Mar. Ecol. Prog. Ser. 345, 1–12. Taggart, C.T., Ruzzante, D.E., Cook, D., 1998. Localised stocks of cod (Gadus morhua) in the northwest Atlantic: the genetic evidence and otherwise. In: Hunt von Herbing, I., Kornfield, I., Tupper, M., Wilson, J. (Eds.), The Implications of Localized Fishery Stocks. Natural Resource, Agriculture and Engineering Service, Ithaca, NY, pp. 65–90. Wirgin, I., Berlinsky, D., Nirmal, R., Maceda, L., Kovach, A., 2007. Stock identification of Atlantic cod in U.S. waters using microsatellite and single nucleotide polymorphism DNA analyses. Trans. Am. Fish. Soc. 136, 375–391. Wise, J.P., 1963. Cod groups in the New England area. Fish. Bull. 63, 189–203. Wright, P.J., Neat, F.C., Gibb, F.M., Gibb, I.M., Thordarson, H., 2006. Evidence for metapopulation structuring in cod from the west of Scotland and North Sea. J. Fish Biol. 69, 181–199. Zar, J.H., 1999. Biostatistical Analysis, fourth ed. Prentice Hall. Zeller, D.C., Russ, D.R., 1998. Marine reserves: Patterns of adult movement of the coral trout (Plectropomus leopardus (Serranidae)). Can. J. Fish. Aquat. Sci. 55, 917–924.
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Residency of adult Atlantic cod (Gadus morhua) in the western Gulf of Maine W. Huntting Howell a,∗ , Michael Morin a,1 , Nathan Rennels b , David Goethel c a
Department of Zoology, University of New Hampshire, Spaulding Life Sciences Building, Durham, NH 03824, USA b UNH Coastal Marine Laboratory, P.O. Box 474, New Castle, NH 03854, USA c F/V Ellen Diane, 23 Ridgeview Terrace, Hampton, NH 03842, USA Received 4 April 2007; received in revised form 12 November 2007; accepted 13 November 2007
Abstract A mark and recapture study of cod in the western Gulf of Maine was conducted to study the seasonal movements of fish, particularly as they related to areas closed to commercial fishing. A total of 27,772 cod were tagged, and 1334 (4.8%) were recaptured with sufficiently detailed recapture location and date to be included in the study. Results indicated that the group is resident to the area and sedentary. Although there were a small percentage of fish (2.5%) that traveled long distances (>100 km), most were recaptured near their release location. There was no linear relationship between fish length and linear distances traveled, and linear distance traveled was only weakly related to days-at-large. For all groups of cod tagged and released in particular areas and months, mean distances traveled were small (<65 km), rates of travel were slow (<0.2 km/day), and rates of group dispersion were <50 km2 /day. There were no recognizable spatial or temporal patterns in the mean angles of travel for groups released in various month/area combinations, and the angular deviations associated with the mean angles were quite large. Although there seems to be little pattern in the movement of cod in our study area, temporal and spatial changes in abundance indicate that movements are occurring. The general pattern was a concentration of large cod into one small, inshore area (Area 133) in both the spring and winter, and dispersion from this area in the ensuing months. Monthly percentages of ripe cod in Area 133 provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July. Thus it is likely that the observed spring and winter concentrations of fish in Area 133 were associated with spawning. Fish in the spring group were the largest encountered in the study. The study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Although there is some exchange of fish between most of the closed areas, the timing of the closures appears to protect the largest aggregations of cod. © 2007 Elsevier B.V. All rights reserved. Keywords: Atlantic cod; Gadus morhua; Movement; Natal homing; Spawning
1. Introduction The Atlantic cod (Gadus morhua) is widely distributed on both sides of the North Atlantic, and supports important commercial and recreational fisheries throughout its range. The National Marine Fisheries Service (NMFS) manages Atlantic cod found in U.S. waters as two stocks: Georges Bank and the Gulf of Maine (Mayo and Col, 2006). Because of the broad distribution and historical importance of the species, the
∗ 1
Corresponding author. Tel.: +1 603 862 2109; fax: +1 603 862 3784. E-mail address: [email protected] (W.H. Howell). Current address: 19 St. Louis Avenue, Gloucester, MA 01930, USA.
0165-7836/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2007.11.021
movements and migratory behaviors of a large number of cod populations, including those in the western North Atlantic, have been studied (see review by Robichaud and Rose, 2004). There is a good deal of variation in movement patterns between populations, ranging from sedentary to those that disperse over large areas. These differences have important management implications for two reasons. First, the stock (management unit) may be composed of subpopulations, or spawning components, between which there may be some exchange caused by movements (Smedbol and Stephensen, 2001). Understanding the degree of this exchange is essential in conservation efforts. Second, the management of cod in some regions is tied to their migratory behavior and their resultant temporal and spatial distribution. An example of this is found in the western Gulf of Maine,
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where there are time/area closures, in which commercial fishing is restricted during certain months of the year, and a permanently closed area, the western Gulf of Maine Area Closure (WGMAC). The effectiveness of permanent or rolling closures as a management tool depends on many things, including size and location. Of particular importance are the abundance of fish within them, the degree of fish movement in and out of the areas, and whether the areas are used for spawning (Appeldoorn, 1997; Zeller and Russ, 1998; Kaiser, 2005; Gibb et al., 2007). Mark and recapture techniques have proven effective in studying the temporal and spatial distribution patterns and movements of fish, and they have been used with great effectiveness in studying the movements of fish in and out of closed areas (Appeldoorn, 1997; Lawson and Rose, 2000; Martell et al., 2000; Hunter et al., 2006). The objective of this research was to study the movements of cod in the western Gulf of Maine, particularly in and around the closed areas, using mark and recapture techniques. Catchper-unit-effort data, calculated from tows made to collect fish for tagging, were used to determine if spatial and temporal changes in abundance were associated with observed movements. Although the movements of cod on Stellwagen Bank, to the southeast of our study area have been studied (Groger et al., 2007; Lindholm et al., 2007), to our knowledge the temporal and spatial distribution of cod within these area closures is unknown, so it is hoped that results will provide fisheries scientists and managers with information that will contribute towards an improved understanding of cod movements in the western Gulf of Maine, and the effectiveness of permanent and rolling closures.
2. Methods and materials The study areas included the Western Gulf of Maine Area Closure (WGMAC) and the adjacent time/area closure blocks 124, 132, 133, and 139 (Fig. 1). Both time/area closures and the permanent closure were established in 1998 (New England Fishery Management Council, 1998), with the purpose of reducing fishing mortality rates on Gulf of Maine cod by eliminating fishing in times/areas where cod are aggregated. The WGMAC is located off the coasts of northern Massachusetts, New Hampshire and southern Maine. It is approximately 32 km wide (east–west) and 97 km long (north–south), and covers a large portion of Jeffreys Ledge. The six adjacent time/area (rolling) closures (Areas 124, 125, 132, 133, 139 and 140) are each about 48 km on a side, with some (124, 133 and 139) overlapping the WGMAC (Fig. 1). In these, fishing is restricted during certain times of the year when cod biomass is especially high. The timings of the western Gulf of Maine rolling closures have been changed several times since their implementation; with the intent to further reduce fishing mortality rates. During the course of this study, Areas 124 and 125 were closed in April and May, and again in October and November. Areas 132 and 133 were closed from April through June, and Areas 139 and 140 were closed in May and June. The study was conducted in cooperation with five members of the New England commercial fishing industry. They were selected because of their interest in the project, and because all had extensive fishing experience in the study areas. The National Marine Fisheries Service issued a scientific sampling permit to each vessel that enabled sampling in
Fig. 1. Areas where cod were tagged in the western Gulf of Maine. Numbered blocks are 48 km × 48 km, and are closed to commercial fishing in selected months. The western Gulf of Maine area closure (WGMAC) is 32 km × 97 km, and is closed to fishing year-round.
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Table 1 Summary of sampling effort, number of cod tagged, and number of cod recaptured in each of the study areas Area
No. of sampling days No. of cod tagged No. cod recaptured Percent recaptured Effort adjusted no. recaptures Percent change
124
132
133
139
WGMAC
Totals
16 3,796 197 5.19 47 −75.9
37 5,962 223 3.74 75 −66.6
45 14,186 834 5.88 177 −78.7
35 1,846 23 1.25 23 0.0
21 1,982 57 2.88 25 −56.5
154 27,772 1,334 4.8 347
‘Effort adjusted’ numbers of recaptures are based on the percentage of recaptures in each area, and percent change reflects the difference between the actual number recaptured and the effort adjusted number of recaptures.
the closed areas. Fish were collected by commercial vessels using otter trawls (15.2 cm codend mesh) in short duration tows (15–30 min). Fish were collected as shallow as possible to minimize decompression of the swimbladder. Towing depths ranged from 30 to 270 m, but more than 90% of the tows were less than 180 m in depth. Upon completion of a tow, fish to be tagged were placed into onboard tanks supplied with flowing seawater. The primary species tagged was Atlantic cod (Gadus morhua), but haddock (Melanogrammus aeglefinus), pollock (Pollachius virens), American plaice (Hippoglossoides platessoides), yellowtail flounder (Limanda ferruginea), winter flounder (Pseudopleuronectes americanus), witch flounder (Glyptocephalus cynoglossus), goosefish (Lophius americanus) and wolffish (Anarhichas lupus) were also tagged as time permitted. Fish were measured to the nearest centimeter, and a 7 cm FloyTM T-bar tag was inserted into the musculature at the base of the first dorsal fin. The tag was either bright orange or yellow in color and included an identification number and telephone number to call when the fish was recaptured. Gender and reproductive condition were noted when obvious, i.e. if the fish extruded milt or eggs as it was handled. Once tagged, the fish was placed headfirst back into the water. The time it took to tag a fish and return it to the ocean was typically less than 30 s. The date, Loran C coordinates, start and stop time, and depth of the tow were recorded. A poster that contained a brief description of the project, and information on how to report a tag return, was distributed at commercial fishing cooperatives in Maine, New Hampshire, and Massachusetts. Fishermen involved in the project also encouraged other fishermen to return tag information. When a tag return was reported, the tag number, date, and position of capture were recorded. Only those recaptures with complete date and positional information were used. All release and recapture data were compiled into Microsoft ExcelTM worksheets for analyses. We used POSAID2TM software to convert all Loran C positions into latitude and longitude coordinates. For each recapture, straight-line distance traveled (km) and vector angle, measured as degrees from true north, were obtained by plotting the release and recapture position into a Maptech® digital Chartkit 2001 software program. This information was then used to calculate, for groups of fish, north–south and east–west velocities (km/day), a mean-vector angle (degrees relative to true north), angular deviation (degrees), and dispersion rates
(km2 /day), using the equations given in Saila and Flowers (1968). Rayleigh’s test (Zar, 1999) was used to examine the distribution of recapture locations of groups of interest (e.g. fish released in a particular place and time). If the z-statistic was insignificant (P > 0.05), the recapture locations were uniformly distributed around the release location, and movement was considered random. Alternatively, if the z-statistic was significant, the distribution of recapture locations was not uniform around the release location, and movement away from the release location was considered to be directed. Because fishing effort data on the small spatial scale reported in this study were not available, we used the percentage of recaptures in each area as a proxy for fishing effort, and adjusted the number of recaptures in each area accordingly into an ‘effort adjusted number of recaptures’ (Table 1). This adjustment was done by scaling the percentages to a common number (the lowest percentage of 1.25 found in area 139), and multiplying the actual number of recaptures by this scaled ratio. This adjustment procedure resulted in reductions of the number of recaptures ranging from 0% in the area with the lowest ‘effort’ (Area 139) to 78.7% in the area (133) with the highest ‘effort’ (Table 1). To determine spatial changes in the abundance of Atlantic cod in the western Gulf of Maine over time, catch-per-unit-effort (CPUE, kg/h) was calculated for each tow made to collect fish for tagging. Since fish were not weighed, weight was estimated from the following length-weight relationship (O’Brien and Munroe, 2001): loge weight (kg, live) = −11.7231 + 3.0521 loge length (cm) Because catch-per-unit-effort data are ratios and are asymmetric around the arithmetic mean, geometric means were calculated for each data set. Geometric means are more suitable for calculating confidence intervals for lognormal data (McConnaughey and Conquest, 1993). Mean lengths were calculated to examine changes in fish length over time within an area. Because in most cases the gender of the fish was not determined, mean lengths represent a combination of males and females. Differences in mean length between areas within month were analyzed using analysis of variance tests, followed by a Tukey’s test if significant differences (P < 0.05) were found.
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Fig. 2. Frequency distribution of number of recaptures vs. days at large.
3. Results A total of 154 days, between February 2001 and June 2003, were spent collecting and tagging. The distribution of days among areas and the number of cod tagged and recaptured are given in Table 1. Months when tagging occurred included February through December 2001, January through October 2002, and April through June 2003. Towing locations and months in the WGMAC were limited because a large portion of the area is unsuitable for trawling, and because of the seasonal abundance of lobster gear. A total of 27,772 cod were tagged in the 870 tows made. Of these, 1334 (4.8%) were recaptured prior to 31 December 2004, and reported with sufficiently detailed recapture location and date. It is these cod that are included in this study. Days at large ranged from 0 (recaptured the same day) to 1343. Most (59.7%) were recaptured within 120 days of being tagged and released (Fig. 2). The maximum linear distance between release and recapture was 362 km, but most (73.8%) were recaptured less than 30 km from their tagging location. There was no linear relationship (P = 0.48) between fish length and linear displacement (Fig. 3a), but displacement and days-atlarge was linearly related (P < 0.01). The relationship, however, was not strong, and days-at-large explained only 5.6% of the variation in linear displacement (Fig. 3b). Mean total lengths of tagged cod, for all combinations of sampling month and tagging area, are given in Table 2. Virtually all fish were adults, and there was some variation in mean size over time within all areas. Mean values ranged from 43.6 cm (Area 124 in June) to 80.8 cm (Area 133 in March). The simplest way to consider the movement of fish is to determine if they were recaptured within the same general location where they were tagged, or if they were recaptured in some other location. Although it was not possible to calculate effort adjusted recapture percentages for fish tagged in one area and recaptured in another, Table 3 shows the unadjusted percentage of cod released in each area that were recaptured in the same or different areas. In 3 of the 5 tagging areas (124, 132 and 133), the majority of tagged fish were recaptured in the area where
they were released. A small percentage (16.8%) of fish tagged in Area 124 moved into Area 132 to the north, but few if any moved into Areas 139 and the WGMAC. Although some fish tagged in Areas 132 and 133 were recaptured in each of the other areas, the greatest exchange was between these two areas, in an east-west direction. In areas 139 and the WGMAC, an equal or greater percentage of fish were recaptured outside the tagging area. Almost equal percentages of the fish tagged in Area 139 were recaptured in the tagging area (25.8%), in Area 133 to
Fig. 3. (a) Relationship between total length (cm) and linear displacement (km); (b) days at large and linear displacement by Atlantic cod tagged in the western Gulf of Maine.
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Table 2 Mean total length (cm) and standard error of cod tagged in the indicated area and months Month
Area 124
132
Mean January February March April May June July August September October November December
S.E.
133
Mean 66.0b
59.1b
0.7
56.1b 57.0b 43.6d 65.6a 48.2c 65.1a
0.3 0.3 1.0 1.1 0.9 0.9
56.5b
0.5
46.0d 70.2a 65.8ab 60.1c 58.6c 65.2b 56.6cd 52.3d 66.5ab 66.9ab
139
S.E.
Mean
S.E.
0.9 0.6 1.5 0.5 0.3 0.5 0.5 0.8 1.4 0.4 0.5
71.0b
0.4 0.7 1.1 0.4 0.2 0.6 1.4 3.6 1.6 2.9 0.6 0.5
70.0bc 80.8a 79.2a 79.3a 79.3a 76.6a 51.2d 68.1bc 72.0abc 66.5bc 66.5c
WGMAC
Mean
S.E.
56.0ab 68.8ab 68.7ab 77.8a 62.8ab 46.9c
8.7 3.1 0.6 0.7 1.7 2.9
59.2bc
4.2
70.4a
3.2
Mean
S.E.
67.3a 69.4a 65.8a 58.7b 50.0c 59.9b 60.2abc
0.7 0.6 1.1 0.9 1.4 1.3 4.8
Means within columns with the different letter are not significantly different (P > 0.05).
the southwest (25.8%), and in Area 132 to the south (29.0%). Data from the WGMAC were quite different than other areas. A relatively small percentage (2.3%) of fish were recaptured in the tagging area, presumably because most of the recaptures in the study came from commercial harvesters, and this area is closed to commercial fishing year-round. Some fish tagged in the WGMAC were recaptured in each of the other areas. Exchanges were greatest with adjacent areas to west (Area 132), southwest (Area 124), and northwest (Area 139). Taken together, the data in Table 3 suggest that although some exchange occurs between most of the areas, the movement of cod in the southwestern Gulf of Maine is quite limited. Movement statistics for groups of cod tagged and released in selected month/area combination, all years combined, and irrespective of recapture location, are provided in Table 4. Combinations from which we had <10 recaptures were not analyzed. Mean distances moved between release and recapture for the various groups ranged from 7.2 km (Area 132, August) to 65 km (WGMAC, April). Both N–S and E–W velocities for the groups were low, ranging from 0.0011 to 0.1956 km/day north–south, and from 0.0032 to 0.1509 km/day east–west. Dispersion coefficients (km2 /day) for the groups also varied by month and area of tagging, ranging from 1 (Area 132 released in August) to >49 km2 /day (Area 139 released in May). There were no spatial or temporal patterns in the mean angles of travel, and the angular deviations associated with the mean angles were quite large. Table 3 Percent recaptures in various areas associated with each tagging area Tagging area
124 132 133 139 WGMAC
Recapture area 124
132
133
139
WGMAC
“Out”
65.8 2.8 6.8 6.4 16.3
16.8 70.8 18.8 29 27.9
0.6 18.8 59.8 25.8 4.6
0 1.6 4.3 25.8 11.6
0 0.8 3 6.4 2.3
16.8 5.2 7.2 6.4 37.2
“Out” refers to recaptured made outside the general study area.
Only 10 of the 20 month/area tagging combinations had a significant Raleigh Z value (Table 4), and the results were almost certainly influenced by the distribution of commercial fishing effort. For example, directed movements away from the release location were most common among fish tagged in Area 133. Five of the seven groups showed directional movement, and in all cases it was towards the southeast quadrant. It is likely that these results may have been due, in part, to the lack of fishing to the west of the area, which includes inshore state waters where commercial fishing is prohibited. The data reported in Table 4 were calculated from those groups of fish where sufficient numbers were recaptured, independent of recapture date. To interpret movement patterns in the 30 days following tag and release, returns from specific months and areas of release were grouped (Table 5). There were six tagging month/area combinations from which enough fish (>10) were recaptured to conduct quantitative analyses of their movement during the 30 days after release. Mean distances traveled by the six groups ranged from 7.5 to 19.5 km over the 30 days, and directional movement was evident in only two of the six groups; those released in May and June in Area 133. In both cases the mean angle of travel was towards the southeast. One group of cod, tagged in Area 133 in May, provided enough recaptures to examine their position, relative to their release location, at 30 days intervals (Table 5). These analyses were done in an attempt to identify temporal changes in direction, distance, and dispersion. In each of the five 30 days intervals examined, the fish were located to the southeast of their release location. Distance from their release location increased with time, but even after 150 days, this particular group of fish was <65 km from their release location. Only 34 of the 1334 (2.5%) recaptured fish moved >100 km from their release location. Some of these long distance movers came from each of the five tagging areas, and in each of the areas they were recaptured in all directions from their point of release. Of the 34, three were recaptured off the west coast of Nova Scotia, 3 on or near Georges Bank, 7 along the northern Maine or New Brunswick, Canada coasts, 6 from the central
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W.H. Howell et al. / Fisheries Research 91 (2008) 123–132
Table 4 Summary of movement statistics of all cod released in the indicated month and area, regardless of recapture location Tagging month
Tagging area
Mean distance Mean angle of (km) travel (◦ )
Angular deviation (◦ )
North–south velocity (km/day)
East–west velocity Dispersion coefficient (km/day) (km2 /day)
Raleigh Z value
n
April May January April May June July August October January February April May June November December May June April May
124 124 132 132 132 132 132 132 132 133 133 133 133 133 133 133 139 139 WGMAC WGMAC
29.6 23 13.8 17.9 10.6 12 46.4 7.2 21.5 24.9 25.5 26.1 25.7 17 63.2 45.8 64.6 41.8 65 53.7
72.8 68.9 66.8 76.7 74.2 59.6 75.1 61.2 57.5 50.6 67.9 79.2 57.2 62.5 38.4 45.4 48.3 51.7 50.9 69.1
0.0437 S 0.0401 N 0.0132 S 0.0011 S 0.0129 S 0.0330 S 0.0794 S 0.0015 S 0.0390 S 0.0403 S 0.0036 S 0.0077 S 0.0424 S 0.0333 S 0.1648 S 0.1624 S 0.1956 S 0.1209 S 0.0900 N 0.0253 N
0.0585 E 0.0476 E 0.0035 W 0.0090 E 0.0097 W 0.0083 W 0.1182 E 0.0290 W 0.0174 W 0.1509 E 0.0203 E 0.0367 E 0.0838 E 0.0460 E 0.0860 E 0.0869 E 0.0476 W 0.0399 W 0.0707 W 0.0901 E
2.35 3.45* 1.22 0.37 2.13 12.97*** 0.4 2.2 5 8.16*** 0.88 0.3 131.43*** 6.21** 6.02*** 5.65** 6.65*** 4.21** 8.81*** 0.96
64 45 12 34 83 57 20 12 10 22 10 149 521 38 10 12 16 12 24 13
186.5 21.8 283.3 223.5 255.1 176.6 248.3 220.6 229.3 103.9 126.1 53.9 120.1 133.6 160 148.6 196.3 196.2 320.9 62.9
34.5 17.1 3.2 4.1 3.6 4.0 20.8 1.0 7.2 16.0 16.6 18.3 28.2 19.7 14.2 19.1 49.3 40.0 44.7 23.0
Tabulated results are a combination of all years combined. n is the number of recaptures from those released in the indicated month/area. Significant Raleigh Z values are indicated (* P < 0.05; ** P < 0.01; *** P < 0.001).
Gulf of Maine, and the remainder from locations east or south of Cape Cod, Massachusetts. Although there seems to be little pattern in the directional movement of cod in our study area, the temporal and spatial changes in abundance indicate that some movements are occurring. Catch-per-unit-effort (kg/h) values for the various month/area combinations are given in Fig. 4. Because of the small number of tows, and the inherent patchy distribution of schooling species such as cod, mean CPUE values were quite variable. Seasonal coverage was incomplete in all areas except 133, so we cannot be certain of seasonal patterns in CPUE in the other areas. There are, however, sufficient data to speculate on the seasonality of CPUE in all areas (Fig. 4). Mean values ranged from as low as 2.3kg/h (Area 139 in March), to more than 1400 kg/h (Area 133 in May). In general, the lowest values were
seen in Areas 139 and the WGMAC, and the highest values were seen in Areas 132 and 133. In Area 132, which was sampled in all months except December, there appeared to be two seasonal peaks in abundance. The first, and highest, was seen in October and November, and this was followed by a dramatic decline from January through March. CPUE then increased again in April, fluctuated through August, and declined to nearly its lowest level of the year in September. A bimodal pattern was also seen in Area 133, which was sampled in every month. Here, the highest mean CPUE values were found in April, and especially May, and these fish had the largest mean lengths of any of the groups we caught (Table 2). Mean CPUE in Area 133 then declined in June, and fluctuated at relatively low levels through October. Mean CPUE then rose again in November, to reach a second, but smaller annual peak in December and
Table 5 Data for cod tagged and released in the indicated month and area, and recaptured within a relatively short time Tagging month
Tagging area
Recaptured within (day)
Mean angle of travel
Mean distance Angular (km) deviation
North–south velocity
East–west velocity
Dispersion coefficient
Raleigh Z Value
n
April April May May May June May May May May May
124 133 124 132 133 133 133 133 133 133 133
0–30 0–30 0–30 0–30 0–30 0–30 0–30 31–60 61–90 91–120 121–150
179.8 312.3 37.7 279.7 128.5 155.4 128.5 118.8 119 124.8 116.5
12.1 13.1 19.5 7.5 10.1 11.6 10.1 31.2 39.3 40.8 64.3
0.0851 S 0.2145 N 1.1832 N 0.1055 S 0.3884 S 0.6140 S 0.3884 S 0.1928 S 0.2467 S 0.1549 S 0.0963 S
0.4954 E 0.1506 W 0.4324 W 0.2802 W 0.3366 E 0.4299 E 0.3366 E 0.4365 E 0.4011 E 0.2532 E 0.3788 E
21.0 4.4 0.0 3.3 16.9 13.5 16.9 3.6 1.3 0.8 1.8
2.4 0.3 0.2 2.1 131.4*** 6.2** 131.4*** 1.8 0.9 0.6 0.2
22 35 23 22 252 16 252 48 26 23 11
77.9 76.1 71.3 78.4 74.9 72.9 74.9 78.6 79.3 79.6 80.3
Mean angle is relative to due north, velocities are km/h, dispersion coefficients are km2 /day, and n is the number of recaptures. ** P < 0.01; *** P < 0.001.
W.H. Howell et al. / Fisheries Research 91 (2008) 123–132
129
Fig. 4. Mean catch-per-unit-effort (kg/h) in the indicated months and areas. Vertical bars represent 2SE. Shaded bars represent months that the area was closed to commercial fishing.
January. It is clear from the CPUE data that there are temporal and spatial changes in cod abundance within our study area. There is no clear correspondence however, between the directional movements of the fish, as indicated by mark-recapture, and their changing abundance. The monthly percentages of ripe cod in Area 133, which is well known as an important cod spawning ground (Bigelow and Schroeder, 2002; Ames, 2004), are shown in Fig. 5. Results provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July (Fig. 5). The presence of two cod spawning groups utilizing this area has long been suggested by fishermen, and it has been recently been shown that the two groups are genetically distinct (Wirgin et al., 2007). Morin (unpublished data) also found that the winter group had a significantly shorter mean length than the spring group. This study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Seven hundred and thirty-three fish, tagged during the spawning months of April and May in Area 133 were
recaptured. A plot of mean distance from the tagging location against days at large is shown in Fig. 6. Although there is some variation, mean distances from the spawning ground tended to increase during the first 6–8 months, to reach approximately 70 km. Mean distance then decreased over the following 4–6
Fig. 5. Percentage of ripe cod in Area 133 in the indicated sampling month.
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W.H. Howell et al. / Fisheries Research 91 (2008) 123–132
Fig. 6. Distance from release location vs. days-at-large for those cod released in Area 133 in April and May.
months, such that the mean distance from the spawning ground where the fish were tagged was only about 20 km one year after tagging. A similar pattern was shown in cod at large for more than a year (Fig. 6). This movement away from, and then back towards, the spawning location is interpreted as natal homing behavior. A similar cyclic pattern may be indicated in Fig. 3b, which includes all cod, independent of tagging location and date. It should be noted, however, that the pattern seen in Fig. 3b is influenced by the large number of fish, in the more appropriate subset, depicted in Fig. 6. 4. Discussion The movements of scores of cod populations around the world have been examined. Several studies, including two from Stellwagen Bank in the southwestern Gulf of Maine (Groger et al., 2007; Lindholm et al., 2007), have found that groups of cod in different localities are composed of a mixture of both resident and migratory fish (Neat et al., 2006; Wright et al., 2006; Svedang et al., 2007). The fact that a small portion of the fish tagged in this study moved long distances, suggests that this is probably true in our study area as well. After reviewing many of these studies, Robichaud and Rose (2004) proposed four migratory behavioral categories based on the degree of site fidelity and homing. “Sedentary” populations are found year-round in a relatively small geographical area. “Accurate homers” display seasonal movements and home to a relatively small area, and “inaccurate homers” display seasonal movements and home to a much broader area. The fourth category (“dispersers”) includes populations that move and spawn in a haphazard pattern over large geographical areas. Results of this study indicate that the group of cod in the southwestern Gulf of Maine can be categorized as “sedentary resident”. In most areas, the cod were recaptured within the same area they were released (Table 3), and most (73.8%) traveled less than 30 km. Further, only 3% of the recaptures were from distances >100 km from the point of release. It should be noted, however, that these finding might have been influenced by the short time at large for many of the fish. Another indication of the sedentary nature of these fish was the low average rate of linear displacement (∼0.2 km/day).
This is lower than the range of rates (2–66 km/day) reported by Groger et al. (2007) for cod on Stellwagen Bank in the western Gulf of Maine. Their rates, however, were calculated from swimming speed rather than observed displacement, while our rates were calculated from linear displacements of groups of fish. Our low average rate of displacement may have been due to two factors. One was the inclusion of fish that were at large for a limited number of days. The other was that many of our fish were tagged on the spawning grounds, and it has been shown that schools of spawning cod remain relatively immobile, exhibiting a daily displacement of only 1–2 km (Rose, 1993). Although it could be argued that time at large, which was relatively short for many tagged fish, influenced these findings, we found only a very weak positive relationship between linear displacement and time at large (Fig. 3b). Similar local movements have been found in many groups of cod studied, and are often associated with reproductive cycles, feeding, site fidelity, and/or seasonally changing water temperatures (Wise, 1963; Campana and Simpson, 1984; Rose, 1993; Taggart et al., 1998; Hunt et al., 1999; Robichaud and Rose, 2001, 2002, 2004; Neat et al., 2006; Wright et al., 2006; Groger et al., 2007; Lindholm et al., 2007). The limited movements of most of the fish in this study, the small percentage that moved greater distances, and their probable homing, indicates that this group of cod probably has limited spatial and temporal overlap with other groups of cod in the western Gulf of Maine, and thus constitutes a subpopulation. Although this subpopulation appears to remain in the general area year-round, there are local movements that result in small-scale seasonal changes in distribution. The CPUE data indicates that the fish become concentrated in particular areas and times, and then disperse. Thus it is likely that cod movements in the western Gulf of Maine, at least at certain times of the year, are directed. Based on seasonal changes in abundance, the area undergoing the greatest seasonal change is 133, with a strong movement into the area in both the spring (April/May) and winter (December/January), followed by movement out of the area in the succeeding months. Because these movements are associated with spawning (Fig. 5), and because some cod populations return to the same spawning areas each year (Rose, 1993; Robichaud and Rose, 2001, 2002; Svedang et al., 2007), it seems likely that pre-spawning movement into the area is directed. Post-spawning movements out of the area may be either dispersive or directed. Distance from the spawning area increases over the following 6–8 months (Table 5, Fig. 6), to a mean distance of 70–90 km. In the following 4–6 months mean distance decreases, which we interpret as a homing migration associated with reproduction. Similar homing behavior, combined with site fidelity, has led to a mosaic of small, genetically distinct cod populations in Norway (Fevoldon and Pogson, 1997), Newfoundland (Ruzzante et al., 1998, 1999; Beacham et al., 2002), Labrador (Morris and Green, 2002), and Iceland (Jonsdottir et al., 1999). Results of this study, as well as those of Wirgin et al. (2007), suggest that a similar mosaic may exist in the western North Atlantic. The time/area closures referred to in this study were established to reduce the fishing mortality of Gulf of Maine cod by
W.H. Howell et al. / Fisheries Research 91 (2008) 123–132
protecting dense aggregation of fish. At present, Area 124 is closed to commercial fishing in April and May, and again in October and November. Although our study lacks data for October, CPUE was relatively high in April, May and November (Fig. 4), and thus these months of closure appear to be appropriate. Areas 132 and 133 are presently closed in April, May and June. In Area 133, CPUE was relatively high in April and May, but not in June (Fig. 4), which suggests that the June closure may not be contributing as much to the desired conservation effects as the other two months. We note, however, that this is an important spawning area for cod in the spring, so the June closure is almost certainly protecting some spawning fish. CPUE in Area 132 (Fig. 4) was relatively low during the months of closure (April, May and June). In fact, mean CPUE values were 2–4 times higher in October and November. Thus there is an apparent disconnect between months in which the grounds were closed and months of highest abundance. Protecting the cod in this area in April, May and June, may however, be important because this area may serve as a pre-spawning staging area (Morin, unpublished data). Area 139 is closed in May and June. These two months correspond with the months of highest CPUE (Fig. 4), and thus the temporal closure appears appropriate. It is worth noting, however, that mean CPUE values in this area (<175 kg/h) were low relative to other areas, suggesting that the conservation value of this temporal closure is less than those of other areas. The WGMAC is closed to commercial groundfish fishing year-round. We were unable to sample the WGMAC in all months, and thus we do not have a clear idea of seasonal changes in abundance. In the months we did sample (April through October), both CPUE and mean length decreased steadily as the year proceeded. Very few ripe cod were caught in the area in the months we sampled, leading us to conclude that the WGMAC is not an important cod spawning area in the southwestern Gulf of Maine; particularly relative to the very large aggregations of spawning fish found in Area 133 in the spring. The size of closed areas relative to the movements of the species they are designed to protect is also important (Appeldoorn, 1997; Ingram and Patterson, 2001). The time/area closures in this study are each about 2300 km2 , and the WGMAC is about 3100 km2 . Because the dispersion coefficients (0–21 km2 /day) and mean distances traveled (7.5–19.5 km) in the 30 days following release (Table 5) were both small relative to closed area size, it is likely that the time/area and permanent closures are large enough to protect the aggregations of cod. 5. Conclusion In summary, the group of cod in the southwestern Gulf of Maine can be classified as sedentary and resident, staying within the general area year around. There are however, local seasonal movements, almost certainly associated with spawning, that result in dense aggregations of fish in certain areas and times. Time/area closures currently in place appear to be appropriate, in that they are closed to fishing in those months when cod abundance is highest.
131
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