Effects of a codend mesh size increase on size selectivity and catch rates in a small-mesh bottom trawl fishery for longfin inshore squid, Loligo pealeii

Fisheries Research 108 (2011) 42–51

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Effects of a codend mesh size increase on size selectivity and catch rates in a small-mesh bottom trawl fishery for longfin inshore squid, Loligo pealeii Lisa C. Hendrickson ∗ U.S. National Marine Fisheries Service, Northeast Fisheries Science Center, Woods Hole, MA 02543, United States

a r t i c l e

i n f o

Article history: Received 25 March 2010 Received in revised form 22 November 2010 Accepted 23 November 2010 Keywords: Codend mesh selectivity Loligo pealeii Illex illecebrosus Peprilus triacanthus Merluccius bilinearis Bycatch reduction

a b s t r a c t Loligo pealeii (longfin inshore squid) co-occurs with Atlantic butterfish (Peprilus triacanthus) throughout the year and discarding in the L. pealeii bottom trawl fishery is the primary source of fishing mortality on the butterfish stock. Consequently, a codend mesh size increase in the Loligo fishery has been proposed as a management measure to minimize discarding of butterfish and other bycatch species. A paired-tow study was conducted using a Loligo twin trawl to assess the effects of a codend mesh size increase, from 50 mm to 65 mm (inside stretched mesh), on catch rates and size selection of the target and bycatch species. Relative mesh selection factors estimated from a SELECT model were: 1.7, 1.5, 2.2, and 3.0 for L. pealeii; P. triacanthus; Illex illecebrosus (Northern shortfin squid); and Merluccius bilinearis (silver hake), respectively. Catches of butterfish and silver hake in the 65 mm codend were reduced by 58% and 41% in terms of numbers, respectively. However, a larger mesh size would be necessary to allow 50% escapement of the median sizes of mature silver hake and butterfish. A trade-off associated with the bycatch reductions is a 29% loss in the catch weight of the target species. However, the reduction in ex-vessel value of Loligo catch is probably not proportional to the percent loss in Loligo catch because most of the loss consisted of squid from the smallest market size categories which have the least value and are primarily discarded. In addition, a greater percentage of large, more valuable squid was caught in the 65 mm codend. The September study results likely represent a worst-case scenario with respect to Loligo catch loss because the monthly mean body size of Loligo tends to be smallest during September. Most catches of the three bycatch species evaluated herein and Loligo smaller than 10 cm are discarded, most likely dead, in the Loligo fishery. Therefore, a codend mesh size increase to 65 mm should provide some conservation benefits to these stocks if a portion survive escapement. On a fleet-wide basis, the magnitude of bycatch reductions and Loligo catch loss will vary depending on seasonal changes in mean body size, vesselspecific gear characteristics and fishing practices. Published by Elsevier B.V.

1. Introduction A small-mesh bottom trawl fishery for longfin inshore squid, Loligo pealeii, occurs throughout the year on the continental shelf of the east coast of the United States between Cape Cod, Massachusetts and Cape Hatteras, North Carolina (approximately 35◦ –42◦ N). L. pealeii co-occurs with Atlantic butterfish, Peprilus triacanthus, throughout the year (Lange and Waring, 1992; MAFMC, 2009). There is currently no directed butterfish fishery and discarding of butterfish has been exacerbated due to the lack of a stable market. Discarding in the L. pealeii fishery is the primary source of fishing mortality on the butterfish stock (NEFSC, 2010). Approximately 26%, by weight, of the total catch in the L. pealeii fishery is discarded (Glass et al., 1999), including commercially fished species

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such as: Merluccius bilinearis (silver hake); Illex illecebrosus (Northern shortfin squid); and Urophycis chuss (red hake); in addition to P. triacanthus (MAFMC, 2009). The Loligo fishery is also the primary source of L. pealeii discards, which averaged between 3% and 6% of the Loligo landings during 1989–2000 and consisted primarily of small individuals <10 cm in dorsal mantle length (NEFSC, 2002). Historically, a minimum codend mesh size of 60 mm (inside stretched mesh) was implemented as a bycatch reduction measure in the foreign L. pealeii fishery that occurred along the eastern USA coast, along with limiting squid fishing (i.e., for Illex illecebrosus and L. pealeii) to offshore areas during specific months (ICNAF, 1978). These foreign squid fleets were also subject to a butterfish bycatch quota (Lange and Waring, 1992). After 1987, the squid fisheries became entirely domestic (NEFSC, 2006), and since 1997, the L. pealeii fishery has been subject to minimum mesh sizes of 48 mm and 114 mm (inside stretched mesh) for the codend and strengthener, respectively (MAFMC, 2009). An increase in the minimum codend mesh size is slated for implementation as a management

L.C. Hendrickson / Fisheries Research 108 (2011) 42–51

measure designed to minimize discarding of butterfish and other bycatch species in the Loligo fishery (MAFMC, 2009). The mesh size increase to 54 mm is small because of concerns about the amount of Loligo catch loss associated with a larger mesh size increase. There are no published mesh selection factors (length at 50% retention divided by mean codend mesh size) for catches of L. pealeii or any of the bycatch species in L. pealeii bottom trawls and the effects of a codend mesh size increase on catch rates of the target and bycatch species are unknown. A codend mesh size increase in L. pealeii fishery may be an effective and feasible gear-based solution to bycatch reduction if the loss of marketable Loligo catch is low and the retention of butterfish and other bycatch species is substantially reduced. In reality, the effectiveness of a codend mesh size increase as a finfish bycatch reduction measure will primarily be limited by the amount of Loligo catch loss that is acceptable to Loligo fishermen. Butterfish are short-lived and grow rapidly; most individuals do not live beyond age three (Cross et al., 1999). The median age at maturity is one year and the median length at maturity is 120 mm (O’Brien et al., 1993). L. pealeii is also short-lived, with a lifespan of less than a year and rapid growth rates which vary by seasonal cohort (Brodziak and Macy, 1996; Macy and Brodziak, 2001). Spawning occurs throughout the year, but it is primarily the summer-hatched squid that support the winter, offshore fishery and the winter-hatched squid that support the summer, inshore fishery (Macy and Brodziak, 2001). Median length-at-maturity for this semelparous species varies widely depending on hatch season and summer-hatched squid mature at smaller sizes than winterhatched squid (Macy and Brodziak, 2001). There are no minimum landings sizes established for either the target or bycatch species in the L. pealeii fishery. Therefore, given management concerns about butterfish discarding, an experimental codend mesh size close to the median length at maturity for butterfish was selected for testing during the subject study. A mesh selection factor of 1.8, determined to be constant across a codend mesh size range of 61–75 mm (Myer and Merriner, 1976), indicates that a codend mesh size of 67 mm would be required to allow 50% escapement of 120 mm butterfish from a pound net. However, a larger codend mesh size may be necessary to allow this same level of escapement from a Loligo trawl because, unlike pound nets, diamond meshes in trawl codends become constricted under load (Robertson and Stewart, 1988). Given these concerns and practical considerations, an experimental codend mesh size of 70 mm (nominal, diamond mesh) was chosen for testing against the predominant codend mesh size currently in use by the Loligo fishery (60 mm, nominal mesh size). The objectives of the study were to estimate mean contact-selectivity curves for the target and bycatch species and to quantify bycatch reductions and Loligo catch loss associated with the codend mesh size increase. An additional objective was to determine how the mesh size increase might affect the Loligo fleet with respect to Loligo landings and ex-vessel value.

2. Materials and methods 2.1. Sea trials A paired-tow study was conducted with a 24-m twin trawler from the L. pealeii fishing fleet, the F/V Karen Elizabeth, during September 22–October 2, 2008. The twin trawl method was used because of the technical difficulties associated with using the covered codend method on large, high-opening Loligo nets. Both bottom trawls were identical two-seam, two-bridle “rope trawls” used in the L. pealeii fishery and were constructed by Superior Trawl of Point Judith, Rhode Island. Gear and vessel characteristics are provided in Table 1. Detachable codends, manufactured specifi-

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Table 1 Characteristics of the two-seam, Loligo pealeii twin trawl and vessel used to conduct the study.a Doors Bridles Backstraps Ground cables Headrope Footrope Net mouth circumference Wings Belly Extension Control codend Experimental codend Codend strengthener Chafing gear Gross registered tonnage, vessel length overall (m) Engine horsepower Trawl monitoring system a

Thyboron Type IV 213.4 cm, 720 kg per door 73.2 m 9.1 m 36.6 m of 22 mm wire with 6 cm rubber cookies 42.64 m Tenex with 92 20.3-cm floats 48.25 m of 16 mm SS wire with 6 cm rubber cookies 288 meshes × 40 cm; 240 cm spectra face 240 cm of 11 mm Polytron First bottom belly is 240 cm of 11 mm Polytron graduating to 60 mm of 1.1 mm Dyneema None 60 mm diamond polyethylene (3.0 mm) hanging ratio = 0.4 70 mm diamond polyethylene (2.5 mm) hanging ratio = 0.4 10.6 m × 4.8 m of 16 cm square mesh 14 mm Polytron wrapped with 19 mm polyethylene 40 × 50 meshes of 165 mm diamond double 5 mm polyethylene 156 GRT, 24 m 705 Simrad ITI

All mesh sizes are nominal knot-center values.

cally for the study, were laced onto the body of each net with a rope woven through plastic rings to facilitate switching the control and experimental codends between port and starboard sides of the vessel in a pseudo-random fashion. The control codend (Net C) and experimental codend (Net E) were constructed of 60 mm and 70 mm diamond mesh polyethylene (nominal, knot-center), respectively. Both codends were 3.4 m in diameter and 12.0 m in length. The codend strengtheners were identical and constructed of 16 cm square mesh Polytron (nominal knot-center) wrapped with 19 mm polyethylene twine. Actual mesh sizes of the codends and strengtheners were determined as the mean of ten random, inside stretched mesh measurements obtained to the nearest mm with vernier calipers. Hereafter, mesh sizes refer to inside stretched mesh measurements unless otherwise noted. Sampling stations were selected by the captain with the objective of simultaneously capturing butterfish and Loligo during each tow. All tows were conducted by the same individual to ensure consistent setting and hauling of the gear. A total of 70 paired tows were conducted between sunrise and sunset. Sampling stations were located between Montauk, New York and Ocean City, Maryland (38◦ 15 –40◦ 45 N) at depths ranging between 60 m and 134 m. The gear was towed for 1 h, between net touchdown and lift-off, at 5.4 km per hour (2.9 knots). Speed over the bottom, measured to the nearest 0.1 knot, was recorded from a Furuno GPS unit at tensecond intervals. Net touchdown and lift-off were determined from the Simrad ITI screen display of the trawl location, the position of which was logged at 10-s intervals. Port and starboard doorspread readings from the Simrad ITI were updated and recorded, to the nearest 0.1 m, at 10-s intervals. Total catch weights of each species were recorded for each net, to the nearest 0.01 kg, using a Marel M1100 scale. L. pealeii catches were boxed for sale and the weight per tow was recorded as the sum of the tared weights of the boxed catches. Length composition data were collected for the following commercial species: L. pealeii, P. triacanthus, M. bilinearis, and I. illecebrosus. Hereafter, the four species will be referred to as Loligo, butterfish, silver hake and Illex. Length measurements were collected during every tow, by net, from random subsamples of approximately 200 individuals, for butterfish and Loligo, and approximately 100 individuals for Illex and silver hake. Lengths were measured to the nearest mm using

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L.C. Hendrickson / Fisheries Research 108 (2011) 42–51

a Limnoterra digital measuring board connected to a laptop computer. Fork length (FL) was recorded for butterfish and silver hake and dorsal mantle length (DML) was recorded for the two squid species. For each tow and net, species-specific catch numbers-at-length were computed as the subsampled numbers-at-length multiplied by the species-specific ratio of catch weight to subsample weight. Catch numbers per tow were computed for each net as the sum of the expanded numbers-at-length of each species. The catch rates of each net were standardized for the area swept by the trawl during each tow. Swept area was computed as the product of the average doorspread, between net touchdown and lift-off, and tow distance. Tow distance was computed as the sum of the GPS tow distance measurements recorded between net touchdown and lift-off. 2.2. Comparison of experimental and Loligo fleet towing regimes and codend mesh sizes Data collected by fishery observers from the Northeast Fisheries Observer Program (NEFOP), during 1997–2008, were summarized to compare tow duration and towing speed, as well as codend and strengthener mesh characteristics of the Loligo fleet, with those used in the study. NEFOP observers collected ten random mesh size measurements of the codend strengthener and one to four measurements of the codend (inside stretched) measured to the nearest mm with vernier calipers. The means of these mesh size measurements were used in the analysis. The Loligo fleet was defined using the regulatory definition of a directed trip which includes vessels landing greater than 1134 kg of Loligo per trip. Loligo catches from these trips were graphed as proportions, by 2 mm and 5 mm mesh size intervals, to characterize codend and strengthener mesh size characteristics of the fleet, respectively. The codend mesh size composition of the fleet was also compared to the codend mesh size composition of all bottom trawl trips in the NEFOP database which caught butterfish during 1997–2008. 2.3. Estimation of selectivity parameters The SELECT model (Share Each LEngth’s Catch Total) was used to estimate selectivity parameters and the relative fishing intensity of the experimental net (Millar, 1992; Millar and Walsh, 1992). The model uses a maximum likelihood estimation approach where the expected proportion of the total catch (in both nets), for length class l, that was caught in the experimental codend, (l), is modeled as a function of the parameters a, b, ı, and the relative fishing efficiency (p) of the gear (often called the “split” parameter) such that: (l) =



p exp(a + bl) (1 − p) + exp(a + bl)

1/ı (1)

where the parameter ı quantifies the amount of asymmetry and is equal to 1 for a logistic curve. Selectivity parameter estimates were obtained by fitting the SELECT model to the combined hauls catch-at-length data, binned as the midpoint of 5 mm intervals, with the use of the “ttfit” function in the “Trawlfunctions” programs for R (Millar et al., 2004). Model fits were assessed using model deviances and degrees of freedom calculated using length classes with expected catches greater than three in each net and from examination of the deviance residuals plotted by length class. The combined hauls approach was used to account for between haul variability and to estimate the standard errors of the selectivity parameter estimates. The model was fit, using the “Rep.ttfit” function in the “Trawlfunctions” programs (Millar et al., 2004), to the stacked haul data to calculate a replication estimate of overdispersion called REP, computed as the Pearson chi-square statistic from the model divided by the degrees of freedom (McCullagh

and Nelder, 1989). When present, between-haul variation was accounted for by√multiplying the standard errors of the parameter estimates by REP (Millar et al., 2004). Other methods can also be used to account for between-haul variability (Millar and Fryer, 1999). Nonlinear mixed-effects models similar to those described in Millar et al. (2004) were investigated but are not presented here because of convergence problems. This difficulty has been noted by others (Millar et al., 2004; Millar, 2010). In cases where mixed-effects models could be fit, standard errors of the selectivity parameter estimates were slightly lower, but the parameter estimates were similar to those from the combined hauls approach. In view of these problems, and because the final estimates were robust to the modeling approach, the combined hauls method was used in the subject study. The combined hauls approach was robust to the variability in the data, allowed weighting of individual hauls by the size of the catch, and satisfied the study objective of estimating a mean contact–selectivity curve that relates to the Loligo fishery (Millar and Fryer, 1999; Millar, 2010). 2.4. Evaluation of catch rate differences Paired-comparisons t-tests were conducted using PROC TTEST in SAS (SAS Institute Inc., 1999) to determine whether the mean differences in standardized catch rates, between the control and experimental codends, were significantly different from zero for each of the four species. Catch rates were standardized for area swept by the trawl then log-transformed prior to analysis. In addition, Wilcoxon signed rank tests for paired differences were also conducted using the Wilcox.test function in R. For species that exhibited reductions in bycatch, the effectiveness of the codend mesh size increase was evaluated in terms of the percent reductions at sizes above and below the median length at maturity for females; 120 mm for butterfish and 232 mm for the southern stock of silver hake, respectively (O’Brien et al., 1993). Three additional analyses were conducted to evaluate potential impacts of the codend mesh size increase on the Loligo fishery. The analyses utilized data from the study as well as Loligo landings and length data obtained from the Commercial Fisheries Database System maintained by the Northeast Fisheries Science Center of the National Marine Fisheries Service. The analyses included data for 1997–2008 because reporting of Loligo landings and purchase prices (i.e., ex-vessel value) were mandatory during this period. As described in Section 2.2, the Loligo fleet was defined using the regulatory definition of a directed trip. Throughout the year, field staffs from the National Marine Fisheries Service randomly sample the length composition of Loligo landings by market size category. Samples consist of at least 100 squid per market size category and averaged one sample per 98 mt during the 1997–2008 study period. Dorsal mantle lengths were measured to the nearest cm. Although there is some overlap in the length ranges of market size categories, Loligo frozen at sea are generally classified by squid dealers as: #6 (<8 cm), #5 (8–12 cm), #4 (12–16 cm), #3 (16–20 cm) and #2 (>20 cm). Taking into account market size categories and the predominant size range of Loligo discarded by the directed fishery, individuals <10 cm DML, the loss of Loligo catch was evaluated based on the following length bins: <10 cm, 10–12 cm, 13–16 cm, 17–20 cm and >20 cm. Length samples which were not classified by market size category were prorated to assign a market category, using a Loligo length–weight equation derived from the Northeast Fisheries Science Center (NEFSC) autumn bottom trawl surveys (Lange and Johnson, 1981). The first of the three analyses to determine the effects of the codend mesh size increase on the Loligo fishery involved quantification of Loligo catch loss, by market size category, using data from the study. Loligo catch weights from the study were converted to weight-at-length data using the Lange and Johnson (1981)

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length–weight equation noted above. Catch numbers- and weightat-length data were partitioned into the five market category length bins previously described then summed by station, net, and market category. Paired-comparisons t-tests were conducted on the log-transformed catches. A second analysis involved estimation of the theoretical loss in Loligo landings of the directed fishery if a codend mesh size of 65 mm had been in effect during September–October of 2004–2008. The Loligo selectivity curve from the study was applied to the September–October length composition of the 2004–2008 Loligo landings from the fishery (N = 18,914 length samples) to compare the landings length composition for the current codend mesh size of 50 mm with the theoretical length composition for the 65 mm codend mesh size. To evaluate one component of the economic impacts that a 65 mm codend mesh size may have on the Loligo fleet, the ex-vessel value of Loligo landings from the study were compared with the realized average ex-vessel values of Loligo landings by the fleet, by market size category, during September–October of 2004–2008. A third analysis evaluated the association between percent loss in Loligo catch and monthly mean body size by comparing the monthly mean body size of landed Loligo during the September study period with mean body size during the rest of the year. During 2000–2008, the Loligo fishery was closed one or more times per year because the quarterly or trimester quota allocations were harvested. Therefore, in order to avoid potential biases in monthly mean body size calculations, due to a lack of length samples during fishery closures, landings data for 2004 and 2007 were used in the analysis because the fishery was closed for the least amounts of time, during portions of March and April, respectively (MAFMC, 2009).

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Fig. 1. Proportions of Loligo catch in the directed bottom trawl fishery and butterfish catch in all bottom trawl trips sampled by fishery observers during 1997–2008, by codend mesh size (top panel), and proportions of Loligo catch in the directed fishery by strengthener mesh size and shape (bottom panel).

database also indicates that girth ropes and chafing gear are used in the fishery. 3. Results 3.2. Selectivity parameter estimates 3.1. Comparison of experimental and Loligo fleet towing regimes and codend mesh sizes The modal towing speed of the F/V Karen Elizabeth during the field trials (5.6 km/h) was identical to that of the Loligo fleet. A onehour tow duration lies near the low end of the commercial tow duration range, 1.0–5.2 h with a mode of 3.0 h, but was used during the study to maximize the number of tows sampled during daylight. The directed fishery occurs primarily during daylight because Loligo schools are located near the seabed and most available to bottom trawls (Sissenwine and Bowman, 1976; Brodziak and Hendrickson, 1999). During the study, the average tow distance was 5.7 km and the average area swept by the trawl was 0.42 km2 . The mean inside stretched mesh sizes of the control and experimental diamond mesh codends measured 50.4 mm (SD = 1.58) and 64.9 mm (SD = 0.88), respectively. Fishery observer data indicate that most of the Loligo catch by the directed fishery is harvested with 49–54 mm diamond mesh codends and that this is the same codend mesh size range within which most of the butterfish catch occurred in all types of bottom trawl trips sampled (Fig. 1). The mesh size of the control codend used in the subject study is similar to the modal codend mesh size used by the Loligo fleet (51–52 mm). Both diamond and square mesh strengtheners are used in the Loligo fishery and most mesh sizes are larger than the regulatory minimum of 114 mm. Most of the Loligo catch is taken with 141–160 mm, double-twine diamond mesh strengtheners, but 20% of the catch is also taken using square mesh strengtheners which are predominately of the type and size (245 mm, SD = 1.8) used in the study. These large, square mesh strengtheners are purportedly used: (a) to increase water flow through the codend in an effort to reduce bycatch, and therefore reduce catch sorting time, and (b) because the wrapped meshes increase the functional longevity of the strengthener and are therefore more cost-effective. The NEFOP

A total of 69 out of 70 paired tows were successfully conducted. Tow 33 was not completed due to a major doorspread problem. Mean port and starboard doorspread values, 72.3 m (SE = 0.65) and 75.3 m (SE = 0.73), respectively, were not significantly different. However, tows 14, 22, 26, and 58 were omitted from all analyses because the differences between the port and starboard doorspread values exceeded the standard deviation of the doorspread differences, 6.1 m. The numbers of tows included in the selectivity analyses were 65 for Loligo and 61 for butterfish. Illex and silver hake were caught less frequently and analyses of these species were based on 58 and 19 tows, respectively. Analyses of silver hake catches did not include tows 22–70 because catches in both nets were very low, ranging between 0 and 21 individuals. The numbers of butterfish, silver hake and Loligo caught in the 65 mm codend were reduced over those in the 50 mm codend, but the Illex catches were slightly greater (Fig. 2). Catch length compositions of all species except butterfish were unimodal for both nets. Although the size ranges of butterfish were similar for catches in both codends (35–202 mm), the length composition was unimodal for the control codend and bimodal for the experimental codend. For all four species, the 65 mm codend caught a smaller proportion of small individuals and a greater proportion of large individuals than the 50 mm codend. The 65 mm codend caught a higher proportion of butterfish larger than 140 mm and Loligo larger than 120 mm. For silver hake and Illex, the 65 mm codend caught a slightly greater proportion of individuals larger than 217 mm and 160 mm, respectively. For all four species, logistic models provided the best fits to the data. Maximum likelihood parameter estimates for the logistic selectivity curves and goodness of fit measures from the Rep.ttfit models runs are presented in Table 2. The reported selection param-

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Fig. 2. Length frequency distributions of Loligo, butterfish, Illex and silver hake catches in the control (Net C, 50 mm diamond mesh) and experimental codends (Net E, 65 mm diamond mesh).

eter estimates should be interpreted as relative (Wileman et al., 1996) rather than absolute (i.e., the estimated selection factors should not be used to estimate retention lengths for other mesh sizes) because the control net did not exhibit 100% retention of all size classes. For Loligo, truncation of the length classes to those ≤297.5 mm reduced the positive trend in the deviance residuals for the rarer, largest size classes. The remaining residuals trends were generally positive and values exceeded 2 for multiple length classes (Fig. 3A). Both nets fished with nearly equal fishing efficiency (p = 0.49). The length at 50% retention (L50 ), selection factor (SF), and selection range (SR) estimates were: 112 mm, 1.7 mm, and 36 mm, respectively. The best model fit to the Illex data included a length class range of 87.5–262.5 mm. There was a large negative trend in the deviance residuals for most length classes >222.5 mm, in addition to several

large values (Fig. 3B). The relative fishing efficiency of the 65 mm codend was 0.61. L50 , SF, and SR estimates were: 143 mm, 2.2 mm, and 42 mm, respectively. For the silver hake data, the model with the best fit included a length class range of 172.5–282.5 mm. There was no trend in the deviance residuals, but there were several large values (Fig. 3C). Relative fishing efficiency of the 65 mm codend was 0.43. L50 , SF and SR estimates were: 196 mm, 3.0 mm, and 16 mm, respectively. For butterfish, estimation of the parameter p was not possible without applying a correction factor of 1.3, the ratio of the two codend mesh sizes, to the selectivity equation for the control codend. The application of this correction factor is based on the assumption of geometric similarity, whereby the selection curve of the control codend is assumed to be a scaled version of the selection curve of the experimental codend when selection depends only on the relative geometry of the mesh and the fish (Millar and Fryer,

Table 2 Maximum likelihood estimates of logistic selectivity curve parameters for a 65 mm diamond mesh codend, relative to a 50 mm control codend, and SELECT model goodnessof-fit measures for Loligo pealeii, Peprilus triacanthus, Illex illecebrosus, and Merluccius bilinearis catches in a twin trawl. With the exception of butterfish, standard errors shown in parentheses were adjusted for between-haul variability (refer to Section 2).

N tows N length classes Length class rangea (mm) a b p (relative fishing efficiency) L25 c (mm) L50 L75 Selection range Selection factor Model deviance df p-value a b c

Loligo pealeii

Peprilus triacanthus

Illex illecebrosus

Merluccius bilinearis

65 57 17.5–297.5 −6.87 0.061 0.49 (0.008) 94.2 (1.15) 112.1 (1.67) 130.1 (2.48) 36.0 (1.92) 1.7 26915.89 1783 <0.0001

61 31 37.5–187.5 −8.15 0.085 0.30b 83.3 (0.56) 96.3 (0.30) 109.3 (0.20) 26.0 (0.58) 1.5 24.97 20 0.20

58 35 87.5–262.5 −7.50 0.053 0.61 (0.022) 121.7 (4.76) 142.6 (5.36) 163.5 (8.50) 41.8 (8.66) 2.2 9737.94 498 <0.0001

19 23 172.5–282.5 −27.05 0.138 0.43 (0.015) 187.0 (1.60) 195.9 (1.87) 203.9 (3.00) 15.9 (3.03) 3.0 2330.55 191 <0.0001

Length classes represent the midpoint of 5 mm length bins. Fixed parameter. Lengths at 25%, 50%, and 75% retention.

L.C. Hendrickson / Fisheries Research 108 (2011) 42–51

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Fig. 3. Deviance residuals, by length class, from logistic SELECT model fits for: (A) Loligo, (B) Illex, (C) silver hake, and (D) butterfish.

1999). For butterfish, the geometric similarity assumption is appropriate given the linear relationship between fish length (L) and maximum girth (G) defined as: G = −2.433 + 0.949L, r2 = 0.89 (Myer and Merriner, 1976). When the correction factor was applied, relative fishing efficiency was estimated as 0.30 for a logistic model fitted to catch data for the length class range of 37.5–187.5 mm. The fit resulted in a very large and positive trend in the deviance residuals for length classes greater than 137.5 mm and a smaller and negative trend in the 117.5–132.5 length classes (Fig. 3D). However, a curve fit through the catches of length classes greater than 137.5 mm resulted in unrealistically high estimates of p and L50 because catches composed of these larger length classes did not reach an asymptote. L50 , SF and SR estimates were: 96 mm, 1.5 mm, and 26 mm, respectively. Selection curves for the two finfish species were steeper, resulting in narrower selection ranges than for the two squid species (Fig. 4, Table 2). Additionally, the 50% retention lengths for both butterfish and silver hake were less than their median lengths at maturity, by 24 mm and 36 mm, respectively.

Fig. 4. Logistic selectivity curves for Loligo pealeii, Illex illecebrosus, Peprilus triacanthus (butterfish), and Merluccius bilinearis (silver hake) catches in a L. pealeii bottom trawl with a 65 mm diamond mesh codend. Median lengths at maturity (LM50 ) are also shown for butterfish and silver hake.

3.3. Catch rate differences The same tows included in the selectivity analyses were included in the comparison of catch rate differences between the experimental and control codends. Catch rates were highly variable. Unstandardized catch rates of Loligo and butterfish in both nets combined averaged 261 kg per tow (SD = 142.7) and 519 kg per tow (SD = 861.0), respectively, and averaged 159 kg per tow (SD = 305.6) and 125 kg per tow (SD = 163.6) for Illex and silver hake, respectively. The results from the nonparametric Wilcoxon signed rank tests led to the same inferences as the results from the paired t-tests, so only the latter test results are presented. Differences in the mean standardized catch rates between the two nets were significantly different from zero, at the 5% alpha level, for butterfish but not silver hake. Butterfish catch in the 65 mm codend was significantly reduced (p < 0.0001), by 51% and 58% on average in terms of weight and number, respectively (Table 3). More than half (54%) of the reduction in butterfish catches consisted of fish smaller than the median length at maturity (120 mm). The lack of significance for the reduction in silver hake catch, 40% in weight and 41% in number, was likely due to the small number of tows included in the analysis and the highly variable catch rates of this schooling species. For example, catches in the experimental net exceeded those in the control net during 58% of the tows. Most of the reduction in silver hake catch (86%) consisted of fish smaller than the median size at maturity (232 mm). The increase in Illex catch in the 65 mm codend, 28% in number and 37% in weight, was significant for number (p = 0.027) but not for weight (p = 0.087). The loss of Loligo catch in the 65 mm codend was highly significant (p < 0.0001), averaging 29% and 50% in terms of weight and number, respectively (Table 3). The loss in weight was only significant for squid in the smallest three market size categories: <10 cm (p < 0.0001), 10–12 cm (p < 0.0001), and 13–16 cm (p = 0.009) and

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L.C. Hendrickson / Fisheries Research 108 (2011) 42–51

Table 3 Results of paired t-tests for differences in mean standardized catch rates between the control net (Net C = 50 mm diamond mesh codend) and experimental net (Net E = 65 mm diamond mesh codend). Species

Butterfish Loligo Illex Silver hake

Mean kg per km2

Mean number per km2

Net C

Net E

Difference (%)

df

p

809 358 163 194

400 254 223 117

−51 −29 +37 −40

60 64 57 18

0.0001 <0.0001 0.087 NS

Net C

Net E

Difference (%)

p

19,274 5007 1469 2510

8086 2497 1879 1485

−58 −50 +28 −41

<0.0001 <0.0001 0.002 NS

NS indicates not significant at the 5% alpha level.

4. Discussion

Fig. 5. Length compositions of L. pealeii landings, in mt (top panel) and numbers (bottom panel), in the directed fishery during September–October of 2004–2008 for the current codend mesh size of 50 mm diamond (thick line) versus the theoretical landings length compositions for 65 mm diamond mesh when the logistic selectivity curve from the study is applied.

percent loss declined with increasing market size category. Applying the Loligo selectivity curve from the study to the length composition of the 2004–2008 landings resulted in a theoretical loss, during September–October, of 37%, 39%, and 18% in Loligo catch weight for the <10 cm, 10–12 cm, 13–16 cm market size categories, respectively (Fig. 5). Squid in the same three market size categories accounted for 58%, 31%, and 8%, respectively, of the theoretical loss in Loligo landings in terms of numbers. These three market size categories represented 3%, 14%, and 38%, respectively, of the total Loligo landings during September–October of 2004–2008 and increases in ex-vessel value were associated with increasing market size category (Fig. 6).

Fig. 6. L. pealeii landings and mean ex-vessel values ($/lb) by market size category, during September–October of 2004–2008, in relation to the ex-value values of L. pealeii from the September study.

When evaluating the effectiveness of a codend mesh size increase, it is important to consider conservation benefits to the target and bycatch species, as well as negative and positive economic effects on the fishery. In comparison to the codend mesh size currently in use by a majority of the Loligo fleet, 50 mm, the study results suggested that an increase to 65 mm would reduce the bycatch (in number) of butterfish and silver hake by 58% and 41%, respectively, on average. However, a codend mesh size larger than 65 mm would be necessary to allow 50% escapement of the median sizes of mature fish because the lengths at 50% retention for butterfish and silver hake were below their median lengths at maturity, by 24 mm and 36 mm, respectively. The difficulty for Loligo fishery managers lies in selecting an appropriate mesh size which maximizes bycatch reduction for multiple species while concurrently minimizing catch loss of the target species, the latter which can often be of smaller size than the bycatch species. Nevertheless, the reported bycatch reductions will have conservation benefits if escapement-related mortality is low in relation to discard mortality. In this regard, the conservation benefits of the codend mesh size increase to the butterfish and silver hake stocks are expected to be high because the Loligo fishery is the primary source of butterfish discards and most of the silver hake bycatch in the Loligo fishery is also discarded (MAFMC, 2009). In addition, survival rates of most species discarded in the L. pealeii fishery are presumably low due to lengthy tow durations which are generally three hours or longer. Ultimately, fishermen are most interested the effects of a codend mesh size increase on the quantity of their landings, by market size category, particularly for the target species. The bycatch reduction trade-off associated with use of the 65 mm codend is an average loss of 29% in the catch weight of Loligo. However, the associated loss in ex-vessel value is probably not proportional to the amount of catch loss for several reasons. Firstly, Loligo catch loss decreases as the market size category and ex-vessel value of Loligo increases. Most of the catch loss in weight (76%) consisted of squid from the two smallest market size categories (i.e., <10 cm and 10–12 cm) which have the least value. Secondly, a sizeable portion of squid in the <10 cm size category (which accounted for 37% and 58% of the total catch loss in weight and number, respectively) is discarded by the fleet (NEFSC, 2002), although a small percentage is also landed (e.g., 3% of the total landings during September–October in 2004–2008). Thirdly, the study showed that the mesh size increase resulted in higher catches of larger, more valuable squid. In addition, monthly trends in the mean body size of landed Loligo suggest that the September study results probably represent a worst-case scenario with respect to Loligo catch loss because mean body size tends to be smallest during September (Fig. 7). Applying the selectivity curve from the study to the 2004–2008 Loligo landings resulted in a theoretical loss of 18% for the most valuable market size category for which catch loss was statistically significant (i.e., 13–16 cm). The short, sub-annual life cycles of most squid species are associated with rapid growth rates which vary by season and are

L.C. Hendrickson / Fisheries Research 108 (2011) 42–51

Fig. 7. Monthly trends in the mean dorsal mantle length (±2 SE) of Loligo landings in the directed fishery during 2004–2007 in relation to the study period (circled).

strongly influenced by water temperature (Boyle and Rodhouse, 2005). Therefore, selectivity (and losses or gains in Loligo catches) is also likely to vary by season depending on mean body size. L. pealeii growth rates are rapid, averaging 19 mm and 13 g per month, and summer-hatched squid (i.e., hatched during June–October), which are caught in the winter fishery, have faster growth rates than winter-hatched squid (Brodziak and Macy, 1996). Consequently, the loss of Loligo catch associated with a codend mesh size increase will likely be greater in the summer Loligo fishery than in the winter fishery. When mean body size is small, Loligo fishing mortality may increase if fishing effort is increased to compensate for any catch loss associated with a codend mesh size increase. However, other bottom trawl selectivity studies suggest that the amount of squid catch loss declines rapidly as squid increase in body size over their lifespan (Amaratunga et al., 1979; Fonseca et al., 2002). For I. illeceborus, mean body size increases rapidly between June and October (Dawe and Beck, 1997; Hendrickson, 2004; NEFSC, 2006). A seasonal bottom trawl selectivity study showed that Illex catch loss associated with the use of a 60 mm diamond mesh codend declined from 13% to 0% between June and October, respectively (Amaratunga et al., 1979). During the subject study period, when the spring cohort approaches maximum body size, Illex catches in the 65 mm codend exceeded those in the 50 mm codend, illustrating how the rapid growth rates of squid species can compensate for catch loss during portions of the year. The 65-mm codend retained a greater proportion of large individuals than the 50 mm codend. This selectivity pattern was evident for all four of the species analyzed, but was most prominent for butterfish larger than 140 mm and Loligo larger than 120 mm. The occurrence of this common phenomenon in selectivity studies has been attributed to a lower catch efficiency of the small-mesh codend with respect to large individuals which, as a result of reduced flow rates within the small-mesh codend, may be able to out-swim the net (He, 1993) or to avoid capture as a result of increased turbulence (Millar, 1992; Cadigan and Millar, 1992; Millar and Walsh, 1992). For butterfish, the size at which increased catch rates of large individuals occurred in the large-mesh codend coincided with the theoretical maximum size of an individual that can physically escape through 65 mm mesh, based on their lengthmaximum girth relationship (Myer and Merriner, 1976). Use of the SELECT model, however, avoids any bias in selectivity parameter estimates which may occur as a result of this phenomenon because the model accounts for any differences in relative fishing efficiencies between the two nets. It is possible that the 0.5 mm reduction in twine thickness of the experimental codend may have affected the study results. Experimental and simulation studies have found that increases in the codend twine thickness can result in significant reductions in codend selectivity (Lowry and Robertson, 1996; Kynoch et al., 1999; Herrmann and O’Neill, 2006; Sala et al., 2007). However, the magnitudes of the reductions were variable and dependent on twine material, twine stiffness, catch weight, species caught, and uniden-

49

tifiable confounding factors. Based on the aforementioned studies, the potential effect on the subject study results would presumably be an increase in the selection factor estimates. The magnitude of such an effect was low for other studies involving codends constructed of polyethylene, similar to the subject study. Herrmann and O’Neill (2006) showed that selection factors decreased by 0.18–0.19 for each mm increase in twine thickness. Their simulation study results closely approximated the results from two experimental studies which tested the effects of twine thickness on the selectivity of polyethylene codends (Lowry and Robertson, 1996; Kynoch et al., 1999). Based on the linear relationship between polyethylene twine thickness and selection factor from Herrmann and O’Neill (2006), the potential effect of the 0.5 mm decrease in twine thickness on the subject study results is an increase of 0.09 in the estimated selection factors. A study by Sala et al. (2007) found that a 20–31% decrease in L50 (dependent on the species) was associated with a 0.5 mm increase in twine thickness. However, their results are not directly comparable to those from the subject study because polyamide twine was tested, which has greater selective properties than polyethylene twine (Stewart, 2001), and a much smaller mesh size (i.e., 44 mm versus 65 mm) was used. In any case, Sala et al. (2007) importantly note that the selectivity benefits associated with codend mesh size increases and the use of square mesh codends may be reduced if specifications for codend twine thickness are not included in codend mesh regulations. The butterfish selection factor estimate of 1.5 for the 65 mm diamond mesh codend was lower than the pound net selection factor estimate of 1.8 for 68 mm and 75 mm diamond mesh (Myer and Merriner, 1976). For reasons discussed in the Introduction, this finding is not unexpected for the two different gear types. The bimodal length composition of butterfish catches in the experimental codend when compared with the unimodal length composition of the control codend suggested that the control net did not exhibit 100% retention of the smallest individuals, resulting in model convergence difficulties prior to adjusting catches in the control codend based on the assumption of geometric similarity and fixing the p parameter. It is most likely that the maximum size of butterfish that escaped from the control codend was 107 mm because theoretically, individuals of this size cannot escape through 50 mm diamond meshes (Myer and Merriner, 1976). This escapement resulted in an overestimation of the retention of a portion of the fish smaller than 107 mm in the experimental codend. However, the amount of such escapement was likely small given the fact that butterfish smaller than 107 mm were caught in the control codend at most stations (72%) and the overall catches of these fish remained greater in the control codend (20% of the total number) than the experimental codend (14%). At most stations, butterfish size compositions in both nets were characterized by narrow, normal distributions with a modal size range in the control net of either 110–120 mm or 130–150 mm. This length composition pattern emphasizes the fact that butterfish school by size and the importance of sampling their entire size range in order to avoid biased selectivity parameter estimates. Despite being relative, codend mesh selection factors from the subject study are remarkably similar to selection factors from other bottom trawl selectivity studies which tested diamond mesh codends of similar mesh size (Table 4). The selection factor for M. bilinearis from the subject study (3.0) is similar to 2.9, the selection factor estimated for the same species (Halliday and Cooper, 2000) as well as for the European hake, M. merluccius with 60 mm polyethylene, diamond mesh codends (Campos et al., 2003). Likewise, the Illex selection factor from the subject study (2.2) is similar to the selection factors of 2.1 (Amaratunga et al., 1979) and 2.3 (Clay, 1979) for 58 mm and 60 mm diamond mesh codends, respectively. The Illex coindetii selection factor of 1.5 for a 65 mm diamond mesh codend (Fonseca et al., 2002) is much lower. However, there are

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L.C. Hendrickson / Fisheries Research 108 (2011) 42–51

Table 4 Comparison of relative codend mesh selection factors from the study, which tested 50 mm diamond mesh against 65 mm diamond mesh, with selection factors from the literature for Peprilus triacanthus, Loligo sp., Illex sp., and Merluccius sp. Multiple selection factors within a study are listed in order of the experimental codend mesh size tested. “BT” represents bottom trawl. Species

Selection factor

Gear type tested

Study location

Experimental codend mesh size (mm), shape and materiala

Length range (mm)

Reference

Peprilus triacanthus Peprilus triacanthus Loligo pealeii Loligo vulgaris Loligo vulgaris Loligo vulgaris Loligo forbesi Illex illecebrosus Illex illecebrosus

L. pealeii BT Pound net L. pealeii BT Multispecies BT Multispecies BT Multispecies BT Survey BT L. pealeii BT I. illecebrosus BT

NW Atlantic, US Chesapeake Bay, US NW Atlantic, US W. Mediterranean Aegean Sea N. Portugal NE Atlantic, Scotland NW Atlantic, US Scotian Shelf, CA

Silver hake BT

Scotian Shelf, CA

35–189 75–155 15–299 20–120 20–170 90–370 60–440 85–264 154–168c 232–240 100–270

Illex coindetii Illex coindetii Merluccius bilinearis Merluccius bilinearis Merluccius bilinearis

1.0, 1.3, 1.9 1.5 3.0 2.9 3.8, 3.1, 3.2

Multispecies BT Multispecies BT L. pealeii BT Silver hake BT Silver hake BT

Aegean Sea N. Portugal NW Atlantic, US Scotian Shelf, CA Scotian Shelf, CA

20–190 50–200 170–284 80–340 120–400

Tosuno˘glu et al. (2009) Fonseca et al. (2002) This study Halliday and Cooper (2000) Clay (1979)

Merluccius merluccius Merluccius merluccius

2.7, 3.8 2.9, 2.9

Multispecies BT Survey BT

W. Mediterranean, NE Atlantic, Portugal

65 D PE 61, 68 and 75 D nylon 65 D PE 40 D and S PEb 45 D, 43 H, 42 S PE 80 and 90 dbl PEb 75 D PEb 65 D PE 49, 58 and 86 D PE 86, 96 and 136 D PE 60, 70 and 124 D kapron 45 D, 43 H, 42 S PE 65 dbl PEb 65 D PE 60 D PE 60, 70 and 124 D kapron 40 D and S PEb 55 and 60 D PE

This studyd Myer and Merriner (1976) This study Ordines et al. (2006) Tosuno˘glu et al. (2009) Fonseca et al. (2002) Hastie (1996) This study Amaratunga et al. (1979)

Illex illecebrosus

1.5 1.7, 1.8, 1.8 1.7 0.9, 1.5 1.0, 1.8, 1.5 1.3, 1.3 1.9–2.0 2.2 2.4, 2.1, 1.4 2.0, 2.0, 1.5 2.3, 2.6, 1.9

100–400 50–700

Ordines et al. (2006) Campos et al. (2003)

a b c d

Clay (1979)

Inside stretched mesh size unless designated otherwise; D = diamond, S = square, and H = hexagonal mesh; PE = polyethylene. Nominal mesh measurement. Mean dorsal mantle lengths. Covered by a 245 mm S codend strengthener.

several reasons this might be expected. The Fonseca et al. study tested a nominal codend mesh size of 65 mm that was constructed of double rather than single twine, so the effective mesh size was likely much smaller than 65 mm. In addition, the study pertained to a squid population from the northeast Atlantic with a smaller modal size (130 mm) than was sampled during the subject study (172.5 mm) and the catch rates were much lower (i.e., a total catch of 35 kg in 14 tows versus an average catch of 125 kg per tow, for both nets combined, in the subject study). As previously noted, diel differences in availability to bottom trawls, as well as large seasonal and spatial differences in squid body size make comparisons of squid mesh selection factors difficult. Similar to Illex species, a broad range of selection factors for diamond mesh codends has been reported for loliginid squid (i.e., 0.9–2.0), depending on the species, codend mesh size, and seasonality of the study. The L. pealeii selection factor estimated from the subject study, 1.7, is near the upper end of the range and most similar to Hastie’s estimate of 1.9–2.0 for 75 mm diamond mesh (Table 4). The process by which the size selection of squid occurs in trawls is unclear. Video camera recordings of bottom trawl capture behavior indicate that L. pealeii tires shortly after encountering the net, suggesting that escapement from the codend is limited. Individuals swim for approximately three minutes at a towing speed of 5.4 km per hour (the same towing speed used in the subject study), then rise upward in the net, turn toward the codend, cease swimming and allow the net to overtake them (Glass et al., 1999). Scandol et al. (2006) also concluded that the escapement of squid from codends is probably limited because they found no significant reduction in squid catches despite the presence of a 30 mm square-mesh escape panel installed in the top anterior section of the codend. In order to determine how squid are size-selected in bottom trawls, additional camera studies are needed to further document their behavior. In summary, a majority of the catches of the three bycatch species evaluated herein and Loligo smaller than 10 cm in mantle length are discarded in the Loligo fishery and are most likely dead. Therefore, a codend mesh size increase to 65 mm should provide some conservation benefits to these stocks if a portion sur-

vive escapement. On a fleet-wide basis, the magnitude of bycatch reduction and loss of Loligo catch will vary depending on seasonal changes in mean body size, vessel-specific gear characteristics such as codend mesh size and shape (Robertson and Stewart, 1988; Ordines et al., 2006; Tosuno˘glu et al., 2009) and strengthener mesh size and shape (Kynoch et al., 2004; Özbilgin and Tosuno˘glu, 2003), as well as vessel-specific fishing practices. In this regard, a bioeconomic analysis would be useful to assess the overall economic impacts of the mesh size increase on the Loligo fleet in relation to stock conservation benefits for the target and bycatch species. Such an analysis could determine on a seasonal basis whether the cost of a potential increase in fishing effort in this quota-managed stock is offset by economic gains from the increased catches of larger, more valuable squid and reductions in bycatch sorting time.

Acknowledgements The am grateful for funding from the NMFS Northeast Cooperative Research Partners Program and to the captain of the F/V Karen Elizabeth, Chris Roebuck, and his crew as well as the following scientific field staff: Paul Kostovik, Katie Anderson, Jason Dean, Eric Matzen, and Lauren Marcinkiewiecz. Rob Johnston and Nathan Keith kindly helped stage and offload the vessel. I also thank Larry Jacobson, Henry Milliken, Fred Serchuk, Paul Rago, and the journal’s anonymous reviewers, for thorough reviews that greatly improved the manuscript quality.

References Amaratunga, T., Kawahara, S., Kono, H., 1979. Mesh selection of the short-finned squid, Illex illecebrosus, on the Scotian shelf using a bottom trawl: a joint Canada–Japan 1978 research program. In: ICNAF Res. Doc. 79/II/35, Ser. No. 5361 , p. 29. Boyle, P., Rodhouse, P., 2005. Cephalopods: Ecology and Fisheries, p. 452. Brodziak, J., Hendrickson, L., 1999. An analysis of environmental effects on survey catches of squids Loligo pealei and Illex illecebrosus in the northwest Atlantic. Fish. Bull. 97, 9–24. Brodziak, J.K.T., Macy III, W.K., 1996. Growth of long-finned squid, Loligo pealei, in the northwest Atlantic. Fish. Bull. 94, 212–236.

L.C. Hendrickson / Fisheries Research 108 (2011) 42–51 Cadigan, N.G., Millar, R.B., 1992. Reliability of selection curves obtained from trouser trawl or alternate haul experiments. Can. J. Fish. Aquat. Sci. 49, 1624–1632. Campos, A., Fonseca, P., Erzini, K., 2003. Size selectivity of diamond and square mesh codends for four by-catch species in the crustacean fishery off the Portuguese south coast. Fish. Res. 60, 79–97. Clay, D., 1979. Mesh selection of silver hake (Merluccius bilinearis) in otter trawls on the Scotian Shelf with reference to selection of squid (Illex illecebrosus). In: ICNAF Res. Doc. 79/II/3 , p. 36. Cross, J., Zetlin, C.A., Berrien, P.L., Johnson, D.L., McBride, C., 1999. Essential fish habitat source document: butterfish, Peprilus triacanthus, life history and habitat characteristics. In: NOAA Tech. Memo. NMFS-NE-145 , p. 42. Dawe, E.G., Beck, P.C., 1997. Population structure, growth and sexual maturation of short-finned squid (Illex illecebrosus) at Newfoundland. Can. J. Fish. Aquat. Sci. 54, 137–146. Fonseca, P., Campos, A., Garcia, A., 2002. Bottom trawl codend selectivity for cephalopods in Portuguese continental waters. Fish. Res. 59 (1–2), 263–271. Glass, C.W., Sarno, B., Milliken, H.O., Morris, G.D., Carr, H.A., 1999. Bycatch reduction in Massachusetts inshore squid fisheries. Mar. Technol. Soc. J. 33 (2), 35–42. He, P., 1993. Swimming speeds of marine fish in relation to fishing gears. ICES Marine Science Symposium 196, 183–189. Halliday, R.G., Cooper, C.G., 2000. Size selection of silver hake (Merluccius bilinearis) by otter trawls with square and diamond mesh codends. Fish. Res. 49, 77–84. Hastie, L.C., 1996. Estimation of trawl codend selectivity for squid (Loligo forbesi) based on Scottish research vessel survey data. ICES J. Mar. Sci. 53, 741–744. Hendrickson, L.C., 2004. Population biology of northern shortfin squid (Illex illecebrosus) in the Northwest Atlantic Ocean and initial documentation of a spawning area. ICES J. Mar. Sci. 61, 252–266. Herrmann, B., O’Neill, F.G., 2006. Theoretical study of the influence of twine thickness on haddock selectivity in diamond mesh cod-ends. Fish. Res. 80, 221–229. International Commission for the Northwest Atlantic Fisheries (ICNAF), 1978. Report of Standing Committee on Research and Statistics (STACRES). In: Special Meeting on Squid, February, 1978 , ICNAF Redbook, pp. 29–30. Kynoch, R.J., Ferro, R.S.T., Zuur, G., 1999. The effect of juvenile haddock by-catch of changing codend twine thickness in EU trawl fisheries. Mar. Technol. Soc. J. 33 (2), 61–72. Kynoch, R.J., O’Dea, M.C., O’Neill, F.G., 2004. The effect of strengthening bags on cod-end selectivity of a Scottish demersal trawl. Fish. Res. 68, 249–257. Lange, A.M.T., Johnson, K., 1981. Dorsal mantle length-total weight relationships of squids Loligo pealei and Illex illecebrosus from the Atlantic coast of the United States. In: NOAA Tech. Report, NMFS SSRF-745 , p. p. 17. Lange, A.M., Waring, G.T., 1992. Fishery interactions between long-finned squid (Loligo pealei) and butterfish (Peprilus triacanthus) off the Northeast USA. J. Northw. Atl. Fish. Sci. 12, 49–62. Lowry, N., Robertson, J.H.B., 1996. The effect of twine thickness on codend selectivity of trawls for haddock in the North Sea. Fish. Res. 26, 353–363. Macy III, W.K., Brodziak, J.K.T., 2001. Seasonal maturity and size at age of in waters of southern New England. ICES J. Mar. Sci. 58, 852–864. Mid-Atlantic Fishery Management Council (MAFMC), 2009. Amendment 10 to the Atlantic mackerel, squid, and butterfish fishery management plan. In: Final environmental impact statement and essential fish habitat assessment , p. 623. McCullagh, P., Nelder, J.A., 1989. Generalized Linear Models , 2nd edition. Chapman and Hall, London, p. 511. Millar, R.B., 1992. Estimating the size-selectivity of fishing gear by conditioning on the total catch. J. Am. Stat. Assoc. 87 (420), 962–968. Millar, R.B., Walsh, S.J., 1992. Analysis of trawl selectivity studies with an application to trouser trawls. Fish. Res. 13, 205–220.

51

Millar, R.B., Fryer, R.J., 1999. Estimating the size selection curves of towed gears, traps, nets, and hooks. Rev. Fish Biol. Fisher. 9, 89–116. Millar, R.B., Broadhurst, K.M., Macbeth, W.G., 2004. Modelling between-haul variability in the size selectivity of trawls. Fish. Res. 67, 171–181. Millar, R.B., 2010. Reliability of size-selectivity estimates from paired-trawl and covered-codend experiments. ICES J. Mar. Sci. 67, 530–536. Myer, H.L., Merriner, J.V., 1976. Retention and escapement characteristics of pound nets as a function of pound-head mesh size. Trans. Am. Fish. Soc. 3, 370–379. Northeast Fisheries Science Center (NEFSC), p. 383 2010. 49th Northeast Regional Stock Assessment Workshop (49th SAW) Assessment Report. In: Northeast Fish. Sci. Cent. Ref. Doc. 10-03 , http://www.nefsc.noaa.gov/publications/crd/crd1003/. Northeast Fisheries Science Center (NEFSC), p. 284 2006. 42nd Northeast Regional Stock Assessment Workshop (42nd SAW) stock assessment report, part A: silver hake, Atlantic mackerel, and northern shortfin squid. In: Northeast Fish. Sci. Cent. Ref. Doc. 06-09a , http://www.nefsc.noaa.gov/nefsc/publications/crd/crd0609/index.htm. Northeast Fisheries Science Center (NEFSC), p. 346 2002. Report of the 34th Northeast Regional Stock Assessment Workshop (34th SAW): Stock Assessment Review Committee (SARC) consensus summary of assessments. In: Northeast Fish. Sci. Cent. Ref. Doc. 02-06 , http://www.nefsc.noaa.gov/nefsc/publications/crd/crd0206/crd0206.pdf. O’Brien, L., Burnett, J., Mayo, R.K., 1993. Maturation of nineteen species of finfish off the northeast coast of the United States, 1985–1990. In: NOAA Tech. Report, NMFS 113 , p. 66. Ordines, F., Masutti, E., Guijarro, B., Mas, R., 2006. Diamond vs. square mesh codend in a multi-species trawl fishery of the western Mediterranean: effects on catch composition, yield, size selectivity and discards. Aquat. Living Resour. 19, 329–338. Özbilgin, H., Tosuno˘glu, Z., 2003. Comparison of the selectivities of double and single codends. Fish. Res. 63, 143–147. Robertson, J.H.B., Stewart, P.A.M., 1988. A comparison of size selection of haddock and whiting by square and diamond mesh codends. J. Cons. Explor. Mer. 44, 148–161. Sala, A., Lucchetti, A., Buglioni, G., 2007. The influence of twine thickness on the size selectivity of polyamide codends in a Mediterranean bottom trawl. Fish. Res. 83, 192–203. SAS Institute Inc., 1999. SAS/STAT User’s Guide, Version 8. SAS Institute Inc., Cary, NC. Scandol, J.P., Underwood, T.J., Broadhurst, M.K., 2006. Experiments in gear configuration to reduce bycatch in an estuarine squid-trawl fishery. Fish. Bull. 104, 533–541. Sissenwine, M.P., Bowman, E.W., 1976. An analysis of some factors affecting the catchability of fish by bottom trawls. ICNAF Res. Bull. 13, 81–87. Stewart, P.A.M., p. 75 2001. A review of studies of fishing gear selectivity in the Mediterranean. In: FAO COPEMED , http://www.faocopemed.org/old copemed/vldocs/0000317/rev sel.pdf. Tosuno˘glu, Z., Aydin, C., Salman, A., Fonseca, P., 2009. Selectivity of diamond, hexagonal and square mesh codends for three commercial cephalopods in the Mediterranean. Fish. Res. 97, 95–102. Wileman, D.A., Ferro, R.S.T., Fonteyne, R., Millar, R.B. (Eds.), 1996. ICES Coop. Res. Rep. No. 215. , p. 126.