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Trawl net modifications to reduce the bycatch of eulachon (Thaleichthys pacificus) in the ocean shrimp (Pandalus jordani) fishery
Fisheries Research 110 (2011) 277–282
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Trawl net modifications to reduce the bycatch of eulachon (Thaleichthys pacificus) in the ocean shrimp (Pandalus jordani) fishery Robert W. Hannah a,∗ , Stephen A. Jones a , Mark J.M. Lomeli b , W. Waldo Wakefield c a
Oregon Department of Fish and Wildlife, Marine Resources Program, 2040 SE Marine Science Drive, Newport, OR 97365, USA Pacific States Marine Fisheries Commission, 2032 SE OSU Drive, Newport, OR 97365, USA Northwest Fisheries Science Center, Fishery Resource Analysis and Monitoring Division, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2032 SE OSU Drive, Newport, OR 97365, USA b c
a r t i c l e
i n f o
Article history: Received 15 March 2011 Received in revised form 21 April 2011 Accepted 21 April 2011 Keywords: Bycatch reduction devices Nordmøre grate Ocean shrimp Shrimp trawl Eulachon Fish behavior Trawl footrope
a b s t r a c t Two trawl gear modifications for reducing fish bycatch (weight) in ocean shrimp (Pandalus jordani) trawls were tested in June and August–September 2010. The primary focus of the study was evaluating trawl system modifications for reducing bycatch of eulachon (Thaleichthys pacificus) below levels already achieved via mandatory use of bycatch reduction devices (BRDs). An experimental footrope, modified by removing the central one third of the trawl groundline, reduced eulachon bycatch by 33.9%. It also reduced bycatch of slender sole (Lyopsetta exilis), other small flatfishes and juvenile darkblotched rockfish (Sebastes crameri) by 80% or more, but had no effect on bycatch of whitebait smelt (Allosmerus elongatus) or Pacific herring (Clupea pallasii). The experimental groundline also reduced the catch of ocean shrimp (weight) by 22.2% in hauls yielding commercial quantities of shrimp (>194 kg/haul) and by 23.2% in all hauls. Reducing bar spacing in a rigid-grate BRD from 25.4 mm to 19.1 mm reduced eulachon bycatch by 16.6%, with no reduction in ocean shrimp catch. It also reduced bycatch of slender sole, other small flatfish and juvenile darkblotched rockfish by 36.8%, 71.8% and 76.3%, respectively with no effect on bycatch of whitebait smelt or young-of-the-year (YOY) Pacific hake (Merluccius productus). Although both trawl modifications reduced eulachon bycatch, the footrope modification tested, if developed further, has the potential to also avoid trawl entrainment for some demersal fishes, as well as reduce bottom impacts from trawling. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since 2003, vessels participating in the ocean shrimp (Pandalus jordani, also known as smooth pink shrimp) trawl fishery that operates in the United States coastal shelf waters (water depths of about 80–300 m) off Washington, Oregon and California have been required to use bycatch reduction devices (BRDs) for all fishing. The BRDs allowed included both soft-panel (127 mm maximum stretch mesh size) and rigid-grate BRDs (51 mm maximum spacing of vertical bars) very similar in function to the Nordmøre grate (Hannah and Jones, 2007; Isaksen et al., 1992). As of 2005, the BRD requirements, which were enacted by the states of Washington, Oregon and California primarily to reduce the bycatch of canary rockfish (Sebastes pinniger), had reduced total fish bycatch in the Oregon component of the fishery by 66–88% by weight from historical levels, leaving bycatch at about 7.5% of total catch (Hannah and Jones, 2007). The BRD requirement also shifted the species and size composition of the bycatch from one heavily dominated by adult and
∗ Corresponding author. Tel.: +1 541 867 0300x231; fax: +1 541 867 0311. E-mail address: [email protected] (R.W. Hannah). 0165-7836/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2011.04.016
juvenile Pacific hake (Merluccius productus), smelts (Osmeridae), adult yellowtail rockfish (Sebastes flavidus) and lingcod (Ophiodon elongatus) to one composed mostly of fish with very low or no commercial value such as juvenile Pacific hake, smelts, slender sole (Lyopsetta exilis), small rex sole (Glyptocephalus zachirus) and juvenile rockfishes (Sebastes spp., Hannah and Jones, 2007). The BRDs used in this fishery recently are almost exclusively rigid-grate designs (used in 99% of fishing trips in 2009, Oregon Department of Fish and Wildlife, unpublished data) and since 2005, bar spacing in these BRDs has been steadily dropping (Fig. 1), suggesting that current levels of fish bycatch may now be even lower than 7.5% of total catch. Comprehensive data on BRD designs used by participating vessels is limited to the Oregon fishery. A typical BRD used in this fishery in 2006 was a rigid-grate device with bar spacing of more than 31.8 mm (Fig. 1). As of 2009, the majority of shrimp trawl trips employed rigid-grate BRDs with bar spacing ranging from 22.2 mm to 28.6 mm, and about 7% of trips used BRDs with bar spacing as small as 19.1 mm (Fig. 1). Although fish bycatch in the ocean shrimp trawl fishery has been greatly reduced by the use of BRDs, some conservation concerns remain regarding the residual fish bycatch. In March of 2010, the southern distinct population segment (DPS) of eulachon
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Fig. 1. Distribution of bar spacing in rigid-grate bycatch reduction devices (BRDs) used in the Oregon trawl fishery for ocean shrimp, by percentage of trips, 2006–2009.
(Thaleichthys pacificus) was listed as “threatened” under the United States Endangered Species Act (NMFS, 2010). Eulachon is an anadromous smelt (Osmeridae) species that spawns in many rivers on the U.S. and Canadian west coasts, from California northward through parts of Alaska (Hay and McCarter, 2000). The southern DPS includes stocks spawning in rivers ranging from California to the Nass River in British Columbia, Canada. The stock composition of eulachon captured in the ocean shrimp trawl fishery in U.S. waters is unknown. However eulachon can at times constitute a substantial component of the residual bycatch (NWFSC, 2009) and it’s likely that many of these fish are from the southern DPS. Therefore, techniques to further reduce eulachon bycatch in the shrimp trawl fishery are needed to reduce impacts and aid in the recovery of eulachon. Current shrimp fishery bycatch also includes juvenile darkblotched rockfish (Sebastes crameri), a slope rockfish species that was declared “overfished” and is now in a rebuilding phase in U.S. west coast waters (Miller et al., 2009; Wallace and Hamel, 2009), suggesting another conservation benefit that could result from further reducing shrimp fishery bycatch. Additional field studies to reduce bycatch of eulachon are warranted for several reasons. First, most earlier studies did not identify smelts to the species level and thus cannot be directly applied to eulachon (Hannah and Jones, 2000, 2003; Hannah et al., 1996). Also, because of the decline in eulachon abundance, most of the more recent research on bycatch reduction has been conducted in areas or years in which eulachon were either not abundant or totally absent. Lastly, new research was needed to further evaluate bycatch reduction techniques for which only preliminary data
had been generated, but which showed some potential for reducing eulachon bycatch (Hannah and Jones, 2007). We report here on two 2010 field studies that evaluated gearbased approaches to further reduce fish bycatch in the ocean shrimp trawl fishery, with an emphasis on reducing the bycatch of eulachon. The first experiment tested the effect on bycatch of completely removing a large central section of groundline from a shrimp trawl. This study was conducted as one component of a larger study designed to evaluate how changes in footrope configuration influence impacts on benthic macroinvertebrates, some of which have been negatively affected by chronic shrimp trawling (Hannah et al., 2010). Ocean shrimp trawls are considered semipelagic, in that the fishing line of the net is suspended 35–70 cm above the sea floor, while a separate, parallel groundline is in contact with the bottom (Fig. 2). It has previously been shown that the relative positions of the fishing line and groundline in ocean shrimp trawls can greatly influence bycatch of some species, and that removing or disabling the central portion of the groundline can reduce bycatch of juvenile rockfish and small flatfish (Hannah and Jones, 2000, 2003). However, the effect of removing the central section of the groundline on bycatch of eulachon has not been previously studied. The second experiment we conducted tested the effect on bycatch of reducing bar spacing in rigid-grate BRDs below that most commonly used in the fishery, specifically a comparison of bycatch with rigid-grate BRDs with 19.1- and 25.4-mm bar spacing. This comparison was conducted to follow up on preliminary data (Hannah and Jones, 2007) suggesting that rigid-grate BRDs with 19.1-mm bar spacing could be used to further reduce fish bycatch in this fishery while causing little or no reduction in shrimp catch.
2. Methods We evaluated bycatch reduction by comparing catches from the port and starboard nets of a double-rigged shrimp trawl vessel, with one net incorporating the treatment of interest and the other serving as a control. Haul duration was limited to about 45–60 min to avoid shrimp catches that were too large to sort and weigh with the field staff available. Towing speed was typical for ocean shrimp trawling, varying between 3.0 km h−1 and 3.3 km h−1 (1.6–1.8 knots). Three techniques were used to compensate for potential differences in catch efficiency between the two nets. First, both trawl nets were inspected to make sure they were rigged as similarly as possible. Second, the treatment was
Fig. 2. Schematic drawing of an ocean shrimp trawl with a “ladder style” footrope (view looking into the net from the front).
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interchanged between the two nets at regular intervals. In the rigid-grate BRD experiment, the BRDs with 19.1- and 25.4-mm bar spacing were alternated between the port and starboard nets every two hauls, utilizing an ABBA pattern. For the groundline experiment, the treatment and control groundlines were interchanged between the port and starboard nets after the first and third days of the four-day cruise, also resulting in an ABBA pattern, but in terms of days fished. The final technique used to control for netspecific differences in catch efficiency was in situ measurement of fishing line height (FLH) and adjustments to equalize this variable between the two nets prior to starting comparative tows. Increased FLH has been shown to reduce both shrimp catch and bycatch of some species in ocean shrimp trawl nets (Hannah and Jones, 2003), so equalizing this variable between port and starboard nets, to the extent possible, reduces between-net variance and makes it easier to detect treatment-related differences in catch composition. We used recording inclinometers at the center of the fishing line on both nets, following Hannah and Jones (2003), to measure FLH for a series of preliminary hauls. Based on these data, adjustments were made to the lengths of chain connecting the groundline to the fishing line in one or both of the nets, followed by additional test hauls. We continued this process of measuring and adjusting FLH until the two nets were as similar as possible in average FLH, given the vessel time available to complete each project. For the groundline experiment, a second set of measurements and adjustments to FLH was made after the treatment effect was moved from one net to the other, following the first full day of comparative hauls. Then on the fourth day of that experiment, the groundline configurations were returned to their settings from the first day. Data analysis was similar for both experiments. Catches from each net were emptied into one side of a divided hopper and sorted to species and weighed at sea. The combined catch of smelt species and in some instances, juvenile rockfish, from each net was weighed at sea, placed into labeled sample bags and frozen for later identification in the laboratory. Lengths (mm) were measured for eulachon during lab analysis. For the groundline experiment, we measured fork length of eulachon, but switched to total length for the rigid-grate comparison as it proved easier to measure consistently for eulachon. Fork length measurements were converted to total lengths using the equation of Buchheister and Wilson (2005). For a few hauls in the groundline experiment, the smelt catch was too large to bag and freeze all of it. In those instances a subsample (approximately 1 kg) was bagged and frozen and the remaining smelt were weighed and discarded at sea. Weight, by species, from laboratory analysis was used to expand subsamples to estimate total catch, by haul and net, for eulachon and any other smelt species captured. Catch weight data were analyzed as a multi-factor ANOVA, with haul, side of gear (port or starboard) and the treatment as main effects without interaction, following Hannah et al. (1996). To account for remaining differences in FLH, we also included FLH as a covariate. To determine the statistical significance of a particular treatment, we utilized a one-tailed P-value. For some species or species groups, transformations were utilized to achieve normality of model residuals. Length data for eulachon were expected to be multi-modal and were therefore compared between treatment and control nets using the nonparametric Wilcoxon two-sample test (Sokal and Rohlf, 1981). 2.1. Groundline experiment The groundline experiment was conducted in June 2010 utilizing the 22 m double-rigged shrimp trawler F/V Kylie Lynn, out of Newport, OR. The study area we selected was located just north of the Umpqua River, OR, centered on the shoreward edge of the main shrimping grounds, at depths ranging from 99 m to 148 m.
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This area was chosen because it often holds quantities of both eulachon and ocean shrimp, as well as sufficient abundances of sea whips (Halipteris spp.) and other benthic invertebrates that were important for other questions being addressed during the cruise. The footrope on the control net was configured as shown in Fig. 2. The experimental footrope was configured similarly, except that the central section of the groundline (about 1/3 of the total groundline) was removed, leaving just a few simple drop chains attached to the central section of the fishing line to stabilize it. Both nets incorporated rigid-grate BRDs with 19.1-mm bar spacing. 2.2. Comparison of rigid-grate BRDs with 19.1- and 25.4-mm bar spacing The rigid-grate BRD experiment was conducted in AugustSeptember 2010 utilizing the 21 m double-rigged shrimp trawler F/V Miss Yvonne, out of Newport, OR. The study area was the same as the one used for the groundline experiment, however fishing was conducted at a slightly deeper depth range, from 108 m to 157 m. The groundlines of both trawls were configured as shown in Fig. 2 (control groundline). The BRDs we compared were of identical construction, except for bar spacing and the diameter of the round stock used for the vertical bars. They were both constructed using an outer circular frame of 107 cm diameter, made of 32-mm (outside diameter) aluminum tubing. The BRD with 25.4-mm bar spacing utilized 8-mm diameter solid aluminum rod for the vertical bars while the BRD with 19.1-mm spacing used 6.4-mm diameter solid aluminum rod. This difference allowed for approximately the same total open area between the bars for both BRDs, to equalize water flow into the codend. Neither BRD incorporated a guiding panel to concentrate catch at the bottom of the grate (for a diagram of typical rigid-grate BRDs, see Hannah and Jones, 2007). 3. Results 3.1. Groundline experiment We completed 35 double-rigged hauls comparing ocean shrimp trawl nets with and without the central section of the groundline and analyzed catch data from 26 hauls. The first 3 hauls and hauls 11–15 were spent measuring and adjusting FLH between the two nets (Fig. 3A). Haul 20 was excluded because of zero catch in both nets. Despite using eight hauls to evaluate and adjust FLH, it was never completely equalized between the two nets used in this experiment (Fig. 3A). However, FLH averaged about 7.8 cm higher on the starboard net (paired t-test, P < 0.001). FLH varied considerably during this experiment, most likely as a result of variability in sea conditions and currents. Although FLH was not generally highly variable within a haul, removing the central section of groundline did make FLH more variable in that net (P < 0.05), with more of an effect on the port net. The lack of a central groundline section increased the standard error of FLH by 63% in the port net versus 25% in the starboard net, when each was compared to the opposing net with a central groundline section. The absence of a central groundline section significantly (P < 0.05) decreased eulachon bycatch by 33.9%, by weight, in relation to the control net. This footrope modification also decreased residual bycatch of slender sole, other small flatfish and darkblotched rockfish by 95.9% (P < 0.0001), 96.5% (P < 0.0001) and 79.6% (P < 0.001), respectively (Table 1). No effect on the bycatch of whitebait smelt (Allosmerus elongatus) or Pacific herring (Clupea pallasii) was observed (P > 0.05). The effect of removing the central section of the groundline on shrimp catch was more difficult to discern from these data. The mean catch of ocean shrimp in the net with no central groundline
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Table 1 Comparison of mean catch by species and weight (kg haul−1 for all species except eulachon, whitebait smelt and darkblotched rockfish which are reported as g haul−1 ) between ocean shrimp trawl nets with and without the central section of groundline. SE = standard error. Species name
Complete groundline (control) (SE)
Central section of groundline removed (SE)
Reduction with groundline removed (%)
P-valuea
Ocean shrimp Eulachon Whitebait smelt Pacific herring Slender sole Other small flatfish Darkblotched rockfish
176.12 (58.32) 2858.10 (519.02) 29.00 (11.95) 1.11 (0.22) 3.92 (0.55) 1.72 (0.37) 61.07 (15.62)
135.22 (41.74) 1890.06 (371.14) 17.81 (7.05) 1.04 (0.26) 0.16 (0.10) 0.06 (0.02) 12.46 (2.50)
23.2 33.9 11.2 6.3 95.9 96.5 79.6
0.066 0.017 0.367 0.481 0.00005 0.00005 0.0001
a
Multi-factor ANOVA, 1-tailed P-value for treatment effect (see text).
section was 23.2% less than in the control net. However, catches were so variable among hauls that this mean difference was not statistically significant (P > 0.05). Considering just the hauls with commercial quantities of ocean shrimp (>194 kg/haul, 12 hauls total, equally balanced between having the treatment effect on port and starboard sides) resulted in a highly significant reduction in ocean shrimp catch of 22.2% due to the absence of the central section of groundline (P < 0.001). FLH was not statistically significant as a covariate for any of the species or species groups (P > 0.05). For ocean shrimp, the “side” effect (port versus starboard net) was statistically significant (P < 0.05), with the port net accounting for about 18% more catch weight than the starboard net. The “side” effect was also significant for darkblotched rockfish (P < 0.05) with the port net again having higher catches. Interestingly, there was no evidence for a “side” effect for eulachon, as the mean catches between port and starboard nets differed by less than 1%.
Fig. 3. Mean fishing line height (cm, measured at the center of the fishing line) in port and starboard nets, by treatment, in fishing experiments comparing catches in ocean shrimp trawl nets with and without the central section of the groundline (A) and in nets with rigid-grate BRDs with bar spacing of 19.1- and 25.4-mm (B).
Comparison of eulachon length frequency data from the treatment and control nets showed a slight reduction (5 mm) in mean total length of eulachon captured in the net with the central section of groundline removed (Wilcoxon test, P < 0.01, Fig. 4). This suggests that removing the central section of groundline from ocean shrimp trawls reduces entrainment into the trawl slightly more for large eulachon than for smaller ones. Given the difference in FLH between the two nets used in this experiment, this result should be considered preliminary. 3.2. Comparison of rigid-grate BRDs with 19.1- and 25.4-mm bar spacing We analyzed 30 hauls comparing bycatch with the rigid-grate BRDs with 19.1- and 25.4-mm bar spacing (Fig. 3B). FLH was better equalized between the two nets in this experiment, a result of utilizing numerous initial hauls for iterative measurement and adjustment of FLH (not shown in Fig. 3B). After all adjustments, FLH was still slightly higher in the port net (t-test, P < 0.001), with a mean difference of 4.2 cm, however, this difference is probably very close to the real limits of accuracy of the recording inclinometers. FLH was less variable during this experiment, possibly as a result of different sea conditions. For reasons unknown, FLH was increased in the port net for hauls 23 and 24. This type of variation can result from debris becoming entangled on the footrope. Since this type of event happens frequently during normal shrimping activity, we chose not to exclude the data from these two hauls. The net incorporating the rigid-grate BRD with 19.1-mm bar spacing caught 16.6% less eulachon, by weight, than the control net (P < 0.05, Table 2). Bycatch of slender sole, other small flatfish and darkblotched rockfish was also reduced 36.8%, 71.8% and 76.3%, respectively, by the narrower bar spacing (P < 0.01, Table 2). The effect of reduced bar spacing caused no reduction in shrimp catch (P > 0.05, Table 2). Bycatch of whitebait smelt and young-ofthe-year (YOY) Pacific hake was not influenced by the reduction in
Fig. 4. Length frequency of eulachon (total length, mm) captured in ocean shrimp trawl nets with and without the central section of groundline present.
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Table 2 Comparison of mean catch by species and weight (kg haul−1 for all species except eulachon, whitebait smelt and darkblotched rockfish which are reported as g haul−1 ) between ocean shrimp trawl nets equipped with rigid-grate bycatch reduction devices (BRDs) with 19.1- and 25.4-mm bar spacing. SE = standard error. Species name
25.4-mm spacing (SE)
19.1-mm spacing (SE)
Ocean shrimp Eulachon Whitebait smelt YOY Pacific hake Slender sole Other small flatfish Darkblotched rockfish
45.49 (24.21) 382.47 (66.47) 50.11 (13.24) 16.33 (6.58) 0.63 (0.14) 0.51 (0.10) 89.21 (2.07)
46.00 (24.51) 319.17 (55.75) 54.68 (14.96) 17.65 (6.81) 0.40 (0.08) 0.14 (0.03) 21.17 (0.61)
a
Percent reduction with 19.1-mm spacing (%) −1.1 16.6 −9.1 −8.1 36.8 71.8 76.3
P-valuea 0.473 0.023 0.113 0.465 0.00005 0.00005 0.0019
Multi-factor ANOVA, 1-tailed P-value for treatment effect (see text).
bar spacing (P > 0.05, Table 2). Neither the “side” effect nor FLH as a covariate significantly influenced catches (P > 0.05), as would be expected with very similar FLH between the two nets. The length frequency of eulachon captured in the nets incorporating the different BRDs showed no difference in mean total length with respect to grid spacing or “side” of gear (P > 0.05, Fig. 5). Similar reductions in catch were evident across the wide range of eulachon sizes encountered, suggesting that a rigid-grate BRD with 19.1-mm bar spacing is simply more efficient at excluding eulachon of all sizes from the trawl net, rather than differentially excluding larger eulachon. 4. Discussion The findings from these experiments suggest that there are at least two gear-based strategies that may be effective at further reducing fish bycatch in the ocean shrimp trawl fishery, and particularly, the residual bycatch of eulachon. Consistent with some prior work testing rigid-grate BRDs with 19.1-mm bar spacing (Hannah and Jones, 2007), our rigid-grate BRD comparison showed that the use of rigid-grate BRDs with 19.1-mm bar spacing can reduce both total fish bycatch and eulachon bycatch with minimal, if any, effect on ocean shrimp catch rates. The degree of fishery-scale reduction of eulachon and total bycatch from such a requirement is difficult to predict. Generally, fishery-scale bycatch reduction in shrimp fisheries has been less than predicted based on research data (Foster, 2004; Richards and Hendrickson, 2006) although the ocean shrimp fishery has fared better in this regard (Hannah and Jones, 2007). Also, because the bar spacing currently being used in rigid-grate BRDs in the ocean shrimp fishery varies widely (Fig. 1), the additional bycatch reduction for each vessel that would result from a restric-
Fig. 5. Length frequency of eulachon (total length, mm) captured in ocean shrimp trawl nets equipped with rigid-grate bycatch reduction devices with 19.1- and 25.4mm bar spacing.
tion on maximum bar spacing would also vary. Furthermore, these experiments were very limited. They were not conducted across a wide range of environmental conditions or fish densities, both of which have been shown to influence fish behavior inside trawls (Godø et al., 1999; Winger et al., 1999). Our data suggest that restricting the use of groundlines in the central section of shrimp trawl nets could also be effective at reducing bycatch in the ocean shrimp fishery, including that of eulachon. An additional benefit of trawling without a central section is the reduction of trawl-related impacts on some benthic invertebrates. However, to make trawling for shrimp without a central groundline section an economically viable option for vessels, more research is needed to determine if the bycatch reduction we observed in this study can be maintained after FLH is reduced to improve catch rates of ocean shrimp and also the variability in shrimp loss under different fishing conditions. It is possible that reducing FLH, in the absence of a central groundline section, may increase ocean shrimp and eulachon catch proportionately. However, it’s also possible that changes in FLH influence ocean shrimp and eulachon catch rates in different ways, suggesting the possibility of “fine-tuning” footrope modifications to reduce eulachon bycatch with less loss of ocean shrimp. Predicting the level of fishery-scale bycatch reduction that could be achieved through restricting the use of central groundline sections is complicated by several factors. How fish escapement might differ as environmental conditions and fish densities change, as was mentioned above for BRDs, is also an unknown factor with regard to fish entrainment at the footrope of ocean shrimp trawls. Also, as noted above, if central groundline sections are not allowed in ocean shrimp trawls, fishers would probably respond to reduced shrimp catches by reducing FLH, a factor that requires additional study. Another complicating factor is that there are no comprehensive data on the styles of groundlines currently in use in the fishery. The footrope style utilized here and in prior studies as a control to evaluate footrope-based bycatch effects (Fig. 2, Hannah and Jones, 2000, 2003) is only one of several styles of footrope used in this fishery. An unknown number of vessels utilize footropes that incorporate groundlines that are completely covered with 7.5 cm diameter, or larger, rubber disks. It’s likely that these bulkier footropes are more efficient at causing trawl entrainment of eulachon and other small demersal fishes than the control footrope tested here. To further complicate matters, there are already several vessels that currently fish without the use of a central section of groundline, routinely fishing a footrope very similar to the experimental footrope tested in this study. Although it’s clear that BRDs have been highly effective at reducing bycatch in the ocean shrimp fishery (Hannah and Jones, 2007), the survival of the various fish species that are excluded is unknown. Research with Nordmøre grates has shown very high survival rates of 1-year old northern cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) after exclusion from shrimp trawls in Norwegian waters (Soldal
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and Engås, 1997). Underwater video observations of fish exiting ocean shrimp trawls after interacting with BRDs showed a wide range of behaviors for different species and sizes of fish, indicative of variation in the degree of exhaustion and in species- and sizerelated swimming abilities (Hannah et al., 2003). Fish species that are strong swimmers and that are mostly encountered as larger, adult fish on the ocean shrimp grounds, like Pacific halibut (Hippoglossus stenolepis) and lingcod, probably have high survival rates following exclusion via BRDs. Video observations of these types of fishes exiting BRDs in ocean shrimp trawls shows that they appear to be in very good condition and are capable of vigorous swimming (Hannah et al., 2003). However, smaller-bodied species and juvenile fishes with weaker swimming abilities may not survive as well. If footrope-based gear modifications can be developed to reduce residual bycatch in ocean shrimp trawls with acceptable shrimp loss, they will have the added benefit of allowing many small fishes to avoid trawl entrainment entirely, likely reducing physiological stress and potential subsequent mortality. Although this study provides no direct evidence regarding postexclusion survival, it does show some interesting differences in exclusion efficiency by species that may be related to swimming ability and thus to the potential for post-exclusion survival. Eulachon are small-bodied fish that can easily fit through the spaces in a rigid-grate BRD with 19.1-mm spacing. However, they were excluded more efficiently here by the BRD with 19.1-mm bar spacing and the effect was not length-based (Fig. 5), suggesting that the underlying mechanism is an optomotor or avoidance response to the BRD (Kim and Wardle, 2003). This suggests that eulachon have some residual swimming ability as they encounter the BRD and may not have high post-exclusion mortality. In contrast, whitebait smelt and YOY Pacific hake showed no reduction in catch with the BRD with narrower bar spacing, as might be expected for small fishes with little remaining ability to swim as they encounter the BRD. More direct study of the swimming behavior of eulachon and other small fishes as they interact with BRDs in ocean shrimp trawls is needed to further evaluate this hypothesis. Acknowledgements The owners and operators of the fishing vessels Miss Yvonne and Kylie Lynn provided technical expertise and served as sampling platforms for bycatch reduction studies. The Oregon shrimp fleet provided data on BRD use via fishery logbooks. Funding for these studies was provided, in part, by NOAA Fisheries Bycatch Reduction Engineering Program (BREP).
References Buchheister, A., Wilson, M.T., 2005. Shrinkage correction and length conversion equations for Thergra chalcogramma, Mallotus villosus and Thaleichthys pacificus. J. Fish Biol. 67, 541–548. Foster, D.G., 2004. 1999–2003 North-Central and Western Gulf of Mexico BRD Performance. Report to the Gulf of Mexico Fishery Management Council. National Oceanic and Atmospheric Administration, Southeast Fisheries Science Center, Pascagoula, MS. Godø, O.R., Walsh, S.J., Engås, A., 1999. Investigating density-dependent catchability in bottom-trawl surveys. ICES J. Mar. Sci. 56, 292–298. Hannah, R.W., Jones, S.A., 2000. Bycatch reduction in an ocean shrimp (Pandalus jordani) trawl from a simple modification to the trawl footrope. J. Northwest. Atl. Fish. Sci. 27, 227–234. Hannah, R.W., Jones, S.A., 2003. Measuring the height of the fishing line and its effect on shrimp catch and bycatch in an ocean shrimp (Pandalus jordani) trawl. Fish. Res. 60, 427–438. Hannah, R.W., Jones, S.A., 2007. Effectiveness of bycatch reduction devices (BRDs) in the ocean shrimp (Pandalus jordani) trawl fishery. Fish. Res. 85, 217–225. Hannah, R.W., Jones, S.A., Hoover, V.J., 1996. Evaluation of fish excluder technology to reduce finfish bycatch in the pink shrimp trawl fishery. Oregon Dept. Fish Wildl., Info. Rep. Ser., Fish. No. 96-4. p. 46. Hannah, R.W., Jones, S.A., Matteson, K.M., 2003. Observations of fish and shrimp behavior in ocean shrimp (Pandalus jordani) trawls. Oregon Dept. Fish Wildl., Info. Rept. Ser., Fish. No. 2003-03. p. 28. Hannah, R.W., Jones, S.A., Miller, W., Knight, J.S., 2010. Effects of trawling for ocean shrimp (Pandalus jordani) on macroinvertebrate abundance and diversity at four sites near Nehalem Bank. Oreg. Fish. Bull. 108, 30–38. Hay, D.E., McCarter, P.B., 2000. Status of the eulachon Thaleichthys pacificus in Canada. Department of Fisheries and Oceans Canada, Canadian Stock Assessment Secretariat, Ottawa, Ontario (Research Document 2000-145). Isaksen, B., Valdemarsen, J.W., Larsen, R.B., Karlsen, L., 1992. Reduction of finfish bycatch in shrimp trawl using a rigid separator grid in the aft belly. Fish. Res. 13, 335–352. Kim, Y.H., Wardle, C.S., 2003. Optomotor response and erratic response: quantitative analysis of fish reaction to towed fishing gears. Fish. Res. 60, 455–470. Miller, S.D., Clarke, M.E., Hastie, J.D., Hamel, O.S. (Eds.), 2009. Our living oceans: Report on the Status of U.S. Living Marine Resources., 6th ed. U.S. Dept. of Commer., pp. 211–222, NOAA Tech. Memo. NMFS-F/SPO-80, p. 369. NMFS (National Marine Fisheries Service), 2010. Endangered and threatened wildlife and plants: threatened status for southern distinct population segment of eulachon. Federal register (Docket No. 080229343-0039-03; March 18, 2010) 75 (52), 13012–13024. NWFSC (Northwest Fisheries Science Center), 2009. Data Report and Summary Analyses of the California and Oregon Pink Shrimp Trawl Fisheries. West Coast Groundfish Observer Program. NWFSC, 2725 Montlake Blvd E., Seattle, WA 98112. Richards, A., Hendrickson, A.L., 2006. Effectiveness of the Nordmøre grate in the Gulf of Maine northern shrimp fishery. Fish. Res. 81, 100–106. Sokal, R.R., Rohlf, F.J., 1981. Biometry, 2nd ed. W.H. Freeman, New York. Soldal, A.V., Engås, A., 1997. Survival of young gadoids excluded from a shrimp trawl by a rigid deflecting grid. ICES J. Mar. Sci. 54, 117–124. Wallace, J.R., Hamel, O.S., 2009. Status and future prospects for the darkblotched rockfish resource in waters off Washington, Oregon, and California as updated in 2009. Pacific Fishery Management Council, 7700 NE Ambassador Place, Suite 200, Portland, OR 97201. Winger, P.D., He, P., Walsh, S.J., 1999. Swimming endurance of American plaice (Hippoglossoides platessoides) and its role in fish capture. ICES J. Mar. Sci. 56, 252–265.
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Trawl net modifications to reduce the bycatch of eulachon (Thaleichthys pacificus) in the ocean shrimp (Pandalus jordani) fishery Robert W. Hannah a,∗ , Stephen A. Jones a , Mark J.M. Lomeli b , W. Waldo Wakefield c a
Oregon Department of Fish and Wildlife, Marine Resources Program, 2040 SE Marine Science Drive, Newport, OR 97365, USA Pacific States Marine Fisheries Commission, 2032 SE OSU Drive, Newport, OR 97365, USA Northwest Fisheries Science Center, Fishery Resource Analysis and Monitoring Division, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2032 SE OSU Drive, Newport, OR 97365, USA b c
a r t i c l e
i n f o
Article history: Received 15 March 2011 Received in revised form 21 April 2011 Accepted 21 April 2011 Keywords: Bycatch reduction devices Nordmøre grate Ocean shrimp Shrimp trawl Eulachon Fish behavior Trawl footrope
a b s t r a c t Two trawl gear modifications for reducing fish bycatch (weight) in ocean shrimp (Pandalus jordani) trawls were tested in June and August–September 2010. The primary focus of the study was evaluating trawl system modifications for reducing bycatch of eulachon (Thaleichthys pacificus) below levels already achieved via mandatory use of bycatch reduction devices (BRDs). An experimental footrope, modified by removing the central one third of the trawl groundline, reduced eulachon bycatch by 33.9%. It also reduced bycatch of slender sole (Lyopsetta exilis), other small flatfishes and juvenile darkblotched rockfish (Sebastes crameri) by 80% or more, but had no effect on bycatch of whitebait smelt (Allosmerus elongatus) or Pacific herring (Clupea pallasii). The experimental groundline also reduced the catch of ocean shrimp (weight) by 22.2% in hauls yielding commercial quantities of shrimp (>194 kg/haul) and by 23.2% in all hauls. Reducing bar spacing in a rigid-grate BRD from 25.4 mm to 19.1 mm reduced eulachon bycatch by 16.6%, with no reduction in ocean shrimp catch. It also reduced bycatch of slender sole, other small flatfish and juvenile darkblotched rockfish by 36.8%, 71.8% and 76.3%, respectively with no effect on bycatch of whitebait smelt or young-of-the-year (YOY) Pacific hake (Merluccius productus). Although both trawl modifications reduced eulachon bycatch, the footrope modification tested, if developed further, has the potential to also avoid trawl entrainment for some demersal fishes, as well as reduce bottom impacts from trawling. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since 2003, vessels participating in the ocean shrimp (Pandalus jordani, also known as smooth pink shrimp) trawl fishery that operates in the United States coastal shelf waters (water depths of about 80–300 m) off Washington, Oregon and California have been required to use bycatch reduction devices (BRDs) for all fishing. The BRDs allowed included both soft-panel (127 mm maximum stretch mesh size) and rigid-grate BRDs (51 mm maximum spacing of vertical bars) very similar in function to the Nordmøre grate (Hannah and Jones, 2007; Isaksen et al., 1992). As of 2005, the BRD requirements, which were enacted by the states of Washington, Oregon and California primarily to reduce the bycatch of canary rockfish (Sebastes pinniger), had reduced total fish bycatch in the Oregon component of the fishery by 66–88% by weight from historical levels, leaving bycatch at about 7.5% of total catch (Hannah and Jones, 2007). The BRD requirement also shifted the species and size composition of the bycatch from one heavily dominated by adult and
∗ Corresponding author. Tel.: +1 541 867 0300x231; fax: +1 541 867 0311. E-mail address: [email protected] (R.W. Hannah). 0165-7836/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2011.04.016
juvenile Pacific hake (Merluccius productus), smelts (Osmeridae), adult yellowtail rockfish (Sebastes flavidus) and lingcod (Ophiodon elongatus) to one composed mostly of fish with very low or no commercial value such as juvenile Pacific hake, smelts, slender sole (Lyopsetta exilis), small rex sole (Glyptocephalus zachirus) and juvenile rockfishes (Sebastes spp., Hannah and Jones, 2007). The BRDs used in this fishery recently are almost exclusively rigid-grate designs (used in 99% of fishing trips in 2009, Oregon Department of Fish and Wildlife, unpublished data) and since 2005, bar spacing in these BRDs has been steadily dropping (Fig. 1), suggesting that current levels of fish bycatch may now be even lower than 7.5% of total catch. Comprehensive data on BRD designs used by participating vessels is limited to the Oregon fishery. A typical BRD used in this fishery in 2006 was a rigid-grate device with bar spacing of more than 31.8 mm (Fig. 1). As of 2009, the majority of shrimp trawl trips employed rigid-grate BRDs with bar spacing ranging from 22.2 mm to 28.6 mm, and about 7% of trips used BRDs with bar spacing as small as 19.1 mm (Fig. 1). Although fish bycatch in the ocean shrimp trawl fishery has been greatly reduced by the use of BRDs, some conservation concerns remain regarding the residual fish bycatch. In March of 2010, the southern distinct population segment (DPS) of eulachon
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Fig. 1. Distribution of bar spacing in rigid-grate bycatch reduction devices (BRDs) used in the Oregon trawl fishery for ocean shrimp, by percentage of trips, 2006–2009.
(Thaleichthys pacificus) was listed as “threatened” under the United States Endangered Species Act (NMFS, 2010). Eulachon is an anadromous smelt (Osmeridae) species that spawns in many rivers on the U.S. and Canadian west coasts, from California northward through parts of Alaska (Hay and McCarter, 2000). The southern DPS includes stocks spawning in rivers ranging from California to the Nass River in British Columbia, Canada. The stock composition of eulachon captured in the ocean shrimp trawl fishery in U.S. waters is unknown. However eulachon can at times constitute a substantial component of the residual bycatch (NWFSC, 2009) and it’s likely that many of these fish are from the southern DPS. Therefore, techniques to further reduce eulachon bycatch in the shrimp trawl fishery are needed to reduce impacts and aid in the recovery of eulachon. Current shrimp fishery bycatch also includes juvenile darkblotched rockfish (Sebastes crameri), a slope rockfish species that was declared “overfished” and is now in a rebuilding phase in U.S. west coast waters (Miller et al., 2009; Wallace and Hamel, 2009), suggesting another conservation benefit that could result from further reducing shrimp fishery bycatch. Additional field studies to reduce bycatch of eulachon are warranted for several reasons. First, most earlier studies did not identify smelts to the species level and thus cannot be directly applied to eulachon (Hannah and Jones, 2000, 2003; Hannah et al., 1996). Also, because of the decline in eulachon abundance, most of the more recent research on bycatch reduction has been conducted in areas or years in which eulachon were either not abundant or totally absent. Lastly, new research was needed to further evaluate bycatch reduction techniques for which only preliminary data
had been generated, but which showed some potential for reducing eulachon bycatch (Hannah and Jones, 2007). We report here on two 2010 field studies that evaluated gearbased approaches to further reduce fish bycatch in the ocean shrimp trawl fishery, with an emphasis on reducing the bycatch of eulachon. The first experiment tested the effect on bycatch of completely removing a large central section of groundline from a shrimp trawl. This study was conducted as one component of a larger study designed to evaluate how changes in footrope configuration influence impacts on benthic macroinvertebrates, some of which have been negatively affected by chronic shrimp trawling (Hannah et al., 2010). Ocean shrimp trawls are considered semipelagic, in that the fishing line of the net is suspended 35–70 cm above the sea floor, while a separate, parallel groundline is in contact with the bottom (Fig. 2). It has previously been shown that the relative positions of the fishing line and groundline in ocean shrimp trawls can greatly influence bycatch of some species, and that removing or disabling the central portion of the groundline can reduce bycatch of juvenile rockfish and small flatfish (Hannah and Jones, 2000, 2003). However, the effect of removing the central section of the groundline on bycatch of eulachon has not been previously studied. The second experiment we conducted tested the effect on bycatch of reducing bar spacing in rigid-grate BRDs below that most commonly used in the fishery, specifically a comparison of bycatch with rigid-grate BRDs with 19.1- and 25.4-mm bar spacing. This comparison was conducted to follow up on preliminary data (Hannah and Jones, 2007) suggesting that rigid-grate BRDs with 19.1-mm bar spacing could be used to further reduce fish bycatch in this fishery while causing little or no reduction in shrimp catch.
2. Methods We evaluated bycatch reduction by comparing catches from the port and starboard nets of a double-rigged shrimp trawl vessel, with one net incorporating the treatment of interest and the other serving as a control. Haul duration was limited to about 45–60 min to avoid shrimp catches that were too large to sort and weigh with the field staff available. Towing speed was typical for ocean shrimp trawling, varying between 3.0 km h−1 and 3.3 km h−1 (1.6–1.8 knots). Three techniques were used to compensate for potential differences in catch efficiency between the two nets. First, both trawl nets were inspected to make sure they were rigged as similarly as possible. Second, the treatment was
Fig. 2. Schematic drawing of an ocean shrimp trawl with a “ladder style” footrope (view looking into the net from the front).
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interchanged between the two nets at regular intervals. In the rigid-grate BRD experiment, the BRDs with 19.1- and 25.4-mm bar spacing were alternated between the port and starboard nets every two hauls, utilizing an ABBA pattern. For the groundline experiment, the treatment and control groundlines were interchanged between the port and starboard nets after the first and third days of the four-day cruise, also resulting in an ABBA pattern, but in terms of days fished. The final technique used to control for netspecific differences in catch efficiency was in situ measurement of fishing line height (FLH) and adjustments to equalize this variable between the two nets prior to starting comparative tows. Increased FLH has been shown to reduce both shrimp catch and bycatch of some species in ocean shrimp trawl nets (Hannah and Jones, 2003), so equalizing this variable between port and starboard nets, to the extent possible, reduces between-net variance and makes it easier to detect treatment-related differences in catch composition. We used recording inclinometers at the center of the fishing line on both nets, following Hannah and Jones (2003), to measure FLH for a series of preliminary hauls. Based on these data, adjustments were made to the lengths of chain connecting the groundline to the fishing line in one or both of the nets, followed by additional test hauls. We continued this process of measuring and adjusting FLH until the two nets were as similar as possible in average FLH, given the vessel time available to complete each project. For the groundline experiment, a second set of measurements and adjustments to FLH was made after the treatment effect was moved from one net to the other, following the first full day of comparative hauls. Then on the fourth day of that experiment, the groundline configurations were returned to their settings from the first day. Data analysis was similar for both experiments. Catches from each net were emptied into one side of a divided hopper and sorted to species and weighed at sea. The combined catch of smelt species and in some instances, juvenile rockfish, from each net was weighed at sea, placed into labeled sample bags and frozen for later identification in the laboratory. Lengths (mm) were measured for eulachon during lab analysis. For the groundline experiment, we measured fork length of eulachon, but switched to total length for the rigid-grate comparison as it proved easier to measure consistently for eulachon. Fork length measurements were converted to total lengths using the equation of Buchheister and Wilson (2005). For a few hauls in the groundline experiment, the smelt catch was too large to bag and freeze all of it. In those instances a subsample (approximately 1 kg) was bagged and frozen and the remaining smelt were weighed and discarded at sea. Weight, by species, from laboratory analysis was used to expand subsamples to estimate total catch, by haul and net, for eulachon and any other smelt species captured. Catch weight data were analyzed as a multi-factor ANOVA, with haul, side of gear (port or starboard) and the treatment as main effects without interaction, following Hannah et al. (1996). To account for remaining differences in FLH, we also included FLH as a covariate. To determine the statistical significance of a particular treatment, we utilized a one-tailed P-value. For some species or species groups, transformations were utilized to achieve normality of model residuals. Length data for eulachon were expected to be multi-modal and were therefore compared between treatment and control nets using the nonparametric Wilcoxon two-sample test (Sokal and Rohlf, 1981). 2.1. Groundline experiment The groundline experiment was conducted in June 2010 utilizing the 22 m double-rigged shrimp trawler F/V Kylie Lynn, out of Newport, OR. The study area we selected was located just north of the Umpqua River, OR, centered on the shoreward edge of the main shrimping grounds, at depths ranging from 99 m to 148 m.
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This area was chosen because it often holds quantities of both eulachon and ocean shrimp, as well as sufficient abundances of sea whips (Halipteris spp.) and other benthic invertebrates that were important for other questions being addressed during the cruise. The footrope on the control net was configured as shown in Fig. 2. The experimental footrope was configured similarly, except that the central section of the groundline (about 1/3 of the total groundline) was removed, leaving just a few simple drop chains attached to the central section of the fishing line to stabilize it. Both nets incorporated rigid-grate BRDs with 19.1-mm bar spacing. 2.2. Comparison of rigid-grate BRDs with 19.1- and 25.4-mm bar spacing The rigid-grate BRD experiment was conducted in AugustSeptember 2010 utilizing the 21 m double-rigged shrimp trawler F/V Miss Yvonne, out of Newport, OR. The study area was the same as the one used for the groundline experiment, however fishing was conducted at a slightly deeper depth range, from 108 m to 157 m. The groundlines of both trawls were configured as shown in Fig. 2 (control groundline). The BRDs we compared were of identical construction, except for bar spacing and the diameter of the round stock used for the vertical bars. They were both constructed using an outer circular frame of 107 cm diameter, made of 32-mm (outside diameter) aluminum tubing. The BRD with 25.4-mm bar spacing utilized 8-mm diameter solid aluminum rod for the vertical bars while the BRD with 19.1-mm spacing used 6.4-mm diameter solid aluminum rod. This difference allowed for approximately the same total open area between the bars for both BRDs, to equalize water flow into the codend. Neither BRD incorporated a guiding panel to concentrate catch at the bottom of the grate (for a diagram of typical rigid-grate BRDs, see Hannah and Jones, 2007). 3. Results 3.1. Groundline experiment We completed 35 double-rigged hauls comparing ocean shrimp trawl nets with and without the central section of the groundline and analyzed catch data from 26 hauls. The first 3 hauls and hauls 11–15 were spent measuring and adjusting FLH between the two nets (Fig. 3A). Haul 20 was excluded because of zero catch in both nets. Despite using eight hauls to evaluate and adjust FLH, it was never completely equalized between the two nets used in this experiment (Fig. 3A). However, FLH averaged about 7.8 cm higher on the starboard net (paired t-test, P < 0.001). FLH varied considerably during this experiment, most likely as a result of variability in sea conditions and currents. Although FLH was not generally highly variable within a haul, removing the central section of groundline did make FLH more variable in that net (P < 0.05), with more of an effect on the port net. The lack of a central groundline section increased the standard error of FLH by 63% in the port net versus 25% in the starboard net, when each was compared to the opposing net with a central groundline section. The absence of a central groundline section significantly (P < 0.05) decreased eulachon bycatch by 33.9%, by weight, in relation to the control net. This footrope modification also decreased residual bycatch of slender sole, other small flatfish and darkblotched rockfish by 95.9% (P < 0.0001), 96.5% (P < 0.0001) and 79.6% (P < 0.001), respectively (Table 1). No effect on the bycatch of whitebait smelt (Allosmerus elongatus) or Pacific herring (Clupea pallasii) was observed (P > 0.05). The effect of removing the central section of the groundline on shrimp catch was more difficult to discern from these data. The mean catch of ocean shrimp in the net with no central groundline
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Table 1 Comparison of mean catch by species and weight (kg haul−1 for all species except eulachon, whitebait smelt and darkblotched rockfish which are reported as g haul−1 ) between ocean shrimp trawl nets with and without the central section of groundline. SE = standard error. Species name
Complete groundline (control) (SE)
Central section of groundline removed (SE)
Reduction with groundline removed (%)
P-valuea
Ocean shrimp Eulachon Whitebait smelt Pacific herring Slender sole Other small flatfish Darkblotched rockfish
176.12 (58.32) 2858.10 (519.02) 29.00 (11.95) 1.11 (0.22) 3.92 (0.55) 1.72 (0.37) 61.07 (15.62)
135.22 (41.74) 1890.06 (371.14) 17.81 (7.05) 1.04 (0.26) 0.16 (0.10) 0.06 (0.02) 12.46 (2.50)
23.2 33.9 11.2 6.3 95.9 96.5 79.6
0.066 0.017 0.367 0.481 0.00005 0.00005 0.0001
a
Multi-factor ANOVA, 1-tailed P-value for treatment effect (see text).
section was 23.2% less than in the control net. However, catches were so variable among hauls that this mean difference was not statistically significant (P > 0.05). Considering just the hauls with commercial quantities of ocean shrimp (>194 kg/haul, 12 hauls total, equally balanced between having the treatment effect on port and starboard sides) resulted in a highly significant reduction in ocean shrimp catch of 22.2% due to the absence of the central section of groundline (P < 0.001). FLH was not statistically significant as a covariate for any of the species or species groups (P > 0.05). For ocean shrimp, the “side” effect (port versus starboard net) was statistically significant (P < 0.05), with the port net accounting for about 18% more catch weight than the starboard net. The “side” effect was also significant for darkblotched rockfish (P < 0.05) with the port net again having higher catches. Interestingly, there was no evidence for a “side” effect for eulachon, as the mean catches between port and starboard nets differed by less than 1%.
Fig. 3. Mean fishing line height (cm, measured at the center of the fishing line) in port and starboard nets, by treatment, in fishing experiments comparing catches in ocean shrimp trawl nets with and without the central section of the groundline (A) and in nets with rigid-grate BRDs with bar spacing of 19.1- and 25.4-mm (B).
Comparison of eulachon length frequency data from the treatment and control nets showed a slight reduction (5 mm) in mean total length of eulachon captured in the net with the central section of groundline removed (Wilcoxon test, P < 0.01, Fig. 4). This suggests that removing the central section of groundline from ocean shrimp trawls reduces entrainment into the trawl slightly more for large eulachon than for smaller ones. Given the difference in FLH between the two nets used in this experiment, this result should be considered preliminary. 3.2. Comparison of rigid-grate BRDs with 19.1- and 25.4-mm bar spacing We analyzed 30 hauls comparing bycatch with the rigid-grate BRDs with 19.1- and 25.4-mm bar spacing (Fig. 3B). FLH was better equalized between the two nets in this experiment, a result of utilizing numerous initial hauls for iterative measurement and adjustment of FLH (not shown in Fig. 3B). After all adjustments, FLH was still slightly higher in the port net (t-test, P < 0.001), with a mean difference of 4.2 cm, however, this difference is probably very close to the real limits of accuracy of the recording inclinometers. FLH was less variable during this experiment, possibly as a result of different sea conditions. For reasons unknown, FLH was increased in the port net for hauls 23 and 24. This type of variation can result from debris becoming entangled on the footrope. Since this type of event happens frequently during normal shrimping activity, we chose not to exclude the data from these two hauls. The net incorporating the rigid-grate BRD with 19.1-mm bar spacing caught 16.6% less eulachon, by weight, than the control net (P < 0.05, Table 2). Bycatch of slender sole, other small flatfish and darkblotched rockfish was also reduced 36.8%, 71.8% and 76.3%, respectively, by the narrower bar spacing (P < 0.01, Table 2). The effect of reduced bar spacing caused no reduction in shrimp catch (P > 0.05, Table 2). Bycatch of whitebait smelt and young-ofthe-year (YOY) Pacific hake was not influenced by the reduction in
Fig. 4. Length frequency of eulachon (total length, mm) captured in ocean shrimp trawl nets with and without the central section of groundline present.
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Table 2 Comparison of mean catch by species and weight (kg haul−1 for all species except eulachon, whitebait smelt and darkblotched rockfish which are reported as g haul−1 ) between ocean shrimp trawl nets equipped with rigid-grate bycatch reduction devices (BRDs) with 19.1- and 25.4-mm bar spacing. SE = standard error. Species name
25.4-mm spacing (SE)
19.1-mm spacing (SE)
Ocean shrimp Eulachon Whitebait smelt YOY Pacific hake Slender sole Other small flatfish Darkblotched rockfish
45.49 (24.21) 382.47 (66.47) 50.11 (13.24) 16.33 (6.58) 0.63 (0.14) 0.51 (0.10) 89.21 (2.07)
46.00 (24.51) 319.17 (55.75) 54.68 (14.96) 17.65 (6.81) 0.40 (0.08) 0.14 (0.03) 21.17 (0.61)
a
Percent reduction with 19.1-mm spacing (%) −1.1 16.6 −9.1 −8.1 36.8 71.8 76.3
P-valuea 0.473 0.023 0.113 0.465 0.00005 0.00005 0.0019
Multi-factor ANOVA, 1-tailed P-value for treatment effect (see text).
bar spacing (P > 0.05, Table 2). Neither the “side” effect nor FLH as a covariate significantly influenced catches (P > 0.05), as would be expected with very similar FLH between the two nets. The length frequency of eulachon captured in the nets incorporating the different BRDs showed no difference in mean total length with respect to grid spacing or “side” of gear (P > 0.05, Fig. 5). Similar reductions in catch were evident across the wide range of eulachon sizes encountered, suggesting that a rigid-grate BRD with 19.1-mm bar spacing is simply more efficient at excluding eulachon of all sizes from the trawl net, rather than differentially excluding larger eulachon. 4. Discussion The findings from these experiments suggest that there are at least two gear-based strategies that may be effective at further reducing fish bycatch in the ocean shrimp trawl fishery, and particularly, the residual bycatch of eulachon. Consistent with some prior work testing rigid-grate BRDs with 19.1-mm bar spacing (Hannah and Jones, 2007), our rigid-grate BRD comparison showed that the use of rigid-grate BRDs with 19.1-mm bar spacing can reduce both total fish bycatch and eulachon bycatch with minimal, if any, effect on ocean shrimp catch rates. The degree of fishery-scale reduction of eulachon and total bycatch from such a requirement is difficult to predict. Generally, fishery-scale bycatch reduction in shrimp fisheries has been less than predicted based on research data (Foster, 2004; Richards and Hendrickson, 2006) although the ocean shrimp fishery has fared better in this regard (Hannah and Jones, 2007). Also, because the bar spacing currently being used in rigid-grate BRDs in the ocean shrimp fishery varies widely (Fig. 1), the additional bycatch reduction for each vessel that would result from a restric-
Fig. 5. Length frequency of eulachon (total length, mm) captured in ocean shrimp trawl nets equipped with rigid-grate bycatch reduction devices with 19.1- and 25.4mm bar spacing.
tion on maximum bar spacing would also vary. Furthermore, these experiments were very limited. They were not conducted across a wide range of environmental conditions or fish densities, both of which have been shown to influence fish behavior inside trawls (Godø et al., 1999; Winger et al., 1999). Our data suggest that restricting the use of groundlines in the central section of shrimp trawl nets could also be effective at reducing bycatch in the ocean shrimp fishery, including that of eulachon. An additional benefit of trawling without a central section is the reduction of trawl-related impacts on some benthic invertebrates. However, to make trawling for shrimp without a central groundline section an economically viable option for vessels, more research is needed to determine if the bycatch reduction we observed in this study can be maintained after FLH is reduced to improve catch rates of ocean shrimp and also the variability in shrimp loss under different fishing conditions. It is possible that reducing FLH, in the absence of a central groundline section, may increase ocean shrimp and eulachon catch proportionately. However, it’s also possible that changes in FLH influence ocean shrimp and eulachon catch rates in different ways, suggesting the possibility of “fine-tuning” footrope modifications to reduce eulachon bycatch with less loss of ocean shrimp. Predicting the level of fishery-scale bycatch reduction that could be achieved through restricting the use of central groundline sections is complicated by several factors. How fish escapement might differ as environmental conditions and fish densities change, as was mentioned above for BRDs, is also an unknown factor with regard to fish entrainment at the footrope of ocean shrimp trawls. Also, as noted above, if central groundline sections are not allowed in ocean shrimp trawls, fishers would probably respond to reduced shrimp catches by reducing FLH, a factor that requires additional study. Another complicating factor is that there are no comprehensive data on the styles of groundlines currently in use in the fishery. The footrope style utilized here and in prior studies as a control to evaluate footrope-based bycatch effects (Fig. 2, Hannah and Jones, 2000, 2003) is only one of several styles of footrope used in this fishery. An unknown number of vessels utilize footropes that incorporate groundlines that are completely covered with 7.5 cm diameter, or larger, rubber disks. It’s likely that these bulkier footropes are more efficient at causing trawl entrainment of eulachon and other small demersal fishes than the control footrope tested here. To further complicate matters, there are already several vessels that currently fish without the use of a central section of groundline, routinely fishing a footrope very similar to the experimental footrope tested in this study. Although it’s clear that BRDs have been highly effective at reducing bycatch in the ocean shrimp fishery (Hannah and Jones, 2007), the survival of the various fish species that are excluded is unknown. Research with Nordmøre grates has shown very high survival rates of 1-year old northern cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) after exclusion from shrimp trawls in Norwegian waters (Soldal
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and Engås, 1997). Underwater video observations of fish exiting ocean shrimp trawls after interacting with BRDs showed a wide range of behaviors for different species and sizes of fish, indicative of variation in the degree of exhaustion and in species- and sizerelated swimming abilities (Hannah et al., 2003). Fish species that are strong swimmers and that are mostly encountered as larger, adult fish on the ocean shrimp grounds, like Pacific halibut (Hippoglossus stenolepis) and lingcod, probably have high survival rates following exclusion via BRDs. Video observations of these types of fishes exiting BRDs in ocean shrimp trawls shows that they appear to be in very good condition and are capable of vigorous swimming (Hannah et al., 2003). However, smaller-bodied species and juvenile fishes with weaker swimming abilities may not survive as well. If footrope-based gear modifications can be developed to reduce residual bycatch in ocean shrimp trawls with acceptable shrimp loss, they will have the added benefit of allowing many small fishes to avoid trawl entrainment entirely, likely reducing physiological stress and potential subsequent mortality. Although this study provides no direct evidence regarding postexclusion survival, it does show some interesting differences in exclusion efficiency by species that may be related to swimming ability and thus to the potential for post-exclusion survival. Eulachon are small-bodied fish that can easily fit through the spaces in a rigid-grate BRD with 19.1-mm spacing. However, they were excluded more efficiently here by the BRD with 19.1-mm bar spacing and the effect was not length-based (Fig. 5), suggesting that the underlying mechanism is an optomotor or avoidance response to the BRD (Kim and Wardle, 2003). This suggests that eulachon have some residual swimming ability as they encounter the BRD and may not have high post-exclusion mortality. In contrast, whitebait smelt and YOY Pacific hake showed no reduction in catch with the BRD with narrower bar spacing, as might be expected for small fishes with little remaining ability to swim as they encounter the BRD. More direct study of the swimming behavior of eulachon and other small fishes as they interact with BRDs in ocean shrimp trawls is needed to further evaluate this hypothesis. Acknowledgements The owners and operators of the fishing vessels Miss Yvonne and Kylie Lynn provided technical expertise and served as sampling platforms for bycatch reduction studies. The Oregon shrimp fleet provided data on BRD use via fishery logbooks. Funding for these studies was provided, in part, by NOAA Fisheries Bycatch Reduction Engineering Program (BREP).
References Buchheister, A., Wilson, M.T., 2005. Shrinkage correction and length conversion equations for Thergra chalcogramma, Mallotus villosus and Thaleichthys pacificus. J. Fish Biol. 67, 541–548. Foster, D.G., 2004. 1999–2003 North-Central and Western Gulf of Mexico BRD Performance. Report to the Gulf of Mexico Fishery Management Council. National Oceanic and Atmospheric Administration, Southeast Fisheries Science Center, Pascagoula, MS. Godø, O.R., Walsh, S.J., Engås, A., 1999. Investigating density-dependent catchability in bottom-trawl surveys. ICES J. Mar. Sci. 56, 292–298. Hannah, R.W., Jones, S.A., 2000. Bycatch reduction in an ocean shrimp (Pandalus jordani) trawl from a simple modification to the trawl footrope. J. Northwest. Atl. Fish. Sci. 27, 227–234. Hannah, R.W., Jones, S.A., 2003. Measuring the height of the fishing line and its effect on shrimp catch and bycatch in an ocean shrimp (Pandalus jordani) trawl. Fish. Res. 60, 427–438. Hannah, R.W., Jones, S.A., 2007. Effectiveness of bycatch reduction devices (BRDs) in the ocean shrimp (Pandalus jordani) trawl fishery. Fish. Res. 85, 217–225. Hannah, R.W., Jones, S.A., Hoover, V.J., 1996. Evaluation of fish excluder technology to reduce finfish bycatch in the pink shrimp trawl fishery. Oregon Dept. Fish Wildl., Info. Rep. Ser., Fish. No. 96-4. p. 46. Hannah, R.W., Jones, S.A., Matteson, K.M., 2003. Observations of fish and shrimp behavior in ocean shrimp (Pandalus jordani) trawls. Oregon Dept. Fish Wildl., Info. Rept. Ser., Fish. No. 2003-03. p. 28. Hannah, R.W., Jones, S.A., Miller, W., Knight, J.S., 2010. Effects of trawling for ocean shrimp (Pandalus jordani) on macroinvertebrate abundance and diversity at four sites near Nehalem Bank. Oreg. Fish. Bull. 108, 30–38. Hay, D.E., McCarter, P.B., 2000. Status of the eulachon Thaleichthys pacificus in Canada. Department of Fisheries and Oceans Canada, Canadian Stock Assessment Secretariat, Ottawa, Ontario (Research Document 2000-145). Isaksen, B., Valdemarsen, J.W., Larsen, R.B., Karlsen, L., 1992. Reduction of finfish bycatch in shrimp trawl using a rigid separator grid in the aft belly. Fish. Res. 13, 335–352. Kim, Y.H., Wardle, C.S., 2003. Optomotor response and erratic response: quantitative analysis of fish reaction to towed fishing gears. Fish. Res. 60, 455–470. Miller, S.D., Clarke, M.E., Hastie, J.D., Hamel, O.S. (Eds.), 2009. Our living oceans: Report on the Status of U.S. Living Marine Resources., 6th ed. U.S. Dept. of Commer., pp. 211–222, NOAA Tech. Memo. NMFS-F/SPO-80, p. 369. NMFS (National Marine Fisheries Service), 2010. Endangered and threatened wildlife and plants: threatened status for southern distinct population segment of eulachon. Federal register (Docket No. 080229343-0039-03; March 18, 2010) 75 (52), 13012–13024. NWFSC (Northwest Fisheries Science Center), 2009. Data Report and Summary Analyses of the California and Oregon Pink Shrimp Trawl Fisheries. West Coast Groundfish Observer Program. NWFSC, 2725 Montlake Blvd E., Seattle, WA 98112. Richards, A., Hendrickson, A.L., 2006. Effectiveness of the Nordmøre grate in the Gulf of Maine northern shrimp fishery. Fish. Res. 81, 100–106. Sokal, R.R., Rohlf, F.J., 1981. Biometry, 2nd ed. W.H. Freeman, New York. Soldal, A.V., Engås, A., 1997. Survival of young gadoids excluded from a shrimp trawl by a rigid deflecting grid. ICES J. Mar. Sci. 54, 117–124. Wallace, J.R., Hamel, O.S., 2009. Status and future prospects for the darkblotched rockfish resource in waters off Washington, Oregon, and California as updated in 2009. Pacific Fishery Management Council, 7700 NE Ambassador Place, Suite 200, Portland, OR 97201. Winger, P.D., He, P., Walsh, S.J., 1999. Swimming endurance of American plaice (Hippoglossoides platessoides) and its role in fish capture. ICES J. Mar. Sci. 56, 252–265.