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Measuring the height of the fishing line and its effect on shrimp catch and bycatch in an ocean shrimp (Pandalus jordani) trawl
Fisheries Research 60 (2003) 427–438
Measuring the height of the fishing line and its effect on shrimp catch and bycatch in an ocean shrimp (Pandalus jordani) trawl Robert W. Hannah∗ , Stephen A. Jones Oregon Department of Fish and Wildlife, Marine Resources Program, 2040 S.E. Marine Science Drive, Newport, OR 97365, USA Received 6 December 2001; received in revised form 17 May 2002; accepted 26 May 2002
Abstract The relationship between fishing line height (FLH), shrimp catch and bycatch in a semi-pelagic ocean shrimp (Pandalus jordani) trawl was investigated using a newly developed recording inclinometer. The inclinometer was effective at measuring FLH and indicating trawl performance deficits. FLH was determined to be stable during a haul and also between hauls within a given footrope and groundline configuration. FLH was readily adjusted with simple modifications to the footrope “dropper” chains. Inclinometer data showed that FLH can be unequal between double-rigged nets of identical configuration. Shrimp catch and the bycatch of flatfish and juvenile rockfish varied inversely with FLH, suggesting FLH can be adjusted to equalize the catch of shrimp, flatfish and juvenile rockfish between two double-rigged shrimp nets. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bycatch; Footrope; Semi-pelagic shrimp trawl; Mensuration
1. Introduction The Pacific trawl fishery for ocean shrimp (Pandalus jordani) operates mostly between Cape Mendocino, California and the west coast of Vancouver Island, British Columbia, in water depths of between 120 and 240 m. At these depths, ocean shrimp are found mostly in areas with green mud bottoms (Zirges and Robinson, 1980). The regional catch of ocean shrimp is highly variable (Hannah, 1999) and in recent years has ranged from about 4500 t to over 36,000 t. The typical ocean shrimp vessel is between 20 and 27 m in length and is double-rigged (Zirges and Robinson, 1980). Typical ocean shrimp nets are high rise box trawls with footrope lengths of between 20 and 27 m, closely ∗ Corresponding author. Tel.: +1-541-867-4741; fax: +1-541-867-0311. E-mail address: [email protected] (R.W. Hannah).
coupled to rectangular wooden trawl doors approximately 2.1 m high by 1.8 m wide (Jones et al., 1996). The fishery has relatively low levels of fish bycatch in comparison to most other large-scale shrimp fisheries (Alverson et al., 1994). However, high levels of Pacific whiting (Merluccius productus) bycatch have caused operational problems in the fishery at times (Hannah and Jones, 2000), leading to research on the effectiveness of various bycatch reduction devices (BRDs) in the codend (Hannah et al., 1996). More recently, mandatory codend BRDs have been used as a management tool to limit canary rockfish (Sebastes pinniger) catch in this fishery to meet allocation goals set by the Pacific Fishery Management Council. Tests of BRDs in the ocean shrimp fishery have generally compared catches of shrimp and bycatch in each of two double-rigged nets deployed from a shrimp vessel. With both nets fished simultaneously, one incorporating the BRD and the other acting as a control,
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a very powerful statistical design is achieved (Hannah et al., 1996). Using two nets simultaneously is so effective that, in at least one published experiment, significant bycatch reduction was demonstrated in as few as 17 hauls (Hannah and Jones, 2000). By also varying which net incorporates the functioning BRD (Hannah et al., 1996), differences in net efficiency are accounted for statistically and a latin square design is achieved, as recommended for comparative fishing experiments by Burridge and Robins (2000). However, with some types of effects, such as differences in footrope configuration, switching the effect of interest from net to net simply is not practical and net mensuration has been used successfully to help correct for differences in net spread and rise (Hannah and Jones, 2000). Power analysis of comparative fishing experiments suggests that modest number of hauls can detect large effects, such as those expected for fish catch reduction from a BRD. However, even with an efficient experimental design, small effects, such as the shrimp loss caused by a BRD, can still be hard to reliably measure without very large number of hauls (Burridge and Robins, 2000). This problem can be exacerbated greatly by any increase in variance from two nets on a double-rigged vessel that have different efficiencies for shrimp. One aspect of shrimp trawl performance that can greatly influence efficiency for ocean shrimp is the height of the fishing line above bottom (Hannah et al., 1996). Ocean shrimp nets could perhaps be best described as “semi-pelagic” in that they are rigged to maintain a particular fishing line height above bottom, usually between 35 and 70 cm. This height is maintained by a groundline attached to the fishing line of the trawl, with dropper chains in either a “tickler chain” or “ladder chain” configuration (Hannah and Jones, 2000). In general, ocean shrimp nets that run closer to the bottom catch more shrimp, and often also catch more flatfish and juvenile rockfish (Sebastes sp.). In this study, we developed and tested a device that measures and records data on fishing line height (FLH) above bottom while trawling. Using this device to monitor changes in FLH, we evaluated how changes in FLH and other aspects of trawl footrope design influenced shrimp catch and the bycatch of selected species of demersal fish in an ocean shrimp trawl. Although fish bycatch is moderate in the ocean shrimp fishery, the juveniles of some commercially
important species, such as darkblotched rockfish (Sebastes crameri) and Dover sole (Microstomus pacificus) are caught. Concern over the catch of juvenile darkblotched rockfish is warranted because this species is now classified as “overfished” by the US National Marine Fisheries Service (PFMC, 2000). While codend BRDs are effective at removing large fish from shrimp trawls (for example, see Isaksen et al., 1992; Mounsey et al., 1995; Broadhurst et al., 1996; Hannah et al., 1996; Brewer et al., 1998) they are generally less effective at removing small fishes. Also, concerns remain about survival of fish escaping through BRDs or through large diamond or square meshes (for example, Chopin and Arimoto, 1995; Suuronen et al., 1995, 1996; Sangster et al., 1996; Broadhurst et al., 1997; Farmer et al., 1998; Ryer, 2002). Trawl gear configurations that keep fish from entering a shrimp trawl have the potential to reduce bycatch mortality as opposed to just reducing bycatch (Ryer, 2002). If ocean shrimp trawls without continuous groundlines can be developed and proven, the potential exists to reduce both trawl entrainment of small demersal fish and to reduce potential benthic habitat impacts from trawling (Van Dolah et al., 1987; Collie, 1998; Rogers et al., 1998). Another objective of this study was to begin the research needed to develop ocean shrimp footrope designs that entrain fewer small demersal fish and that create less bottom contact than conventional footropes.
2. Methods This experiment was conducted aboard the 21 m double-rigged shrimp vessel Miss Yvonne, out of Newport, OR, USA. The nets were of standard 4-seam design with 22.9 m headropes and footropes, coupled to rectangular 1.8 m high, wooden doors. The footropes were of the ladder configuration (Fig. 1, also described in Hannah and Jones, 2000). The groundline was composed of three sections, each about 7.6 m long. The two sections closest to the wings were constructed of 8 mm chain, while the center section was made of 6.4 cm rubber disks strung on a 7.6 m section of 13 mm steel cable. This groundline was attached at intervals to the fishing line, with “dropper” chains that increased in length from the wings to the center of the footrope, creating the ladder configuration (Fig. 1).
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Fig. 2. Schematic drawing of the recording inclinometer used to measure FLH.
FLH was measured while trawling with a recording inclinometer attached to the footrope (Fig. 2). We designed and built the inclinometer, largely basing our design on the bottom contact sensor technology developed at the Alaska Fisheries Science Center of the US National Marine Fisheries Service. The device consisted of an underwater housing containing an electronic pendulum, coupled to a small data recording device. The housing was attached to the trawl footrope with an aluminum bracket and protective shoe. A “telltale” made of various lengths of 8 mm aluminum rod with a terminal weight attached was mounted to the bottom of the protective shoe. With the terminal weight dragging along the bottom, the angle recorded by the incinometer was used to calculate FLH. The inclinometer was constructed of lightweight materials so that it would have minimal impact on FLH. The effectiveness of the inclinometer at indicating changes in FLH was evaluated in some experimental tows prior to using the device for fishing experiments. We also measured net spread at the wing ends and headrope height for both nets using acoustic net mensuration equipment (Simrad ITI).
Data from the inclinometer were processed in several steps. First, they were graphed to determine the time at which the inclinometer reached bottom and again when it left bottom on trawl retrieval. Then, based on a polynomial relation fitted to calibration data, the voltages corresponding to various angles of the pendulum were replaced with FLH values. The “on-bottom” FLH data were then segregated by inspection of graphs of FLH versus elapsed time. Finally, the on-bottom FLH values were averaged to produce an estimate of FLH throughout the haul. ANOVA and Fisher’s “protected least significant difference” criteria were used to test for differences in mean FLH between treatments. Catch data is generally not normally distributed, so we tested for differences in catch between the port and starboard nets using the Wilcoxon signed rank test. We evaluated FLH and catch composition for five different configurations of the ladder style groundline (Fig. 1). In what we termed the “standard” configuration, the eight central dropper chains on both the port and starboard nets were shortened to 51 cm. This was chosen as the standard configuration so that both
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a longer and a shorter dropper length could be tested without cutting the dropper chains used by the vessel. FLH was measured for both nets in this configuration for at least two hauls to see if identical configurations produced equal FLH. On subsequent hauls, the starboard net was left in the standard configuration while various changes were made to the port net to see how changes in configuration were reflected in changes in measured FLH as well as relative catches of ocean shrimp and bycatch. The second and third treatments consisted of shortening the central dropper chains on the port net to 41 cm, and releasing the dropper chains back to 61 cm, respectively. In the fourth treatment, the central drop chains were left in the “released” position (61 cm) and the two pairs of 46 cm chains at the “corners” of the net were also cut, in an effort to increase FLH further. The final treatment consisted of removing completely the center section of groundline and adding some segments of chain hanging straight down from the central portion of the net to try and reduce FLH, in the absence of a continuous groundline. This treatment was included to see if a low FLH could be used to maintain shrimp catches, while using the absence of a central groundline to reduce fish bycatch and bottom contact of the trawl. Haul data were processed for port and starboard nets separately, using a specially divided hopper constructed by the operator of the Miss Yvonne. Shrimp catch was measured by counting full baskets and converting to weight using an average weight of 28.76 kg per basket. This average was determined by weighing the first 46 full baskets processed. Weights and counts were collected for all flatfish and juvenile rockfish. Other fish, such as Pacific whiting (Merluccius productus), sablefish (Anoplopoma fimbria) and lingcod (Ophiodon elongatus) were discarded live when possible and not counted or weighed to reduce haul processing time. Based on prior experimentation with footrope effects on bycatch, it was considered unlikely that bycatch of these other species would vary much with the groundline changes being tested (Hannah and Jones, 2000).
3. Results Measurement of the port and starboard nets, in the standard configuration, showed that although they
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were rigged with identical footropes, FLH was not equal between the two nets (Fig. 3, P < 0.001). The port net FLH was determined to average 33.7 cm while the starboard net FLH averaged 41.4 cm, a difference of 7.7 cm. Data on spread and rise showed little difference between the two nets. Both nets had headrope to bottom measurements of about 3.7 m and spread measurements between the wing ends of about 11.8 m. More precise statements about rise and spread are not possible because within a single tow spread varied from 11.6 to 12.0 m and rise varied from 3.1 to 4.0 m. Data from the recording inclinometer showed that FLH was generally consistent between tows within a single net and treatment (Figs. 3–5). The inclinometer also readily showed differences in FLH between treatments. Fig. 4 shows the extremes of variation in FLH across the treatments tested. With the port net chained down, FLH was reduced to 26.0 cm. With the central dropper chains released and the corner chains cut, FLH increased to an average of 46.8 cm in the port net. The inclinometer also proved useful in detecting hauls in which something had gone wrong, adversely affecting trawl configuration. When haul number 4 was retrieved it was discovered that the “lazy line” on the port net had become snagged on top of a headrope float, preventing the net from rising and spreading properly. When the inclinometer data was retrieved, it showed that this problem had reduced FLH in the port net to just over 20 cm, well below the other FLH data from the standard configuration (Fig. 5). After the fourth haul shown in panel B of Fig. 4 was retrieved, very large masses of sea pens (Ptilosarcus sp.) were found to be snagged on the fishing line of the port net, despite a relatively high FLH. When the inclinometer data was retrieved it showed that FLH was reduced and variation in FLH was increased relative to the other hauls within this treatment. We excluded both these tows from our analysis. Average FLH data for the standard net configuration and four treatments showed that simple changes in footrope configuration could readily change FLH and that the change in FLH remained stable between tows within each treatment (Fig. 5). All changes in FLH were in the predicted direction for that treatment. These findings suggest that with modest effort and a recording inclinometer, FLH can be equalized between
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Fig. 3. FLH measurements of the port and starboard shrimp nets in standard configuration (see text) versus elapsed time for 4 hauls.
two shrimp nets. Between haul variation is sufficient to indicate also that while FLH can be adjusted, it cannot be “fine-tuned”. All of the treatments resulted in significantly different mean FLH (Fig. 5, P < 0.05), with two exceptions. Approximately equal FLH was attained with the dropper chains on the starboard net held at 51 cm, with droppers on the port net released to 61 cm in length (P > 0.05). After removal of the center section of the port groundline, adding some simple drag chains reduced FLH to about 25.0 cm, roughly equivalent to the chained down configuration (P > 0.05), showing that shrimp nets can be tuned to maintain low FLH without a continuous groundline. The ratio of shrimp catch between the port and starboard nets was inversely correlated with port net FLH
(Fig. 6) within the four treatments in which the central groundline was not removed (P < 0.05). This shows that controlling FLH is critical in matching the shrimp catching efficiency of two similarly rigged ocean shrimp nets. In standard configuration, the port net caught about 8% more shrimp than the starboard net (P < 0.05). When chained down to decrease FLH, the port net caught about 11.5% more shrimp than the starboard net (P < 0.05). Rough equivalency in shrimp catch rates was attained between the two nets with the starboard net in standard configuration and the port net with dropper chains at 61 cm, the same relative configurations that achieved equal FLH (P > 0.05). Rough equivalency between the two nets was also attained with the port net dropper chains at 61 cm and the
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Fig. 4. FLH measurements (cm) for the port net from six hauls from each of two treatment configurations (A and B).
Fig. 5. Mean FLH for each net, haul and footrope configuration measured with the recording inclinometer.
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Fig. 6. Ratio of shrimp catch in the port net to shrimp catch in the starboard net versus port net FLH for five port net footrope configurations.
corner chains cut, despite the fact that this treatment produced even higher FLH in the port net (P > 0.05). With the central portion of groundline removed and additional chain added, shrimp catch rates between the two nets were also roughly equal, despite very different FLH values (P > 0.05). This suggests that high
shrimp catch rates can be maintained without a continuous groundline by decreasing FLH to compensate. The bycatch of flatfish in the port net, relative to the starboard net, was increased with decreased port net FLH. The catch of flatfish was much higher in the port net under the standard and chained down
Fig. 7. Number of flatfish captured in the port and starboard nets for each port footrope configuration tested.
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Fig. 8. Number of juvenile rockfish captured in the port and starboard nets for each port footrope configuration tested.
configurations, in which port net FLH was also lower (Figs. 5 and 7, P < 0.05). Flatfish catches were not significantly different in the port net with full length dropper chains or with the corner chains cut (Fig. 7, P > 0.05). Catch data for juvenile rockfish followed a very similar pattern to the flatfish data (Fig. 8). For two of the treatments with reduced FLH in the port net (standard and chained down), relative rockfish catch was increased (P < 0.05). With 61 cm drop chains in the port net, catches were not significantly different (P > 0.05). The two nets were most comparable in juvenile rockfish catch when the port net was fished with 61 cm drop chains and the corner chains cut (Fig. 8, P > 0.05). With the central section of groundline removed, the bycatch data were inconclusive but suggestive of different results for flatfish and juvenile rockfish. Flatfish catch was not significantly different between the two nets (P > 0.05), although the graph shows, if anything, flatfish bycatch might tend to be increased by this treatment. The nets did not catch significantly different amounts of juvenile rockfish either (P > 0.05). The graph (Fig. 8) shows that the port net catches were lower, and the comparison was marginally non-significant (P = 0.068), suggesting that with a larger sample size significant reduction in juvenile rockfish catch might have been measured.
4. Discussion vskip-2pt This study shows that a recording inclinometer can be used effectively to measure FLH in semi-pelagic shrimp trawls, allowing changes to the footrope to adjust FLH. Although the data show a lot of variability in FLH within a tow, we believe the inclinometer overstates this variation. The inclinometer is expected to register any roughness in the sea bottom and any bouncing of the telltale as well as any true “within haul” variation in FLH. Underwater video footage of the fishing line of shrimp trawls also shows them to be quite stable during a tow (ODFW, unpublished data). FLH measurements in this study responded consistently and somewhat predictably to footrope configuration adjustments, giving confidence to FLH measurements obtained with the inclinometer. The data collected in this study also show that measuring and controlling FLH can help to accurately evaluate gear changes aimed at reducing bycatch in the ocean shrimp fishery. Having nets with the same construction is apparently not sufficient to equalize shrimp catch rates between the two nets, as nets with the same configuration can have different FLH, and different catch rates for shrimp and small demersal fish. This finding is consistent with problems encountered in previous studies evaluating BRDs (Hannah et al., 1996) and with field observations that
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well matched double-rigged shrimp nets often fish differently. The findings of this study are generally consistent with the findings of other studies that evaluated footrope height and fish catches in semi-pelagic trawls (Ramm et al., 1993; Brewer et al., 1996). Brewer et al. (1996) tested trawls with two different FLH measurements against a standard demersal trawl and reported decreased catches of flatfish and of “small fishes” with increased FLH, as well as reduced catches of a variety of other non-target species. Previous work with ocean shrimp trawls shows that in addition to FLH, the relative “fore-aft” position of the fishing line and groundline may also be important in influencing entrainment of flatfish into these trawls (Hannah and Jones, 2000). When the groundline is held below and slightly aft of the fishing line, as opposed to well forward of the fishing line (used as a tickler chain), flatfish and juvenile rockfish catch is decreased. Our findings of decreased flatfish catch with increased FLH also make sense in light of the described behavior of flatfish encountering trawl footropes. Bublitz (1996) showed that the most common behavior of flatfish in response to an approaching trawl footrope was an inverted or sideways rolling maneuver that kept the fish very close to the bottom as they entered the trawl. In Bublitz (1996) study, the distance of the fish off bottom was usually less than 1 m and with the most common entrance behavior observed, the distance averaged only about 35 cm. With FLH varying between 25 and 45 cm in this study, it is easy to see why flatfish entrainment could be decreased with increased FLH. The data and analysis presented here indicate that adjusting FLH with simple footrope modifications can eliminate much of the difference in shrimp, flatfish and juvenile rockfish catch rates between two double-rigged nets. Clearly, data on FLH can help in interpreting catch and bycatch data from doublerigged nets that are not fishing equally and can also be useful in monitoring shrimp hauls for trawl performance deficits. For shrimp trawls without tickler chains, the exact mechanisms that cause fish and shrimp to be entrained in the trawl near the footrope are still poorly understood. Studies focused on the behaviour of small fish and shrimp in response to the approaching groundline might help shed light on why equal FLH does not produce equal catches when
footrope configurations differ. We measured FLH only at the center of the fishing line. Measurements of FLH out in the wing portions of the trawl, in combination with observations of fish behavior, might help explain some of the important details influencing the entrainment of fish and shrimp into the trawl. The relationship between FLH and bycatch of flatfish and juvenile rockfish shows that maintaining some minimum FLH has the potential to reduce bycatch in shrimp trawls. The shrimp catch ratio data suggest, however, that some reduction in shrimp catch would accompany the reduction of bycatch attained by keeping fishing lines higher off bottom. Additionally, given the mix of single-rigged and double-rigged vessels and the various footrope styles currently fished in the ocean shrimp fishery, regulating gear to maintain a certain minimum FLH would be very difficult. Removal of the central portion of the groundline and adding chain to reduce FLH in the port net equalized shrimp catch rates between the nets. This suggests that the lower FLH in the port net was compensating for the reduced catch of shrimp caused by the lack of a central groundline. The data on bycatch reduction from this approach was inconclusive and the trends conflicting for flatfish and juvenile rockfish. This would seem to suggest that this approach might not be useful in reducing bycatch. However, the slightly higher bycatch in the port net after FLH had been equalized by releasing the central drop chains to 61 cm (Figs. 7 and 8) indicates that the port net may have been entraining more small fish out in the wing area than the starboard net, for some unknown reason. If this was the case, then elimination of the entire groundline may be much more successful at reducing bycatch, while maintaining shrimp catch, than removing only the central portion of the groundline, as was tried in this experiment. Further testing of shrimp trawls without groundlines could clarify this and also help show how eliminating continuous groundlines to protect benthic habitats might change the bycatch of small demersal fish in this fishery.
5. Conclusions 1. Height of the fishing line of an ocean shrimp trawl with a “ladder” style footrope can be measured effectively with a recording inclinometer.
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2. The recording inclinometer can also indicate trawl performance deficits. 3. FLH in an ocean shrimp trawl is relatively stable during a haul and also between hauls within a given footrope and groundline configuration and can be adjusted with simple modifications to the footrope “dropper” chains. 4. FLH can be unequal between double-rigged nets of identical configuration. 5. Within a given footrope style, shrimp catch and the bycatch of flatfish and juvenile rockfish vary inversely with FLH. 6. Removal of the central portion of the groundline with added weight to reduce FLH produced inconclusive data on bycatch reduction suggesting additional testing, perhaps with more of the groundline removed.
Acknowledgements This paper was funded in part by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its sub-agencies. This project was financed in part with Federal Interjurisdictional Fisheries Act funds (75% federal, 25% state of Oregon funds) through the US National Marine Fisheries Service (contract# NA16FI1106—total 2001 project funds—$59,121 federal, $19,708 state). Jeff Boardman, the skipper of the F/V Miss Yvonne, supervised all footrope modifications. Scott McEntire of the Alaska Fisheries Science Center of the US National Marine Fisheries Service helped with the design of the recording inclinometer.
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Measuring the height of the fishing line and its effect on shrimp catch and bycatch in an ocean shrimp (Pandalus jordani) trawl Robert W. Hannah∗ , Stephen A. Jones Oregon Department of Fish and Wildlife, Marine Resources Program, 2040 S.E. Marine Science Drive, Newport, OR 97365, USA Received 6 December 2001; received in revised form 17 May 2002; accepted 26 May 2002
Abstract The relationship between fishing line height (FLH), shrimp catch and bycatch in a semi-pelagic ocean shrimp (Pandalus jordani) trawl was investigated using a newly developed recording inclinometer. The inclinometer was effective at measuring FLH and indicating trawl performance deficits. FLH was determined to be stable during a haul and also between hauls within a given footrope and groundline configuration. FLH was readily adjusted with simple modifications to the footrope “dropper” chains. Inclinometer data showed that FLH can be unequal between double-rigged nets of identical configuration. Shrimp catch and the bycatch of flatfish and juvenile rockfish varied inversely with FLH, suggesting FLH can be adjusted to equalize the catch of shrimp, flatfish and juvenile rockfish between two double-rigged shrimp nets. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bycatch; Footrope; Semi-pelagic shrimp trawl; Mensuration
1. Introduction The Pacific trawl fishery for ocean shrimp (Pandalus jordani) operates mostly between Cape Mendocino, California and the west coast of Vancouver Island, British Columbia, in water depths of between 120 and 240 m. At these depths, ocean shrimp are found mostly in areas with green mud bottoms (Zirges and Robinson, 1980). The regional catch of ocean shrimp is highly variable (Hannah, 1999) and in recent years has ranged from about 4500 t to over 36,000 t. The typical ocean shrimp vessel is between 20 and 27 m in length and is double-rigged (Zirges and Robinson, 1980). Typical ocean shrimp nets are high rise box trawls with footrope lengths of between 20 and 27 m, closely ∗ Corresponding author. Tel.: +1-541-867-4741; fax: +1-541-867-0311. E-mail address: [email protected] (R.W. Hannah).
coupled to rectangular wooden trawl doors approximately 2.1 m high by 1.8 m wide (Jones et al., 1996). The fishery has relatively low levels of fish bycatch in comparison to most other large-scale shrimp fisheries (Alverson et al., 1994). However, high levels of Pacific whiting (Merluccius productus) bycatch have caused operational problems in the fishery at times (Hannah and Jones, 2000), leading to research on the effectiveness of various bycatch reduction devices (BRDs) in the codend (Hannah et al., 1996). More recently, mandatory codend BRDs have been used as a management tool to limit canary rockfish (Sebastes pinniger) catch in this fishery to meet allocation goals set by the Pacific Fishery Management Council. Tests of BRDs in the ocean shrimp fishery have generally compared catches of shrimp and bycatch in each of two double-rigged nets deployed from a shrimp vessel. With both nets fished simultaneously, one incorporating the BRD and the other acting as a control,
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a very powerful statistical design is achieved (Hannah et al., 1996). Using two nets simultaneously is so effective that, in at least one published experiment, significant bycatch reduction was demonstrated in as few as 17 hauls (Hannah and Jones, 2000). By also varying which net incorporates the functioning BRD (Hannah et al., 1996), differences in net efficiency are accounted for statistically and a latin square design is achieved, as recommended for comparative fishing experiments by Burridge and Robins (2000). However, with some types of effects, such as differences in footrope configuration, switching the effect of interest from net to net simply is not practical and net mensuration has been used successfully to help correct for differences in net spread and rise (Hannah and Jones, 2000). Power analysis of comparative fishing experiments suggests that modest number of hauls can detect large effects, such as those expected for fish catch reduction from a BRD. However, even with an efficient experimental design, small effects, such as the shrimp loss caused by a BRD, can still be hard to reliably measure without very large number of hauls (Burridge and Robins, 2000). This problem can be exacerbated greatly by any increase in variance from two nets on a double-rigged vessel that have different efficiencies for shrimp. One aspect of shrimp trawl performance that can greatly influence efficiency for ocean shrimp is the height of the fishing line above bottom (Hannah et al., 1996). Ocean shrimp nets could perhaps be best described as “semi-pelagic” in that they are rigged to maintain a particular fishing line height above bottom, usually between 35 and 70 cm. This height is maintained by a groundline attached to the fishing line of the trawl, with dropper chains in either a “tickler chain” or “ladder chain” configuration (Hannah and Jones, 2000). In general, ocean shrimp nets that run closer to the bottom catch more shrimp, and often also catch more flatfish and juvenile rockfish (Sebastes sp.). In this study, we developed and tested a device that measures and records data on fishing line height (FLH) above bottom while trawling. Using this device to monitor changes in FLH, we evaluated how changes in FLH and other aspects of trawl footrope design influenced shrimp catch and the bycatch of selected species of demersal fish in an ocean shrimp trawl. Although fish bycatch is moderate in the ocean shrimp fishery, the juveniles of some commercially
important species, such as darkblotched rockfish (Sebastes crameri) and Dover sole (Microstomus pacificus) are caught. Concern over the catch of juvenile darkblotched rockfish is warranted because this species is now classified as “overfished” by the US National Marine Fisheries Service (PFMC, 2000). While codend BRDs are effective at removing large fish from shrimp trawls (for example, see Isaksen et al., 1992; Mounsey et al., 1995; Broadhurst et al., 1996; Hannah et al., 1996; Brewer et al., 1998) they are generally less effective at removing small fishes. Also, concerns remain about survival of fish escaping through BRDs or through large diamond or square meshes (for example, Chopin and Arimoto, 1995; Suuronen et al., 1995, 1996; Sangster et al., 1996; Broadhurst et al., 1997; Farmer et al., 1998; Ryer, 2002). Trawl gear configurations that keep fish from entering a shrimp trawl have the potential to reduce bycatch mortality as opposed to just reducing bycatch (Ryer, 2002). If ocean shrimp trawls without continuous groundlines can be developed and proven, the potential exists to reduce both trawl entrainment of small demersal fish and to reduce potential benthic habitat impacts from trawling (Van Dolah et al., 1987; Collie, 1998; Rogers et al., 1998). Another objective of this study was to begin the research needed to develop ocean shrimp footrope designs that entrain fewer small demersal fish and that create less bottom contact than conventional footropes.
2. Methods This experiment was conducted aboard the 21 m double-rigged shrimp vessel Miss Yvonne, out of Newport, OR, USA. The nets were of standard 4-seam design with 22.9 m headropes and footropes, coupled to rectangular 1.8 m high, wooden doors. The footropes were of the ladder configuration (Fig. 1, also described in Hannah and Jones, 2000). The groundline was composed of three sections, each about 7.6 m long. The two sections closest to the wings were constructed of 8 mm chain, while the center section was made of 6.4 cm rubber disks strung on a 7.6 m section of 13 mm steel cable. This groundline was attached at intervals to the fishing line, with “dropper” chains that increased in length from the wings to the center of the footrope, creating the ladder configuration (Fig. 1).
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Fig. 2. Schematic drawing of the recording inclinometer used to measure FLH.
FLH was measured while trawling with a recording inclinometer attached to the footrope (Fig. 2). We designed and built the inclinometer, largely basing our design on the bottom contact sensor technology developed at the Alaska Fisheries Science Center of the US National Marine Fisheries Service. The device consisted of an underwater housing containing an electronic pendulum, coupled to a small data recording device. The housing was attached to the trawl footrope with an aluminum bracket and protective shoe. A “telltale” made of various lengths of 8 mm aluminum rod with a terminal weight attached was mounted to the bottom of the protective shoe. With the terminal weight dragging along the bottom, the angle recorded by the incinometer was used to calculate FLH. The inclinometer was constructed of lightweight materials so that it would have minimal impact on FLH. The effectiveness of the inclinometer at indicating changes in FLH was evaluated in some experimental tows prior to using the device for fishing experiments. We also measured net spread at the wing ends and headrope height for both nets using acoustic net mensuration equipment (Simrad ITI).
Data from the inclinometer were processed in several steps. First, they were graphed to determine the time at which the inclinometer reached bottom and again when it left bottom on trawl retrieval. Then, based on a polynomial relation fitted to calibration data, the voltages corresponding to various angles of the pendulum were replaced with FLH values. The “on-bottom” FLH data were then segregated by inspection of graphs of FLH versus elapsed time. Finally, the on-bottom FLH values were averaged to produce an estimate of FLH throughout the haul. ANOVA and Fisher’s “protected least significant difference” criteria were used to test for differences in mean FLH between treatments. Catch data is generally not normally distributed, so we tested for differences in catch between the port and starboard nets using the Wilcoxon signed rank test. We evaluated FLH and catch composition for five different configurations of the ladder style groundline (Fig. 1). In what we termed the “standard” configuration, the eight central dropper chains on both the port and starboard nets were shortened to 51 cm. This was chosen as the standard configuration so that both
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a longer and a shorter dropper length could be tested without cutting the dropper chains used by the vessel. FLH was measured for both nets in this configuration for at least two hauls to see if identical configurations produced equal FLH. On subsequent hauls, the starboard net was left in the standard configuration while various changes were made to the port net to see how changes in configuration were reflected in changes in measured FLH as well as relative catches of ocean shrimp and bycatch. The second and third treatments consisted of shortening the central dropper chains on the port net to 41 cm, and releasing the dropper chains back to 61 cm, respectively. In the fourth treatment, the central drop chains were left in the “released” position (61 cm) and the two pairs of 46 cm chains at the “corners” of the net were also cut, in an effort to increase FLH further. The final treatment consisted of removing completely the center section of groundline and adding some segments of chain hanging straight down from the central portion of the net to try and reduce FLH, in the absence of a continuous groundline. This treatment was included to see if a low FLH could be used to maintain shrimp catches, while using the absence of a central groundline to reduce fish bycatch and bottom contact of the trawl. Haul data were processed for port and starboard nets separately, using a specially divided hopper constructed by the operator of the Miss Yvonne. Shrimp catch was measured by counting full baskets and converting to weight using an average weight of 28.76 kg per basket. This average was determined by weighing the first 46 full baskets processed. Weights and counts were collected for all flatfish and juvenile rockfish. Other fish, such as Pacific whiting (Merluccius productus), sablefish (Anoplopoma fimbria) and lingcod (Ophiodon elongatus) were discarded live when possible and not counted or weighed to reduce haul processing time. Based on prior experimentation with footrope effects on bycatch, it was considered unlikely that bycatch of these other species would vary much with the groundline changes being tested (Hannah and Jones, 2000).
3. Results Measurement of the port and starboard nets, in the standard configuration, showed that although they
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were rigged with identical footropes, FLH was not equal between the two nets (Fig. 3, P < 0.001). The port net FLH was determined to average 33.7 cm while the starboard net FLH averaged 41.4 cm, a difference of 7.7 cm. Data on spread and rise showed little difference between the two nets. Both nets had headrope to bottom measurements of about 3.7 m and spread measurements between the wing ends of about 11.8 m. More precise statements about rise and spread are not possible because within a single tow spread varied from 11.6 to 12.0 m and rise varied from 3.1 to 4.0 m. Data from the recording inclinometer showed that FLH was generally consistent between tows within a single net and treatment (Figs. 3–5). The inclinometer also readily showed differences in FLH between treatments. Fig. 4 shows the extremes of variation in FLH across the treatments tested. With the port net chained down, FLH was reduced to 26.0 cm. With the central dropper chains released and the corner chains cut, FLH increased to an average of 46.8 cm in the port net. The inclinometer also proved useful in detecting hauls in which something had gone wrong, adversely affecting trawl configuration. When haul number 4 was retrieved it was discovered that the “lazy line” on the port net had become snagged on top of a headrope float, preventing the net from rising and spreading properly. When the inclinometer data was retrieved, it showed that this problem had reduced FLH in the port net to just over 20 cm, well below the other FLH data from the standard configuration (Fig. 5). After the fourth haul shown in panel B of Fig. 4 was retrieved, very large masses of sea pens (Ptilosarcus sp.) were found to be snagged on the fishing line of the port net, despite a relatively high FLH. When the inclinometer data was retrieved it showed that FLH was reduced and variation in FLH was increased relative to the other hauls within this treatment. We excluded both these tows from our analysis. Average FLH data for the standard net configuration and four treatments showed that simple changes in footrope configuration could readily change FLH and that the change in FLH remained stable between tows within each treatment (Fig. 5). All changes in FLH were in the predicted direction for that treatment. These findings suggest that with modest effort and a recording inclinometer, FLH can be equalized between
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Fig. 3. FLH measurements of the port and starboard shrimp nets in standard configuration (see text) versus elapsed time for 4 hauls.
two shrimp nets. Between haul variation is sufficient to indicate also that while FLH can be adjusted, it cannot be “fine-tuned”. All of the treatments resulted in significantly different mean FLH (Fig. 5, P < 0.05), with two exceptions. Approximately equal FLH was attained with the dropper chains on the starboard net held at 51 cm, with droppers on the port net released to 61 cm in length (P > 0.05). After removal of the center section of the port groundline, adding some simple drag chains reduced FLH to about 25.0 cm, roughly equivalent to the chained down configuration (P > 0.05), showing that shrimp nets can be tuned to maintain low FLH without a continuous groundline. The ratio of shrimp catch between the port and starboard nets was inversely correlated with port net FLH
(Fig. 6) within the four treatments in which the central groundline was not removed (P < 0.05). This shows that controlling FLH is critical in matching the shrimp catching efficiency of two similarly rigged ocean shrimp nets. In standard configuration, the port net caught about 8% more shrimp than the starboard net (P < 0.05). When chained down to decrease FLH, the port net caught about 11.5% more shrimp than the starboard net (P < 0.05). Rough equivalency in shrimp catch rates was attained between the two nets with the starboard net in standard configuration and the port net with dropper chains at 61 cm, the same relative configurations that achieved equal FLH (P > 0.05). Rough equivalency between the two nets was also attained with the port net dropper chains at 61 cm and the
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Fig. 4. FLH measurements (cm) for the port net from six hauls from each of two treatment configurations (A and B).
Fig. 5. Mean FLH for each net, haul and footrope configuration measured with the recording inclinometer.
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Fig. 6. Ratio of shrimp catch in the port net to shrimp catch in the starboard net versus port net FLH for five port net footrope configurations.
corner chains cut, despite the fact that this treatment produced even higher FLH in the port net (P > 0.05). With the central portion of groundline removed and additional chain added, shrimp catch rates between the two nets were also roughly equal, despite very different FLH values (P > 0.05). This suggests that high
shrimp catch rates can be maintained without a continuous groundline by decreasing FLH to compensate. The bycatch of flatfish in the port net, relative to the starboard net, was increased with decreased port net FLH. The catch of flatfish was much higher in the port net under the standard and chained down
Fig. 7. Number of flatfish captured in the port and starboard nets for each port footrope configuration tested.
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Fig. 8. Number of juvenile rockfish captured in the port and starboard nets for each port footrope configuration tested.
configurations, in which port net FLH was also lower (Figs. 5 and 7, P < 0.05). Flatfish catches were not significantly different in the port net with full length dropper chains or with the corner chains cut (Fig. 7, P > 0.05). Catch data for juvenile rockfish followed a very similar pattern to the flatfish data (Fig. 8). For two of the treatments with reduced FLH in the port net (standard and chained down), relative rockfish catch was increased (P < 0.05). With 61 cm drop chains in the port net, catches were not significantly different (P > 0.05). The two nets were most comparable in juvenile rockfish catch when the port net was fished with 61 cm drop chains and the corner chains cut (Fig. 8, P > 0.05). With the central section of groundline removed, the bycatch data were inconclusive but suggestive of different results for flatfish and juvenile rockfish. Flatfish catch was not significantly different between the two nets (P > 0.05), although the graph shows, if anything, flatfish bycatch might tend to be increased by this treatment. The nets did not catch significantly different amounts of juvenile rockfish either (P > 0.05). The graph (Fig. 8) shows that the port net catches were lower, and the comparison was marginally non-significant (P = 0.068), suggesting that with a larger sample size significant reduction in juvenile rockfish catch might have been measured.
4. Discussion vskip-2pt This study shows that a recording inclinometer can be used effectively to measure FLH in semi-pelagic shrimp trawls, allowing changes to the footrope to adjust FLH. Although the data show a lot of variability in FLH within a tow, we believe the inclinometer overstates this variation. The inclinometer is expected to register any roughness in the sea bottom and any bouncing of the telltale as well as any true “within haul” variation in FLH. Underwater video footage of the fishing line of shrimp trawls also shows them to be quite stable during a tow (ODFW, unpublished data). FLH measurements in this study responded consistently and somewhat predictably to footrope configuration adjustments, giving confidence to FLH measurements obtained with the inclinometer. The data collected in this study also show that measuring and controlling FLH can help to accurately evaluate gear changes aimed at reducing bycatch in the ocean shrimp fishery. Having nets with the same construction is apparently not sufficient to equalize shrimp catch rates between the two nets, as nets with the same configuration can have different FLH, and different catch rates for shrimp and small demersal fish. This finding is consistent with problems encountered in previous studies evaluating BRDs (Hannah et al., 1996) and with field observations that
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well matched double-rigged shrimp nets often fish differently. The findings of this study are generally consistent with the findings of other studies that evaluated footrope height and fish catches in semi-pelagic trawls (Ramm et al., 1993; Brewer et al., 1996). Brewer et al. (1996) tested trawls with two different FLH measurements against a standard demersal trawl and reported decreased catches of flatfish and of “small fishes” with increased FLH, as well as reduced catches of a variety of other non-target species. Previous work with ocean shrimp trawls shows that in addition to FLH, the relative “fore-aft” position of the fishing line and groundline may also be important in influencing entrainment of flatfish into these trawls (Hannah and Jones, 2000). When the groundline is held below and slightly aft of the fishing line, as opposed to well forward of the fishing line (used as a tickler chain), flatfish and juvenile rockfish catch is decreased. Our findings of decreased flatfish catch with increased FLH also make sense in light of the described behavior of flatfish encountering trawl footropes. Bublitz (1996) showed that the most common behavior of flatfish in response to an approaching trawl footrope was an inverted or sideways rolling maneuver that kept the fish very close to the bottom as they entered the trawl. In Bublitz (1996) study, the distance of the fish off bottom was usually less than 1 m and with the most common entrance behavior observed, the distance averaged only about 35 cm. With FLH varying between 25 and 45 cm in this study, it is easy to see why flatfish entrainment could be decreased with increased FLH. The data and analysis presented here indicate that adjusting FLH with simple footrope modifications can eliminate much of the difference in shrimp, flatfish and juvenile rockfish catch rates between two double-rigged nets. Clearly, data on FLH can help in interpreting catch and bycatch data from doublerigged nets that are not fishing equally and can also be useful in monitoring shrimp hauls for trawl performance deficits. For shrimp trawls without tickler chains, the exact mechanisms that cause fish and shrimp to be entrained in the trawl near the footrope are still poorly understood. Studies focused on the behaviour of small fish and shrimp in response to the approaching groundline might help shed light on why equal FLH does not produce equal catches when
footrope configurations differ. We measured FLH only at the center of the fishing line. Measurements of FLH out in the wing portions of the trawl, in combination with observations of fish behavior, might help explain some of the important details influencing the entrainment of fish and shrimp into the trawl. The relationship between FLH and bycatch of flatfish and juvenile rockfish shows that maintaining some minimum FLH has the potential to reduce bycatch in shrimp trawls. The shrimp catch ratio data suggest, however, that some reduction in shrimp catch would accompany the reduction of bycatch attained by keeping fishing lines higher off bottom. Additionally, given the mix of single-rigged and double-rigged vessels and the various footrope styles currently fished in the ocean shrimp fishery, regulating gear to maintain a certain minimum FLH would be very difficult. Removal of the central portion of the groundline and adding chain to reduce FLH in the port net equalized shrimp catch rates between the nets. This suggests that the lower FLH in the port net was compensating for the reduced catch of shrimp caused by the lack of a central groundline. The data on bycatch reduction from this approach was inconclusive and the trends conflicting for flatfish and juvenile rockfish. This would seem to suggest that this approach might not be useful in reducing bycatch. However, the slightly higher bycatch in the port net after FLH had been equalized by releasing the central drop chains to 61 cm (Figs. 7 and 8) indicates that the port net may have been entraining more small fish out in the wing area than the starboard net, for some unknown reason. If this was the case, then elimination of the entire groundline may be much more successful at reducing bycatch, while maintaining shrimp catch, than removing only the central portion of the groundline, as was tried in this experiment. Further testing of shrimp trawls without groundlines could clarify this and also help show how eliminating continuous groundlines to protect benthic habitats might change the bycatch of small demersal fish in this fishery.
5. Conclusions 1. Height of the fishing line of an ocean shrimp trawl with a “ladder” style footrope can be measured effectively with a recording inclinometer.
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2. The recording inclinometer can also indicate trawl performance deficits. 3. FLH in an ocean shrimp trawl is relatively stable during a haul and also between hauls within a given footrope and groundline configuration and can be adjusted with simple modifications to the footrope “dropper” chains. 4. FLH can be unequal between double-rigged nets of identical configuration. 5. Within a given footrope style, shrimp catch and the bycatch of flatfish and juvenile rockfish vary inversely with FLH. 6. Removal of the central portion of the groundline with added weight to reduce FLH produced inconclusive data on bycatch reduction suggesting additional testing, perhaps with more of the groundline removed.
Acknowledgements This paper was funded in part by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration. The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its sub-agencies. This project was financed in part with Federal Interjurisdictional Fisheries Act funds (75% federal, 25% state of Oregon funds) through the US National Marine Fisheries Service (contract# NA16FI1106—total 2001 project funds—$59,121 federal, $19,708 state). Jeff Boardman, the skipper of the F/V Miss Yvonne, supervised all footrope modifications. Scott McEntire of the Alaska Fisheries Science Center of the US National Marine Fisheries Service helped with the design of the recording inclinometer.
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