© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Do Pacific cod (Gadus macrocephalus) and walleye pollock (Theregra chalcogramma) lack a herding response to the doors, bridles, and mudclouds of survey trawls?
Resource Assessment and Conservation Engineering, Alaska Fisheries Science Center 7600 Sand Point Way NE, Seattle, WA, USA
*Tel: +1 206 526 4116; fax: +1 206 526 6723. e-mail: david.somerton{at}noaa.gov.
Pacific cod and walleye pollock were subjected to herding experiments in which trawl hauls are conducted repeatedly in an area with the bridles varied among three distinct lengths. For the flatfishes in these studies, catch per unit of area swept (cpue) by the trawls increased greatly with increasing bridle length, indicating that flatfish are stimulated to herd into the path of the net by the action of the bridles. In contrast, the cpue of Pacific cod and walleye pollock did not increase significantly with increasing bridle length. This lack of significance indicates that these two species respond only weakly to any herding stimuli produced by the 83–112 Eastern and Poly Nor'eastern trawls used to conduct groundfish trawl surveys in the North Pacific Ocean.
Keywords: bridle herding, Pacific cod, trawl efficiency, trawl survey, walleye pollock
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Introduction |
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 Top Introduction Materials and methodsResultsDiscussionReferences |
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The herding of fish by the otter doors, mudclouds, and bridles of a bottom trawl into the path of the net can make substantial contributions to the catches of survey trawls (Dickson, 1993). Knowledge of this process is therefore important for determining the catchability of trawls (Somerton et al., 1999) as well as for reducing the variability in catchability due to haul-to-haul variability in the stimuli eliciting the herding behaviour. One method of quantitatively studying the herding process is to conduct herding experiments designed to estimate the changes in catch per unit of swept area (cpue) that can accompany changes in the length of the bridles and sweeps (Engås and Godø, 1989; Ramm and Xiao, 1995; Somerton and Munro, 2001).
Such herding experiments have been applied to the 83–112 Eastern and the Poly Nor'eastern trawls used by the Alaska Fisheries Science Center (AFSC) to conduct bottom trawl surveys in the North Pacific. Based on these experiments, flatfishes display a strong herding response to these trawls because cpue significantly increased with bridle length for all species examined (Somerton and Munro, 2001; unpublished data). In this paper, I examine experimental evidence for a herding response of two Pacific gadids, Pacific cod (Gadus macrocephalus), and walleye pollock (Theregra chalcogramma) to potential stimuli produced by the 83–112 Eastern and the Poly Nor'eastern trawls.
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Materials and methods |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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Description of the trawls
The 83–112 Eastern trawl is a two-seam flatfish trawl with a 25-m headrope and a 34-m footrope constructed of a simple rubber-wrapped steel cable (net plans provided in Armistead and Nichol, 1993). The Poly Nor'eastern trawl is a four-seam high-rise trawl with a 27-m headrope and a 25-m footrope equipped with 36-cm diameter bobbins (net plans are provided in Britt and Martin, 2001). During normal survey operations, both trawls use 54.6-m bridles, constructed of 1.6-cm steel cable, connected to "V" doors measuring 1.83 m by 2.74 m and weighing 818.2 kg in air. These trawls differ in two respects that potentially could provide differences in herding stimuli. First, the 83–112 Eastern trawl has two bridles on each side, whereas the Poly Nor'eastern trawl has three bridles. Second, underwater camera observations have revealed that the mudclouds produced by the 83–112 Eastern trawl cover the bridles for the first 30% of the distance between the doors and the wings (Somerton and Munro, 2001), whereas the mudclouds produced by the Poly Nor'eastern trawl cover the bridles for their entire length (Somerton, 2003).
The herding experiment
The herding experiment for the 83–112 Eastern trawl was conducted on 25 July to 2 August 1994 aboard the FV "Arcturus" in the Eastern Bering Sea at a mean depth of 76 m. The herding experiment for the Poly Nor'eastern trawl was conducted on 10–19 May 1998 aboard the FV "Hickory Wind" in the Gulf of Alaska at a mean depth of 155 m. Both experiments were conducted during daylight hours in a similar manner. At each sampling site, three nearby (<0.25 nmi), parallel, 30-min tows were made sequentially using the three bridle lengths (Table 1) in random order. On each tow, net spread, door spread, and headrope height were continuously measured using an acoustic net mensuration system (Table 1). The entire catch of each species was weighed, then all individuals were measured for total length in centimeters. Catch per unit of swept area was computed as the number of individuals caught divided by the product of the average net width and the towed distance from the moment the footrope contacted the bottom until the moment it left the bottom. After completing the three tows at a sampling site, the vessel then moved to the next sampling site.
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Data analysis
If fish are herded by the doors, mudclouds, or bridles into the path of the net, then the cpue should increase as bridle length increases because an increasingly greater area is exposed to the potential herding stimuli. To test if Pacific cod and walleye pollock are herded by the survey trawls, cpue was regressed on total bridle length (bridle length + tail chain length) considering sampling site as a factor variable accounting for between-site differences in fish density and weighting by the inverse of the cpue variance at each bridle length. Significance of the slope in this relationship indicates that herding occurred.
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Results |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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For the 83–112 Eastern trawl experiment, 15 sampling sites were completed, all of which had at least one cod in all three trawl tows comprising a sampling site. For this experiment, a total of 10 684 cod were measured (median = 60 cm; interquartile range = 54–65 cm), but too few pollock were encountered for consideration. For the Poly Nor'eastern trawl study, 20 sampling sites were completed, 18 of which had at least one cod and seven of which had at least one pollock in all three trawl tows. For this experiment, a total of 6899 cod (53 cm; 44–58 cm) and 3814 pollock (57 cm; 47–60 cm) were measured.
The slope of the regression of cpue on bridle length was not significant for cod in the 83–112 Eastern trawl experiment, nor for cod or pollock in the Poly Nor'eastern trawl experiment (Figure 1; Table 2). This indicates that the catches of cod and pollock by the survey nets were not substantially enhanced by the herding of fish into the path of the net.
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Discussion |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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Pacific cod and walleye pollock did not display a strong, consistent herding response to stimuli produced by the doors, mudclouds, or bridles of the two survey trawls. This is in contrast to the flatfishes examined in these experiments. In the 83–112 Eastern trawl experiments, all seven species of flatfish displayed a highly significant increase in cpue with bridle length (Somerton and Munro, 2001). In the Poly Nor'eastern trawl experiments, all four species of flatfish examined likewise displayed a highly significant increase in cpue with bridle length (unpublished data).
The apparent difference in the strength of the Pacific cod and walleye pollock herding response compared to that of flatfish is twofold. First, the two gadid species have more patchy distributions and, consequently, greater variability in cpue. Second, there are likely differences in the stimuli needed to elicit a herding response. For flatfish, underwater observations suggest that it is the sight or direct contact with the lower bridles that stimulates movement, whereas for gadids it is the sight of the doors and mudclouds that is believed to be the primary stimulus (Main and Sangster, 1981a, b). If this is true for the species we considered, then perhaps the light level or water clarity was too low during our experiments to allow visual stimuli to be effective at eliciting a response. Tactile stimuli used by the flatfishes, however, would still be effective.
Despite the apparent weak herding response of Pacific cod and walleye pollock to the two survey trawls examined here, other gadids do show a herding response in conditions similar to those in our experiment. For example, Engås and Godø (1989) reported a relatively strong herding response by Atlantic cod and haddock to a survey trawl at depths >280 m in the Barents Sea. Furthermore, Pacific cod and walleye pollock apparently can be induced to herd if the suitable stimulus was applied. Commercial fishers maintain that cod catches are increased when rubber disks are attached to the lower bridles of their trawls to provide a greater visual stimulus and pollock catches are increased when net spreads are increased with extremely large mesh netting.
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References |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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Armistead C. E. and Nichol D. G. (1993) 1990 bottom trawl survey of the eastern Bering Sea continental shelf. U.S. Department of Commerce, NOAA Technical Memorandum, NMFS-AFSC-7, 190 pp.
Britt L. L. and Martin M. H. (2001) Data report: 1999 Gulf of Alaska bottom trawl survey. U.S. Department of Commerce, NOAA Technical Memorandum, NMFS-AFSC-121. 249 pp.
Dickson W. (1993) Estimation of the capture efficiency of trawl gear. II: Testing a theoretical model. Fisheries Research 16:255–272.[CrossRef][Web of Science]
Engås A. and Godø O.R. (1989) The effect of different sweep lengths on the length composition of bottom-sampling trawl catches. Journal du Conseil International pour l'Exploration de la Mer 45:263–268.
Main J. and Sangster G.I. (1981) A study of the fish capture process in a bottom trawl by direct observations from a towed underwater vehicle. Scottish Fisheries Research Report 23:1–23.
Main J. and Sangster G.I. (1981) A study of the sand clouds produced by trawl boards and their possible effect on fish capture. Scottish Fisheries Research Report 20:1–20.
Ramm D.C. and Xiao Y. (1995) Herding in groundfish and effective pathwidth of trawls. Fisheries Research 24:243–259.[CrossRef][Web of Science]
Somerton D.A. (2003) Bridle efficiency of a survey trawl for flatfish: measuring the length of the bridles in contact with the bottom. Fisheries Research 60:273–279.[CrossRef][Web of Science]
Somerton D., Ianelli J., Walsh S., Smith S., Godø O.R., Ramm D. (1999) Incorporating experimentally derived estimates of survey trawl efficiency into the stock assessment process: a discussion. ICES Journal of Marine Science 56:200–302.
Somerton D.A. and Munro P. (2001) Bridle efficiency of a survey trawl for flatfish. Fishery Bulletin 99:641–652.[Web of Science]
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