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Groundfish Assessment Program

Estimating Capture Probability of a Survey Bottom Trawl for Bering Sea Skates and Other Fishes

  Figure 3, see caption
Figure 3.  Probability of capture estimated by the parametric model is shown along with 95% bootstrapped confidence regions. Circles represent observed capture probabilities. Size of the circle is proportional to number of fish observed in each centimeter length bin.

Capture probabilities for skates, three species of flatfish, and seven other species were estimated for the 83-112 Eastern bottom trawl used to conduct the eastern Bering Sea bottom trawl survey.

Capture probability data were collected by towing with an experimental trawl that consisted of a standard survey trawl with an auxiliary net attached beneath the footrope of the survey trawl. It was assumed that the auxiliary net caught all fish escaping under the survey trawl footrope. Capture probability was then estimated for each 1 cm of fish length as ratio of fish caught in the survey trawl to the sum of fish caught in both survey trawl and auxiliary net.

Four competing models describing different capture processes were fitted to the data using the maximum likelihood method and the best model was chosen by likelihood ratio test. Capture probability for skates (Bathyraja spp.) increased monotonically with length from ~ 0.65 for 30-cm skates to ~0.8 for skates > 100 cm (Fig. 3 above).

Capture probability for flatfish (arrowtooth flounder (Atheresthes stomias), rex sole (Glyptocephalus zachirus), and Pacific halibut (Hippoglossus stenolepis)), sturgeon poacher (Podothecus acipenserinus), wattled eelpout (Lycodes palearis), and great sculpin (Myoxocephalus polyancanthocephalus) was constant across lengths and close to unity > 0.97.

Capture probability was constant, but significantly lower than 1 for sawback poacher (Leptagonus frenatus) (0.89), searcher (Bathymaster signatus) (0.94), and spinyhead sculpin (Dasycottus setiger) (0.69). For spectacled sculpin (Triglops scepticus) capture probability was 1.00 for fish smaller than 15 cm, but decreased for larger fish.

By Stan Kotwicki and Kenneth Weinberg

Effect of Net Spread on Capture Efficiency of a Demersal Survey Trawl Used on the Eastern Bering Sea Shelf

Relative abundance estimates generated from the NMFS annual groundfish trawl surveys in the eastern Bering Sea shelf are based on mean catch per area swept. In the calculation of these estimates, the capture efficiency is assumed to be constant with net spread, which increases with depth throughout the depth range of the survey area and also varies substantially with bottom type and currents. The validity of this assumption was tested using a linear model of catch-per-unit-towed distance versus net spread and depth that was fit to 14 years of survey data for seven species.

Catch rates decreased significantly with increasing net spreads for six of the seven species despite the larger area swept at greater net spreads. The magnitude of the downward trends varied considerably by species. Purple sea star (Asterias amurensis) and northern rock sole (Lepidopsetta polyxystra) exhibited the strongest negative relationships between catch rate and net spread, while Alaska plaice (Pleuronectes quadrituberculatus) exhibited the weakest relationship.

Figure 4, see caption
Figure 4.  Relationship between mean net spread and the clearance between the footrope and bottom for the 83-112 Eastern trawl. The clearance increases with increasing net spread, possibly allowing some fish to escape under the footrope. Data were collected on board the chartered fishing vessel Arcturus during the NMFS annual eastern Bering Sea demersal trawl survey in 2004. N=136.

We attribute this decrease in catch to a decrease in capture efficiency, which is primarily due to increased escapement under the footrope. Calibrated bottom contact sensors indicate that footrope clearance with the bottom increases with net spread (Fig. 4 above). The current assumption of constant capture efficiency at all net spreads may result in biased size distributions and sex ratios for species displaying size and sex segregation by depth. This assumption may also have negative implications for the assessment of highly patchily distributed species as well as those whose depth distribution is a function of changing environmental conditions. Controlling trawl geometry by means of restrictor cables on the warps is a feasible method to address this problem.

By Paul von Szalay


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