288 



Fishery Bulletin 105(2) 



footrope attachment of the lower bridle out to a distance 

 L„,, but not beyond. This is an over simplification of the 

 herding process, but in our previous study (Somerton 

 and Munro, 2001) video recordings substantiated this 

 assumption. Because the bridles are obscured by the 

 mudclouds on the Poly Nor'eastern trawl, not only were 

 we unable to verify the fish reaction to the bridles but 

 the mud clouds themselves may have provided a herd- 

 ing stimulus as they reportedly do for some roundfish 

 (Main and Sangster, 1981a, 1981b). If so, the effective 

 length of the bridle over which herding occurs could be 

 longer than L ,„. We attempted to answer this question 

 by positioning a video camera to allow us to observe 

 flatfish behavior at the inner edge of the mud cloud, but 

 were unsuccessful. 



Another assumption is that the estimates of L^,, can 

 be extrapolated from the bridle measurement experi- 

 ment to the herding experiment. On all vessels that 

 we use for experiments, the tailchain extensions are 

 adjusted in length depending on the size of the vessel 

 and storage location of the doors when out of water. On 

 the herding and bridle measurement experiments, the 

 difference in the sizes of the vessels was so large that 

 the tailchain length was approximately 10 m longer in 

 the bridle measurement experiment. Because the cable 

 used for the tail chain extensions is quite similar in 

 diameter to that used for the bridles (i.e., 19 vs. 16 

 mm), the difference in tailchain length can be viewed 

 as an equivalent difference in bridle length. Although 

 unsubstantiated, our belief is that the combined forces 

 on the lower bridle are such that a lengthening of the 

 bridle by 10 m would result in a minimal change in the 

 length of the bridle held off the bottom and therefore 

 simply add a 10-m increment to L^^,. 



Whole-gear efficiency 



We are aware of two previous studies in which whole- 

 gear efficiency and the efficiency of the subsidiary 

 trawling processes were experimentally estimated. 

 One of these studies, that of Dickson (1993b), focused 

 on Atlantic cod and haddock and therefore produced 

 results that were not directly comparable to ours. The 

 other, that of Harden Jones et al. (1977), is comparable 

 because it was focused on a flatfish (plaice, Pleuronectes 

 platessa) and used a bottom trawl similar in design to 

 the Poly Nor'eastern trawl. In the latter study, gear 

 efficiency was estimated by determining the fate (i.e., 

 capture or escape) of individual fish tagged with acous- 

 tic transponders which allowed them to be located with 

 a sector scanning sonar. For all fish passing between 

 the doors, 44% were subsequently caught. This result 

 is quite similar to the maximum efficiency of the Poly 

 Nor'eastern trawl for three of the species in our study 

 (mean=43%). In the Harden Jones et al. (1977) study, 

 fish entering the trawl within the bridle path (between 

 the wings and doors) had a 22% chance of being caught 

 and fish entering the trawl within the net path had 

 a 61% chance of being caught. Again, based on the 

 maximum efficiency of the Poly Nor'eastern trawl, there 



was a 19% mean chance of being caught (calculated as 

 ki,k^^) for fish entering the trawl within the bridle path 

 and a 90% chance of being caught for fish entering 

 the net path. Thus, compared with the Harden Jones 

 et al. (1977) study, the bridle efficiency of the Poly 

 Nor'eastern trawl for flatfish was less, net efficiency 

 was greater, but the whole-gear efficiency was nearly 

 the same. 



For some bottom trawl surveys, including those con- 

 ducted by the AFSC, swept area calculations are based 

 on wing spread rather than door spread and the selec- 

 tivity or catchability parameters in stock assessment 

 models are formulated according to this convention. 

 To convert efficiency estimates presented here to the 

 estimates appropriate for the net spread convention, the 

 values must be multiplied by the quotient of the door 

 spread and net spread, which for the Poly Nor'eastern 

 trawl is approximately equal to 3 (47.8 m/16.1 m). 



Such efficiency estimates can be used to estimate 

 survey catchability, which, in turn, can be used to con- 

 strain the survey catchability parameter in stock as- 

 sessment models. In situations when the survey time 

 series is relatively short, constraining the catchability 

 parameter can lead to improved predictions of stock 

 biomass and harvest rate (Somerton et al., 1999). 



Literature cited 



Andrew, N. L., K. J. Graham, S. J. Kennelly, and M. K. Broadhurst 



1991. The effects of trawl configuration on the size and 



composition of catches using benthic prawn trawls off 



the coast of New South Wales, Australia. ICES J. Mar. 



Sci. 48:201-209. 



Bublitz, G. G. 



1996. Quantitative evaluation of flatfish behavior during 

 capture by trawl gear. Fish. Res. 25:293-304. 

 Burnham, K. P., and D. R. Anderson 



1998. Model selection and inference: practical informa- 

 tion-theoretic approach, 353 p. Springer-Verlag, New 

 York, NY. 

 Dickson, W. 



1993a. Estimation of the capture efficiency of trawl gear. 

 I: development of a theoretical model. Fish. Res. 16: 

 239-253. 

 1993b. Estimation of the capture efficiency of trawl 

 gear. II: testing a theoretical model. Fish. Res. 16: 

 255-272. 

 Efron, B., and R. Tibshirani 



1993. An introduction to the bootstrap, 436 p. Chapman 

 and Hall, New York. NY. 

 Engas, A., and O. R. Godo 



1989a. The effect of different sweep lengths on the length 

 composition of bottom-sampling trawl catches. J. Cons. 

 Int. Explor. Mer 45:263-268. 

 1989b. Escape of fish under the fishing line of a Nor- 

 wegian sampling trawl and its influence on survey 

 results. J. Cons. Int. Explor. Mer 45:269-276. 

 Harden Jones, F. R., A. R. Margetts, M. G. Walker, and G. P. 

 Arnold 



1977. The efficiency of the granton otter trawl determined 

 by sector-scanning sonar and acoustic transponding 

 tags. Rapp. P-v. Reun. Cons. Explor. Mer 170:45-51. 



