NOTE Bartoo and Holts Drift gillnet selectivity for Thunnus ala/unga 



377 



right limb of the selectivity curve. This skewing can be 

 troublesome when symmetrical or pre-determined 

 selectivity curves are fit to data. The method used in 

 this study minimizes this problem. 



Assumptions that selectivity curves estimated from 

 the 1980 JAMARC experiment applied to similar nets, 

 such as the NMFS 184 mm mesh and the JAMARC- 

 1982 180 mm mesh, appear acceptable from the data 

 available. The principal factor limiting the application 

 of these selectivity curves to other albacore popula- 

 tions is the girth/length relationships discussed above 

 (Fig. 12). Significant changes in this relationship will 

 change the steepness of the selectivity-curve limbs and 

 the skewness. 



Extrapolation of the mean selectivity curve to smaller 

 mesh sizes than those providing the initial data may 

 be improper based on the data observed for the 118 mm 

 mesh. Although the sample size was relatively small, 

 the apparent increase in tangling of larger fish caught 

 in smaller mesh sizes compared with larger mesh sizes 

 suggests that the right portion of selectivity curves for 

 meshes < 130 mm are more skewed. Tangled fish did 

 not appear to be a problem in the NMFS or JAMARC 

 1980 datasets, presumably because all mesh sizes ei- 

 ther gilled or wedged fish in the largest mode. Addi- 

 tionally, there may have been fewer larger fish avail- 

 able in the survey areas, as shown by troll-caught 

 samples. 



The absence of larger, tangled fish in both net-caught 

 and troll-caught samples could result from reduced 

 availability or vulnerability of larger fish in the areas 

 sampled. Surface fisheries (i.e., drift net, troll, and pole- 

 and-line) for albacore in the North Pacific produce 



catches with similar length-frequencies in the same 

 areas (Bartoo & Foreman In press). However, albacore 

 taken by subsurface longline gear at the same time 

 and area are generally larger in length than those 

 taken in the surface fisheries. This indicates albacore 

 show differential availability or vulnerability between 

 surface and subsurface gears. This could be due to 

 spatial separation or behavior which may change with 

 fish size. The results presented here assume that vul- 

 nerability to DGNs is relatively static over time. 



Acknowledgments 



We acknowledge contributions to this work by several 

 individuals. Thanks to Tony West and the crew of the 

 F/V Steelfin II for their assistance during the cruises. 

 Thanks to Earl Weber for helping conduct the NMFS 

 drift gillnet experiment, Sonee Sonu for providing Japa- 

 nese-English translations, and to the many reviewers 

 for constructive comments on the manuscript. Finally, 

 thanks to Karen Handschuh for production of the 

 manuscript. 



Citations 



Bartoo, N., & T. J. Foreman 



In Press A synopsis of the biology and fisheries for 

 north Pacific albacore tuna. In Shomura, R. (ed). 

 Report of the FAO Expert Consultation on Interac- 

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 Tech Pap. 

 Hamley, J. M. 



1975 Review of gillnet selectivity. J. Fish. Res. Board 

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 Ishida, T. 



1962 On the gill-net mesh selectivity curve. Bull. 

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 JAMARC (Japan Marine Fishery Resource Center) 



1983 Newly developing resources survey report, north 

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1985 Newly developing resources survey report, east- 

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1986 Newly developing resources survey report, east- 

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Regier, H. A., & D. S. Robson 



1966 Selectivity of gill nets, especially to lake 

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