376 



Fishery Bulletin 91(2). 1993 



NMFS 1985 experiment 



Length-frequency data for albacore collected in the 

 184 mm mesh during the 1985 NMFS experiment are 

 shown in Fig. 8. The estimated selectivity curve is 

 shown in Fig. 9 and, as expected, is almost identical to 

 the 180mm mesh selectivity curve (Fig. 7). For com- 

 parison, length-frequencies from the NMFS experiment 

 taken by both trolling jigs and drift netting in the 

 same area are shown in Fig. 8. As predicted by the 

 selectivity curve (Fig. 9), few fish <50cm length were 

 sampled by the 184 mm mesh even though the fish 

 were assumed available to the net, as indicated by the 



FORK LENGTH (CM) 



Figure 10 



Percent frequency-of-occurrence by fork-length interval for 

 albacore Thunnus alalunga for 180mm stretch mesh sampled 

 by the 1982 JAMARC survey (JAMARC 1985). 



118 mm 

 N =87 



go 



FORK LENGTH (CM) 



Figure 1 1 



Percent frequency-of-occurrence by fork-length interval for 

 albacore Thunnus alalunga for 118mm stretch mesh sampled 

 by the 1983 JAMARC survey (JAMARC 19861. 



size-frequency sampled by the trolling gear fished in 

 the same area and time. 



JAMARC 1 982 experiment 



The 1982 JAMARC South Pacific experiment col- 

 lected data for a 180 mm mesh size. Length- 

 frequency data obtained are shown in Fig. 10. The 

 estimated selectivity curve for this mesh is the same 

 as shown for the 180 mm mesh in Fig. 7. From in- 

 spection of the length-frequencies for 180 mm and 

 184 mm mesh sizes from the North Pacific, it was 

 apparent that the South Pacific experiment sampled 

 an albacore population with a different size struc- 

 ture. The results show a much greater proportion of 

 fish in the 70-80 cmFL range than in the North Pa- 

 cific sampling. This is also seen in samples taken 

 from the U.S. troll fishery that operated in the South 

 Pacific (Rensink 1991) 



JAMARC 1 983 experiment 



Length-frequency data obtained from the 1983 

 JAMARC South Pacific experiment using 118 mm mesh 

 are shown in Fig. 11. The selectivity relationships de- 

 veloped from the North Pacific data do not accurately 

 predict the expected selectivity curve for the 118 mm 

 mesh. This is a result of small sample sizes and an 

 observed peak size mode well below the sizes of alba- 

 core captured in the mesh sizes used to derive the 

 selectivity curves. 



Discussion 



Selectivity curves derived from the JAMARC 1980 ex- 

 perimental data appear similar in shape to selectivity 

 curves presented for salmon Onchorynchus nerka and 

 O. keta, slender tuna Allothunnus fallai, and mackerel 

 Scomberomorus niphonius, and other fusiform fishes 

 (Taguchi 1961, Ishida 1964, Shimazaki et al. 1984). 

 The asymmetry of each selectivity curve (i.e., steeper 

 on the left limb and flatter on the right limb) is simi- 

 lar to that observed for selectivity curves of other fusi- 

 form fishes (Hamley 1975) and is explained by the 

 mechanics of capture. The smallest fish, retained by 

 the net at some particular girth/mesh-perimeter ratio, 

 tend to be caught near maximum girth. Also, the range 

 of sizes providing the ratio is relatively narrow. For 

 larger fish, the same ratio can be found over a larger 

 range offish sizes because the point of capture may be 

 anywhere from the maximum girth forward to the 

 snout tip. For example, maximum girth of albacore 

 increases more rapidly than opercular girth as a func- 

 tion of length (Fig. 12) which extends or skews the 



