SELECTIVITY OF TRAPS FOR WHELKS NEPTUNEA- ARTHRITICA

Fishing of whelks Neptunea arthritica in Shiriuchi, Hokkaido is carrie d out using three types of trap. However, the size selectivity of thes e traps is not known. Size selectivity significantly depends on the me sh size and hanging ratio of the bottom net of the trap. In this study the size selectivity curve of traps for whelks was estimated by calcu lating the probability of retention (P-n) that entrapped whelks can no t pass through the bottom net of a trap. P-n is expressed as a functio n of mesh bar length l, hanging ratio alpha and shell length L'. The r esults of an examination of 48 individuals with alpha values of either 0.50 or 0.29 indicate that P-n can be expressed as a partial sigmoid curve. The regression curve can be expressed approximately as: When al pha=0.05, P-n=2/1+e(2.7-6.40(L'/2l) -1, When alpha=0.29, P-n=2/1+e(3.7 8-7.50(L'/2l) -1. where L' is the measured shell length. The slope of the latter is greater than that of the former and the values of the se lection span (the difference of L'/2l between 25% and 75%) are 0.22 wh en alpha = 0.50 and 0.19 when alpha = 0.29. The resultant selection fa ctors are 0.60 for the former and 0.65 for the latter.