Abstract
Fishing of whelks Neptunea arthritica in Shiriuchi, Hokkaido is carried out using three types of trap. However, the size selectivity of these traps is not known. Size selectivity significantly depends on the mesh 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 calculating the probability of retention (Pn) that en-trapped whelks can not pass through the bottom net of a trap. Pn is expressed as a function of mesh bar length l, hanging ratio α and shell length L'. The results of an examination of 48 individuals with α values of either 0.50 or 0.29 indicate that Pn can be expressed as a partial sigmoid curve. The regression curve can be expressed approximately as: where L' is the measured shell length. The slope of the latter is greater than that of the former and the values of the selection span (the difference of L'/2l between 25% and 75%) are 0.22 when α=0.50 and 0.19 when α=0.29. The resultant selection factors are 0.60 for the former and 0.65 for the latter.
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