Selectivity of Gill Nets, Especially to Lake Whitefish

This paper re-examines five methods previously described for estimating the selectivity of gill nets as influenced by mesh size, and introduces four more. Those that appeared appropriate for lake whitefish on the basis of preliminary analysis were employed to estimate selectivity using data on whitefish from South Bay, Manitoulin Island, Ontario.The methods considered most reliable yielded estimates of selectivity curves for whitefish resembling the normal probability function with a positive skew. The length at which the mode occurred was a significantly nonlinear function of mesh size, the variance was approximately proportional to the square of the mesh size, and the coefficient of skewness not related to mesh size.An implication of these findings is that, for general sampling purposes, standard gangs in which mesh size increased in a certain geometric progression would be more efficient than those with mesh sizes in the usual arithmetic progression. The series for lake whitefish might be 1.40(1.275)x with x = 0, 1, 2, 3, 4, 5.Under the assumption that at peak efficiency all mesh sizes are equally efficient with respect to number of fish, "total efficiency" of a mesh size is defined as the area under the selectivity curve. With respect to number, we estimated that, for whitefish, total efficiency r = 12 + 5.5 (mesh size, stretch measure in inches). Here peak efficiency is taken as 100, with length units as inches. Similarly with respect to weight total efficiency r = 0.80 (mesh size)3.2. Thus with respect to weight, total efficiency increases from about 7.5 for 2-inch mesh to 100 for 4.5-inch mesh to about 200 for 5.5-inch mesh.