Some Effects of Shape and Structure of Fish on Selectivity of Gillnets

Abstract

Selectivity curves for three species of fish taken in South Bay, Manitoulin Island, and in Georgian Bay during 1954–59 with gangs of nylon gillnets differed with respect to mode, skew, and kurtosis. A graphic method was developed to describe one aspect of body shape, namely the relation between girth and distance from the snout, which would affect selectivity. Surprisingly, the shapes of mature (but not gravid) and immature perch and suckers of both sexes could be described by the same curve. Hence, differences in body shape did not account for differences in the selectivity of these species. In contrast, the taper for gravid female perch was markedly different from that for nongravid, as might be expected. Catches in which the positions where caught were carefully identified showed that most of the fish were caught either on the opercle or near the position of maximum girth. That the fish could be caught at different positions accounted for most of the skew in the selectivity curves based on maximum girth. Selectivity curves plotted on the basis of girth where caught, rather than on maximum girth or length, show that efficiency of capture was maximal when the girth of the fish was 1.0–1.2 times as great as the perimeter of the mesh. Efficiency was negligible at girth/perimeter ratios smaller than 0.8 or 0.9 and it declined rapidly at ratios greater than 1.2. However, a few fish were taken at ratios up to 1.5 or 1.6.

A peaked logistic-based selection curve

“…Gray, in review). Asymmetry in size selection can be caused by a volume-based capture allometry (McCombie and Berst 1969) or multiple capture processes (Hamley 1975), and nonzero asymptotic selection can occur because of ontogenetic changes mediating behavioural responses to, or physical exposure to, certain gear types (J.A. Smith, M.D.…”

Bycatch mitigation assessment for sharks caught in coastal anchored gillnets

“…Bycatch mitigation focuses primarily on changes to the design of a gillnet that results in applicable management tools (for example, mesh size). Speciesspecific size selectivity is characterized by an upper and lower size limit and few fish are caught whose lengths differ from the target size by more than 20% for a specific mesh size (Baranov, 1948;Garrod, 1961;McCombie and Berst, 1969). Other gillnet design techniques that influence the catch size distribution include hanging ratio (Samaranayaka et al, 1997;Moth-Poulsen, 2003), twine color (Tweddle and Bowa, 1990;Wardle et al, 1991), twine thickness (Hovgård and Lassen, 2000;He, 2006) and twine material (Balik, 2003;Ayaz et al, 2006;Maki et al, 2006).…”

Overestimation of percid fishes (Percidae) in gillnet sampling

“…doi:10.1016/j.fishres.2007.11.009 (according to Baranov;Hamley, 1975): (i) wedged-when the fish is held tightly by the mesh around the body; most fish are caught in this way (Yokota et al, 2001); (ii) gilled-when the fish is prevented from backing out of the net by the mesh caught behind the gillcover; (iii) tangled-when the fish is held in the net by teeth, opercular spines, maxillaries, or other projections, without necessarily penetrating the mesh. In this respect, the morphology of the fish body is very important (McCombie and Berst, 1969;Pet et al, 1995;Reis and Pawson, 1999) and gillnets tend to be more efficient in capturing fishes adorned with external projections, teeth, etc. (Lagler, 1978).…”

“…are very rarely caught in gillnets (Hammar and Filipsson, 1985;Degerman et al, 1988;Rossier, 1997;Holmgren, 1999;Vetemaa et al, 2006); thanks to their smooth body morphology and motoric abilities. Tangling particularly has a very close association with the properties of the fish body, which may differ among families (McCombie and Berst, 1969;Reis and Pawson, 1999). For example, common European percid species (family Percidae), such as perch Perca fluviatilis, pikeperch Sander lucioperca and ruffe Gymnocephalus cernuus, possess structured body surfaces and have a relatively firm body structure (Kipling, 1963;Hamley and Regier, 1973).…”