Direct estimates of multi‐mesh gillnet selectivity to Oreochromis shiranus chilwae

Abstract

The selectivity of multi‐mesh monofilament nylon gillnets to Oreochromis shiranus chilwae was estimated directly by gillnetting in a small reservoir containing a population marked by size class. Numbers in each size class were estimated using the adjusted Petersen estimate. Pearson Type I curves were fitted to plots of selectivity against the fish length/mesh perimeter ratio, for two modes of capture: wedging and tangling. The generality of the selectivity curves is tested and discussed. The tangling component of the selectivity was unimportant, and for many applications the selectivity curves may be approximated by normal curves obtainable through indirect methods. However, the increase in the amplitude of the curves with mesh size, deserves further study.

Overestimation of percid fishes (Percidae) in gillnet sampling

“…Hamley (1980) pointed out that perch are easily tangled and roach are usually only wedged or gilled. Our percids were more frequently gilled and tangled than cyprinids, but still the majority of all species were wedged, which is in accordance with the results of other studies (Winters and Wheeler, 1990;Henderson and Wong, 1991;Mattson, 1994;Santos et al, 1995;Hansen et al, 1997;Reis and Pawson, 1999;Yokota et al, 2001;Grant et al, 2004). Due to the low number of percids that were tangled, and the similar proportions of percids and cyprinids that were gilled, it could be concluded that the mechanism of enmeshing cannot importantly bias the gillnet catches towards a higher proportion of percid species in the reservoirs studied.…”

Size selectivity of standardized multimesh gillnets in sampling coarse European species

“…In theory, this should guarantee that the catchability of fish does not change when the fish size increases." However, many authors, as well as this study have shown that catchability (∼height of the selectivity curve) increased with the mesh/fish size (Mattson, 1994;Santos et al, 1995;Jensen, 1995a,b;Jensen and Hesthagen, 1996;Santos et al, 1998;Kurkilahti et al, 1998). This is related to the higher probability that larger fish will encounter the gillnet.…”

Abundance and distribution of fish in the lower Mnembo River, Malawi‐Mozambique

“…A multi‐mesh gillnet (5–43 mm mesh size) was used in this study and sampling sites were selected where impacts of gear selectivity were thought to be minimal. While gillnets can be very selective for size and species (Mattson, 1994; van der Mheen, 1995), selectivity was most likely reduced by using the multi‐mesh gillnet with a wide range of mesh sizes. Net location can also affect gear selectivity.…”