Extrusion of fish larvae from SEAMAP plankton sampling nets: a comparison between 0.333-mm and 0.202-mm mesh nets Fishery Bulletin

To assess larval fish and egg extrusion through the standardsize mesh plankton net used during resource surveys of the Southeast Area Monitoring and Assessment Program (SEAMAP), 81 bongo tows with side-by-side nets, each constructed with a different mesh size (0.333 mm and 0.202 mm), were taken during 5 SEAMAP surveys conducted in the Gulf of Mexico during 2005–2007. Retention by length class for the larvae of 6 taxa and an unidentified group was evaluated by using 2 deterministic functions to estimate the number of larvae missed when sampling with the net with standard-size mesh (0.333 mm) compared with sampling with the smaller mesh net (0.202 mm). Smaller larvae, particularly those between 1.5 and 3 mm in body length, were retained in greater numbers in the 0.202-mm-mesh net than in the 0.333-mm-mesh net. Extrusion was most pronounced for small, undeveloped larvae that could be identified only to the suborder Percoidei or that could not be identified. Extrusion was evident also among larvae of taxa in the families Engraulidae, Sciaenidae, and Scombridae, but less so for Clupeidae and Lutjanidae; the latter result was most likely attributable to a mismatch between the timing of sampling and spawning seasons. The functional relationships presented here, based on larval abundance ratios and body lengths, represent the first empirically derived estimates of extrusion and size bias in SEAMAP ichthyoplankton samples.