Comparison of plankton net and light trap methodologies for sampling larval and juvenile fishes at offshore petroleum platforms and a coastal jetty off Louisiana

Abstract

            While sampling ichthyoplankton associated with artificial structures across the continental shelf (3–230 m depth) of the northern Gulf of Mexico (GOM), we compared passive, 60 cm diameter plankton net and light-trap methodologies at three offshore petroleum platforms (continental shelf slope, mid-shelf, and inner shelf) and a 1m x 1m plankton pushnet and a light-trap at a coastal jetty. Clupeiform fishes dominated collections for all gears (59%–97% of the total catch for nets and lighttraps combined). Plankton nets collected more fish than light-traps at the shelf slope and innerplatforms (plankton net vs. light-trap: 1,404 vs. 659; 3,076 vs. 12,474; and 1,689 vs.1,193), and the pushnet collected more fish than the light-trap (33,1477 vs. 849) at the jetty. Plankton nets collected individuals from more families than light-traps at the shelf slope and mid-shelf platforms (plankton net vs. light-trap: 43 vs. 35; 38 vs. 35; and 32 vs. 32), but only collected more taxa (genus level) than light-traps at the shelf slope platform (plankton net vs. light-trap: 56 vs. 47; 75 vs. 78; and 50 vs. 56). At the jetty, the pushnet collected more families (39 vs. 19) and taxa (77 vs. 34) than the lighttrap. Results of Kolmogorov-Smirnov length-frequency comparisons of fish collected in plankton nets vs. light-traps indicated light-traps generally overlapped the net’s smaller sizes, but alsocollected significantly larger individuals. At the jetty, greater overlap in size distributions was observed for comparisons of the pushnet and light-trap. Schoener’s Index of Niche Overlap values for light-trap and plankton net samples indicated low similarity between gears at two platforms (0.32–0.38), but higher similarity (0.63) at the inner shelf platform which was most dominated by clupeiforms. At the jetty, pushnet and light-trap samples had relatively high taxonomic similarity (0.61). Few significant differences were detected between Shannon-Weiner Diversity Indices for platform light-trap and plankton net samples, while at the jetty, pushnet samples had significantly higher diversity than light-trap samples. When significant differences in mean total densities and mean total CPUEs were found between new vs. full moon phases, four out of five instances had greater new moon catches.