Comparison of two plankton net mesh sizes for ichthyoplankton collection in the northern Gulf of Mexico

Abstract

Comparisons were made between ichthyoplankton samples collected in the northern Gulf of Mexico using a Bedford Institute of Oceanography Net Environmental Sampling System (BIONESS) fitted with 0.202 mm and 0.333 mm mesh plankton nets. Tow duration, tow speed, filtration efficiency and filtration rate were monitored in real time during paired oblique (1–18 m) tows. General tow characteristics and larval assemblages were compared for samples characterized by filtering efficiencies >85%. Plankton tows characterized by relatively short durations (approximately 4.5 min) and low filtered volumes (approximately 70 m 3) were largely overlapping in fish egg densities, larval densities, and length frequency distributions. Higher taxonomic richness (number of families) and diversity (exponential of Shannon Entropy) were observed in the 0.333 mm mesh net. No significant differences in larval fish densities were observed between mesh sizes for seven families of fishes; only unidentified clupeiform fishes were more common in the 0.333 mm mesh net. Mean lengths for total fish larvae, total non-clupeiform fish larvae, sciaenids and unidentified larvae were significantly larger in the 0.333 mm collections. However no significant differences were noted in the length frequencies for these groups. We conclude that under controlled tow conditions, ichthyoplankton samples collected in our region with these two mesh sizes are largely comparable.