A comparison of bridled and bridle-less ichthyoplankton nets in a variable flow estuarine environment

The effectiveness of bridled and bridle-less plankton nets for sampling ichthyoplankton from a fixed structure in a variable flow environment was examined. From 22 June to 19 September 2011, larval and juvenile fishes were collected weekly during night flood tides from the Moon River, an estuarine tidal creek south of Savannah, Georgia, USA. Passive horizontal tows were made from a fishing pier using two ichthyoplankton nets, each with a 1 m diameter hoop and 1 mm mesh. A bridled plankton net was configured with a tow rope attached to a traditional three-point bridle anchored to the hoop and a closed, 1 L collection jar at the cod end. A bridle-less net was configured with a tow rope and a float tied to the top of the hoop while a weight was tied to the bottom of the hoop. The cod end of the bridle-less net had an open collection bucket with 1 mm mesh. Nets were deployed simultaneously in the water column, just below the surface, for three consecutive 30-min tows. For each sampling date, pair-wise comparisons of nets were performed on mean values of fish abundance, species richness, species diversity, and species evenness. Mean abundance was the only consistent difference between bridled and bridle-less nets, with the bridle-less net having on average 4.870 times more fishes. Species richness (as measured by both number of species and Margalef's species richness index) tended to be greater in the bridle-less net, but not significantly greater. Species diversity (Shannon-Wiener diversity index) and species evenness (Pielou's evenness index) were also not significantly different between the two nets.