Can optimal trap mesh size be predicted from body depth in a laterally-compressed fish species?

We used fish body depth to predict trap center-to-center mesh sizes that would optimize size selection of black sea bass Centropristis striata for both current and proposed minimum size limits for this species. We fished trap types of five different square mesh sizes/configurations: (1) 38.1 mm mesh, (2) 38.1 mm mesh on five sides and 50.8 mm mesh on one side (back panel trap), (3) 50.8 mm mesh, (4) 57.2 mm mesh, and (5) 63.5 mm mesh. The 38.1 mm mesh trap was the control trap type. Back panel traps are the minimally legal mesh configuration in this region while 50.8 mm mesh traps are commonly used in this fishery to further reduce culling of sub-legal black sea bass by fishers. Two previously untested mesh sizes, 57.2 and 63.5 mm, were evaluated because a previously published morphometric relationship between black sea bass body depth and total length (TL) predicted that the diagonal openings of these two respective mesh sizes would retain black sea bass close to the current (279 mm) and proposed minimum TL limits (305 mm). We estimated size selection of each experimental trap type by comparing catches of control and experimental traps. All but the back panel trap displayed relatively steep selection around the fish length at which 50% of individuals were selected (l50). Initial size at retention (l10) by the 57.2 and 63.5 mm trap types were nearly identical to current and proposed minimum fish sizes, respectively. Predictions from the body depth: TL relationship were very similar to estimates of l10 and l50, based on uncompressed body depth and compression to 93%. The 57.2 mm mesh and 63.5 mm mesh trap types maintained catches of legal fish but reduced catches of sub-legal fish compared to the back panel and 50.8 mm mesh trap types. Relative to back panel and 50.8 mm mesh traps, use of 57.2 mm mesh traps would reduce rates of discard and discard mortality given current size limits in this fishery.