Intake screens for sampling fish populations. The size-selectivity problem.

The size selectivity of the intake screen at the P.H. Robinson generating station on Galveston Bay, Texas [USA] was compared with that of a 3-m otter trawl hauled in the intake waters. A means of predicting size selectivity of the intake screens was evaluated for brown shrimp (Penaeus aztecus), white shrimp (P. setiferus), gulf menhaden (Brevoortia patronus), bay anchovy (Anchoa mitchilli), sand seatrout (Cynoscion arenarius) and Atlantic croaker (Micropogonias undulatus). The trawl generally caught smaller individuals than the intake screens, with the relative frequency of catch by size gradually increasing to a peak at an intermediate length of a species, then decreasing as its length increased. Conversely, the relative frequency of collection at the intake screens increased abruptly at a predictable threshold length. Above this threshold, catches decreased gradually, presumably because larger individuals were both less abundant and less susceptible to capture than smaller ones. Intake screens are a practical means of sampling fish populations and provide data that are as useful as trawl data for most fishery analyses.