Comparison of an enclosure drop trap and a visual diving census technique to estimate fish populations in eelgrass habitats

Abstract

In shallow vegetated aquatic habitats exceeding 2 m depth, conventional methods to determine fish abundances are of limited success. In this study, the sampling qualities of 2 methods for investigating the fish community associated with the eelgrass Zostera marina were tested: a newly developed enclosure drop trap (ET) operated from the water surface and a visual diving strip transect technique. The construction and the sampling procedure of the enclosure trap are described in detail. Both sampling procedures were performed simultaneously at the same eelgrass sampling site at depths between 2 and 5 m. The comparison revealed deficiencies in relative catch efficiency for both methods. Limitations with respect to bottom-dwelling forms are obvious for the ET and with respect to the Syngnathidae and Gasterosteidae for the visual diving census. Abundance estimates of pelagic, schooling species were highly variable with both methods. Correction coefficients for the different behavioral categories of eelgrass fish were calculated. The fish species composition showed good agreement between the 2 methods, but the number of species on each sampling occasion was consistently higher in the ET catches. The length class frequency distributions estimated by the divers corresponded approximately with direct measurements of fish from the ET catches. The methods complemented one another and can be recommended for simultaneous deployment to get accurate quantitative estimates of eelgrass fish populations. The relative capture efficiency of the enclosure trap can be incorporated to improve abundance measurements of small fish in Zostera marina beds without the help of divers.