Influence of adaptive stations in a transect-based sampling design for a multispecies fish survey

A multispecies transect-based survey with an adaptive sampling design was evaluated to determine the import of adding stations adaptively at sea in order to better resolve the cross-shelf distribution of species in regions of rapid bathymetric change. The dataset came from a series of transects occupied in November, January, March, and May from May 2003 to November 2005 along transects northeast of Hudson Canyon and northeast of Baltimore Canyon in the Mid-Atlantic Bight region of the western Atlantic. The transect survey results demonstrate the necessity of high sample density in this region of the continental shelf where fish aggregate patchily over scales of a few kilometers. Species cross-shelf distributions were often bimodal. The adaptive sampling protocol demonstrated the expected tendency of fish to be underestimated or overestimated given inadequate sampling density. However, on the average, low sample density led to an underestimation of biomass or abundance and often strongly so. Modeling of the transect design reveals that the overestimates and underestimates, as well as the bias towards underestimation, originate from variations in patch location and, even more importantly, patch shape, with the sampling design. The transect model shows that extreme overestimates and underestimates can occur when patch size is small relative to the distance between stations, but the occurrence of routine, predictable, persistent underestimates in some species is not so easily explained. The model clearly resolves the importance of bimodality and a patch form represented by a dome shape in determining the vulnerability of a species to a biomass or abundance underestimate. Cases where the fixed stations alone provide data clearly inadequate for the estimate of abundance or biomass occur when sampling density is inadequate to identify the center of the patch or to identify the shape of the patch. It is the improved understanding of patch shape that is the single most important contribution of the adaptive sampling protocol, not a better knowledge of the location of the patches. Patch shape is a principal determinant of the adequacy of sample density.