Independent Distributions of Fish Larvae and Their Prey: Natural Paradox or Sampling Artifact?

Abstract

Several recent studies have concluded that larval fish distributions are independent of the abundance and distribution of their prey. All of these studies used coarse-mesh (> 250 μm) nets, incapable of retaining the edible zooplankton for fish larvae, to provide quantitative estimates of the larval food resource. The assumption that zooplankton captured by coarse-mesh nets provided a reliable index of the edible zooplankton for fish larvae was tested and unsupported by the analysis of several independent data sets. In the waters off southwestern Nova Scotia the biomass of edible zooplankton for young larval fish was highly concentrated in the nearshore region, progressively lower levels were evident offshore on the shelf, and the mesoscale distributional pattern did not accurately reflect the total zooplankton biomass retained by a 333-μm-mesh net. Independent spatial distributions of discrete size groups of zooplankton are characteristics similar to zooplankton distributions reported for other geographic regions. A more logical explanation for the reports of "paradoxical" distributions of fish larvae and their prey is to be found in the inefficiency and bias in the sampling methods used to evaluate the larval food resource. Failure to properly evaluate the larval food resource has led to inappropriate testing of some longstanding hypotheses in fisheries biology.