Biological characteristics, factors affecting catchability, and abundance indices of capelin in the southern and northern Gulf of St. Lawrence multi species bottom trawl surveys

Abstract

The main objective of this study was to investigate, based on new analyses and a conceptual model of capelin summer-feeding habitat use, if biologically sound capelin abundance indices could be derived from the southern and northern Gulf of St. Lawrence (GSL) multispecies bottom trawl surveys. A series of predictions generated by the conceptual model, which states that capelin is associated with the cold intermediate layer (CIL) and performs diel vertical migrations making it more available to bottom trawls during the day, were supported by analyses of survey data. Size frequency distributions of capelin caught in the multispecies bottom trawl surveys indicated regional differences in size structure. Compared to length frequencies from the commercial fishery targeting spawning fish, the surveys captured on average smaller sized (and younger) capelin. The hypothesized negative effect of predation risk on capelin catchability in bottom trawls was quantified while accounting for other habitat descriptors. Results showed that, for the southern GSL (sGSL), capelin catchability was negatively related with predation risk but with a small overall effect relative to other factors. Among the abundance indices presented, an index computed from hauls performed in capelin preferred thermal habitat is promising because of its consideration of capelin behaviour and lower associated variability. This index, however, assumes that capelin density is horizontally homogenous in the CIL and this assumption has yet to be validated. An estimate of the order of magnitude of fishing mortality is also presented based on newly developed abundance indices and is considered low when compared to conservative estimates of capelin natural mortality, although the risk of local depletion where the fishery is concentrated was not investigated. These new analyses bring useful information in support of the assessment of NAFO Divisions 4RST capelin stock assessment.