Effects of Biomass–Range Interactions on Catchability of Migratory Demersal Fish by Mobile Fisheries: An Example of Atlantic Cod (Gadus morhua)

Abstract

We used daily acoustic estimates of the abundance and range of a migratory demersal species, Atlantic cod (Gadus morhua), coupled with catch statistics from a mobile gillnet fishery in the northern Gulf of St. Lawrence, to verify two predictions of the Paloheimo–Dickie hypothesis: (1) that biomass and range of fish distributions are positive covariates and (2) that catchability (q) is negatively correlated with biomass and occupied range. Fishing power changed little during this study (1985–86) and was thus not a major cause of variation in q. The daily biomass (b) of cod surveyed on the fishing ground was significantly and positively correlated with the range occupied (a) in both 1985 and 1986 (b = 86a^1.05; R² = 0.71; P < 0.05). Local cod densities assessed at small scales (1 km) were relatively stable. Daily catchability was a negative multiplicative function of both biomass (q = 0.095b^−0.787; R² = 0.70; P < 0.05) and occupied range (q = 0.003a^−0.747; R² = 0.56; P < 0.05). The q–range relationship derived in 1986 was used to hindcast q in 1985. Hindcast and observed 1985 q's were significantly correlated (r = 0.74; P < 0.05).