Causes of Density-Dependent Catchability for Georges Bank Haddock Melanogrammus aeglefinus

Abstract

We used population estimates (VPA) fishing effort and catch data for Georges Bank haddock, Melanogrammus aeglefinus, from 1964 through 1984 to test Paloheimo and Dickie's hypothesis which predicts: (1) that the catchability coefficient (q) from the commercial fishery is inversely related to haddock stock size (N) and stock area (Area); (2) that commercial catch per effort (C/f) data are curvilinearly related to stock abundance; and (3) that search time in the Georges Bank haddock fishery is inversely related to stock abundance. The catchability coefficients for the Georges Bank trawl fishery were inversely related to haddock stock size (r = −0.82, P < 0.001) and stock area (r = −0.75, P < 0.001). There was also a significant curvilinear relationship between commercial C/f and absolute and relative haddock stock biomass from 1964 through 1984. The slope estimate (B + 1) from this relationship was less than 1.0 (r = 4.18 to 8.95, P < 0.001), indicating nonlinearity between C/f and N. Finally, the relative search time expended by the haddock commercial fishery was inversely related to stock size from 1964 through 1984 (r = −0.75, P < 0.001). These results are consistent with the three predictions of the Paloheimo and Dickie hypothesis, suggesting that fishing mortality rates on Georges Bank haddock are a depensatory function of stock biomass.