Changes in catchability of Atlantic cod (Gadus morhua) to an otter-trawl fishery and research survey in the southern Gulf of St Lawrence

For each year from 1971 to 1991 and age from 3 to 8+, we estimated catchability of Atlantic cod (Gadus morchua) to an otter-trawl fishery (q) and a research survey (k) using SPA-based abundance estimates (N) and catch rates to the fishery (U) or survey (R). q tended to be highest early in the time series for young cod (ages 3 and 4), late in the time series for old cod (ages 7 and 8+), and at the two extremes of the time series for cod of intermediate ages (5 and 6). k tended to be higher later in the time series for all ages of cod. We tested for density dependence of q and k by comparing InU or InR to InN. The slope of the regression of InU versus InN was significantly less than 1.0 for ages 3 to 5 and greater than 1.0 for ages 8+, indicating an inverse relationship between q and N for the younger ages and a positive relationship for the older age. The slope of the regression of InR on InN was significantly greater than 1.0 for ages 5 to 7, indicating a positive relationship between k and N for these ages of cod. An SPA calibration model assuming a non-linear relationship between catch rate and abundance led to the same conclusions about the density dependence of q and k. We discuss possible causes of these apparent changes in q and k, and relate density-dependent changes in k to previous reports of density-depedent changes in spatial distribution for this population.