Natural Mortality and Fishing Mortality in a Coastal Cod Population: A Release-Recapture Experiment

As with most teleosts, the life cycle of the cod (Gadus morhua) is characterized by high productivity of eggs leading to high concentration of larvae. These stages are known to be associated with high mortality rates. Mortality of older stages (juveniles and adults) has been relatively poorly studied. We use capture-mark-recapture (CMR) methodology to estimate time and age variation of natural mortality and fishing mortality in a coastal cod population within the Norwegian Skagerrak coast. A total of 36 728 reared 6-mo-old, and 2415 wild-caught mostly 18-mo-old, individually tagged cod were released in four consecutive years, leading to 4155 recaptures by fishermen. Tag loss and mortality due to tagging appeared negligible for reared young fish, but up to 60% of the wild-caught tagged fish disappeared just after release. Tag return by fishermen was ~50-60%. Natural mortality was found to be high in the 6-12 mo range, but subsequently it decreased rapidly and was no longer age-dependent. Fishing mortality was negligible for fish younger than 1-yr-old, intermediate for 1-yr-old fish, and high for older fish. The seasonal pattern of fishing varied much between age classes, with 2-yr-old fish mostly taken by recreational fishermen in summer, and older fish mostly taken by professional fishermen in winter. Because it is directed toward relatively young fish, recreational fishing may have a strong impact on the population; we specifically point out the coincidence between the increase of tourism and the decrease of the cod stock in the Risør area during the last 20 yr. Total mortality varied little between years, except during ~5 mo after a major algae bloom in May 1988, when fish disappeared at a high rate whatever their age (no emigration was observed in connection with the algae bloom). We conclude that mortality of adult fish may be an important determinant of the stock size.