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 characte rized by high productivity of eggs leading to high concentration of larvae. These stages are known to be associated with high mortality rates. Mortali ty of older stages (juveniles and adults) has been relatively poorly studie d. We use capture-mark-recapture (CMR) methodology to estimate time and age variation of natural mortality and fishing mortality in a coastal cod popu lation 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 re leased 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 r elease. Tag return by fishermen was similar to 50-60%. Natural mortality wa s found to be high in the 6-12 mo range, but subsequently it decreased rapi dly and was no longer age-dependent. Fishing mortality was negligible for f ish younger than 1-yr-old, intermediate for 1-yr-old fish, and high for old er fish. The seasonal pattern of fishing varied much between age classes, w ith 2-yr-old fish mostly taken by recreational fishermen in summer, and old er fish mostly taken by professional fishermen in winter. Because it is dir ected 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 Risor are a during the last 20 yr. Total mortality varied little between years, excep t during similar to5 mo after a major algae bloom in May 1988, when fish di sappeared 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.