Life history co-variation in a fishery depleted Atlantic cod stock

The northern stock of Atlantic cod (Gadus morhua) once supported one of the world's largest fisheries, but since the early 1990s has been in a collapsed and post-industrial state. Extreme life history trait changes have occurred in this stock, most evident in early maturation and increased fecundity and mortality. Using historical data from the early 1960s (pre-industrial) and previously unpublished data from 1999 and 2003 (post-industrial), we show co-variation of these traits. Few fish in the later period reached 6 years of age or >60 cm (formerly many reached >12 years and >80 cm), but fecundity at size in the early maturing fish was higher than ever recorded in any cod stock. Post-industrial cod at 4 years had a potential fecundity of about 0.5 million eggs, which did not occur until age 6 in pre-industrial times. Coincidentally, mortality was low during the pre-industrial period but increased dramatically in the post-industrial period. Using an egg-per-recruit model, fit to age and length structure data from the two periods, we also show how reduced survivorship, earlier age at maturity and increased fecundity resulted in a compensated fecundity per recruit that was >75% of pre-industrial levels. The cause of these changes, whether they represent traits selected for by the fishery, responses to reduced densities, or a combination of these factors is discussed, but remains uncertain.