Using tagging experiments to evaluate the potential of closed areas in protecting migratory Atlantic cod (Gadus morhua)

About 5200 cod (Gadus morhua) were tagged on three fishing grounds and within two marine protected areas (MPAs) northwest of Iceland. Two of the fishing grounds were closed to otter trawling, and the third was open to all fishing gear. Tagged fish were divided into two size classes, below (small) and above (large) a reference length of 55 cm, and the following variables were estimated: (i) proportion of the recaptured small cod that did not reach the reference size, (ii) size increase from tagging to recapture, (iii) time at liberty, and (iv) spatial distribution of recaptures. For the small cod, the results suggest that not only protection status but also the distance to areas of intense fishing mainly account for differences in the variables among tagging areas. No differences were observed for the large cod among tagging areas. The results show that immature cod are relatively stationary, but perform seasonal migrations between feeding and spawning grounds after reaching maturity. The estimated distribution of tagged cod generally changed after standardizing the recapture data with fishing effort (catch per unit area). Our results suggest that area closures on nursery grounds can be useful in protecting immature cod, but the MPAs studied are of little use in protecting highly migratory adults.