Assessing the Effectiveness of Releasing Cod Larvae for Stock Improvement With Monitoring Data

Beginning in the 1880s, management of marine fisheries by hatching and releasing yolk-sac-stage larvae was advocated in both the United States and Norway. Major cod hatchery programs were popular in both countries until the mid-20th century, despite lack of evidence that cod abundance increases with release of hatchery-reared fish larvae; the potential value for such management procedures was repeatedly advocated throughout the 20th century. In Norway, a beach-seine monitoring program was begun in the early 1900s to collect data on fall abundance of 6-mo-old demersal fish in 21 fjords along the Norwegian Skagerrak coast and is still going on. We used these data in conjunction with hatchery data on numbers of yolk-sac larval cod released each spring in several fjords to test for an effect of the releases on the abundance of fjord cod populations. Using both a permutation test and a statistically derived time-series model for the cod's population dynamics, we found a slight, but statistically significant, dependence of 6-mo-old cod abundance on the number of yolk-sac larvae released in four of the 16 fjords (for which we had adequate release and beach-seine data needed for carrying out the testing). However, using the time-series model, we did not find evidence of long-term increases in the abundance of mature cod in any of the fjords. We discuss our findings on the basis of the literature on marine fish population enhancement programs worldwide.