A century of manipulating recruitment in coastal cod populations: the Flødevigen experience

Beginning late in the 19th century and continuing throughout the 20th century, attempts have been made to augment recruitment of free-living fish populations through the release of larval and juvenile fish. Especially in Norway and the United States, such stock enhancement has been argued to be useful in the management of fisheries. This paper reviews the historical and scientific settings of four studies on the effects of manipulating recruitment of cod in southern Norway and presents new analyses of available data on young fish releases and population monitoring. Analyses suggest that the effects of the larval cod released from the Flødevigen hatchery were, in fact, detectable in beach-seine samples of 6-month-old cod. This is in contradiction to earlier analyses. Further, the behaviour o f a population dynamics model fit to the beach-seine survey data is consistent with the experimental results from release of 6-month-old fish. Taken together, these results suggest that, although the release of larval cod may influence the immediate abundance of juvenile cod. the effects on the mature population are minuscule. This paper suggests that the claims of the supporters of the "Marine Fish Hatchery Movement" of the late 19th century are not supported, but points out that the Norwegian scientific dialogue, although at times heated, contributed much to our understanding of the nature of coastal cod populations and their dynamics. Although ICES did not play a major role in the "Marine Fish Hatchery Movement", many early ICES participants were involved. Scientific studies undertaken to determine the effectiveness of marine fish hatching had a direct effect on the scientific basis underlying ICES, especially the question of the variability of year classes.