The dynamics of a recovering fish stock: Georges Bank herring

The decline and subsequent recovery of Georges Bank Atlantic herring (Clupea harengus) provides a rare opportunity to examine the dynamics of a recovering fish population. Moreover, the near absence of a commercial fishery on Georges Bank between 1978 and 1995 removes the confounding effects of exploitation during the recovery period. Herring abundance on Georges Bank increased and the distribution of adult spawning fish evolved from a few isolated locations to most of the northern fringe during the period 1983-1995. The distribution of recently hatched larvae also expanded in a manner consistent with progressive occupation of historical spawning grounds. Changes in the size composition and age structure of herring during the spawning season broadened from the dominance of a single age class to multiple year classes as the stock recovered and expanded. Growth, as reflected by length-at-age, decreased significantly and was correlated with the number of fish estimated to be in the stock. This and the observed difference in mean length and length at first spawning during the recovery provide strong evidence of density-dependent growth. In particular, there is a highly correlated (p < 0.01) relationship between the number of 4+ herring in the stock at the start of the year and the mean length of herring aged 3 recruiting to the spawning stock in autumn of the same year. A mechanism based on an extended period of prespawning interaction is proposed to explain the density-dependence.