The ecology of the two-spot goby (Gobiusculus flavescens Fabricius): the potential for cod enhancement

Abundance, growth, production, and biomass of the 1986 and 1987 year classes of the two-spot goby (Gobiusculus flavescens) are reported. Both year classes comprised high numbers of individuals in late summer as a result of recruitment; abundance decreased during autumn and reached a minimum in winter. Abundance increased again in May/June, due to immigration from deeper water, followed by a decrease once more in the autumn. The year classes disappeared from the catches during their second autumn. Rapid growth of juveniles resulted in a high production and rapid build-up o f biomass during July-September, with 60-80% of the production of a year class occurring in this period. Gadus morhua, Pollachius pollachius, P. virens, and Trisopterus minutus were the main predators on G. flavescens. Predator pressure was most intense during July-December, coinciding with a maximum in biomass and production of G. flavescens. The highest production per metre of shoreline was found in the outer area of the fjord in both 1986 and 1987, suggesting that the shallow areas of the outer fjord have the highest potential for supporting released cod with food. Distribution o f G. flavescens is discussed in relation to the distribution of macroalgae and food availability. Co-variance between years in abundance of G. flavescens, 0-group cod, and zooplankton is discussed briefly. The food chain, zooplankton-G. flavescens-cod, seems to be an important and effective way for the cod to utilize parts o f the zooplankton, although the amount of energy that is transported through this path from year to year is highly variable.

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