Diatom production in the marine environment: implications for larval fish growth and condition

To test the effects of diatom production on larval fish growth and condition, laboratory experiments were performed with larval North Sea cod reared on different algal food chains. These food chains were based on cultures of (a) the diatoms Skeletonema costatum and Thalassiosira weissflogii; (b) the dinoflagellate Heterocapsa triquetra; (c) the flagellate Rhodomonas baltica; (d) a diet composed of both Skeletonema and Heterocapsa food chains (1:1), and (e) a starvation group. These algae were fed to cultures of adult Acartia tonsa. Copepod eggs were collected, hatched, and the N1 nauplii (200l−1) were fed to post-yolk-sac larval cod. Results indicate that larval growth rates are significantly influenced by the content of essential fatty acids of the algal food source: growth rates were positively correlated with the content of DHA (C22:6ω3) and negatively with EPA (C20:5ω3). The ratio of ω3/ω6 fatty acids in the algal source had no significant effect. The highest and lowest growth rates were observed in food chains based on H. triquetra and T. weissflogii, respectively (means for days 14–16 of 4.0 and −4.7). The mixed diatom/dinoflagellate diet resulted in intermediate growth rates and condition. Regressions of growth rates against EPA and DHA content indicated no inhibitory effect of diatom production on growth in larval cod.