Lipid biomarkers: linking the utilization of frontal plankton biomass to enhanced condition of juvenile North Sea cod

The effects of physical mixing processes on phytoplankton production in the marlne environment are well established. However, the effects of these processes on growth and condition of zooplankton and larval fish are at present poorly understood. In this study, w e utilized phytoplankton group-specif~c fatty acid content to trace the phytoplankton group and mixing regime contributing to the condition of individual juvenile North Sea cod. In order to establish a relationship between lipid tracer content and algal utilization, post yolk-sac larval North Sea cod were reared in the laboratory on food chains based on monocultures of either the diatom Skeletonema costaturn or the dinoflagcllate Heterocapsa trjquetra (algae dominating in the mixed and stratified reglons of the North Sea). In the laboratory, these algae were fed to cultures of adult Acartjd tonsa, the copepod eggs were collected, hatched dnd the K1 naupl i~ from these different feeding regimes fed to post yolk-sac larval North Sea cod. Post yolk-sac larval cod required 8 d on either a Heterocapsaor Skeletonema-based food chain before tracer lipid signals (the ratio of the lipids lG:lw7 to 16 0) in the larvae began to change from their original values to those similar to the algae at the base of their respective food chains The cod larvae displayed a lipid tracer content similar to that of their algal food source after 13 d on their respective feedlng regimes During a cruise in May 1992 to examine the distribution of larval and juvenile North Sea cod, a subsample of 100 juvenile cod from the stratified, mixed and frontal regimes of the northeastern North Sea were examined for their content of lipid biomarkers and condition (as determined by the ratlo of total hpid content to total length). Juvenile cod displaying a lipld tracer content indicating utilization of dlatom-based food webs (found in proximity to regions of frontal mlxing) were observed to be in significantly better condition (p 5 0.05) than those containing a lipid signal indicative of utilization of flaycllate-based food webs (found in stratified regions of the North Sea)