Differences in growth and survival between cod and herring early stages co-reared at variable prey density

Abstract

A number of studies have tried to establish the feeding levels at which survival and growth is optimized or severely reduced for early life stages (ELS) of marine fish. Experimental studies have generally been carried out at a constant prey concentration. However, in reality prey is usually patchy distributed in space and time, and different life history strategies have most likely adapted to survive different regimes depending on the timing of spawning and oceanographic conditions. Along the Norwegian coast, co-occurring herring (Clupea harengus) and cod (Gadus morhua) have different spawning strategies presenting the larvae with different challenges during ELS. We hypothesise that the survival and growth strategies of herring is adapted to coping with longer periods of prey deprivation (i.e. more variable prey availability), in contrast to cod which is adapted to match growth and survival to high prey concentrations during spring bloom. To test this hypothesis we present experimental larval growth and survival data for the two species while reared combined or in single species settings. Two food regimes were offered, either ad libitum (daily adjusted) level or in a pulsed manner where the concentrations were allowed to drop to a third of the initial nominal concentration. Individuals were sampled at regular weekly intervals and all added and sampled fish were counted to allow accurate mortality estimates. The pulsed feeding significantly reduced the initial growth of cod larvae co-reared with herring, while no such effect was seen for the herring. During the latter half of the larval stage the cod larvae in the pulsed feeding treatment grew at the same rate as those offered the constant prey concentration. Herring ELS reared without cod present, tended to be larger at age than when reared together with cod, and also displayed a significantly higher total survival at the end of the six week experiment (59% vs. 42%). For cod ELS, on the other hand, the survival was lower when reared without herring present (24 vs. 31%), suggesting that the two species cope differently to each others presence. Relatively large sample sizes (over 100 per sampling and 4000 in total) revealed differences in size distributions between treatments. The results are discussed in relation to the differences in life history strategies between the faster growing cod larvae and the more aptly surviving herring larvae.