Role of heterotrophic protists in first feeding by cod (Gadus morhua) larvae

The present study evaluated whether the presence of autotrophic phytoplankton and/or heterotrophic dinoflagellates (Oxyrrhis marina) influenced first-feeding success and early larval growth and development of Baltic cod Gadus morhua L. Newly hatched cod larvae were maintained in salt water ('clear water', CW), greened water using Nannochloropsis sp. (NA) or a combination of Nannochloropsis sp. and O. marina (NO) from 2 to 11 d post-hatch. On each day, larvae from each group were placed into feeding tanks and provided potential prey (nauplii of the calanoid copepod Acartia tonsa). Feeding performance was measured both as the percentage incidence of feeding and gut fullness index. The onset of first feeding, point of no return and the magnitude of feeding were derived from the feeding incidence. The study clearly demonstrated precocious, exogenous feeding by cod larvae in the presence of Nannochloropsis sp. and O. marina by 1.2 and 2.2 d (10 degrees C), respectively. Gut fullness was also significantly improved in the presence of both phytoplankton and protists over the entire window of opportunity (defined as the period between onset of first feeding and point of no return). However, the nutritional capacity of these unicellular organisms was not sufficient to alter the somatic growth characteristics in NA and NO groups relative to starving yolk-sac larvae maintained in only salt water (CW group). These findings expand the current understanding of the degree of interaction between fish larvae and unicellular plankton communities and indicate a lack of nutritional benefits from feeding on components of the pelagic microbial loop but a clear benefit in terms of 'priming' first-feeding capabilities.