Exploring the Protozooplankton-Ichthyoplankton Link: Direct and Indirect Benefits of Microalgae and Protists on Early Life Stages of Marine Fish

Recent evidence suggests that microalgae and protists may be critical for successful first feeding and rapid growth of marine fish larvae in nature. We examined effects of microalgae (Rhodomonas baltica, RB) and heterotrophic protists (Oxyrrhis marina, OX) on the growth, condition and foraging success of Atlantic herring (Clupea harengus) larvae from hatch to exogenous feeding. Larvae were reared in the absence (Exp1) and presence (Exp2) of copepods prey and in the presence and absence of RB and/or RB+OX. When provided access to copepods, naïve larvae (Exp1) reared with microalgae exhibited precocious (2 days earlier) feeding and ≥60% more feeding activity compared to larvae in only SW, but survival and growth trajectories of all larvae were similar and trypsin activity markedly declined directly after yolk-sac depletion. When larger zooplankton were present (Exp2), 16 day-old exogenously feeding larvae in RB and RB+OX treatments had 1.6-times greater dry mass, 1.25-times higher nutritional condition (RNA/DNA) and were 60% more active than larvae reared in only SW. The direct (physiological) and indirect (trophodynamic) benefits of microplankton demonstrated here indicate that field programs should place greater emphasis on the protozooplankton-ichthyoplankton link to better understand match-mismatch dynamics and bottom-up drivers of year class success in marine fish.