Can Microzooplankton Support Atlantic Herring Larval Growth and Survival in Wintertime?

Traditionally copepod nauplii have been considered the natural prey of fish larvae in the wild, meanwhile the role of microzooplankton (20 – 200 µm) has been disregarded. However these small protists are crucial for first feeding, and, depending upon the conditions, may support larval growth for short periods of time. Little is known about the prey environment experienced by larvae of autumn and winter spawning Atlantic herring (Clupea harengus) which overwinter in the North Sea. Given the low abundance of mesozooplankton during this time period the question arises whether a diet mostly consisting of dinoflagellates and ciliates is sufficient for larvae to grow at rates observed in situ? Samples were taken in September 2013, January and February 2014 during the ICES-coordinated International Herring Larval Surveys (IHLS) and the International Bottom-Trawl Surveys (IBTS). This field sampling represents the first large-scale (North Sea-wide) snapshot of microzooplankton abundance and distribution prior to and during the winter. Relationships between patterns in the distribution and abundance of microplankton and larval herring were investigated, and the link between in situ nutritional condition of herring larvae and microplankton abundance/biomass will be discussed. Further plans involving physiological larval measurements and trophic level analysis are also presented.