Annual production of fish larvae and their prey in relation to size-fractionated primary production (Scotian Shelf, NW Atlantic)

Abstract

The annual cycles of abundance of fish larvae and their zooplankton prey were analysed in relation to the biomass and production of size-fractionated phytoplankton on the Scotian Shelf (North-west Atlantic). Outside the spring bloom of large (>5 μm) phytoplankton (February to April), the small-sized fraction of phytoplankton (<5 μm) contributed the bulk of total primary production. The production of copepod nauplii and copepodites was sustained throughout the year and fish larvae specializing on copepod prey occurred year-round. Species feeding on a mixed diet of copepods, cladocerans and/or appendicularians were restricted to periods of maximum temperature in summer and early autumn. Two short trophic pathways (two links) from primary producers to fish larvae were identified: the herbivorous food chain (large phytoplankton→calanoid copepods→fish larvae) and the large-microphage shunt of the microbial food web (small phytoplankton→appendicularians/pteropods→fish larvae). The year-round production of fish larvae and the fact that several of their major prey (appendicularians, thecosome pteropods, cladocerans, and cyclopoid copepods) exploit the microbial food web, challenge the tenet that the feeding of marine fish larvae depends primarily on the reproduction of herbivorous calanoid copepods grazing the spring and autumn blooms of large phytoplankton.