The role of prey concentration and size range in the growth and survival of larval cod

Abstract

The effects of food quantity vs. food quality may be influenced by complex interactions that are difficult to isolate in trophic studies of fish larvae in the natural environment. Food quantity, or abundance, is mediated by the proportion of suitable prey available, with ‘suitability’ defined variously as size, species, or nutritional content. Using an experimental approach, food consumption, growth, and survival of larval cod, in response to zooplankton abundance and diversity of zooplankton sizes, were tested in a replicated land-based mesocosm (2500 l) system. Northeast Arctic cod (Gadus morhua L.) larvae were reared for two months with a manipulated zooplankton prey field comprised of narrow (N) or wide (W) prey spectrum (range of prey sizes), combined with high (H) or low (L) prey concentration. In the W treatments, cod larvae were fed both small and large sized zooplankton, while in the N treatments they were fed only small zooplankton. Initial prey concentration was 2000 prey l⁻¹ in the H treatments and 200 prey l⁻¹ in the L treatments, decreasing to 500 and 50 prey l⁻¹ during the course of the experiment. Larval cod grew faster at higher prey concentrations. Prey concentration as well as spectrum affected larval feeding patterns with a wider spectrum prey size leading to the consumption of larger prey, but without a significant effect on larval growth. Survival was significantly higher in high prey concentration treatments, and also tended to be better in treatments with wider prey size diversity.

Keywords:

• Fish larvae
• larval–zooplankton interactions
• mesocosm
• prey selection
• prey size spectrum
• zooplankton

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

We thank V. Lokøy, F. Midtøy, J. Skadal and T. Solbakken for assistance with sampling and laboratory work, Geir Blom and A. Fosshagen for assistance with identification of zooplankton, and Hans Høie for assistance with statistics. Furthermore, thanks are given to Ivar Holmefjord for providing eggs and help with marking and transport. This work was partially funded by the University of Bergen.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark