| COPING WITH THE COLD: OVERWINTERING STRATEGIES IN DOWNS HERRING LARVAE |
| MOYANO, M., Illing, B., Peck, M.A University of Hamburg, Institute for Hydrobiology and Fisheries Science, Olbersweg 24, 22767 Hamburg, Germany, marta.moyano@uni-hamburg.de; bjoern.illing@uni-hamburg.de; myron.peck@uni-hamburg.de |
| Long-term field survey data on the abundance of North Sea herring (Clupea harengus) larvae suggest that mortality acting on early larval stages controls year class success. We have used Downs herring (winter spawner) to investigate the effects of thermal stress and starvation on young larvae under laboratory conditions. Our aim was to mimic a moderate (7C) and a cold (5C) winter temperature and two prey regimes (high and low). The 7C trial was conducted for larvae that were 18 to 30 days-post-hatch (dph) starting with 12.11.0 mm (meanSD) standard length (SL) The trial at 5C was conducted on 39-48 dph larvae that were 13.41.3 mm SL at the start. A variety of biochemical (RNA:DNA, citrate synthase activity), metabolic (oxygen consumption in anesthetized larvae), somatic (SL, dry mass, growth), and behavioural (e.g., swimming and foraging activity) parameters were measured. At 7C, well-fed larvae grew two times more rapidly than larvae in the low prey treatment. However, differences in standard metabolic rate for same sized-larvae were not clear between the two treatments. Activity differences between feeding regimes started after day 5 (e.g., lower number of feeding strikes, longer pause duration for the low-feeding treatment). Acclimatization of larvae to 5C was difficult and mortality was observed prior to trial. At the end of 5C trial, standard metabolic rates in the low-food treatment were lower than those in the high-food (e.g. 50% reduction for larvae between 14.5-16mm). In addition, those survivors in the 5C low-food treatment were the same size as the largest larvae in treatments at 7C. Results suggest that, large size confers clear survival advantages when larvae experiencing cold temperatures and low feeding conditions. Consequences of different winter scenarios on larval survival and the likelihood of bottom-up (environmental) control of North Sea herring recruitment are discussed. |
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