Spawning strategy and a mechanism for adaptive larval production in Arcto-Norwegian cod
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The Arcto-Norwegian cod is spawning in the temperature stable Atlantic water, and the eggs ascend into the more temperature variable coastal water. Changes in time of peak spawning, up to 10 days within the last 60 years, are caused by changes in the cod´s age composition, peak spawning of first-time spawners being somewhat delayed. In contrast, the peak spawning in the coastal water of Calanus finmarchicus, whose nauplii are the almost exclusively food organism for the first feeding cod larvae, vary with more than 40 days due to temperature variations. It is a clear tendency for the majority of nauplii abundance peaks to be situated ahead of the peak first feeding cod larvae, especially during warm years. The temperature covariate in the spawning areas in the Norwegian coastal waters and the feeding areas of cod in the Barents Sea. High temperatures during the egg and larval stages favour the feeding condition of the larvae hatched first, since the concentration of nauplii is higher in the first part of the larval period. These larvae from the larger, high fecund cod females, are the largest and probably the most viable larvae produced during the season. During periods of decreasing temperatures in the feeding areas and the spawning sites, the production of viable larvae is more variable. High temperatures also improve and extend the feeding areas tn the Barents Sea, giving support for larger year-classes. In both temperature regimes the age compostition of the spawning population of Arcto-Norwegian cod will effect the degree of match between the production cycle of nauplii and first feeding cod larvae. The spawning strategy of Arcto-Norwegian cod in relation to Calanus finmarchicus can be named a modified match/mismatch hypothesis, ensuring a adaptive larval production to varying temperature regimes of the feeding areas of the cod.