Researchers reveal how sea temperatures affect fish physiology
Researchers have demonstrated for the first time how warmer sea temperatures affect fish physiologically and so lead to reductions in population size. The scientists, from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, looked at the relationship between seasonal sea temperatures and the population of the eelpout (Zoarces viviparus) in the southern North Sea. Z. viviparus is a bio-indicator fish used for environmental monitoring purposes in both the North and Baltic Seas. The researchers studied the physiology of the fish to determine how increased temperatures affect its internal systems. Their findings are published in the latest edition of the journal Science. Over the last 40 years, temperatures in the southern North Sea have risen by over 1°C, and are predicted to increase further over the next century. The researchers noticed that in warmer summers, the eelpout population size fell. Laboratory experiments demonstrated that above certain temperatures, the ability of the fish to transport oxygen around its body is seriously compromised. This affects muscular activity, behaviour, growth and reproduction and could also make the fish more sensitive to predation, starvation and disease. In addition to this, warmer water contains less dissolved oxygen in the first place, compounding the problem. In the paper, the scientists warn that as different species have different 'thermal windows', sea temperature changes could lead to disruptions in food webs, something which has already happened to the Atlantic cod. 'The shift from larger (Calanus finmarchicus) to smaller (C. helgolandicus) copepod fauna in the southern North Sea caused reduced food availability for the Atlantic cod (G. morhua). This regime shift was largely determined by different thermal windows of the two copepod species,' the authors write. 'Warming-induced reductions of cod abundance are thus caused both directly [via thermal sensitivity of cod] and indirectly [via the food web] but based on the same physiological principles.'
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