Warming worries for cod

Abstract

New insights into haemoglobin polymorphisms may add to the threat of climate change on this fish species. Nigel Williams reports. New insights into haemoglobin polymorphisms may add to the threat of climate change on this fish species. Nigel Williams reports. While bluefin tuna in the eastern Atlantic and Mediterranean now appear to be under serious threat from overfishing (see page R1110), the populations of cod in the Atlantic have been under threat of collapse for many years, again from overfishing. While some measures have been taken in an effort to restore stocks, the future of the fishery generally remains gloomy. And cod have another problem; some populations may be susceptible to ocean warming that may occur as a result of climate change. It has long been known that there are three different haemoglobin phenotypes in populations of North Atlantic cod. The fish are widely dispersed from the North and Baltic seas and across the north Atlantic to the eastern American coast. The fish are believed to comprise more or less reproductively isolated populations of seasonal migrants and sedentary residents which seem to be adapted to the varying conditions across the species' range. The haemoglobin phenotypes are represented as HbI 1/1, HbI 2/2 and HbI 1/2. The homozygote HbI 2/2 predominates in the colder northern waters, whereas the homozygote HbI 1/1 type is present at lower latitudes along the coast of Norway and in the North Sea. And a similar pattern of distribution of these types, although less clear, can be seen along the north-eastern American coast. Previous studies have shown that fish with the HbI 2/2 type prefer colder waters of around 8°C, compared to a preference of warmer waters (15°C) by fish with the HbI 1/1 type. The heterozygotes appear to show intermediate properties between these two homozygotes. In a new study by Oivind Andersen at the Centre of Integrative Genetics in Aas, Norway, and colleagues in Norway, Italy and Sweden, published in the Proceedings of the National Academy of Sciences (online), the researchers have revealed the genetic basis of the haemoglobin polymorphism in Atlantic cod and present molecular mechanisms underlying the adaptation to the different environments inhabited by the two homozygote forms and also show that the allelic distribution of the haemoglobin polymorphisms in the cod populations is related to environmental temperature and oxygen saturation. The new work throws light on cod distribution; the brackish water of the Baltic Sea restricts cod to deeper waters with higher salinity, but this strategy is compromised by the low oxygen levels near the sea bed due to water stagnation. Cod here are also challenged by long periods of low temperatures during winter and spring that might, however, be an advantage in lowering the metabolic rate and increasing the oxygen solubility. From the molecular analysis of the haemoglobin types and their potential oxygen-binding abilities, Andersen and his team therefore propose that the Baltic cod stock is adapted to the environment there by possession of the HbI 2/2 form of haemoglobin. Cod populations have been observed to have responded remarkably to recent increases in sea temperatures as a consequence of global warming. Based on the strong relationship between the haemoglobin polymorphisms and temperature preference, the researchers propose that the observed northward migration of more southerly Atlantic and North Sea cod is due to increased temperatures exceeding the levels preferred by the HbI 1/1 fish. But prospects for Baltic cod under a warming situation may be more challenging. A combination of increased water temperature and low oxygen levels would be even more unfavourable for the Baltic cod, which predominantly possess the HbI 2 allele. The present frequency of the HbI 2 allele in the Baltic cod is about 97 per cent. “The distinct distributions of the functionally different haemoglobin variants indicate that the present biogeography of this ecologically and economically important species might be seriously affected by global warming,” the authors warn. Such problems could further complicate any efforts to manage cod fisheries, if the populations are a shifting entity under the influence of climate change and any ocean warming.