An increasing awareness of the effects of climate change on fish stocks – some of them dramatic, and occurring in our lifetime – was expressed by scientists at a conference in Belfast in July.
Even though overfishing may be the dominating global fisheries problem, increasing evidence point to warming waters, an effect of climate change, as a cause for fluctuations in fish populations, with consequences for marine ecosystems.
Many aquatic ecosystems have proved to warm up even faster than the earth itself, indications being that temperatures in parts of the North Sea, for instance, have risen by more than one degree over the past 30 years.
“In the southern part of the North Sea we see a process we could call to some extent a ‘Mediterranisation’”, a German scientist told the BBC.
Such changes in temperature have made warm-water, southern stocks move north, and cold-adapted northern fish populations either migrate even further north, or search out habitats on greater depths.
The timing of fish migrations appear to be affected as well, for some species as much as two months, which may have substantial consequences for the survival of young fish.
The overall effects vary – some populations profit from the warming, while other meet increasing problems – but ecosystems are always affected, and many scientists stress the need for taking these effects into consideration as well in the fisheries management process.
American scientists earlier this year presented a model for forecasting the impact of climate change on fish stocks, but underlined at the time that more research was urgently needed to enable more short-term forecasts than their own 50-100 year perspective.
As for now, it seems that fish in colder habitats, now slowly warming, are most affected by the changing conditions.
“We already have evidence that cold water fish like trout and charr are beginning to show poor performance, and in many streams in the European and North American area we have major declines in the abundance and distribution of these cold water species”, John Magnuson of the University of Wisconsin told BBC.
Dr. Chris Harrod of Queens University in Belfast, organising the conference, admitted that the decline in many fish stocks was primarily caused by overfishing, but added that “fluctuations in some exploited warm-water species, such as the sardine, have been closely related to changing water temperature.”
“Northern Ireland is of particular interest”, he said. “Many of the fish there, often referred to as glacial relics, are cold water species which are adapted to the colder conditions of more northern latitudes. As our waters heat up these cold-adapted fish such as Atlantic salmon, Arctic charr and pollan are under particular threat. Populations that depend on these fish and value them for their biodiversity will also have to adapt to change”.
An added, more peculiar, effect of climate change is that some species react to temperature change with massive gender swings: as the water warms up, the embryos are almost all male, with devastating consequences for the stock’s reproduction.