Complex Effects of Climate Change on Fisheries Studied

by Erik Chapman

Every day, we hear something in the news about the effects of climate change — warming oceans, in-creasing numbers of severe storms, melting polar ice caps. But we also know that life on this planet has adjusted to changes in climate in the past. As we consider the future of our fisher-ies, we need to ask ourselves: How does climate change influ-ence populations that we depend on? What changes can we expect to see over the next several decades? And, is there anything we can do to prepare for these changes?

Changing temperature

Unlike mammals, fish are cold blooded and most are unable to maintain a constant body temperature. As a result, fish in the Gulf of Maine respond to increasing water temperature by moving to new locations to avoid overheating. “We have already observed that many species of fish are moving northward in our area and into deeper waters as temperatures have warmed,” explains Dr. Janet Nye, a scientist at the Northeast Fisheries Science Center. “For example, red hake and silver hake are now found mainly in the Gulf of Maine. In the 1960s and 1970s, these species were found in high abundances as far south as Delaware Bay, but very few are found there now.” Dr. Nye also suggests that the warmer winters have caused the range of Atlantic croaker to extended further north.

The American Lobster has also been affected by warming waters. The American lobster is a “subarctic species that does best in cold water” explains Dr. Robert Steneck, a professor at the University of Maine. “Warm water affects the distribution of lobsters and is a stressor that may weaken their immune system.” Although the recent warm water temperatures in the Gulf of Maine are not necessarily the result of climate change, Dr. Steneck points out that researchers have seen a significant decline in lobster populations in southern New England where water temperatures have become the warmest. Maine so far has fared well.

Because metabolism increases with temperature, warm water may also lead to faster growth for fish and lobsters. This might benefit some fisheries in some areas as temperatures warm.

Changing currents, phytoplankton patterns and ocean acidity

Climate change may also alter currents and the timing of phytoplankton blooms in ways that influence both fish and lobster populations. Dr. Win Watson, a University of New Hampshire marine biologist, says these types of changes are difficult to detect, but may already be affecting lobsters. “Here and in New Zealand there has been very poor settlement lately and we have seen very few new recruits.” He continues, “One possible explanation for this is that [lobster] larvae are drifting to the wrong areas due to changing ocean currents driven by climate change. Another hypothesis is that the plankton on which they feed might be in different places and different times than before. Thus, they starve.”

Increases in atmospheric carbon dioxide will also influence the ocean’s acidity. “Carbon dioxide in the atmosphere creates carbonic acid when it comes in contact with the ocean”, explains Dr. Steneck. “Ocean acidification is making it more difficult for all creatures with limestone shells to build and maintain those shells. This too may be stressing lobsters.”

Models and uncertainty

Unfortunately, we still have much to learn about the Earth’s climate system and how it will respond to chemical changes in the atmosphere caused by human activities. But we do know a lot and “oceans are predicted to get warmer and more acidic and the range of northern hemisphere animals are expected to shift northward”, says Dr. Steneck. Dr. Nye adds, “We cannot control the effects of climate change except by lowering greenhouse gases.” The majority of scientists agree that even if greenhouse gas emissions are immediately drastically reduced we would still see some of the effects of climate change. “At this point, the effects of climate change seem inevitable and we cannot control climate variability or climate change. However, we can adapt to these changes and try to mitigate the effects of climate change by reducing other pressures such as pollution and overfishing,” said Nye.

However, there is a great deal of uncertainty in the models that are used to forecast the future for our fisheries. “Oceans often surprise us,” explains Steneck. “There are complex ocean currents that may change how they ‘behave’ so some regions could actually get colder. Some changes will be hard to predict.” Dr. Nye elaborates, “The way in which global climate change interacts with natural climate variability and other pressures on our fisheries such as overfishing and pollution may lead to unexpected consequences.”

The future….

This uncertainty puts fisheries managers and the communities that depend on fishing for their livelihood in a difficult situation as they prepare for the future. The stakes are high, and we can either listen to the warnings of climate modelers and other scientists or we can decide that we do not know enough to believe their predictions. Either choice is risky, and the best thing we can do is listen, ask questions, and think critically and openly as we consider these issues.

One thing we can expect, it seems, is that things are not likely to stay the same for very long. Dr. Nye explains, “climate change will most certainly affect the species that fishermen target. As species move out of the area or become less abundant, fishermen will have to target different species.” Therefore, fishermen who are flexible and target several species will fare best as species adjust to a changing climate. Those who are not able to adapt, may suffer. “I’m very concerned about how we’ve put all of our eggs in one basket”, says Robert Steneck. “The lucrative monoculture (lobster fishery) we have today is an economic boon for the region but it carries such a high risk that I think we need to think of ways to bring diversity back. If we had multiple species of groundfish along with lobsters, shrimp and other seafoods, we’d have a more secure future.” So, at this stage, adaptability appears to be the best insurance against an uncertain future for our fisheries as our climate changes.

Erik Chapman is affiliated with the NH Sea Grant and Cooperative Extension