Fish may end up in hot water as climate warms the ocean

Cod: in search of cold waters. August Linnman, CC BY-NC-SA

The rate at which the world has warmed over the past 50 years and is likely to continue to do so in the future poses problems for life on land and in the ocean. Most species have a defined range of temperatures within which they can live – and when temperatures exceed the upper limit for a particular species in a particular location, that species will struggle to survive there. Likewise, places can become newly habitable when temperatures become warmer than a species’ lower limit.

The complex patterns and distribution of these ecological niches with specific temperatures and the varying rates at which they are changing around the globe make it hard to predict where these changes will be felt, and what consequences they will have. Our study, published in Nature, has taken a new look at these changes and their likely effects on biodiversity.

Our research group examined how fast and in which direction local temperatures have moved over the past 50 years. Based on the projections from current climate models we also looked projected forward oceanic changes to the year 2100 under two scenarios, one the “business-as-usual” scenario of largely unchanged greenhouse gas emissions and another modelling a global increase in temperature of 1.75°C.

Assuming that species move toward areas that suit their preferences for heat or cold, our findings provide some insight into how species have migrated over time, and where they may end up after years, decades or even a century of changing, warming world. This is a big assumption. Some species may adapt to the change in temperature and not move, while others may not be able to disperse and colonise new habitats fast enough, and may die off.

But in general, shifts in temperature have proved to be a good predictor for past changes in species distributions. Around the coasts of North America, for example, Pacific cod in the Gulf of Alaska, big skate on the US west coast and American lobster in the north-east have all shifted in the direction and over the distance predicted from thermal shifts.

Over the past few decades, numerous fish and invertebrate species have moved toward cooler regions, in some cases with important consequences for local biodiversity. For example, in the North Sea the areas where cod are found have shifted up to 100km northwards over the the 40 years between 1961-2001. In the same period, plankton in the North East Atlantic have similarly shifted 1,000km poleward.

Temperature gradients, 1960-2009: where will species move to seek relief from the heat? Burrows et al./Nature

However, a key finding of this study is that geographic features such as coastlines create barriers for marine life that may be tracking shifting temperatures. By mapping where these barriers influence potential migrations around the globe it may be possible to identify places where the movement of species biodiversity might be blocked.

These naturally existing blockages might cut some areas off from species migrating from warming seas. Blocked-off areas included the North Sea and the southern Mediterranean in the period since 1960, places without direct connections to warmer environments. In other places it might prevent migrating species from escaping, with potentially catastrophic results – blocked in to a region in which it cannot thrive, extinction becomes likely unless it can adapt to the new conditions.

Knowing where vulnerable areas occur can help focus and inform conservation efforts. Areas where temperature shifts are slow may be better suited to protecting endemic species or long-lived structural species, for example those forming forests or land or coral reefs underwater. Moving species from those warming places without cool refuges might be an option to prevent climate-driven extinctions, although care must be given to the potential negative ecological consequences of introducing these species to new places. For those species unable to move, conservations may need to find ways of enhancing species’ natural adaptation wherever possible.

We are far from advocating that distribution shifts are the only issues for deciding what to do in the face of climate change. Biological factors such as a species’ capacity to adapt and disperse need to be considered. But in an unprecedented period of climate change, economic development and fast-growing demand on an already pressured planet, we need to act fast to make sure the world’s living resources survive that change.

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