Winners and losers: warming oceans already affecting fish stocks

Modelling finds some fish populations are benefitting, but more are under threat. Andrew Masterson reports.

Intensive harvesting in the past correlates with temperature-sensitive fish populations.

Daniel Grill/Tetra Images/Getty Images

Increasing ocean temperatures are already having a marked, but mixed, effect on global fish stocks, researchers have found.

In a study that used current fish population numbers to infer the effects of marine warming since early in the twentieth century, scientists led by Christopher Free from Rutgers University in the US found that while some species have actually benefitted from rising temperatures, more have been negatively impacted.

Several commercially important populations are already under temperature-induced stress, they found, with the situation set to worsen as waters continue to warm.

The researchers modelled population dynamics to measure the effects of changes in ocean temperature on 235 fish and invertebrate populations from around the globe.

The populations comprised 124 species, spread across 38 ecoregions, and accounted for about 33% of the worldwide marine protein harvest. Using a technique called hindcasting, the data was used to estimate temperature-driven changes in yield between 1930 and 2010.

The results were very interesting. Over all, the maximum sustainable yield across the populations decreased across the 80 years by 4.1%. The fine-grain, however, produced significantly more varied results.

Nine of the populations “responded significantly positively” to ocean warming, while 19 experienced a marked negative effect. The five worst affected ecoregions experienced population drops of between 15 and 35%.

When all the figures were averaged out, “the influence of temperature was not significantly different from zero,” Free and colleagues write, “indicating that populations benefitting from ocean warming were roughly offset in number and magnitude by those that were negatively impacted.”

The devil, however, is very much in the detail. The researchers found that many species of very high commercial or ecological value, such as cod and sand-eels, were already experiencing declines due to ocean warming. These two genera, in particular, being largely native to the North Atlantic, “will be especially susceptible to the continued rapid warming predicted for this region”.

But even here the picture is more complex. While cod in warmer waters are thought to be very close to the upper-limit of their heat tolerance, and therefore vulnerable to population collapse, those in colder waters are currently getting a boost from slight increases in temperature.

However, Free and colleagues say, “such benefits may be expected to decline with further warming”.

They also noted that species with “faster life histories” – that is with more rapid maturing and shorter lives – were more significantly affected by ocean warming, sometimes positively and sometimes negatively, than species with longer lifespans and breeding cycles. Populations that had already been subjected to intense harvesting were also more susceptible to temperature change.

The picture painted by the researchers is a lot more complex than the one that arises in earlier modelling exercises that have used only correlations between ocean temperatures and fish harvests in particular locations.

Marine protein sources are becoming increasing important for human populations. In some coastal areas and several developing countries, food from the sea constitutes as much as 50% of protein intake. Maintaining the productivity of ocean resources, the researchers say, is a key challenge already facing the world.

“Outcomes of fisheries management — including long-term food provisioning — will be improved by accounting for changing productivity in a warmer ocean,” they conclude.

The research is published in the journal Science.

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