Smaller adults and bigger juveniles: how global warming is affecting our fish populations.

There is mounting evidence that climate change is affecting commercial fish species. In their latest research, Ikpewe and colleagues investigate the effect of temperature on fish sizes in two neighbouring regions, displaying contrasting trends in warming.

The warming of our oceans due to climate change is affecting marine life in numerous ways.  Fish populations, in particular, are experiencing changes in productivity, distribution, growth and the timing of key events such as spawning. We need to understand these changes and account for them if we are to manage marine living resources sustainably in future. 

Laboratory experiments have demonstrated that cold-blooded organisms develop faster at warmer temperature and reach smaller body sizes: this is known as the Temperature Size Rule (TSR). TSR has since been observed in other animals, plants, protozoans and bacteria. In fish, a few empirical studies have shown that increases in temperature resulted in faster growth and smaller adult body size which is consistent with the TSR. Few, if any studies, have considered the impact on juvenile fish.

In our study, we investigated how fish of different ages have been changing in size over the last 30 years.  We examined fish survey data over this time period, with measurements of length at age of four commercially-important fish species: cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus) and saithe (Pollachius virens).

The surveys covered two large fishery management areas of northern Europe, the West of Scotland and North Sea, which have both experienced consistent warming since the 1970s, although at different rates. These areas represent major economic and social issues for several European countries.

Using a statistical technique to detect common temporal trends, we found that juvenile fish have been getting bigger, and adults getting smaller, in both regions.  We then looked at the temperature at the bottom of the sea, where these fish live.

Sea bottom temperatures in the North Sea warmed at an average rate of 0.35°C per decade; while those on the west of Scotland warmed at a lower rate of 0.15°C per decade. The changes in body size at the youngest (age 1) and oldest (age 7) ages were correlated with rising sea temperatures, providing empirical evidence that warming is affecting the growth and body-size of commercial fish species.  

In between these ages, fish experienced intermediary but complementary changes in size.  Identical patterns were observed in both the fast warming North Sea, which is consistent with existing literature, and the slower warming West of Scotland, where such findings have not been reported before.

Previously, changes in adult body size have been associated with fishing pressure.  We also examined this, but found no significant correlation between fishing pressure and mean length at age 7, in both the West of Scotland and the North Sea, even when including a range of time lags to account for delayed effects.

The effects of these changes on the productivity of fish populations and yield from fisheries now requires further investigation. Although it is clear that a reduction in adult body size (and hence body weight) will adversely affect fisheries yields, it is not clear how much, if at all, this might be compensated for by that fact that juvenile fish, which are more numerous, are getting bigger.  Evaluating this trade-off is the subject of our next investigation. 

What is clear, is that temperature changes should be considered when making long term forecasts of fisheries yields which inform how fisheries are managed.  This may go part of the way to mitigate for the impact of global warming and maximize the sustainable yields that our fisheries can provide. 

Read the full article Bigger juveniles and smaller adults: Changes in fish size correlate with warming seas in Journal of Applied Ecology

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