Nuclear Fish on Heat

But first, warmer conditions, including those produced by climate change, are known to affect the rates at which some fish species grow, and how large they ultimately become. This could affect food chains, because if animals mature at different times and reach different sizes, former food sources may no longer be suitable. As Chris Smith hears, these temperature effects are hard to test in the real world though, unless you’re Max Lindmark, from the Swedish University of Agricultural Sciences…

Max -  A common problem in many studies where we want to look at fish growth rates and body size in the world is that we don't have a very large temperature contrast yet. I mean, there has been warming for decades, but to detect any signals on fish, you would prefer a larger sort of gradient of temperature that you're evaluating against. And we are fortunate enough to have a study system on the coast of Sweden where we have a nuclear power plant that takes in water from the coast. And when they built this in the 1980s, they wanted to investigate the effects of warm water pollution from nuclear power plants. So they took the water that was heated by the nuclear power plants and they didn't release it just on the coast, but they built a manmade lake and put all the warming water in there. So that lake has been, since the 1980s, about five to 10 degrees warmer on average than the surrounding area. And what they did that was really good was also that they did test fishing both outside the reference area and within the lake to see, like, how much fish do we catch? How big are they? How do they grow? So we have this unique time series of 24 years of data.

Chris -  But all the other factors remain relatively comparable, as in it's the same geography, it's the same fish, it's the otherwise the same section of coastline. It's chiefly the temperature that differs between the two sites?

Max -  Yeah. So it was the same population and then they just enclosed a little bit of it.

Chris -  And what's that done to the fish?

Max -  They grow faster. The warming has probably led to a longer growing season. So fish, they don't grow when it's too cold. And in temperate regions, like in our part of the world, they grow in the warm summertime. So with the longer growing season, they can grow for longer to reach a larger size. They also grow faster in this time, so they reach a larger size. At age, the mortality is also higher. So fish die at a younger age. We don't have as many really old fish in this lake. And the third response that is very clear is that they also reach a bigger size in this heated biotest lake that we call it.

Chris -  It's almost like the fish equivalent of being a rock star - I suppose you could say a "rock pool" star - they live fast and die young, but also big!

Max -  Yeah, exactly! It is really a faster sort of pace of life in this warm lake.

Chris -  Are we comfortable that there's no impact other than the temperature? I mean, many people are sensitive to nuclear power and so on and the environmental impacts of that. It's one of the most regulated and one of the safest industries, but there are accidents. There are also discharges and leaks. Nonetheless, are we comfortable that that is not in any way playing a role here?

Max -  I would say this is a fairly small lake, one kilometre across and during its lifetime it's seen a lot of stress. Let's say that. For instance, once a seal came into this lake and it would just eat so much fish. You've had time periods of quite large survey catches. You've had stocking of eel, you've had a total wipe out of benthic production. So we had all these very large disturbances in the system, but you don't pick up those signals in the growth data.

Chris -  The temperature difference that you're dealing with is very large and way beyond what we expect on average across the planet. But do you think you're therefore seeing a bit of an artificial situation here or are we expecting some patches of the Earth's oceans to, to change in temperature by this extent because of climate change?

Max -  This is a very extreme warming scenario. The lower end of the temperature difference that we observe is probably not too different from what we're seeing in the Arctic sometimes. And not in the too distant future. The warming in the world is very heterogeneous. Some areas are warming much faster and others are not warming that much.

Chris -  What might be the implications of what you found then? If we extrapolate from this nuclear pond to the world's ocean systems, what could the effect of what you've seen if it were more manifest more widely across the ecosystem?

Max -  First of all, with faster growth rates, species interactions will happen faster. Predation flow of biomass through the ecosystem might be faster in terms of fish. If they grow faster, they can sustain a higher fishing mortality. But on the other hand, you might then also not catch as much because the sort of background mortality also increases as we show here.

Chris -  Could there be knock on ramifications where if some species are more sensitive to these effects than others, you could get animals that grow at different rates and animals that feed other animals are therefore becoming too big to be eaten by things that would previously be preying on smaller feed. So could there be knock-on effects beyond just the physical growth rates?

Max -  I think so because large fish or large organisms eat small organisms when they're born, they eat zooplankton and they need to be within a specific size window because they can fit perfectly in, in a small larvae mouth, for instance. So it's very important that you always have prey of a specific right size as you go and grow through life. And if you start seeing these mismatches where they don't have the right prey to feed on, that can have sort of knock on effects.