Fish are the most diverse group of vertebrates, ranging from tiny gobies and zebrafish to gigantic tunas and whale sharks. They provide vital sustenance to billions of people worldwide via fisheries and aquaculture, and are critical parts of aquatic ecosystems.
However, many fish species worldwide are shrinking as their habitats get warmer. For example, important commercial fish species in the North Sea declined in size by around 16% in the 40 years leading up to 2008, while the water temperature increased by 1–2℃. This ‘shrinking’ trend is expected to significantly exacerbate the impacts of global warming on marine ecosystems.
The link between warmer water and smaller size is well known but poorly understood. Our experiments, which involved keeping fish in warmer water, offer some crucial clues and may help us prepare for a future with smaller fish.
Fisheries are a potential confounding factor when studying the effect of warmer waters on fish, because fisheries often target large fish. Removing these larger fish from the population benefits the survival of fish that mature quickly and reproduce at a younger age, when they are smaller.
This trait of maturing early can be passed through fish generations. Indeed, it can lead to a phenomenon known as ‘fisheries-induced evolution’, where the exploited species tends to decrease in size over time.
But how do we tell the difference between the impacts of climate warming and those of fisheries?
One way is to examine the body size trends in fish species that are not targeted by fisheries. Several fish species in French rivers, for example, are not exploited by fisheries but have decreased in size over several decades while their environment has grown warmer.
Another way is to examine fish under controlled conditions, by manipulating water temperature and studying the impact on fish size. Such experiments have shown that fish indeed become smaller in warmer conditions, and the trend is so common it has been given a name: the ‘temperature–size rule’.
Warming waters are causing fish to shrink, with profound implications for fisheries and aquaculture. Smaller fish produce fewer offspring and may struggle to sustain populations under increased fishing pressure, threatening ecological balance and food security worldwide.
Warmer water means smaller fish, but why?
The most popular theories suggest the cause is due to a mismatch between how much oxygen a fish needs (to sustain its body’s metabolism) and how much it can get (via its gills).
The argument is that fish gills do not grow at the same pace as the rest of their bodies. Once a fish reaches a certain body size, its gills can only supply enough oxygen to keep its body running – there is no oxygen left over for growth.
What does this have to do with warming? The next part of the theory suggests that fish use more oxygen in warmer water – but their gills don’t grow any bigger. So, fish reach the limit of their growth at a smaller size, leading to the temperature–size rule.
This ‘oxygen mismatch’ theory has sparked heated debate among global scientists, largely because insufficient data exist to confirm or refute it.
To understand how warmer waters affect fish, we conducted long-term experiments exposing them to elevated temperatures. In some trials, we also added extra oxygen to test whether it boosted their growth.
We measured their metabolism regularly and analysed their gill surface area to assess how well fish transport oxygen from water into their bodies.
Our findings challenge the ‘oxygen mismatch’ theory. While fish metabolism does increase in warmer water, their gills grow sufficiently to meet the higher oxygen demand as they grow.
So, why are fish still shrinking as the climate warms?
Fish in warmer waters tend to grow faster and reach reproductive maturity at an earlier age and smaller size. One possibility is that, once fish start reproducing, their energy is diverted from growth to reproduction.
Evidence supporting this comes from a population of fish living in a Swedish lagoon, which offers a glimpse of a warmer future. This lagoon receives warm, non-contaminated water from a nearby nuclear power plant.
Fish in the warm lagoon grow faster and reach reproductive maturity earlier, buy they also die younger and at a smaller body size than their counterparts living in adjacent, cooler waterways. ‘Live fast, die young’, as the saying goes.
While this pattern seems broadly applicable, some conflicting findings suggest their is still much to explore.
As our understanding of the relationship between temperature and fish size continues to evolve, we're working to uncover potential solutions to mitigate these effects.
In research published in June 2023, we explored differences in growth rates between individual fish of the same species.
One of our key questions was whether certain physiological traits might enable some individual fish to better withstand the temperature–size rule and be less affected by climate warming.
Our research found significant variability across individual fish, suggesting that some fish may be more resilient to temperature changes. However, we are still working to understand how this variability can be leveraged to help future-proof fish populations.
As we look ahead, the implications of shrinking fish populations become even more critical. Fisheries and aquaculture industries face new challenges that require proactive measures.
Fish can’t shrink forever, and if they do, the consequences for ecosystems and industries will be severe. There is a minimum size that each species must reach to maintain viable populations.
If species hit their specific thermal limits in particular locations, they may no longer be able to reproduce and could cease to exist in those areas. If their entire habitat range becomes too warm, some species could face extinction.
These considerations of shrinking fish and shifting thermal habitats will be crucial for the sustainability of fisheries and aquaculture industries in a future marked by more extreme climate events. Our ongoing efforts to quantify and forecast these impacts will help industries and resource managers adapt to and prepare for climate-related disruptions.
This article was written by Deakin researcher Associate Professor Timothy Clark and has been republished from The Conversation under a Creative Commons license. Read the original article here.
