We know that fish tend to grow faster in warmer conditions and reach reproductive maturity at an earlier age and smaller size. So, why then are fish shrinking as the climate warms? Is reproduction the key? While the metabolism of fish does increase with warming of the water, we found the gills grow sufficiently to keep up with the increased oxygen demand as fish increase in size. Our results show the "oxygen mismatch" theory doesn't hold up. The link between warmer water and smaller size is well known, but poorly understood. We have regularly taken metabolic measurements, and quantified the gill surface area of the fish to understand how well they can transport oxygen from the water into the body. We also tried providing extra oxygen, to see if it benefited their growth. To get some data, we have carried out long-term experiments keeping fish under warmer water conditions than normal. This "oxygen mismatch" theory has sparked heated debate among global scientists, largely because insufficient data exist to confirm or refute it. So fish reach the limit of their growth at a smaller size, leading to the temperature–size rule. What does this have to do with warming? The next step of the argument says fish use more oxygen in warmer water - but their gills don't get any bigger. 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. The argument is that fish gills do not grow at the same pace as the rest of their bodies. The most popular current 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). Warmer water means smaller fish, but why? This is likely to have substantial ecological and commercial ramifications. So shrinking fish means each fish will have fewer offspring, and more fish being caught. And if fish are shrinking, fisheries that base their catch quotas on weight will be taking a larger number of individual fish. We also know that smaller fish produce proportionally fewer offspring. Such experiments have shown that fish do indeed end up smaller in body size when kept under warm conditions, and the trend is so common it has been given a name: the " temperature-size rule". How do we tell the difference between the impacts of climate warming and those of fisheries? ( Unsplash: Sebastian Pena Lambarri)Īnother way is to examine fish under controlled conditions, by manipulating water temperature and studying the impact on fish size. 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. One way is to examine the body size trends in fish species that are not targeted by fisheries. How do we tell the difference between the impacts of climate warming and those of fisheries? Indeed, it can lead to a phenomenon known as "fisheries-induced evolution", where the exploited species tends to decrease in size over time. This trait of maturing early can be passed through fish generations. 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. The temperature–size ruleįisheries are a potential confounding factor when studying the effect of warmer waters on fish, because fisheries often target large fish. Our experiments keeping fish in warmer water offer some crucial clues - and may help us learn how to prepare for a warmer future with smaller fish. This "shrinking" trend is forecasted to significantly exacerbate the impacts of global warming on marine ecosystems. They provide vital sustenance to billions of people worldwide via fisheries and aquaculture, and are critical parts of aquatic ecosystems.īut fish around the world are getting smaller as their habitats get warmer.įor example, important commercial fish species in the North Sea have declined in size by around 16 per cent in the 40 years to 2008, while the water temperature increased by 1–2C. Fish are the most diverse group of vertebrates, ranging from tiny gobies and zebrafish to gigantic tunas and whale sharks.
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