Sunset (yellow) moon wrasse. Photo by John Turnbull, Flickr.
Fish can look “bigger on average” even while they are actually shrinking due to warming waters caused by climate change.
A new paper in Marine Ecology Progress Series, co-authored by the Sea Around Us Principal Investigator, Dr. Daniel Pauly, explains this conundrum and clarifies a misunderstanding that stemmed from the title of a 2020 paper, where the term ‘fish body sizes’ was used to refer to mean body size, that is, the average size of all fish in a population.
Even though the body of the 2020 article detailed the type of measurement that was being considered, the title caused readers to think that it was referring to maximum body size, which is the largest size ever observed for a species or a population. This misunderstanding caused researchers to think that the paper’s findings denied the Temperature Size Rule (TSR) and the Gill-Oxygen Limitation Theory (GOLT), which posit that maximum size declines as temperatures rise.
The new paper, titled “Increasing mean fish length under warming can be reconciled with the temperature-size rule when mortality-to-growth ratios decline,” looks at data from different populations of alfonsino, swordfish and moon wrasse, and shows that even if a fish species’ maximum length shrinks by 10 per cent due to warming, the average size can still increase by up to 25 per cent. This only happens, however, if fish growth exceeds mortality, which may occur in situations where high temperatures strongly increase predation rates.
When warming and deoxygenated waters cause fish in a population to die at an earlier life stage than they would under normal conditions, the maximum possible size in fish (or asymptotic length) decreases, which means that even the biggest fish in that population will not get as big as they used to. Yet, if all fish in that population are growing fast and fewer are dying young, the population can end up looking “bigger on average,” even though the largest possible fish are shrinking.
In simple terms, the analyses show that fish can shrink in maximum size, but increase in average size. Both things can be true at the same time.
This, thus, reminds us that papers must be carefully read before one can conclude that they overthrew well-established rules, such as the TSR, which is part of the GOLT.
The paper “Increasing mean fish length under warming can be reconciled with the temperature-size rule when mortality-to-growth ratios decline” can be accessed here.