Things are heating up

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It’s a story about the big one that didn’t get away — or, more to the point, the big one that might never be replaced.

Scientists doing ocean modelling in the journal Nature Climate Change have documented an interesting problem with warm oceans and fish metabolism — and it’s one that could have serious repercussions for fishing nations.

For years, scientists have scratched their heads about the impacts of fisheries directed at the largest and most successful members of a species. Fishing male crab only when they’re over a certain size, for example, may allow smaller males to have an unexpected amount of breeding success and lead to future generations of male crab predisposed to be smaller.

But the new research is focused less on changes to the genetic pool, and more on something much smaller — how fish actually work internally, and what effects increasing ocean temperatures might have on them.

There’s already some considerable suggestion that warm temperatures wreak havoc on fish at the food supply stage; increased carbon dioxide could make the oceans more acidic, in the process breaking down calcium carbonate-based plankton that make up a good part of the piscine food supply.

There’s also work showing that fish species are significantly changing their ranges — with species moving further north in the Northern Hemisphere to stay in temperature ranges they are comfortable living in.

The research looks at the fact that individual fish in warmer water have to take in more oxygen in order to keep their metabolism running — and the fact that fish can reach a “ceiling” size beyond which they can’t grow because there simply isn’t enough oxygen in the water to allow further growth. Warmer water moves that ceiling lower — and the study suggested that, overall, fish could be anywhere from 14 per cent to 24 per cent smaller by weight. The biggest decreases, the research suggests, would be in the Indian and Atlantic oceans.

But that’s not all. Scientists also point out that in the two fish species they looked at to test their modelling, North Atlantic cod and haddock, the shrinkage was actually more drastic than their models would have suggested.

It’s a sword with many edges, because it’s more than just diminishing return in terms of the size of individual fish. Scientists point out that fish that cap out at small body weights aren’t as successful at breeding. They have smaller volumes of eggs at maturity — and, given the success rate for larval fish anyway, it tilts the odds even more for overstressed stocks trying to recover.

As always, there are significant wild cards in the research — not the least of them being trying to pin down just exactly what changes in deep-ocean temperatures will be and how quickly they will make themselves known.

All in all, though, in a province that still depends to a large degree on the sea, it’s research well worth watching.

Geographic location: North Atlantic

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