ENVIRONMENT: Shrinking fish?

ARE the fish we regularly catch getting smaller? It’s a conversation my old fishing mates and I often have after a day on the Sydney rocks when we’re cleaning our catch. We target luderick a lot, which are still quite prolific … not as good as they once were, but bag limit catches are still frequent. We also pick up rock blackfish (pigs) and bream regularly, plus the odd tarwhine, silver trevally, tailor and salmon.

It’s not just our memories playing tricks, the average size of all these fish 50 years ago was definitely bigger. With luderick for instance, the average size of a good bag was often 35 to 40cm. Plenty of those fish weighed well over 1kg, 2kg fish were often seen and I weighed two that nudged 2.5kg.

I haven’t seen a 2kg luderick for probably 25 years. Most of the fish we get these days are from around 27 to 34cm. One that hits 40cm is really worth talking about. Similar for bream and silver trevally. Our standard hook size back then when chasing them was 2/0, which we also used for pigs. Now, at size 2, it’s four sizes smaller.

One theory tossed around about a decade ago was that minimum size limits actually encouraged the survival of smaller, slower growing fish, which then passed on the genes responsible to successive generations. If, for instance, luderick at the same age can be 25 or 30cm, and the minimum size for retention is 27cm, then over time you select smaller, released fish to continue breeding. The variation in growth rates is well known in the aquaculture industry, where fish are labelled as “runts” or “shooters”. Naturally aquaculturists like to breed the shooters.

That theory probably relies on too small a sample size, that is it’s based on small fish released by commercial or recreational fishers. Recently, though, there was a brief news item on the effect of rising water temperatures on fish sizes. It suggested that even small increases in water temperature caused some species grow more slowly as their gills did not process oxygen as efficiently in the warmer water.

A quick internet search threw up a swag on articles on declines in fish lengths likely being linked to climate change. Back in 2014, University of Aberdeen researchers reported that over a 38 year period the maximum length of haddock, whiting, herring, Norway pout, plaice and sole in the North Sea decreased by as much as 29%, coinciding with an increase in water temperatures of between 1 and 2 degrees C.

Later North American studies from 2017 on suggest average sizes could in the future reduce by up to 45%. But other scientists dispute the “gill”, or metabolic demand, theory. And it appears some species don’t seem to be affected. But, until a more plausible explanation comes along, the warming water suggestion looks to be a pretty likely answer to our questions on our favourite targets species’ decline in average size.

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