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Shrinking fish more at risk from predators


Predators may amplify human pressures on fish stocks, say researchers (Source: WhitcombeRD/iStockphoto)


Marine biologist, Dr Asta Audzijonyte of the CSIRO's Wealth from Oceans Flagship, and colleagues, report their findings today in the journal Royal Society Biology Letters.

"We found that decrease in size leads to a very large increase in mortality of fish due to predation," says Audzijonyte.

Audzijonyte says there is quite a lot of evidence from Europe that commercially harvested fish have been decreasing in size in recent decades, and that fishing is responsible.

"When we fish we have nets that select bigger, faster-growing fish. Smaller fish manage to escape and the bigger fish get caught in the nets," she says.

There is also some concern that this pressure is causing fish to evolve to become smaller.

"If they are late maturing they may get caught and not reproduce so there is an evolutionary pressure on them to mature earlier," says Audzijonyte.

But, until now, she says, no-one has studied the impact of decreasing fish size on predator-prey interactions in a marine ecosystem.

Audzijonyte and colleagues used a giant computer marine ecosystem model to study the effect size had on the chance of fish being eaten by predators.

The model simulated interactions between 56 groups of organisms including seaweed, prawns, fish and whales.

The key finding was that regardless of fishing, even a small decrease in size led to a large increase in predation of four of the five Australian commercial species studied.

"For some species ... it did not mean anything and for others it led to very large increases in natural predation mortality," says Audzijonyte.

Feedback loops

The researchers found that for tiger flathead, pink ling, morwong and silver warehou, a decrease in fish size of 4 per cent over 50 years, was associated with a 20 to 50 per cent increase in mortality due to predation.

Audzijonyte says this large effect was due to positive feedback loops in the ecosystem.

For example, tiger flatheads eat barracuda juveniles, while adult barracuda eat flatheads.

The smaller the size of the flatheads, the fewer barracuda juveniles they ate. This in turn meant more barracuda grew up and ate the flathead, and the cycle continued.

Audzijonyte says while the idea that fishing leads to the evolution of smaller fish remains controversial, climate change is expected to have a similar effect on fish size.

"When the temperature increases the amount of oxygen in the water decreases and fish get less oxygen so they can't grow as fast," she says.

While Audzijonyte and colleagues' study does not look at the cause of shrinking fish, she says it does point to the importance of considering fish size in fisheries management.

"We have to take it very seriously because it leads to feedback loops in the ecosystem," she says. "So the impacts of fisheries might be much bigger than we expected."

Tags: marine-biology, ecology