Large female fish vital to successful fisheries

May 11, 2018

AN international study led by Monash marine ecologists has found that large fish are much more important to feeding the planet than previously thought.

The new research, published in the prestigious journal ‘Science’, has major implications for how fisheries are managed and the value placed on marine protected areas. Most theories on how reproductive output increases with body size assume that reproductive output increases proportionately with female size. Researchers from the Centre for Geometric Biology at Monash University collected reproductive data on all the world’s fish and found that larger mothers reproduced disproportionately much more than smaller mothers.

The data also showed that larger mothers may produce offspring that perform better and are more likely to survive to adulthood. Director of the Centre for Geometric Biology and senior study author Prof Dustin Marshall said, “We have failed to appreciate the importance of large individuals for sustaining fisheries. Our models of how organisms grow and reproduce are based on the wrong assumptions, and as a consequence we are overharvesting wild populations with calamitous consequences for the world’s fisheries. This is particularly dire because about 20 percent of people on the planet rely on fish for protein.”

Most theories on how reproductive output increases with body size assume reproductive output increases proportionately with female size – that is, for every unit increase in female size, there is a direct proportional increase in reproductive output. Or simply put – the combined reproductive output of two 1kg fish is assumed to be the same as one 2kg fish.

“Biologists have long suspected this assumption is wrong and that fish reproductive output goes up disproportionately so that a 2kg fish has a much greater capacity for reproduction than two 1kg fish,” Prof Marshall said. “This is important because reproductive output drives the replenishment of fisheries.“Getting the relationship between size and reproduction right is essential. Larger fish are more important for the replenishment of marine fish populations than has been previously thought, so most plans for harvesting fish populations sustainably are wrong. We have failed to appreciate how important large individuals are for sustaining fisheries.”

Prof Marshall said climate change is also predicted to cause fish body sizes to decrease. The new research shows this will have severe consequences for fish reproduction and replenishment. Warmer oceans will likely have fewer fish and much less reproductive output.

But the research also points to some good news, suggesting that current conservation strategies are more potent than previously thought, with other research showing that in marine protected areas fish can grow larger by 25 percent on average.

“Our discovery means the benefits of marine protected areas have been massively underestimated,” Prof Marshall said. “They produce far more new fish than unprotected areas of the same size.”