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Where have all the old fish gone?

Older fish have been proven to be more useful to the ecosystem than juveniles Older fish have been proven to be more useful to the ecosystem than juveniles Shutterstock
21 Sep
2017
A study of various fish populations has found dramatic reductions in the proportion of older fish, something that spells trouble for the overall stability of population

Industrial-scale fishing practices have reduced the number of older individuals in around 80 per cent of fish populations around the world, according to scientists at the University of Washington. In fact, a third of populations have experienced a loss of 90 per cent since the beginning of large-scale, commercial fishing.

The fear is that fewer older fish will put whole populations at risk. ‘More age complexity among species can contribute to the overall stability of a community,’ explains Lewis Barnett, a post-doctoral researcher in fisheries and climate at the University of Washington. ‘If you trim away that diversity, you’re probably reducing the marine food web’s ability to buffer against change.’ Because fishing is usually unscrupulous about age, it follows that older fish are disproportionately in danger of being caught.

rockfishA copper rockfish (Image: Shutterstock)

‘Old’ is relative to the species in question – some species of rockfish can live as long as 200 years, while herring are lucky to last a decade. However, across all species, older fish are more successful breeders. They spawn more reliably, at different times of year and in different locations, often with bigger offspring. This increases the chance that hatchlings will emerge with the bloom of algae or zooplankton and survive. Juvenile fish, on the other hand, are less reliable breeders, they are vulnerable to predation and changes in environment such as water temperature and habitat.

‘The success rate of producing baby fish is extremely variable,’ says Trevor Branch, co-author of the study and an associate professor of aquatics at the University of Washington. He describes the older breeders as ‘an insurance policy’ saying that ‘they get you through those periods of bad reproduction by consistently producing eggs.’

The data was collected from 63 populations of fish across five ocean regions. Though it is not the first time fishing has been linked to removing older fish, it is the first study to demonstrate the global spread of the problem.

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