Overfishing Blamed for Ruin of Key Adult Fish Populations

The face of an adult halibut. Adult fish hold the key to a healthy marine ecosystem but are being overfished. (Andrea Pokrzywinski)

(CN) – Older fish are becoming less common across multiple species largely due to overfishing, according to a new study that examines how fishing practices and other threats jeopardize this critical subpopulation.

Published Thursday in the journal Current Biology, the report contextualizes the plight of older fish with the important roles they play in diversifying and stabilizing marine ecosystems.

In particular, older fish have more opportunities to reproduce – a tricky venture in the ocean.

“From our perspective, having a broad age structure provides more chances at getting that right combination of when and where to reproduce,” said lead author Lewis Barnett, a researcher at the University of Washington.

While the ideal time and location of fish reproduction is ambiguous, the damage the reduced presence of older fish causes to the marine ecosystem is fairly clear.

“More age complexity among species can contribute to the overall stability of a community,” Barnett said. “If you trim away that diversity, you’re probably reducing the marine food web’s ability to buffer against change.”

Producing offspring is a process that can take some species up to a decade to successfully complete.

Once a female fish releases her eggs, several factors must align in order for a healthy brood to grow and develop. These variables can stunt fish populations and potentially stagnate species evolution.

“In the marine world, the success rate of producing new baby fish is extremely variable,” said co-author Trevor Branch, a University of Washington associate professor of aquatic and fishery sciences. “I think of old fish as an insurance policy – they get you through those periods of bad reproduction by consistently producing eggs.”

To evaluate the declining presence of older fish, the team reviewed model output data from commercial and recreational fisheries, as well as scientific observations chronicling the state of fish populations over time. The researchers analyzed 63 populations across five ocean regions worldwide, finding that the proportion of fish in the oldest age classes has plummeted in up to 97 percent of the groups. And the magnitude of decline exceeded 90 percent in up to 41 percent of the populations.

The team primarily attributes the dwindling quantity of older fish in these groups to fishing pressure – the longer a fish lives, the higher the likelihood it will be caught. The researchers also note environmental factors like pollution and disease might also contribute to this concerning trend.

These findings could aid the management of fisheries, which often establish limitations based on the total weight of fish caught during a specific season without considering factors like size and age, according to the team.

The researchers also suggest several fishing methods that protect young and old fish by prohibiting the harvest of fish below or above a particular size range. Other options include barring fishing permanently in certain areas, or rotating where fishing is allowed each year in order to allow fish to grow older and larger.

The designation of “older fish” can vary dramatically by species: some types of rockfish living up to 200 years, while herring rarely live more than 10.


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