The proportion of old fish in the ocean has been significantly reduced, according to a new study.
The population of old fish in fish populations worldwide has significantly been reduced due to fishing activities, and this will negatively impact the diversity and stability of marine ecosystems.
This was the finding of a study published in the Current Biology journal recently.
After studying 63 populations in five ocean regions worldwide, scientists from the University of Washington (UW) found out that the proportion of fish in the oldest age ranges had declined significantly in 79 to 97 percent of the populations, compared with historical fishing trends or unfished figures, respectively. In 32 to 41 percent of the groups, the decline was more than 90 percent.
What’s the big deal?
Old fish are integral in ensuring the diversity and stability of marine ecosystems. After a fish successfully lays its eggs, the survival rate of the brood is not guaranteed. Hence, according to co-author Trevor Branch, a UW associate professor of aquatic and fishery sciences, the older fish acts as a sort of insurance policy to get the species through bad reproduction by consistently producing eggs. This would help ensure the species' survival.
Having a broad age structure also provides more chances at getting the right combination of when and where to reproduce, said lead author Lewis Barnett, a UW postdoctoral researcher at the School of Aquatic and Fishery Sciences and the Joint Institute for the Study of the Atmosphere and Ocean.
“More age complexity among species can contribute to the overall stability of a community. If you trim away that diversity, you're probably reducing the marine food web's ability to buffer against change,” he added.
Besides this, older fish may behave differently than younger fish. As they age, some fish change what they eat and where they live in the ocean. They may even take on other roles in the food web, sometimes becoming a more dominant predator.
“Big fish are in a lot of ways different from smaller fish. Having that diversity acts as a hedge against risk and helps stabilize the system a bit,” said co-author Tim Essington, a UW professor of aquatic and fishery sciences.
In light of the findings, the scientists suggest that fisheries management might consider prohibiting the harvesting of fish below and above a specific size range, closing specific areas to fishing permanently, or rotating areas where fishing is allowed to take place to allow the fish there to grow older (similar to crop rotations in agriculture).