Neglecting the capacity of large females can lead to overexploitation

The management of many of the world’s largest fisheries incorrectly assumes that many small fish spawn as well as fewer large ones with similar total masses, a new analysis has found. This can lead to overexploitation of the largest and most prolific fish that can contribute the most to the population.

Better protection of larger, mature females could improve the productivity of major fisheries. This is crucial at a time when fishing plays an increasingly important role in the supply of food resources in the world. The results were published in Proceedings of the National Academy of Sciences this week.

“It’s a fundamental question in fisheries management – how much reproduction can you expect?” said Dustin Marshall of Monash University in Australia, lead author of the research. “When you expect smaller females to produce the same number of eggs per body mass as larger, older females, you won’t get an accurate picture.”

Build on previous research

The new research applies previous findings that challenged long-held assumptions about fisheries management. Traditional thought held that reproduction is a function of biomass. This means that fish representing a certain mass would produce a similar number of offspring, regardless of age or maturity. However, reviews of previous research by some of the same authors have demonstrated that larger, older, and more mature fish produce more offspring. Additionally, previous work suggests that offspring from these older, larger mothers may survive at higher rates.

Management measures, such as creating marine protected areas that provide refuge for fish to grow, can help increase fisheries yields and replenish declining species. They can indeed provide a tank of more mature fish with greater reproductive capacity.

“We have to ask ourselves, ‘How can we get the most out of these more efficiently reproducing fish, both to support the species and to support a sustainable fishery?’ said EJ Dick, a fisheries research biologist at NOAA Fisheries’ Southwest Fisheries Science Center and lead author of the paper.

In contrast, when the fishery takes the most prolific large fish, traditional assumptions tend to overestimate egg production and the ability of the population to recover. This can lead to overfishing, which for many of the largest fisheries could take around twice as many fish as expected, scientists have found.

“In this article, we connect the dots between the early findings that large and old Pacific rockfish produced more eggs per body mass than smaller ones, and Professor Marshall’s more recent work showing that many other species are doing too,” said Professor Marc Mangel. Emeritus of Mathematical Biology at UC Santa Cruz and co-author. “Without recognizing it, scientists and fisheries managers may overestimate the number of spawning fish needed to produce a certain level of recruitment and set fishing mortality levels too high.”

Recognize greater ability

In their new analysis, the scientists looked at whether the world’s biggest fisheries were heeding the results. In many cases, they found, this is not the case with fisheries.

“This systematic error could help explain why some stocks have collapsed despite active management,” the scientists wrote. They recommended that managers recalibrate future species stock assessments to recognize the greater reproductive capacity of larger fish. This could reduce overexploitation and could even increase fishing yields.

“Such reductions could have short-term negative repercussions, both for food security and the economy, but will bring positive long-term benefits,” the scientists wrote. They said better recognition of the capacity of larger fish could help increase catches from the Atlantic cod fishery in the longer term, for example.

“Our work suggests that modern management could respond to this challenge by better exploiting the reproductive potential of larger and older fish in more exploited stocks than is currently the case, using relatively simple policy innovations,” they said.

The research was conducted by scientists from:

  • Monash University
  • Queensland University of Technology
  • University of California Santa Cruz
  • Leibniz Institute of Freshwater and Inland Fisheries Ecology
  • NOAA Fisheries
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