There is no fertility clock for fish. Unlike in mammals, the reproductive ability of most female fish just keeps increasing as they age and grow—bigger fish produce more and more eggs. In many species, the fecundity gains can be especially impressive, creating what might be called “superspawners” that produce disproportionately large numbers of offspring, a new study finds. But these reproductive giants aren’t getting enough protection under fishing regulations, the authors suggest.
Some fisheries scientists disagree with that conclusion. But the finding is “a perfect reminder that in order to rebuild fish stocks and prepare them for global change, we have to increase the proportion of large fish,” says Rainer Froese, a marine ecologist at the Helmholtz Centre for Ocean Research in Kiel, Germany, who was not involved in the study.
It’s been known for decades that fish fertility generally increases with size. A cod that is 70 centimeters long, for example, can produce eight times as many eggs as a younger fish half its size can. But it’s been hotly debated whether larger females are especially important for keeping fish stocks healthy. Most of the models used to manage fisheries assume, because of a lack of comprehensive evidence to the contrary, that the most important factor in sustaining a healthy stock is the total amount of spawning fish, regardless of whether it’s a lot of small fish or the same tonnage of big ones.
There are exceptions: For a few species, such as certain Pacific rockfish, detailed research has shown that big females produce even more eggs than predicted by their size. In addition, some kinds of old females produce extra-nutritious eggs so these offspring are more likely to survive. Many researchers worry about these matriarchs because they are relatively rare and, if killed, it can take a long time for younger fish to mature and replace them. Climate change is also a concern, as fish tend to be smaller when water temperature rises, making bigger females more valuable.
Dustin Marshall, an evolutionary ecologist at Monash University in Melbourne, Australia, wondered how common these disproportionately fertile females might be among marine fish. He and his colleagues compiled data on size and number of eggs for 342 species across various taxonomic groups. “No one had done the brutally boring work we did,” Marshall says. A key data set came from D. Ross Robertson, a taxonomist with the Smithsonian Tropical Research Institute in Panama City, who has spent decades collecting fish eggs of many species and measuring how many calories they contained. Combining egg size, number, and calories revealed the total amount of energy each species put into reproducing. They found that, in 95% of the species, big old females were superspawners, they report in tomorrow’s issue of Science.
“This is a nice meta-analysis and important to highlight,” says Mark Hixon, a marine ecologist at the University of Hawaii in Honolulu, who was not involved in the study.
Marshall and his co-authors also argue the finding has policy implications. In particular, they believe fishing should be restricted for many stocks, in order to allow more young females to become prolific spawners. Over time, this would yield more productive fisheries. Froese and others agree. They say the finding also reinforces the argument that marine protected areas, where fishing is often banned and fish are allowed to grow old, can help boost populations.
Not so, says Ray Hilborn, a fisheries biologist at the University of Washington in Seattle. He argues that because superspawners are relatively rare, even in unfished populations, their overall contribution to the population is not particularly great. Accounting for superspawners would make “little difference to how you manage a fishery,” he says. And Tara Marshall, who researches fisheries ecology at the University of Aberdeen in the United Kingdom, points out that many fisheries models already include safety margins to account for uncertainty, such as unknowns about a stock’s reproductive biology. “It’s not fair to the tremendous effort that goes into fisheries management to say that variability in reproductive potential isn’t accounted for,” she says.
Marshall isn’t persuaded by such arguments. Some fisheries models, depending on how they are set up, demonstrate the significant role of superspawners. In some stocks, he notes, “even very moderate fishing pressures dramatically reduce the abundance of large females and removes their influence.”