Although fishing gear selectivity is important in preventing the overfishing of certain fish stocks, the use of fishing gear with a selective mesh size over time, on some species, is thought to have negative consequences on the overall genetic health of some fish populations (some type of fishing gear are regulated to have a minimum mesh size, thus when used, the gear “selects” only larger fish above a certain size).  Likewise, targeting the largest fish—especially the large, breeding females—seems to be resulting not only in smaller fish sizes but smaller catches too.

The problem with some selective fishing gear (fishing gear can be modified to achieve a wide range of conservation objectives, such as preventing the capture of sea turtles, reducing marine mammal interaction, or selecting larger fish) is that over time, fishing effort will remove too many large female fish, which are thought to be genetically superior and have a higher fecundity than younger fish. The idea of conserving larger, older, breeding female fish is called the “Big Old Fat Fecund Female Fish (BOFFFF) hypothesis.”

To remedy this problem, the importance of marine reserves is being stressed. Rob Inglis at The New Republic has a piece on the problem of shrinking fish:

[O]ne of the most unfortunate (but also most scientifically interesting) consequences of overfishing is that it can cause fish to shrink. Smaller fish are better able to slip through the holes in fish nets and therefore survive to pass on their genes rather than ending up as fish sticks. As a result, heavily-harvested fish populations—especially in places that have minimum net mesh size requirements designed to let a certain fraction of fish escape—tend to evolve toward smaller average body sizes. This is bad for both fishermen and fish eaters, given that larger fish tend to have more usable meat and fetch better prices.

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[E]ven after the selective pressure on a fishery is relaxed, recovery could take decades. And getting rid of fisheries-induced selective pressure towards smaller fish means doing things like establishing no-fishing marine reserves that act as gene banks. In many places, that could turn out to be a real political challenge.

MPA News expands (PDF file) on the idea that conserving larger, older female fish is important to maintaing more healthy and robust fisheries and fish populations.

Modern fisheries management has often guided fishermen to select the large individuals of targeted stocks, either by using size-selective gear or releasing small individuals back to the water. The reasoning has been that this allows smaller, younger individuals to grow up to reproductive age, thereby sustaining the stock.

Recent research, however, shows that removing the larger, older individuals of a population may actually undermine stock replenishment. This appears especially to be the case for removal of larger, older females, which often produce significantly more offspring — and sometimes stronger offspring — than younger females do.

Some researchers have proposed the idea that maintaining old-growth age structure can be important for replenishing fished populations. It is termed the Big Old Fat Fecund Female Fish (BOFFFF) hypothesis.

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The BOFFFF hypothesis arose largely from the recent work of biologists Alan Longhurst on population structure of Atlantic cod and Steven Berkeley on maternal effects in Pacific rockfishes. The hypothesis is based on documented cases of older, larger female fish producing more young per year — often exponentially more — than younger females. The larvae of these older females may also be larger, with greater fat reserves that can aid growth and survival. In several species of rockfish, for example, larvae from older females both grow faster and survive starvation longer than larvae from younger fish. (Rockfish birth their young as larvae, with attached egg yolks; the yolks’ oil serves as the fat reserve.) Older females can also have earlier and/ or longer spawning seasons than younger, smaller females, and the fact of their longer lives may allow them to outlive periods of low larval recruitment.

Timothy M. Caro in “Behavioral Ecology and Conservation Biology” provides further insight:

Fecundity and spawning biomass are key parameters in fisheries management, and are often measured, yet they are vulnerable to misinterpretation without an understanding of the behavioral ecology of the species, population, or individual. Direct exploitation usually places greatest pressures on larger individuals, whether target species or bycatch, if only because of gear selectivity such as the mesh size of nets . . . . Extracting larger females can have a disproportionately large impact on reproductive output relative to change in biomass because egg number tends to increase allometrically with female body size . . . . Moreover, remaining females may mature when younger and smaller, with consequent decrease in fecundity . . . . We therefore need to understand behavior patters that might increase the probability of extracting larger fishes or influence our estimates of egg production and fertilization rates.

Understanding the relationship between large female removal and reproduction often relies on behavioral work. If females of different sizes breed at different times or places, then fishing schedules could greatly affect the impacts of exploitation on populations. . . .

Can these impacts be reversed? From Scientific American:

As people continue to go after the biggest fish in the sea, global fisheries are shrinking—both in number and in the actual body size of their catches. But that rapid evolution can be reversed, according to a new 10-year study published today in the journalProceedings of the Royal Society B.

Previous research has shown that the size of plants and animals harvested from the wild—from cod to ginseng—is actually decreasing two and a half times the rate Mother Nature would dictate. Many scientists pin this on the human tendency to go after the biggest and best food—and our technological ability to do so with extreme efficiency. Although the new study shows the changes are reversible, it also found that the return to normal size was much more gradual, probably taking more than twice as long as the original downsizing.

“There’s a good news story in that the evolutionary changes are not permanent—on a contemporary timescale,” says David Conover, lead study author and a professor of marine sciences at Stony Brook University (S.B.) in Long Island, N.Y. “But the bad news is that it’s slow.”

Images by Buck Denton. Note the tagged Atlantic cod. The flounder species are (from left to right) summer, yellowtail, and winter flounders.

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