Study shows fish are getting smaller
Fish in the ocean are getting smaller.
That is not an opinion, hypothesis, or theory, but a fact that is evident from the Canadian Atlantic to the western Gulf of Mexico.
With each passing year, the average size of many popular game and food fishes is decreasing, prompting fisheries biologists to question current management practices. Jeff Hutchings, associate professor at Dalhousie University in Halifax, Nova Scotia conducted a study on this phenomenon. Using the most thorough fisheries database available, Hutchings has analyzed 90 fish stocks representing 38 species. His study, along with several others recently published, point to rapid, massive changes among numerous popular species.
“There is very little evidence for rapid recovery from prolonged declines, in contrast to the perception that marine fishes are highly resilient to large population reductions,” Hutchings said.
Fish do respond to fishing pressure, but many species do so in a backwards sort of way. Take haddock, for example. Growth and maturation rates of haddock have changed significantly over the past 40 years. During the 1960s, all female haddock age four and older were fully mature, and nearly 75 percent of females age three were mature. In a study conducted in 2002, growth was far more rapid, with nearly all haddock mature by age three and 35 percent of age two females able to reproduce.
A similar problem exists with cod, which federal officials placed under a two-year harvest moratorium in 1992, and still get protection from commercial harvest because their numbers never bounced back.
“The spawning population of northern cod offshore is less than 5 percent of its 1962 level, and shows little improvement eight years after the fishery was declared a disaster,” Hutchings wrote in a past issue of the journal, Nature.
The ability to spawn at earlier ages might seem like an advantage for pressured species like cod and haddock, but that is not the case. According to a study conducted by D. Plicansky in 1993, heavy fishing pressure can change the genetic characteristics of a population by selecting for or against certain genetically inheritable traits such as size at first sexual maturity. This happens when anglers selective harvest the larger fish in a population. Removing the big fish over time results in favoring the survival of smaller fish, which begin maturing at an earlier-than-average age or smaller size. Plicansky wrote that if heavy fishing removes most fish early in their reproductive life, individuals that mature younger or smaller than average are at an evolutionary advantage of sorts.
In other words, the fish that survive and do more of the reproducing (which are now smaller) pass on their genes to future generations. The genetic variability goes from its natural size to a population that genetically encodes itself to begin reproducing at a smaller size. Fishing can inadvertently cause selective breeding toward miniaturization of fish.
Most current fishing regulations aim to protect juvenile fish, so they get a chance to spawn. The idea is that by saving the small individuals, fishermen are protecting the future generation of fish and in the end preserving the species. According to a study conducted by David Conover, Stephen Munch, and Jack Mattice of Stony Brook University, this idea is misguided.
This study shows that well-intentioned management plans that appear to maximize yield may be having the opposite effect after accounting for evolutionary dynamics. It backs up the concept of fish miniaturization proposed by Plicansky.
Again, scientists found that by catching only large individuals and sparing smaller ones, anglers are imposing a selective force on the species, which favors little fish.
Mattice and his co-authors also claimed that populations of fish with a smaller average size could have slower growth and breeding rates, and might be at greater risk of predation. Species like sharks that have naturally slow growth rates would be affected the worst.
In the Western Gulf of Mexico, the average size of many shark species has declined over the last two decades. Blacktips and spinners, two of the most common species found nearshore, average much smaller than they used to, and this may have allowed smaller species to increase in numbers. These species, which migrate into Mexican waters during winter months, are targeted heavily by commercial anglers there.
Officials with the Mote Marine Laboratory in Sarasota, Florida, said populations of small sharks like sharpnose and bonnethead might be increasing because large shark numbers and average size are decreasing. Little sharks no longer get as much pressure from other larger sharks as much as they used to.
A prime example of the fragility of shark populations is in Mexico’s Sea of Cortez, where longliners and gill-netters targeted big sharks for their fins, which restaurants in the Far East make into “shark fin soup.”
In 1985, the killing of over 200,000 sharks began slowly with just three small Mexican fishing boats operating out of La Paz. The next year, four more boats joined them, including harpooners that came in to target the largest of sharks. By the end of 1986, 10 tons of shark had been harvested, and the slaughter continued until about 1995, when there were virtually no sharks to catch—or at least not any big enough to fool with.
Since they targeted large sharks, nearly 40 percent of all sharks caught were pregnant females (the largest sharks). Not only did the region lose its biggest sharks, but also most of its breeders.
Research suggests fisheries managers can avoid these problems in other fisheries. Mattice and his colleagues suggest an upper size limit and a lower size limit (slot limit) for many popular species. This would mean allowing harvest of only mid-range fish.
Unfortunately, such species-specific size limits would not help all fisheries. Some commercial fishing practices are ravaging fish populations at alarming rates, even those practices deemed “environmentally safe.” Take the push for “dolphin safe” tuna, for example. According to Bill Boyce, a former National Marine Fisheries Service purse seine observer, the “dolphin safe” rules developed in the early 1990s promote the killing of non-target fish. A typical haul of 1,000 tons of yellowfin tuna caught primarily by the log method results in the killing and dumping of 200 to 400 tons of undersize yellowfin and skipjack tuna. In addition, the “dolphin safe” methods take out the following per 1,000 tons of tuna: 1,500 sharks, 2,000 to 5,000 dorado, 2,000 to 3,000 wahoo, 50 to 100 marlin, and 200 to 300 sailfish, along with rainbow runners, triggerfish, small jacks, and tripletail.
The tuna fishery went from one that might kill 20 dolphins for every 1,000-ton load of mature yellowfin to one that kills far more non-target species, many of which are highly sought after by recreational anglers. Now tuna stocks in the Atlantic are showing signs of early maturation.
Conscientious anglers have long-promoted releasing the prime breeding-sized fish to pass on their good genes, and now it seems like that is a better idea than ever.