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. This month, MPA News briefly examines this hypothesis and what role marine reserves could play in maintaining old-growth age structure of fishery stocks.
Maternal variability and size-selective fishing
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. See the box at the end of this article for more information on several BOFFFF-related studies.
Mark Hixon, a biologist at Oregon State University (U.S.), says various factors contribute to this maternal variability between BOFFFFs and younger females. “There often are different environmental constraints facing younger vs. older adults,” he says. “Smaller females are more susceptible to predation, and so may be more restricted to safer habitats and thus different food supplies. Smaller females must also devote more energy to growth than larger females, which can devote more energy to reproduction.”
He notes that the BOFFFF hypothesis applies better to some species than to others. The species for which it has been best demonstrated are long-lived and live in temperate waters: Atlantic cod (Gadus morhua) can live over 20 years, and Pacific rockfishes (genus Sebastes) can exceed 200 years in age. In contrast, says Hixon, short-lived and/or tropical species tend not to exhibit the same degree of variability in size of young and spawning season. The hypothesis might also not apply well to species like sharks that birth fully formed juveniles, for which there is no larval life period. Nonetheless, says Hixon, no fish (whether large or small, temperate or tropical) is totally exempt. “At least part of the hypothesis – that older fish can out-live periods unfavorable for recruitment – pertains to all fishes,” he says.
David Conover, a biologist at Stony Brook University (U.S.), has studied the long-term effect of size-selective fishing on population age and size structure. The studies show how removing BOFFFFs and other large adults can result in evolutionary changes in populations. Using studies of captive and wild Atlantic silverside, Conover discovered that not only does average fish size in some stocks get smaller over time, but the populations also evolve characteristics that make it more difficult to survive and reproduce when fishing ends. That is, size-selective fishing removes stronger individuals and leaves slower-growing ones – the opposite of what evolution would normally do. The phenomenon is called size and age truncation.
“Chronic age truncation via size-selective fishing, with its artificial selection for smaller size-at-age, can have a corresponding, genetically based drop in yield that may be difficult to reverse even if fishing ceases,” says Conover. He describes this in an article, “Size Matters”, in the July-September 2007Conservation magazine, available at www.conbio.org/CIP/article30713.cfm.
Implications for management
Hixon and Conover say steps are needed to protect BOFFFFs and guard against the evolutionary changes caused by size-selective harvesting. The answer, however, is not as simple as turning size selection on its head and catching smaller individuals instead. By necessity, it is also important to have smaller, younger fish in populations because they are the source from which larger fish emerge.
In general, according to Hixon, there are three main management methods available to conserve older fish in an exploited population:
- Reduce the rate of exploitation significantly (an economically infeasible option for many fisheries);
- Institute slot size limits in which there is both a minimum and maximum size for retention (an option primarily available for fishes that can readily be released unharmed after capture); or
- Designate marine reserves that set aside areas in which fishing is prohibited and older fish can survive and spawn.
Conover says the marine reserve option makes sense, particularly for non-migratory species. “For some fisheries, we may need no-take zones where the full range of sizes and ages of a given species can thrive,” he writes in Conservation. Hixon, in a paper he co-authored with Steven Berkeley, Ralph Larson, and Milton Love in the journal Fisheries in August 2004, said, “No other method of management, even slot limits, can preserve the potential for longevity as well as marine reserves and allow the unique contributions of older fish to accrue to the population.”
No-take marine reserves can be a hard sell to fishermen. Hixon says that in his experience, however, the BOFFFF concept resonates with fishermen more than any other issue regarding MPAs. “From my perspective, fishermen know the value of large spawners – if for no other reason than the fact that larger females produce more eggs,” he says. This knowledge, he says, comes from watching the huge roe of a BOFFFF hit the deck when cleaning such a fish.
The BOFFFF hypothesis is still new, and there remain relatively few examples of management programs created specifically to protect larger, older females. One such program emerged recently from actions that some fishermen had already adopted informally. In a live fishery of rockfish and other groundfish species in Port Orford, Oregon (U.S.), several local fishermen had begun returning gravid females – full of larvae – to the water. These fishermen wanted their local colleagues to follow their lead.
In May 2007, the Port Orford Ocean Resource Team (POORT), which represents the local fishing community and other stakeholders, responded by formalizing a voluntary conservation measure for the live fishery – to release all gravid and spawning females. Gravid and spawning females are identified by a variety of physiological and behavioral characteristics: gravid individuals, for example, often expel some of their larvae when brought on deck.
Leesa Cobb, POORT executive director, says the Port Orford fishing community is very aware of the BOFFFF hypothesis. (Technically, the voluntary measure covers all gravid and spawning females, not just the largest, oldest females.) Cobb says POORT has considered marine reserves, as well, to protect BOFFFFs – particularly their spawning sites. “In the interest of what we could achieve right away, however, and to get fishermen to start thinking about the role of spawning in healthy populations, marine reserves were not something we felt we could achieve in the short term like we could with this voluntary conservation measure,” she says.
For POORT’s program to be successful, the females need to survive their release back to the water. This is not straightforward. Rockfish and some other fish species have gas-filled swim bladders that help to control buoyancy; on rapid ascent, these swim bladders can burst. For their live fishery, Port Orford fishermen regularly “vent” (poke a hole in) each fish’s swim bladder with a needle to relieve the pressure. This technique is reliable enough that catches regularly remain alive on their way to market in tanks. For the females that are released, however, the venting process could weaken them in the wild, both during descent and onward. Although Port Orford fishermen report re-catching females with venting scars – indicating that survival does occur – there is likely some mortality as well. POORT is now conducting research with Oregon State University to tag females as part of the release program, and measure the survival rate of vented fish in the wild.
“We want to run the program for two years,” says Cobb. “By that point we hope to have the data to support continuing it.”
For more information
Mark Hixon, Department of Zoology, Oregon State University, Corvallis, OR 97331-2914, U.S. Tel: +1 541 737 5364; E-mail: firstname.lastname@example.org
David Conover, Stony Brook University, Stony Brook, NY 11794-5000, U.S. Tel: +1 631 632 8781; E-mail: email@example.com
Leesa Cobb, POORT, P.O. Box 679, Port Orford, OR 97465, U.S. Tel: +1 541 332 0627; E-mail: firstname.lastname@example.org
BOX: Additional sources of information on the BOFFFF Hypothesis
Berkeley, S.A., Chapman, C., and Sogard, S. 2004. “Maternal age as a determinant of larval growth and survival in a marine fish, Sebastes melanops”. Ecology 85(5):1258-1264. http://californiafish.org/maternal_age.pdf
Longhurst, A. 2002. “Murphy’s Law revisited: longevity as a factor in recruitment to fish populations”. Fisheries Research 56:125-131. http://www.cnr.uidaho.edu/fish510/PDF/Longhurst.pdf
Walsh, M.R., Munch, S.B., Chiba, S., and Conover, D.O. 2006. Maladaptive changes in multiple traits caused by fishing: impediments to population recovery”. Ecology Letters 9:142-148. http://www.msrc.sunysb.edu/people/munchpdf/Walsh_et_al06_EcolLet.pdf
BOX: Tribute to Steven Berkeley
This MPA News coverage of the BOFFFF hypothesis pays tribute to the work of Steven Berkeley, credited with developing the hypothesis in his work on Pacific rockfish. Berkeley, a fisheries ecologist at the University of California at Santa Cruz, died of pancreatic cancer on 27 June 2007. He was 60 years old. In his most recent research, he examined how knowledge of maternal age effects in rockfishes could help identify which species were most likely to benefit from protection in marine reserves. The American Fisheries Society is planning a scholarship in his honor.