by Kenneth Stump, Ocean Policy Fellow at The Ocean Foundation
Overfishing (and the use of destructive fishing gear) is often cited as one of the two greatest threats to animals in the ocean. Overfishing occurs when a fishery removes fish from a population faster than the population can replenish itself – in a word, overfishing is overkill. If not quickly controlled, overfishing leads to the eventual decimation of a fish stock and the collapse of the fishery. Scientists and fishery managers strive to identify how big the population of any given species should be to say that it is not overfished.
For well-studied stocks that have been scientifically assessed, it is possible to evaluate the status of the stock relative to overfishing criteria that are based on the ability of a given stock to produce maximum sustainable yield (MSY). Using these conventional measures of fisheries sustainability, Dr. Boris Worm et al. (2009) found that 63% of assessed fished stocks worldwide have a breeding stock size (“biomass,” denoted as “B”) below the level that is estimated to produce MSY (B/Bmsy <1), while a separate study by the FAO (2010) concluded that 32% of globally assessed stocks are overfished (B/Bmsy < 0.5).
In short, most of the world’s assessed fish stocks are fully or overexploited. But only ~20% of the global fish catch (reported landings) comes from assessed species. What about the status of the thousands of data-poor,unassessed fish stocks which account for 80% or more of the global seafood catch every year?
UC Santa Barbara’s Christopher Costello and colleagues have just published a new study of the status of the world’s data-poor stocks in an online edition of Science (September 27, 2012). Using available landings records and indirect evaluation methods, the authors of the new study conclude that most of these fish stocks are likely to be considerably depleted and in serious decline:
64% of unassessed fisheries stocks have a stock biomass less than Bmsy (B/Bmsy <1), which is tantamount to a depletion rate on the order of 60-70% for most stocks.
18% of unassessed stocks are collapsed (B/Bmsy < 0.2) – a level of depletion so severe that a fish population may be only a tiny fraction of its natural, unfished size.
The depleted status of so many fish populations (low B/Bmsy) has consequences for food security: fishery yields are far below their potential if stocks were allowed to recover to the level that will, in theory, produce MSY. Since many of these unassessed fisheries are in poor and developing countries, management approaches to rebuilding stocks that rely on strong governance and monitoring capabilities are not likely to work. But Costello and colleagues also hold out the hope that innovative strategies combining territorial user rights (TURFs), fishing cooperatives, and no-take marine protected areas can restore these populations to healthier, more productive levels – if swift action is taken to reverse the declines.
In the U.S., reforms to the national fisheries law in 1996 and 2006 have reduced overfishing on assessed stocks by about half since the National Marine Fisheries Service began issuing annual status reports in the late 1990s, as shown in Fig. 1. In 2011, U.S. commercial fisheries recorded the highest catch in 17 years, which suggests that efforts to curb overfishing and rebuild overfished stocks are starting to pay off in many (but not all) regions of the country.
But about half of all managed stocks in U.S. waters are still unassessed and the study by Costello et al. finds that some of these data-poor stocks are likely to be in as bad a shape as those in developing countries. For instance, numerous reef fish such as groupers in the South Atlantic and Gulf of Mexico, many species of sharks, halibut in New England, to name a few, are known to be historically depleted even though they have not been formally assessed.
The effects of overfishing are not limited to the decline of individual species of fish. Depletion of commercially valuable species in rapid succession can trigger trophic cascades that change the structure of the food web over time, creating unintended consequences,. The ecological consequences of overfishing rarely receive much consideration in the conventional calculus of overfishing, but one recent analysis by NOAA’s Northeast Fisheries Science Center concluded that the New England region has experienced ecosystem overfishing as a consequence of widespread overfishing and species-selective harvesting patterns that have caused a shift in the fish community composition from a system dominated by species such as cod to one increasingly dominated by lower-value small pelagic fishes such as herring and elasmobranch species (small sharks and skates). Similar effects have been observed in other heavily fished marine ecosystems, such as Europe’s North Sea or the coral reefs of the Caribbean.
As the new study by Costello et al. shows, literally thousands of species are affected by fishing worldwide and most appear to be in decline. The unintended consequences of such widespread impacts on marine ecosystems are not fully known, but ignorance is not bliss. Overfishing threatens food security and local fishing economies, but efforts to sustain the production of wild fish as food for humans will fail if we ignore the functional roles that all these species play in the ecosystem. . As fisheries scientists and managers grapple with ways to end the scourge of overfishing, they must factor these ecological considerations into their calculations of how much fishing is too much. It may mean catching fewer fish, but the alternative may be catching no fish at all.
Christopher Costello, Daniel Ovando, Ray Hilborn, Steven D. Gaines, Olivier Deschenes, and Sarah E. Lester (2012), Status and Solutions for the World’s Unassessed Fisheries, Science Online, September 27, 2012.
NOAA Northeast Fisheries Science Center (2009), Ecosystem Status Report for the NE Continental Shelf Large Marine Ecosystem.
Boris Worm et al. (2009), Rebuilding Global Fisheries, Science 325: 578-585.