Fishing down marine food webs
Image: Greenpeace; White-tipped shark caught in fishers' net
The World's last major "Wild" food resource is in jeopardy. That is the conclusion of an article entitled "Fishing down marine food webs," which was published last month in the journal Science. Here Jennifer English surveys the main findings of that article and interviews Dr. Daniel Pauly the article's senior author. (For further information and links, see References.)
March 16, 1998: Events such as the collapse of the Peruvian anchovy fishery in 1971, and the collapse of North Atlantic cod fishery in 1991 make it clear that the world’s last major "wild" food source is in jeopardy. Unless radical changes in fisheries management occur, the diversity of ocean life will be greatly impoverished. For the fishing industry, the oceans will become aquatic deserts offering little to harvest beside jellyfish and lanternfish.
Such is the conclusion of the article "Fishing Down Marine Food Webs," published last month in Science, the journal of the American Association for the Advancement of Science, by a group including scientists from the University of British Columbia and the International Center for Living Aquatic Resources Management (ICLARM) in the Philippines (1).
In their study, the scientists analyzed United Nations Food and Agricultural Organization fish catch data against food web models (2) to show how, since the 1950’s, global fish harvests have shifted down the food web away from species of higher trophic (food web) level (predators such as halibut and tuna) to species of lower trophic level (plankton-eaters such as anchovies). This shift has occurred as populations of predator fish have been decimated by over-fishing and fishers have been forced to harvest what is left, species of the predators’ prey.
The study demonstrated this shift by means of a scale which assigns top predators such as killer whales a trophic value of 5.0, tuna and cod a value of 4.0, and species at the bottom of the food web, like anchovies, a value of 2.0. Although the study acknowledges that some of the catch data from the Southern Hemisphere may be unreliable, the general trend is evident. Since 1950, the average trophic value of fish caught dropped from 3.4 to 3.1 in oceans and 3.0 to 2.8 in freshwater. The largest decline was off the Eastern coast of Canada and the US where the level has dropped from 3.4 to 2.9.
Alarmingly, this slide has not been accompanied by larger catches. This is contrary to expectation. Normally, as one moves down a food web, biomass increases. Instead, fish catches have stagnated as fishers have moved from top predators to species at lower trophic levels. One reason for this effect is that once a top predator has been depleted or exterminated by fishing, alternative predators, which are of no commercial value, thrive in the absence of competition and thus deplete the biomass of prey species at lower trophic levels.
The Science magazine article shows how this pattern is overlooked in present day fishing management strategies that place moratoriums on the harvest of individual species, such as cod in Atlantic Canada. Reliance on Moratoriums on the harvest of particular species is ineffective if the food source of the protected species is being depleted either by fishers or by alternative predators. To prevent total species collapse, the study suggests, "fisheries management will have to emphasize the rebuilding of fish populations embedded within functional food webs..." (1) instead of focussing on the health of one particular commercially harvested species. This broader approach to conservation could be accomplished with the implementation of more "no-take" Marine Protected Areas (MPAs).
An Interview With Dr. Daniel Pauly
Dr. Pauly, senior author of the Science article discussed above, answers questions about the global fisheries crisis and the concept of "no-take" areas:
There is little published scientific evidence to justify the government-subsidized seal hunt as a means of restoring cod stocks in Canadian waters off the coast of Nova Scotia and Newfoundland. Such was the view of 97 scientists from 15 countries who signed a petition opposing the hunt at the Eleventh Biennial Conference on the Biology of Marine Mammals in Orlando, FL (14-18 Dec. 1995). The petition stated:
Despite a lack of evidence, this year’s hunting quota in Canada is 275,000 seals. Observers report that the actual number of seals killed, including those whose carcasses are not recovered, may exceed the quota by as much as a factor of four (4). This greatly exceeds the 170,000 seals that Canada's Department of Fisheries and Oceans estimates can be killed in a year without depleting the population (5).
(1) Pauly, Daniel, Villy Christensen, Johanne Dalsgaard, Rainer Froese and Francisco Torres, Jr. 1998. Fishing down marine food webs. Science 279:860-863.
(2) Site with links to data used in the study Fishing down marine food webs. Target file now deleted from Web.
(3) The Canadian Harp Seal Dilemma: An analysis of the seal hunt with references.
(4) Mowat, Farley. December 19, 1997. Letter to the Editor, (Toronto) Globe and Mail.
(5) Gowdy, Barbara and Louise Dennys. March 9, 1998. How the hunt earned our seal of disapproval. (Toronto) Globe and Mail.
Marine Protected Areas in the Gulf of Maine: A Survey of Marine Users & Other Interested Parties