FEATURE ARTICLE
Fishing Down Aquatic Food Webs
Industrial fishing over the past half-century has noticeably depleted the topmost links in aquatic food chains
Daniel Pauly, Villy Christensen, Rainer Froese, Maria Palomares
Hold the Anchovies
Exceptions to this general pattern have certainly happened, but they are for the most part easy to explain. For example, average trophic levels of the catches from the South Atlantic and the Western Pacific have climbed somewhat during the past few decades—an effect we ascribe to the development of new fisheries in these regions. And the average trophic level of the fish taken from the South Pacific rose sharply during the 1970s, from about 2.3 to 2.9. This increase corresponds to the collapse of a huge industrial fishery for Peruvian anchoveta, a species with a markedly low trophic level (2.2), and the growth of a more modest fishery for horse mackerel, which occupies a considerably higher position in the marine food web (trophic level 3.3).
Indeed, the rise and fall of the Peruvian anchoveta fishery creates a prominent dip in the curve for the global marine catch, which otherwise shows a largely steady decrease from a value of about 3.4 in the early 1950s to less than 3.1 today.
Might this worrisome trend be merely some sort of statistical artifact? One could imagine, for example, that the decline we noted merely reflects changes in the way the FAO collected its information, say, by using only coarse groupings at first and refining the categories later. To test how our results change when many species are lumped together, we regrouped the relatively detailed totals available for the North Atlantic, so that the accounting distinguished different kinds of fish only at high taxonomic levels. The result of this exercise was to lessen the decline seen. Hence the shift we computed with the full statistics must be, if anything, underestimating the true extent of the drop.
Admittedly, the procedure of assigning a single trophic level to a species rests on a rather shaky scientific foundation, because the feeding habits of many fishes change as they grow. Some begin life eating only plankton and hop up a full trophic level when they begin devouring other fish. Other species maintain roughly the same diet (and trophic level) throughout life. Still other species slide backward a step on the ladder, as larvae feeding on zooplankton and later turning to plants or detritus for sustenance when they reach maturity.
Fortunately, ignoring these complications only makes our estimates of trophic-level decline more conservative. Why? Because some 86 percent of the worldwide marine catch is made up of species that either maintain a constant diet throughout life or increase in trophic level as they grow. In the first case, assigning a single trophic level does not skew the results. The second case creates the possibility of bias, because there is a tendency in many fisheries to catch smaller and smaller specimens over time. But the actual trophic level of the catch from such a fishery would typically be decreasing, whereas our analysis would show it to be constant. That is, our assigning of a single trophic level to a species probably misses some of the real change that is taking place. Here again, we conclude that the trophic level of the global catch might, in actuality, be declining faster than we had documented.
But what if this effect applies only to the fish being netted, not to those remaining in the wild? Perhaps marine ecosystems are not changing at all, only people's choice of what they extract. Although such a selection bias is conceivable for lightly exploited fisheries, we do not consider it likely to be a widespread phenomenon. Indeed, in two cases one can test the hypothesis directly: in the Gulf of Thailand and over the Cantabrian Shelf, offshore from northern Spain. For the Gulf of Thailand, we determined the change in average trophic level (during periods of intense industrial fishing) using scientific trawl surveys of fish populations, rather than relying on the catch statistics. Francisco Sanchez of the Instituto Espanol de Oceanografia in Santander and several of his Spanish colleagues did the same for the Cantabrian Shelf. And both groups found that fishing pressure indeed altered the makeup of the ecosystem enough to depress the average trophic level. So we firmly believe that the average trophic level elsewhere is also truly declining.
It takes very little to convince oneself that this situation is alarming—for seafood lovers as well as for environmentalists. After all, the average trophic level of the global catch has already slipped from 3.4 to 3.1 in just a few decades, and there are not many more appetizing species to be found below this level. (Recall that 2.0 corresponds to copepods and other tiny zooplankton, creatures that are unlikely ever to be filling one's dinner plate.) So if the trend continues, more and more regions are likely to experience complete collapse of their fisheries.
Faultfinders can argued that our interpretation may be overly gloomy. Maybe the shift toward lower values of average trophic level simply reflects an increase in number of plankton feeders in many places, perhaps species that are now thriving from the coastal plankton blooms that take place when fertilizer leached from farmers' fields is carried to the ocean by rivers and streams. We have examined this possibility by comparing the decreases in average trophic level with the increases in amount of seafood being caught. In some places and times (for example, the Mediterranean Sea before 1986, or the South Pacific during the 1960s), declining trophic levels are matched by appropriately large increases in the amount hauled in each year. But for the most part, diminishing trophic levels do not coincide with burgeoning populations of plankton feeders. The overall picture is indeed quite bleak.
» Post Comment