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
On the Level
Historically, the notion of assigning trophic levels in ecology passed through two stages. During the first phase, initiated by Charles Elton of the University of Oxford and Raymond Lindeman of Yale University, trophic levels were seen as pre-defined categories into which organisms could be pigeonholed—perhaps shoehorned is the better metaphor, given that complex feeding habits could not readily be accounted for. Fish that normally consume zooplankton were assigned a trophic level of three, because zooplankton (with a trophic level of two) normally consume phytoplankton (trophic level of one). This procedure ignores the fact that some zooplankton are carnivorous and thus should be assigned a trophic level somewhat higher than two, which implies that their predators should in turn be placed at trophic levels above three.
Such complications forced this overly simplistic scheme to give way in 1975, when the late Frank H. Rigler, a zoologist at the University of Toronto, published an influential critique of the concept. It was then that William E. Odum (of the University of Virginia's Department of Environmental Sciences) and Eric J. Heald (of the University of Miami's Rosenstiel School of Marine and Atmospheric Sciences) pointed out that more precise estimates of trophic level could be obtained from actual observations of diet. Their advance, as followed in current practice, treats trophic level as an empirically determinable property of a species—like average size or metabolic rate.
So how exactly do marine biologists estimate trophic level for a given species? One way is first to determine the average trophic level of the various things that the organism eats and then add one to the value obtained. Following this formula, marine biologists studying Pacific bluefin tuna (Thunnus thynnus orientalis), a carnivore that consumes smaller fish and large invertebrates such as squid, typically assign the adults trophic levels ranging from 4.2 to 4.5, despite wide variation from place to place in the exact composition of their diets. Still, there can be substantial shifts in the value of trophic level that apply to different points in development of this and other species, because dramatic changes in size and behavior take place during the life of most fishes.
Another complication is that it is often quite hard to determine what all goes into the stomach of a fish. This difficulty can be overcome to a large degree by doing what we did: considering the organism as part of an integrated ecosystem and modeling the web of connections between the animals and plants in enough detail to gain a reasonable understanding of what each creature is consuming, at least on average.
A second method for estimating trophic level relies on a curious phenomenon of nitrogen biochemistry. Nitrogen, like the other common building blocks of life, comes in distinct forms. The most common isotope, nitrogen-14, has seven protons and seven neutrons in the nucleus. But a second stable isotope with eight neutrons, nitrogen-15, is also found in nature. Interestingly, biochemical reactions discriminate to a small degree between these two different kinds of atoms. So the nitrogen incorporated into the tissues of a sea creature does not have exactly the same isotopic ratio as is present in its food. Rather, the flesh of the animal tends to collect the heavier isotope. (The same enrichment takes place among terrestrial organisms. In this respect, you are not what you eat.)
Measurements of the nitrogen from various aquatic creatures have shown that the isotopic ratio shifts by a roughly constant amount from one trophic level to the next—no matter what species are involved. So the analysis of nitrogen isotopes from an organism provides a convenient way to ascertain its position in the food web without having to know exactly what it consumes. All that is necessary is to compare the measured isotopic ratio with that obtained from whatever organism sits at the bottom of the local food chain. The difference in these two values (divided by the constant difference between adjacent levels) immediately shows how far above base level the creature should be placed.
Recently, we compared values of trophic level determined in this way to those we obtained from the more traditional approach, using Prince William Sound in Alaska as a sample ecosystem. Happily, the two sets compared favorably (Figure 4). This result adds to our certainty that the 220 estimates of trophic level that we used in our analysis of global fisheries, for which we employed just one method (modeling the food web), were largely correct.
We had, perhaps, less confidence in the accuracy of the FAO statistics for the catch from various parts of the world—a perennial problem for scientists trying to understand the functioning of global fisheries. Some of the concern arises from the breadth of the brush that most countries use in reporting their catches to the FAO. Although the surveys sometimes track individual species (cod, for example), the statistics often lump multiple species together and report values only at the level of genera (such as for hake), families (herring and sardines) or even higher taxonomic groupings (all bony fishes).
Still, with no similarly comprehensive alternatives, we had no choice but to employ the FAO statistics, despite this and other shortcomings. The results were nevertheless clear and reasonably consistent (Figure 5). In most places we looked, the average trophic level of the catches has declined over the years. In the northwest Atlantic, for example, trophic level plunged from a peak of nearly 3.7 in 1965 to 2.8 in 1997 (the last year for which statistics are available), while in the northeast Atlantic it fell from about 3.6 to 3.4. In the Mediterranean, the figure also diminished, but it did so more gradually (slipping from about 3.1 to 3.0 during the same period).
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