How Pervasive Is 'Fishing Down Marine Food Webs'?

In their report (1), and in an earlier paper (2), D. Pauly et al. draw global conclusions about the effects of fishing on world fish stocks with the use of research data fitted to Ecopath models at different sites through the world’s oceans, integrated with data on global fishery landings collected by the Food and Agricultural Organization of the United Nations (FAO). Although Pauly et al. are to be congratulated for giving this important issue high profile, they greatly oversimplify the situation with their hypothesis and may have misinterpreted the FAO statistics. We do not disagree that a general decline in mean trophic level of marine landings is likely to have occurred in many regions, but we are not convinced that the explanation is solely a result of “fishing down the food web” or that the analysis of the FAO data, as undertaken by Pauly et al., substantiates such a thesis. Four considerations significantly qualify the evidence of a “fishing down the food web” phenomenon. (i) Taxonomic resolution. Assigning a trophic level arguably requires knowing at least the related genus or even the species and its age, since trophic level may change by as much as three points from birth to maturity for some top predators. Although the FAO fishery landings data (3) used in their analysis integrate the best estimates by countries, regional fishery organizations, and FAO of the species composition of annual production, it is to be regretted that over 30% of all marine landings cannot be identified to the species level, and about 20% cannot even be assigned to the level of Family (this rises to about 60% for inland capture fishery production). As a consequence, the small drop in mean trophic level they report, from 3.3 in the early 1950s to 3.1 in 1994, appears difficult to substantiate statistically, and its sensitivity to the assumptions necessarily made in allocating coarse data to trophic levels has not been described. (ii) Landing data as ecosystem indicators. The analysis assumes that changes in mean trophic level in the landings reflect changes in the ecosystem, with the use of annual quantities of landings (excluding discarded catch) as abundance indicators. However, the composition of historical landings has been affected by a number of phenomena that are not simply related to increased fishing pressure (for example, natural oscillations in abundance, changes in fishing technology) and that are likely to have seriously influenced mean trophic levels in the landings. Overfishing has seriously affected top predators, and this has already been raised in FAO reports (4). Peaks in predatory demersal fish production and subsequent declines have been registered that differ in timing regionally and among different habitat types (5, 6, 7). Comparing regional landings of demersal fish (generally, long-lived species high in the food chain) with short-lived species such as squid (8) also reveals trophodynamic effects quite clearly. There seem to be few other hypotheses to account for declines in landings of top predators than overfishing. The situation is not the same for species lower in the food chain, where natural medium-term fluctuations of the small pelagic species abundance are likely to quantitatively mask effects that result from declining top predators on the mean trophic level. In addition, long-term changes in strategies of fishing such species add to the difficulty of documenting global trends through a “mean trophic level” for the ecosystem as a whole. This task requires detailed knowledge of local fisheries in order to extrapolate safely from “trophic level of landings” to “trophic level of ecosystems. ” Outside the north boreal area, and except for a few very large oscillating stocks (for example, Peruvian anchoveta), small pelagics were rarely subjected to major exploitation in the 1960s and 1970s because of lower market prices and because technologies for handling and processing the catch were not yet fully developed. During the last two decades, these species have seen a significant increase in their exploitation resulting from the spread of new technologies. One could argue that this increase is indeed due to increase abundance of pelagics resulting from depletion of their predators, but this remains conjecture. The fact is that the interest of industries for small pelagic fish increased, leading to higher landings of these species and shifts in the composition of global landings. A shift in global fishing strategies could be confused with a “fishing down the food web phenomenon. ” (iii) Aquaculture development. FAO landing statistics have traditionally included both capture and aquaculture production, but work is under way to disaggregate them into the two separate components. This has so far been completed for years since 1984. As a rough check, mean trophic levels for species groups as reported in (2) were applied to marine landings of the corresponding species groups to calculate the overall mean trophic level of total production (capture fishery plus aquaculture production) since 1950 and capture fisheries and aquaculture since 1984. In contrast to the decline in mean trophic level reported by Pauly et al., for marine waters the mean trophic level for capture fishery landings has remained stable since 1984 (Fig. 1) at a level similar to that of total production in the early decades when marine aquaculture was insignificant. The decline in mean trophic level in the total production series is entirely a result of the increasing contribution of aquaculture to total production (from 8% in 1984 to 17% in 1996) and the fact that species cultured in the sea (mainly shellfish) have an average trophic level about half that of capture fishery landings (Fig. 1). That the results of Pauly et al. (1) for all marine waters