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D. Pauly and J. Dalsgaard, Fisheries Centre, 2204 Main
Mall, University of British Columbia, Vancouver, British Columbia, Canada
V. Christensen, R. Froese, F. Torres Jr., International Center for Living Aquatic Resources Management, M.C. Post Office Box 2631, 0718 Makati, Philippines.
* To whom correspondence should be addressed. E-mail: email@example.com
The mean trophic level of the species groups reported in Food and Agricultural Organization global fisheries statistics declined from 1950 to 1994. This reflects a gradual transition in landings from long-lived, high trophic level, piscivorous bottom fish toward short-lived, low trophic level invertebrates and planktivorous pelagic fish. This effect, also found to be occurring in inland fisheries, is most pronounced in the Northern Hemisphere. Fishing down food webs (that is, at lower trophic levels) leads at first to increasing catches, then to a phase transition associated with stagnating or declining catches. These results indicate that present exploitation patterns are unsustainable.Exploitation of the ocean for fish and ma-rine invertebrates, both wholesome and valuable products, ought to be a prosperous sector, given that capture fisheries - in con-trast to agriculture and aquaculture - reap harvests that did not need to be sown. Yet marine fisheries are in a global crisis, mainly due to open access policies and subsidy-driven overcapitalization (1). It may be argued, however, that the global crisis is mainly one of economics or of governance, whereas the global resource base itself fluctuates naturally. Contradicting this more optimistic view, we show here that landings from global fisheries have shifted in the last 45 years from large piscivorous fishes toward smaller invertebrates and planktivorous fishes, especially in the Northern Hemi-sphere.
|For all marine areas, the trend over the past 45 years has been a decline
in the mean trophic level of the fisheries landings, from slightly more
than 3.3 in the early 1950s to less than 3.1 in 1994 (Fig. 1A). A dip in
the 1960s and early 1970s occurred because of extremely large catches [>12E6
metric tons (t) per year*] of Peruvian anchoveta with a low trophic level
(12) of 2.2 (+/- 0.42).
Since the collapse of the Peruvian anchoveta fishery in 1972–1973, the global trend in the trophic level of marine fisheries landings has been one of steady decline. Fisheries in inland waters exhibit, on the global level, a similar trend as for the marine areas (Fig. 1B): A clear decline in average trophic level is apparent from the early 1970s, in parallel to, and about 0.3 units below, those of marine catches. The previous plateau, from 1950 to 1975, is due to insufficiently detailed fishery statistics for the earlier decades (10).
Fig. 1. Global trends of mean trophic level of fisheries landings, 1950 to 1994. (A) Marine areas; (B) inland areas.
|In northern temperate areas where the fisheries are most developed, the mean trophic level of the landings has declined steadily over the last two decades. In the North Pacific (FAO areas 61 and 67; Fig. 2A), trophic levels peaked in the early 1970s and have since then decreased rapidly in spite of the recent increase in landings of Alaska pollock, Theragra chalcogramma, which has a relatively high trophic level of 3.8 (+/- 0.24). In the Northwest Atlantic (FAO areas 21 and 31; Fig. 2B), the fisheries were initially dominated by planktivorous menhaden, Brevoortia spp., and other small pelagics at low trophic levels. As their landings decreased, the average trophic level of the fishery initially increased, then in the 1970s it reversed to a steep decline.||
Fig. 2. Trends of mean trophic level of fisheries landings in northern temperate areas, 1950 to 1994. (A) North Pacific (FAO areas 61 and 67); (B) Northwest and Western Central Atlantic (FAO areas 21 and 31); (C) Northeast Atlantic (FAO area 27); and (D) Mediterranean (FAO area 37).
|Similar declines are apparent throughout the time series for the Northeast
Atlantic (FAO area 27; Fig. 2C) and the Mediterranean (FAO area 37; Fig.
2C), although the latter system operates at altogether lower trophic levels.
The Central Eastern Pacific (FAO area 77; Fig. 3A), Southern and Central Eastern Atlantic (FAO areas 41, 47, and 34; Fig. 3B), and the Indo-Pacific (FAO areas 51, 57, and 71; Fig. 3C) show no clear trends over time. In the southern Atlantic this is probably due to the development of new fisheries, for example, on the Patagonian shelf, which tends to mask declines of trophic levels in more developed fisheries.
In the Indo-Pacific area, the apparent stability is certainly due to inadequacies of the statistics, because numerous accounts exist that document species shifts similar to those that occurred in northern tem-perate areas (13).
Fig. 3. Trends of mean trophic levels of fisheries landings in the intertropical belt and adjacent waters. (A) Central Eastern Pacific (FAO area 77); (B) Southwest, Central Eastern, and Southeast Atlantic (FAO areas 41, 34, and 47); and (C) Indo (west)-Pacific (FAO areas 51, 57, and 71).
|The South Pacific areas (FAO areas 81 and 87; Fig. 4A) are interesting in that they display wide-amplitude fluctuations of trophic levels, reflecting the growth in the mid-1950s of a huge industrial fishery for Peruvian anchoveta. Subsequent to the anchoveta fishery collapse, an offshore fishery developed for horse mackerel, Trachurus murphyi, which has a higher trophic level (3.3 +/- 0.21) and whose range extends west toward New Zealand (14).||
Fig. 4. High-amplitude changes of mean trophic levels in fisheries landings. (A) South Pacific (FAO areas
81 and 87); (B) Antarctica (FAO areas 48, 58, and 88).
Antarctica (FAO areas 48, 58, and 88; Fig. 4B) also exhibits high-amplitude
variation of mean trophic levels, from a high of 3.4, due to a fishery
that quickly depleted local accumulations of bony fishes, to a low of 2.3,
due to Euphausia superba (trophic level 2.2 +/- 0.40), a large krill
species that dominated the more recent catches.
The gross features of the plots in Figs. 2 through 4, while consistent with previous knowledge of the dynamics of major stocks, may provide new insights on the effect of fisheries on ecosystems. Further interpretation of the observed trends is facilitated by plotting mean trophic levels against catches.
For example, the four systems in Fig. 5 illustrate patterns different from the monotonous increase of catch that may be expected when fishing down food webs (15). Each of the four systems in Fig. 5 has a signature marked by abrupt phase shifts. For three of the examples, the highest landings are not associated with the lowest trophic levels, as the fishing-down-the-food-web theory would predict. Instead, the time series tend to bend backward. The exception (where landings continue to increase as trophic levels decline) is the Southern Pacific (Fig. 5C), where the westward expansion of horse mackerel fisheries is still the dominant feature, thus masking more local effects.
The backward-bending feature of the plots of trophic levels versus landings, which also occurs in areas other than those in Fig. 5, may be due to a combination of the following:
(i) artifacts due to the data, methods, and assumptions used;Regarding item (i), the quality of the official landing statistics we used may be seen as a major impediment for analyses of the sort presented here. We know that considerable under- and misre-porting occur (16). However, for our analysis, the overall accuracy of the landings is not of major importance, if the trends are unbiased. Anatomical and functional considerations support our assumption that the trophic levels of fish are conservative attributes and that they cannot change much over time, even when ecosystem structure changes (17). Moreover, the increase of young fish as a proportion of landings in a given species that result from increasing fishing pressure would strengthen the reported trends, because the young of piscivorous species tend to be zooplanktivorous (18) and thus have lower trophic levels than the adults.
(ii) large and increasing catches that are not reported to FAO;
(iii) massive discarding of bycatches (16) consisting predominantly of fish with low trophic levels;
(iv) reduced catchability as a result of a decreasing average size of exploit-able organisms; and
(v) fisheries-induced changes in the food webs from which the landings were extracted.
If we assume that fisheries tend to switch from species with high trophic
levels to species with low trophic levels in response to changes of their
relative abundances, then the backward-bending curves in Fig. 5 may be
also due to changes in ecosystem structure, that is, item (v). In the North
Sea, Norway pout, Trisopterus esmarkii, serves as a food source
for most of the important fish species used for human consumption, such
as cod or saithe. Norway pout is also the most important predator on euphausiids
(krill) in the North Sea (3).
Fig. 5. Plots of mean trophic levels in fishery landings versus the landings (in millions of metric tons) in
four marine regions, illustrating typical backward-bending signatures (note variable ordinate and abcissa
scales). (A) Northwest Atlantic (FAO area 21); (B) Northeast Atlantic (FAO area 27); (C) Southeast Pacific
(FAO area 87); (D) Mediterranean (FAO area 37).
We must therefore expect that a directed fishery on this small gadoid (landings in the Northeast Atlantic are about 3E5 t/year) will have a positive effect on the euphausiids, which in turn prey on copepods, a much more important food source for commercial fish species than euphausiids. Hence, fishing for Norway pout may have a cascading effect, leading to a build-up of nonutilized euphausiids. Triangles such as the one involving Norway pout, euphausiids, and copepods, and which may have a major effect on ecosystem stability, are increasingly being integrated in ecological theory (21), especially in fisheries biology (22).
Globally, trophic levels of fisheries landings appear to have declined in recent decades at a rate of about 0.1 per decade, without the landings themselves increasing substantially. It is likely that continuation of present trends will lead to widespread fisheries collapses and to more backward-bending curves such as in Fig. 5, whether or not they are due to a relaxation of topdown control (23). Therefore, we consider estimations of global potentials based on extrapolation of present trends or explicitly incorporating fishing-down-the-food-web strategies to be highly questionable. Also, we suggest that in the next decades fisheries management will have to emphasize the rebuilding of fish populations embedded within functional food webs, within large “no-take” marine protected areas (24).
(*) we've used the notation E for powers of ten: 1E6 = 1 million
22 August 1997; accepted 10 December 1997
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