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THE ECOLOGY OF FISHING DOWN MARINE WEBS

BY DANIEL PAULY

The phenomenon wherein fisheries increasingly target smaller fish lower down in the food web, called "fishing down marine food webs" (FD), and first demonstrated in 1998, is now well documented from a variety of countries and ecosystem types. This is one reason why the Convention on Biological Diversity selected the mean trophic level of fisheries catch, renamed "Marine Trophic Index," as one of eight indicators for "immediate testing" by its over 180 member countries.

FD was an easy transition for the fishing industry to make: moving on from one depleted stock to another traditionally has been its standard operating procedure. And FD does not have a built-in economic break: small fishes and invertebrates, which have low trophic levels, have recently experienced steep increases in their market value, so much so that they may be seen as subsidizing FD.

One aspect of FD that still needs a basic framework, however, is its ecology, or, more precisely, its ecological impact on marine ecosystems. Essentially, FD is a succession, even if it seems to reverse the usual sequence: it consists of a gradual loss of large organisms, species diversity, and structural diversity, and a gradual replacement of recently evolved, derived groups (marine mammals, bony fishes) by more primitive groups (invertebrates, notably jellyfishes, and bacteria). This is best seen when distinguishing three phases of the FD process, and by characterizing, for each phase, (1) the main features of the fishes and other nektonic organism and (2) pelagic-benthic coupling and its effect on processes in the water column.

Three Phases

The first phase, "pristine," prevailed before humans strongly impacted ocean ecosystems. A few parts of the oceans, notably outlying areas of the South Pacific, still may be pristine, But for most of the world, pristine abundances must be recovered-reconstructed-from historical accounts and anecdotes, or inferred from archeological data.

A pristine state invariably is characterized by numerous marine mammals and large fish as top predators, the latter with biomasses often exceeding their present abundance tenfold to hundredfold. Elevated biomass of top predators implies large biomass of small prey fishes and invertebrates, though not necessarily of those opportunistic groups (shrimps, squids) that now support increasingly valuable fisheries.

In the pristine environment, benthic life is dominated by an abundant structure-forming and sessile fauna, composed of filter feeders and deposit feeders, which keep phytoplankton biomass down and prevent resuspension of sediments. As a result, the water column tends to be free of suspended particles and of nutrients leaching from them, or oligotrophic.

The second phase, "exploited," is the phase we are in currently. It is best characterized by declines, notably declining biomasses of large fishes, declining sizes and diversity of fishes in fisheries catches, declining trophic levels of the same (and hence the FD phenomenon), and declining benthos.

Initially, these declines are compensated for by cascades effects, manifest in the emergence of new fisheries for squids and other invertebrates, but these eventually decline as well.

Benthic life is modified: biogenic structures, built over centuries by filter and detritus feeders, are increasingly destroyed by bottom trawling, and replaced by small errant benthic animals and the benthic (polyp) stages of jellyfishes.

This leads to an increased eutrophication of the water column, owing to the increasing scarcity of the animals and structures that were cropping the phytoplankton and consuming the marine snow (detritus), which is now resuspended by storms and by trawling itself.

The third phase, "fully degraded," will follow on the continuation of present trends-although in some places, e.g., estuaries such as the Chesapeake Bay, many of the features associated with this third stage have already developed. In the Chesapeake Bay, fishing not only has eliminated virtually all animals above the size of striped bass, the current top predator, but more importantly has eliminated the benthic filter feeders. Indeed, the oysters that until 150 years ago formed giant reefs are reported as having been capable of filtering the water of Chesapeake Bay in three days. Their absence (again, a result of fishing) is the ultimate reason why pollution from effluents now can have such strong effects, and why harmful algae bloom. This also applies to other water bodies, estuarine or not, which are rendered less resilient by fishing, and easier for invasive species to overwhelm.

The biological endpoint of ecosystem degradation is the "dead zone," a zone free of oxygen and of multicellular life as a result of excess nutrients in the water column and of bacteria, rather than benthic animals, processing the resulting abundance of marine snow and other detritus. There are growing numbers of these dead zones throughout the world, from the northern Gulf of Mexico and the northern Adriatic Sea to the Bohai Sea in China, and there can be no argument that the underlying ecosystems are fully degraded.

Conclusions

These three phases of fishing down, pristine, exploited, and fully degraded, are schematic; they could be further subdivided, and defined more rigorously. Still, even in their present, preliminary form, they provide a coherent framework for many of the changes observed in ocean ecosystems. Our task is to further develop this framework.

Daniel Pauly received SCB's 2005 Edward T. LaRoe III Memorial Award for his leadership, innovation, and effectiveness in conveying the results of his research and their implications for management of fish stocks to fisheries managers and policy makers worldwide.

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