I, Predator
Are Modern Fisheries Reshaping Ocean Ecosystems?

The apex predators, a class of marine heavyweights that includes pinnipeds (seals & sea lions), cetaceans (whales & dolphins), seabirds and sharks, have long enjoyed a unique position atop the marine food web. But after an uninterrupted reign lasting millions of years, apex predators in the 21st century are facing a new contender: humans.

It may be easy to dismiss human fisheries as “just another top predator”, especially given a dearth of scientific data assessing the more subtle impacts of fisheries. The fact remains, however, that the ecological consequences of fisheries are fundamentally different from other predatory impacts, and a recent review by Drs. Andrew Trites, Villy Christensen and Daniel Pauly of the University of British Columbia explores these differences.

Fishing Down the Food Web
Fishing can directly impact ecosystems by removing large quantities of targeted and by-caught species and by physically damaging the ocean floor through activities such as bottom trawling. While these effects are obvious and relatively easy to document, the indirect effects of fishing pressure are far more nebulous and complex.

Historically, fishing started at the top of most food chains by removing the larger, valuable and more easily caught species ---- and then moved down to the next-biggest species as those above were depleted and were no longer easy or economical to catch. The downward shift towards taking species from lower trophic levels is termed fishing down the food web.

Fig. 1.  Schematic of food web competition, illustrating how marine mammals and fisheries may not directly compete (because they consume different species), but could indirectly compete through the primary production required to sustain each of their respective prey populations.

Commercially valuable species in a number of ecosystems are reported to be smaller in size than they once were (at the same age) and appear to be reproducing earlier in life. This is a source of concern to biologists because smaller fish typically produce fewer eggs and have a higher mortality rate than larger fish of the same species. These long-term reductions in size and fecundity (reproductive capacity) due to fishing pressure could translate to evolutionary changes in their genome that may be difficult to reverse.

Further, large-scale ecosystem models (which simulate interactions between species) show that even fisheries targeting prey that is not part of a particular apex predator’s diet may still affect that predator. Such food-web competition occurs when the primary producers at the base of the food web cannot support both the fishery and the apex predators. Thus as catches increase, the primary production available to marine mammals appears to decline; this can translate to a reduction in prey that limits the predator’s survival and reproductive success. Human fisheries are not subject to the same biological controls as marine mammals, which limit what and how much they take. Instead, fisheries are regulated solely by economic incentives which, ironically, often increase (rather than decrease) as a species becomes scarce.

The Human Advantage
While most predators are limited by the size of prey they can consume, humans can feed on any size of organism at any trophic level. Indeed, fisheries consume a size and volume of prey that is unmatched in the natural world, spawning a legacy of exploitation ranging from gigantic right whales to once-abundant Atlantic cod, to cite two infamous examples.

The review points out that humans lack the long history of co-evolution and natural selection with their prey that other predators experienced. Apex predators have developed an arsenal of special adaptations including keen vision and hearing, specialized teeth, and physiological prowess such as extended diving or breath-holding ability. In response, fish and other prey types have come to rely on camouflage, schooling, prolific breeding and diurnal movement through the water column to improve their chances of survival.

Fig. 2.  Global reported catch with (dotted line) and without (solid line) the Peruvian anchoveta.  Total world catches have fluctuated around 82 million tons since the late 1980s due to high catches of anchoveta that compensated for the global decline in landings of all other species combined. (Adapted from Watson and Pauly 2001).

The authors emphasize that such interactions between predator and prey have shaped each other’s life histories and have undoubtedly helped to maintain the integrity and stability of marine ecosystems, whereas “many of the features that have allowed prey to flourish in the face of apex predators now make fish more vulnerable to being caught by fisheries (e.g. schooling behaviour, diurnal movement towards surface light, etc.).”

By operating outside of the natural rules that govern populations and their ecosystems, fisheries may well be the ultimate apex predator. Trites, Christensen and Pauly make no bones about it: “Despite being a relative latecomer to the evolutionary predator-prey game, fisheries play with different sets of rules and different ultimate consequences… [they] represent an abrupt, knife-edged selective force that has potentially destabilizing consequences.”

Will recent improvements in fisheries data and ecosystem modelling deepen our understanding of the indirect impacts of fisheries? Can they help to slow the dangerous trend of fishing down the food web? The sobering alternative—empty nets and jellyfish sandwiches—could be closer than we think.

2 October 2006

Publication:
Effects of fisheries on ecosystems: just another top predator? Trites, A.W., V. Christensen and D. Pauly. 2006. In I.L. Boyd, K. Camphuysen and S. Wanless (eds), Top predators in marine ecosystems: their role in monitoring and management. Cambridge University Press, Cambridge. pp. 11-27. abstract Apex predators such as pinnipeds, cetaceans, seabirds and sharks, are constrained by the sizes of prey they can consume and thus typically feed within a narrow range of trophic levels. Having co-evolved with their prey, they have influenced the behaviors, physiologies, morphologies and life history strategies of the species they target. In contrast, humans can consume prey of any size from all trophic levels using methods that can rapidly deplete populations. On an ecological time scale, fisheries, like apex predators, can directly affect the abundance of other species by consuming, or out-competing them; or they can indirectly affect the abundance of non-targeted species by removing other predators. However, there is growing evidence that the effects of fisheries go well beyond those imposed by apex predators. Theory and recent observations confirm that the continued development and expansion of fisheries over the past half century has led to a decrease in the! size and life spans of targeted species, with reproduction of fish occurring at earlier ages and at smaller sizes. Also, high levels of fishing have altered the makeup of many ecosystems, depressing the average trophic level of heavily fished ecosystems and speeding up the rate of nutrient turnover within them. An inevitable consequence of fishing down the food web is increased ecosystem instability, unsustainable fisheries and an inability for the ecosystem to support healthy abundant populations of apex predators.