Communities are assemblages of species in varying proportions doing different things, and have properties that are the amalgam of the properties of individual populations and interactions among populations. Indeed, it is the interactions that make communities more than the sum of their parts. Fisheries scientists who are rooted in population ecology tend to focus on the dynamics of individual fish populations. While much can be learned from this approach, the structure of communities and thus the dynamics of fish populations within communities cannot be understood by focusing only on single species. It may even be that studies of pairwise interactions (as we do here) between exploited fishes and their prey, competitors or predators may be inadequate. For instance, we might expect the removal of a predator to result in an increase of its prey, and often this occurs (Sih et al. 1985
). However, elimination of predators may lead to decreases in prey biomass (perhaps by increasing the biomass of a competitor). Such indirect effects appear to be common in nature—about one-third of the experimental studies of predation reviewed by Sih et al. (1985)
showed some result that could not be predicted by studying only pairwise interactions. Fisheries typically begin by targeting higher-order predators (Pauly et al. 1998
), and thus human exploitation of fishes could be considered a massive predator removal experiment. In common with smaller-scale ecological manipulations, the prosecution of fisheries is likely to produce changes to the community and to the target species that could not be predicted by investigating the ecology of target populations alone.
The potential unanticipated effects of species removals in concert with changes in the trophic level, species, or even phyla targeted by fisheries requires a shift to community-level thinking if we are to understand or predict effects of fishing. The almost 90% decline of predatory fishes in the Northwest Atlantic over the past century (Christensen et al. 2003
) with the concomitant shifts in target species provide a useful case study (Steneck et al. 2002
). Cod and other large groundfish were abundant and apparently stable, components of coastal zones throughout the Northwest Atlantic for thousands of years (Steneck 1997
). Predation by these fishes upon the dominant subtidal grazer, the green sea urchin (Strongylocentrotus droebachiensis
), reduced herbivory sufficiently to allow abundant kelp forests in coastal zones throughout the region (Steneck et al. 2002
). Because coastal kelp forests appear to be important nursery habitats for cod and other fishes (Tupper & Boutilier 1995
; Levin et al. 1997
), the presence of large numbers of groundfish may have increased the survival of juveniles by sustaining nursery habitats. Increased fishing effort and efficiency in the mid-1900s led to rapid declines of cod abundance and size (Steneck 1997
; Jackson et al. 2001
), the expansion of sea urchin populations, and the regional demise of kelp beds (Steneck et al. 2002
In the late 1980s, an urchin fishery developed in the Gulf of Maine; within a decade urchin populations crashed, and kelp forests recovered (Vavrinec 2003
). Kelp is also an important juvenile habitat for crabs (Cancer
spp.), and with the loss of fish predators, crabs have settled in large numbers to Gulf of Maine kelp beds (Steneck et al. 2002
). These crabs are voracious predators of juvenile and adult urchins, and thus crabs now serve as an apex predator with functionally the same impact that cod and other fish predators had had in the past.
Even with drastic reductions in fishing pressure, cod populations have not recovered (Hutchings 2000
). While Allee effects may be playing a role in the slow recovery of cod (Walters & Kitchell 2001
), the loss of important nursery grounds, once created by the presence of cod themselves, may also be playing a role in the slow recovery of cod. In that case, the increase in the number of crabs and their indirect positive effects on kelp forests should have resulted in an increase in the quality of cod nursery habitat. However, recent increased effort in the crab fishery appears to be leading to a decline in crab numbers and kelp (Vavrinec 2003
). Thus, to the extent that coastal macroalgal habitat is important to cod, the fate of the cod fishery in the Gulf of Maine may lie not only in the hands of cod fishermen, but also urchin and crab fishermen.
Fishing activities may disturb entire communities, and subsequent recovery of communities is essentially an example of secondary succession. By focusing on populations, fishery scientists may be making implicit assumptions about the mechanisms underlying succession in marine communities. For example, after overexploitation, the foundation of models predicting the time to rebuild the stock to acceptable levels may be simple population models (i.e. Ricker or Beverton–Holt stock recruitment curves). Consequently, we are assuming that (i) the succession of the community and thus the rebuilding of its exploited constituents occurs in a predictable manner without interference from the remaining members of the disturbed community (that is, there is no hysteresis) and (ii) the inhibition model of succession (Connell & Slatyer 1977
) is not applicable to fished communities. However, if inhibitory interactions in fish communities are important, they could slow or prevent recovery of overfished stocks. This situation could exist in the southern oceans, where the recovery of smaller (e.g. minke) whales may have inhibited the recovery of the great whales.
Consider a fish assemblage in which all species are good colonists and essentially equal competitors. If several species are able to invade gaps and can successfully hold the gaps against potential competitors, classic succession after fishing is not expected. Instead, the community will reflect chance colonization events by larval fishes (cf. Sale 1977
). By reducing the biomass of target species, fishing could alter the composition of the larval pool, thus promoting a shift in the community species not targeted by fisheries (Kaiser & Jennings 2001
). If prior residency implies great advantage in competitive interactions, individuals occupying habitat, no matter which species, are competitively dominant (e.g. Shulman et al. 1983
); even after fishing mortality is reduced and population models predict a rapid increase in biomass, recovery cannot occur.
The composition of a number of exploited fish communities has recently shifted, with shorter-lived species becoming more prevalent. Along the Pacific coast of the US, for example, short-lived rockfishes (such as greenstriped and splitnose) have greatly increased in abundance, while longer-lived species (such as canary and bocaccio) have declined (). Because smaller species of rockfish may be able to consume or outcompete recruiting juveniles of larger species, and since many rockfishes overlap greatly in their patterns of resource use, it is possible that together environmental change (something we do not control) and fishing (something we do control) have created a perturbation that has shifted the rockfish assemblage to an alternative stable state. If such an alternative stable state has been reached, even severe reductions in fishing pressure may not result in recovery of overfished larger species. Similarly, in the Northeastern Atlantic, Dulvy et al. (2000)
showed a dramatic shift in the assemblage of skates harvested over a 40 year period. Large-bodied species with long generation times have declined, whereas smaller species have increased in abundance. Dulvy et al. (2000)
argued that larger skates historically outcompeted smaller species for food, and that overfishing of larger species released the small skates from competition. Fogarty & Murawski (1998)
also suggested that competitive release resulted in a phase-shift from teleost-dominated to elasmobranch-dominated populations in the Northwestern Atlantic.
Figure 1 Indices of larval abundance of rockfish (Sebastes spp.) along the US Pacific Coast for the period 1977–2001. Short-lived rockfishes such as greenstriped (filled triangles) and splitnose have greatly increased in abundance, while longer-lived species (more ...)
The magnitude of disturbance that can be absorbed by a community before it shifts from one state to another (i.e. its resilience) may be affected the degree to which disturbed sites are linked to undisturbed areas (Duncan & Chapman 1999
). Organisms that move among communities provide ecological memory (sensu Scheffer et al. 2001
) external to the disturbed system that may rapidly restore the lost function resulting from disturbance, thus increasing the resilience of the system (Lundberg & Moberg 2003
). Although the importance of material and energetic flows between distinct communities has received much recent attention by ecologists (e.g. Polis et al. 1997
), the significance of such inputs is often overlooked in population-based fisheries assessments.
Pacific salmon fisheries in the Columbia River Basin in the northwestern US illustrate the consequences of altering links between systems. Salmon runs in the Columbia River once supported a thriving fishery, but overfishing and construction of dams led to the decline of many salmon populations; many are now listed under the US Endangered Species Act (Levin & Schiewe 2001
). Because more than 95% of the body mass of salmon is accumulated while fish are in the sea (Pearcy 1992
), the return of adult salmon results in a transfer of nutrients from marine to freshwater habitats. These marine-derived nutrients are now recognized to play an important role in the ecology of riparian habitats in the Pacific Northwest (Gresh et al. 2000
); consequently, the recent dramatic reduction in salmon abundance has resulted in a nutrient deficit in spawning and rearing streams (Kline et al. 1990
; Bilby et al. 1998
; Wipfli et al. 1999
). Thus, in this system, fishing not only lowers spawning biomass, it also lowers carrying capacity for juveniles (Achord et al. 2003
). As a result of community-level changes in the rearing habitat, juvenile salmon now experience density-dependent mortality even though populations are more than 90% lower than historical levels (Achord et al. 2003
). Recovery of these populations, therefore, does not depend solely on reduction in adult mortality, but also depends critically on changes throughout stream communities that ultimately increase carrying capacity to its previous levels.