Ecosystem services generated by fish populations

Abstract

In this paper, we review the role of fish populations in generating ecosystem services based on documented ecological functions and human demands of fish. The ongoing overexploitation of global fish resources concerns our societies, not only in terms of decreasing fish populations important for consumption and recreational activities. Rather, a number of ecosystem services generated by fish populations are also at risk, with consequences for biodiversity, ecosystem functioning, and ultimately human welfare. Examples are provided from marine and freshwater ecosystems, in various parts of the world, and include all life-stages of fish. Ecosystem services are here defined as fundamental services for maintaining ecosystem functioning and resilience, or demand-derived services based on human values. To secure the generation of ecosystem services from fish populations, management approaches need to address the fact that fish are embedded in ecosystems and that substitutions for declining populations and habitat losses, such as fish stocking and nature reserves, rarely replace losses of all services.

Introduction

Fish constitute one of the major protein sources for humans around the world. There are to date some 25 000 different known fish species of which 15 000 are marine and nearly 10 000 are freshwater (Nelson, 1994). Global capture fisheries harvested 101 million tonnes of fish including 27 million tonnes of bycatch in 1995, and 11 million tonnes were produced in aquaculture the same year (FAO, 1997). Despite the abundance and variation of fish, most western fisheries focus on a few target species. Approximately 75% of the world’s marine fish landings consist of 200 (∼1%) known existing marine fish species (FAO, 1997). Further, recreational fishing in lakes and along coasts is a major tourism activity (FAO, 1996; Postel and Carpenter, 1997). Estimates of the value of fish populations for human societies have predominantly focused on these goods. The fact that such values are derived from ecosystems with complex interactions, and that both economically and non-economically valuable fish populations play active roles in the maintenance of these ecosystems and in the provision of a range of ecosystem services, is seldom taken into account. Ecosystem services have been defined as “conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfill human life” (Daily, 1997, p. 3). This includes the life-support functions (Odum, 1989) of ecosystems and nature’s capacity to provide aesthetic and cultural quality to human life (Daily, 1997).

In 1995, almost 70% of the world’s major marine fish resources were fully- to overharvested, or depleted (World Resources Institute, 1996). Capture fisheries not only reduce the abundance of targeted stocks with cascading responses in the food web and with consequences in other ecological and fishery dependent systems, but also impact an array of other species, including mammals, as bycatch (Dayton et al., 1995; Steneck, 1998). In addition, many nearshore ecosystems are substantially altered through the destruction of benthic habitats by detrimental fishing methods (Malakoff, 1997). Indirect effects of fishing can have more important impacts on aquatic ecosystem structure and function than the removal of the fish (Hammer et al., 1993; Hughes, 1994; Botsford et al., 1997; Estes et al., 1998).

In this paper, we review some of the current knowledge about how fish populations provide ecosystem services for human societies, and the relations between these services and functioning ecosystems in different regions of the world. Available literature relating to large-scale marine systems is limited owing to the obvious difficulty of performing ecosystem experiments here. Small-scale freshwater ecosystems, on the other hand, are better understood in terms of influences of fish on ecosystem structure and function (Carpenter et al., 1995). We also discuss some of the consequences of human impacts on fish populations, such as overfishing in relation to ecosystem resilience. We define resilience here as the amount of change or disruption that is required to transform a system from being maintained by one set of mutually reinforcing processes to a different set of processes (Holling, 1973). Identifying ecosystem services that various fish populations are part of, or generate for human societies, is one step toward holistic, ecosystem-based, resource management with increased understanding of effects on the dynamic, often unpredictable ecosystems by fisheries.