Many vital services by marine and freshwater ecosystems, such as higher production and stability of aquatic resources, are sustained by biodiversity (Figure 8.25). One explanation for higher production is that diverse ecosystems make more efficient use of nutrients and light (Figure 8.26). In addition, higher genetic and species diversity leads to higher stability in the face of disturbance and faster recovery following disturbance. For instance, genetic diversity in populations of seagrass, Zostera marina, is associated with greater resistance to disturbance by grazers and faster recovery following heat-induced mortality (Figure 8.27). When the seagrass is healthy, so, too, are the fisheries that depend on the seagrass. The fact that diverse ecosystems have high productivity and buffer communities from disturbance is a key reason they provide important ecosystem services to humans. Biodiverse and stable ecosystems with complex physical structure sustain higher species and population numbers, which humans can harvest indefinitely with sustainable fisheries management.

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The fishery for sockeye salmon (Oncorhynchus nerka) around Bristol Bay, Alaska, has produced high-quality dietary protein and has brought a stable income to the communities around Bristol Bay for more than a century (Figure 8.28). How has this fishery been sustained, while many other fisheries around the world have collapsed? One of the keys to its sustainability appears to be biodiversity at several levels.

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The high level of biodiversity in the Bristol Bay sockeye salmon system is due largely to the diversity of its environment. The cool summers and mild winters of a maritime climate, as well as the more variable continental climate, mark the area, which includes a large number and diversity of river and lake ecosystems. Overlaying climatic diversity with ecosystem diversity produces another source of biodiversity that arises from the great variety of spawning environments around the bay, including creeks, spring-fed ponds, large rivers, lakeshore beaches, and beaches around islands. As a result of this environmental complexity, the Bristol Bay sockeye fishery includes several hundred distinctive populations, spawning in hundreds of different places. Because adult salmon return to spawn in the streams where they hatched from eggs, they are adapted to those specific stream conditions and, consequently, populations associated with different spawning streams and habitats accumulate genetic differences over time. Therefore, the physical diversity of the Bristol Bay ecosystem produces great genetic diversity among the sockeye salmon populations (Figure 8.29).

Ecosystem and genetic diversity have contributed to the stability of Bristol Bay sockeye salmon fisheries. As shown in Figure 8.30, very high catch levels have been maintained for more than a century. This stability has been maintained even in the face of climatic variation resulting from ocean conditions that produce good and bad years for salmon in their marine life phase. The diversity in sockeye populations has been a critical element to their persistence at the population level, as local environmental changes that might negatively affect one subpopulation might not affect others. For example, notice in Figure 8.30 how the Egegik population of salmon, which contributed very little to the fishery in the early 1900s, was the dominant source of salmon caught after 1990.

An analysis published in the scientific journal Nature in 2010 concluded that the diversity of the Bristol Bay system more than doubles the stability of its salmon yields, compared with what they would be if the fishery consisted of a single salmon population. In addition, a fishery depending on a single salmon population would need to be closed, due to low numbers of returning salmon, 10 times more frequently. Of course, even the very diverse Bristol Bay fishery could not be sustained in the absence of sound management.

Fisheries scientists increasingly recognize that optimal management strategies include information on the whole ecosystem in which commercially important fish populations live and on which they depend. This shift in focus acknowledges how natural systems provide ecosystem services and has given rise to ecosystem-based management of fisheries. This approach to management of natural resources takes into account the entire ecosystem. One of the products of ecosystem-level thinking about fisheries problems has been growing interest in the use of marine protected areas, or marine reserves, where resource use is restricted to help sustain fisheries stocks and to protect entire marine ecosystems and safeguard their many services. Most marine protected areas are designated as permanent no-take zones, but others are seasonally closed during key times of year when fish congregate to spawn.

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Benjamin Halpern of the University of California at Santa Barbara examined the influence of marine protected areas on the density, biomass, size, and diversity of marine organisms. He found that marine protected areas consistently support a higher density and biomass of fish (Figure 8.31). In addition, the fish in marine protected areas are larger and more diverse than in surrounding unprotected areas or than they were before the protected area was established. Furthermore, the growing populations within marine protected areas in the northeastern Atlantic have been shown to “spill over” to surrounding fishing grounds, improving fishing success and fulfilling their promise.

In a study sponsored by The Nature Conservancy, researchers found that recently established marine protected areas had far-reaching effects on communities in Fiji, the Solomon Islands, Indonesia, and the Philippines. As in other regions, fish populations and sizes have grown in these marine protected areas, and the catch has increased in the surrounding fishing grounds. These changes have reduced poverty both directly and indirectly (Figure 8.32). In the Fijian community studied, monthly income from fishing has doubled since the establishment of the marine protected area. While the focal community in Indonesia did not benefit markedly from improved fishing, the marine protected areas have benefited them indirectly. The marine reserves have attracted more tourists, and many local people have taken jobs in the tourism industry, where they earn 2½ times the income of fishers. In addition, by selling produce to resorts that draw tourists to the reefs for snorkeling, local farmers have found a more stable market for their crops and have increased their incomes. The island communities where marine protected areas were established also realized significant social benefits, including better governance, improved diet, and better sanitation.

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