13.5: Fisheries

Because they ultimately depend on primary production for their food, fishes are most abundant in areas where primary productivity is high (Figs. 12-13, 13-16a). Hence, fish population density and productivity are higher in the coastal oceans, especially in upwelling regions, than in open-ocean areas where there is no upwelling.

Figure 13-16. (a) Primary productivity, measured as the total amount of organic carbon produced annually in each square meter of water column, is much higher in upwelling areas than in coastal (nonupwelling) or oceanic areas. (b) The average trophic efficiency is high in upwelling regions, intermediate in coastal regions, and lowest in oceanic regions. (c) Most of the ocean is oceanic in character, whereas upwelling areas constitute only a very small percentage of the total area. (d) Because the area occupied by oceanic regions is so large, the oceanic regions are responsible for most of the worldwide primary production. (e) The total production of fish biomass in oceanic areas is small, despite the high primary production, because this primary production is performed mostly by dinoflagellates and because the average trophic efficiency is low. Dinoflagellates must pass through more trophic levels than the diatoms that dominate in coastal and upwelling regions before they are used to build fish tissue. Thus, more dinoflagellate biomass than diatom biomass is needed to sustain the same fish biomass. Fish production is high in coastal regions because of the relatively high primary productivity and trophic efficiency. Despite their very small area, upwelling regions are responsible for a large proportion of the world’s fish production because of their very high primary productivity and high trophic efficiency.

The average trophic efficiency (CC15) is higher in coastal food webs than in open-ocean food webs and is highest in food webs of upwelling areas (Fig. 13-16b). As a result, the total fish production of upwelling areas is about half of the world’s total (Fig. 13-16e), despite the very small area (0.1% of the total area of the oceans) in which upwelling occurs (Fig. 13-16c). Coastal regions constitute about 10% of the ocean area and account for about 50% of the world’s fish production. Open-ocean regions constitute 90% of the oceans but sustain less than 1% of the world’s fish production (Fig. 13-16e).

The most successful fisheries are herring, anchovy, and sardine fisheries in upwelling regions. These species constitute about 25% by weight of the global catch. Because they feed primarily on zooplankton at the second trophic level, they are extremely abundant, and their harvest represents an efficient use of ocean resources. Much of the catch of these species is used to feed animals and not directly to feed people, so we are using these resources at only about 10% of their potential efficiency. About 40% of the global fishery catch of all species is used as animal food.

Because of the distribution of fish biomass production, the world’s major fisheries are almost all located in the coastal zone, especially in upwelling areas. Most of these major fisheries are being exploited at or near their maximum sustainable yield (CC16), and many are overfished. It is believed that the total world fishery catch cannot be increased substantially from its present level without causing detrimental effects on the entire ocean ecosystem, and particularly on higher carnivores, including the largest fishes, marine mammals, and seabirds, that rely on fish stocks for their food. Further, various anthropogenic influences, including overfishing and increasing ocean water temperatures and acidity, are projected to cause a decline in future fisheries food production potential. Global fishery catch data is hard to estimate due to poor accuracy in data reporting from many fishers and their nations. However, it is believed that the world fishery catch peaked somewhere during the late 1990s and is now declining steadily. Fortunately, the total worldwide production of seafood has remained reasonably stable due to a rapid expansion in aquaculture, but it is not known whether aquaculture can continue to make up for declining wild fisheries into the future. Also, aquaculture can not yet, and perhaps will never be able to, economically produce the high trophic level species such a tuna that are of the highest value, so some shift in species used for human consumption is inevitable if aquaculture is to replace declining wild fisheries resources.

Natural variations in fish stocks are caused by complex interactions of ocean physics, chemistry, and biology. For example, many fish species release their eggs into the plankton community that drifts with the ocean currents. The eggs hatch into larvae that must have the right type of food to grow. For example, anchovy larvae, which eat only phytoplankton that are at least 40 mm in diameter throughout the first few days after hatching. Larval survival of anchovies varies from year to year because the availability of such phytoplankton varies, as do many other factors, including the concentrations of species that compete for food with or prey on the larvae. Survival and success are no less complicated for the larval fish of other species or when they become juveniles, because juveniles also have to find food and are subject to predation, disease, parasitism, and the direct and indirect effects of pollution. Each of these factors for each different fish species varies from year to year, at least in part because of year-to-year climatic variations.

Life cycles and their interactions with physical, chemical, and biological variables are so complex that studies must be conducted over many years to obtain even a limited understanding of the population variations in a single fish species. Even after such extensive studies, it is impossible to predict the future of the fish stock with any certainty because many of the influences on fish stock size and age composition are nonlinear. Consequently, fish stocks of many species are inherently chaotic (CC11) and appear to fluctuate in a random or unpredictable way. It may be impossible to predict future fish stocks accurately, just as it is impossible to forecast the weather accurately more than a few days in advance.