Pelagic Predators

D A Croll and B R Tershy, University of California, Santa Cruz, CA, USA Published by Elsevier B.V.

Further Reading

Pelagic ecosystems are characterized by two major groups of animals: zooplankton and nekton. Pelagic predators are larger nektonic species including elasmo-branchs (e.g., pelagic sharks and rays), teleost fishes (e.g., tuna and billfishes), large squid, and marine tetra-pods (e.g., sea turtles, marine mammals, and seabirds). The majority of pelagic predators congregate in the photic zone, often in regions of increased primary production such as ocean current divergences or convergences, regions of localized upwelling, fronts, or eddies. Such regions, or 'hot spots', serve to enhance primary production rates or concentrate prey resources at higher densities (see Upwelling Ecosystems).

Compared to terrestrial and many near-shore systems, net primary production is low in pelagic ecosystems (see Marine Models). Thus, these predators must adapt to a food web that is a nutritionally dilute three-dimensional system where prey resources are patchy and ephemeral (Figure 1). Pelagic primary producers (e.g., diatoms) constitute a relatively small proportion of total pelagic biomass, but they have high turnover rates - able to respond rapidly to increases in nutrient availability. The prey of pelagic predators similarly has relatively high turnover rates. This results in a situation where the biomass density of the primary producers and prey of pelagic predators may be similar to that of the predators themselves.

Pelagic predators are generally large in body size compared to other marine predators (e.g., geometric mean body size of pelagic predators is 4.7 vs. 0.19 kg in coastal predators) with relatively high metabolic rates. The combination of these factors leads to relatively high average daily prey requirements, and most species range

Pelagic predators Squid, tuna, billfish, sharks, sea turtles, seabirds, marine mammals

Invertebrate carnivores

Pelagic schooling fish

Figure 1 Simplified pelagic predator food web.

Figure 1 Simplified pelagic predator food web.

long distances in search of patchy and ephemeral concentrations of prey. Likewise, they may range long distances in search of mates or breed and forage in widely separated locations (Figure 2). As a result, most are streamlined, efficient swimmers, with low costs of transport. Many species utilize efficient lift-based locomotion strategies (e.g., tuna, billfish, sea turtles, seabirds, and cetaceans such as whales and dolphins). Elasmobranchs and teleosts have evolved rigid bodies with small scales, reduced fins, and keels. In addition, many of these predatory fishes have adaptations to increase the efficiency of aerobic swimming with high proportions of red muscle with complex rete systems to maintain elevated body temperatures. Pelagic seabirds are efficient flyers able to take advantage

Figure 2 Basinwide migrations of three sooty shearwaters equipped with archival geolocation loggers. Although each bird bred at a colony in New Zealand (see multiple foraging trips to Antarctic waters), shearwaters experience a pan-Pacific distribution during the postbreeding migration (May-Sep.). For complete details of the study, see Shaffer SA, Tremblay Y, Weimerskirch H, etal. (2006) Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer. Proceedings of the National Academy of Sciences of the United States of America 103: 12799-12802.

Figure 2 Basinwide migrations of three sooty shearwaters equipped with archival geolocation loggers. Although each bird bred at a colony in New Zealand (see multiple foraging trips to Antarctic waters), shearwaters experience a pan-Pacific distribution during the postbreeding migration (May-Sep.). For complete details of the study, see Shaffer SA, Tremblay Y, Weimerskirch H, etal. (2006) Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer. Proceedings of the National Academy of Sciences of the United States of America 103: 12799-12802.

of global and localized wind patterns (e.g., albatrosses, shearwaters) that significantly reduce their cost of transport. In comparison to other pelagic predators, squid utilize a relatively inefficient jet propulsion-based mode of locomotion that, while less efficient than vertebrates, is relatively efficient in comparison to other invertebrates.

Successful feeding of most pelagic predators depends upon schooling prey or, less commonly, other top predators (e.g., white sharks, killer whales). Pelagic schooling likely evolved as a predator defense strategy and the large body size and generally low cost of transport in pelagic predators allows them to fast for extended periods while ranging long distances in search of prey schools. Large body size is also advantageous in allowing pelagic predators to consume and process larger quantities of prey in a single feeding event. As a means to further increase the efficiency of processing schooling prey, filter feeding has evolved independently in four groups of pelagic predators (seals, whales, sharks, and rays). This allows them to engulf and process many individual prey at once - further increasing the rate at which energy may be ingested when encountered.

As large, conspicuous components of pelagic ecosystems, many pelagic predators are economically or culturally important. Because they are a large and high-quality food resource, many species are directly targeted in fisheries, and the exploitation of pelagic predators was arguably one of the first major impacts of humans on marine ecosystems. Recently, several authors have argued that stocks of pelagic predatory fishes have declined precipitously due to exploitation. For many species it has been estimated that large predatory fish have declined to 10% of unfished levels in the last 50-100 years. For elasmobranchs and marine tetrapods which tend to be long-lived with low fecundity, direct exploitation likely had greater impacts.

Direct exploitation of many marine tetrapod species has led to catastrophic declines (e.g., pinnipeds, cetaceans, sea turtles) and, in some cases, extinction (e.g., Steller's sea cow, great auk). More recently, indirect exploitation via by-catch in commercial fisheries has further added to population declines, particularly in sea turtles and sea-birds (e.g., albatrosses). Declines of colonially breeding tetrapods due to direct and indirect exploitation at sea have been further exacerbated by the introduction of non-native predators (e.g., rats, cats, dogs, pigs, mongooses) at breeding colonies.

Compared to other pelagic predators, squid are unique. They tend to be short-lived with rapid growth and population turnover rates. Thus, unlike other pelagic predators, they are capable of responding rapidly to ecosystem change, and some have speculated that squid populations may rapidly exploit prey resources made available by the removal of other predators. In addition, squid are important as both predators in pelagic ecosystems as well as prey to larger predatory fish and marine tetrapods.

While pelagic predators are conspicuous, important harvest species, the foci of popular attention, and rallying species for environmentalists, our understanding of their role in pelagic ecology is limited. Some have speculated that the removal of these predators may have had significant effects on marine community structure. Given their rapid growth rates and likely high population numbers, the role of squid in top-down regulation of pelagic ecosystems may be significantly underestimated. For whales, it has been debated that declines of large whale biomass in northeast Pacific triggered significant increases in herring and walleye pollock stocks. Because pelagic predators may be relatively abundant, have large body sizes, high metabolic rates, and high trophic status, a significant proportion of ocean primary production indirectly passes through them. Thus, there is the possibility for them to exert strong top-down control in pelagic ecosystems. This may be particularly true for squid given their likely high abundances, metabolic rates, and growth rates. However, evidence for such control is limited, and their top-down impacts may be ameliorated by mismatches in turnover rates of primary producers, consumers, and pelagic predators or the structure of pelagic food webs.

Given their dependence upon ocean (see Marine Models) processes that serve to enhance productivity and concentrate prey, climate change may have important direct and indirect impacts on these predators including changes in movement patterns, species ranges, food web interactions, trophic status, and life history traits (see Global Change Impacts on the Biosphere).

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