EFFECTS OF PREY REMOVAL ON SEA BIRDS

16.3 EFFECTS OF PREY REMOVAL ON SEA BIRDS

Piscivorous sea birds feed primarily on small shoaling fish. In the North Atlantic, these prey species include herring (Clupea harengus), sprat (Sprattus sprattus) and sandeels (Ammodytes spp.). Sandeels are particularly important prey dur- ing the breeding and chick-rearing season, whereas clupeoid and gadoid fish become more important dietary components during the winter period. The International Council for the Exploration of the Sea (ICES) working group on sea bird ecology (Anon. 1994) estimated that the annual consump- tion of fish by 18 sea bird species, based on sea bird

Ecosystem Effects of Fishing

347

100 000 Tonnes per year

50 000 0 Fig. 16.2 Estimated total annual

consumption of fish and discarded

Offal

Sandeels Gadoids Mackerel

Discards

materials by 18 sea bird species in

the North Sea. (Source: data from Sprat and Herring

Large herring

Other prey

Anon. 1994.)

abundance estimates and their energetic require-

16.3.1 Effects of industrial fisheries

ments was 600 000 tonnes y -1 (Fig. 16.2). Of this figure the total consumption of small fish, con-

on sea bird populations

sisting of sandeel, sprat, young herring and small Industrial fisheries in the North Sea have in- gadoids, by sea birds is approximately 250 000 creased substantially since the early 1960s. These tonnes y -1 , which is about one-tenth of the small fisheries target species such as sandeels, which fish consumed by larger predatory fish (Anon. are then converted into animal feed, fertilizer or 1994).

are even used as fuel. This fishery has caused It is thought that the abundance of immature concern among conservationists because many fish in the North Sea has increased due to the over- top predators such as seals, cetaceans and sea birds, fishing of larger size classes of piscivorous fish (e.g. rely on sandeels as the main part of their diet Furness and Monaghan 1987; Daan et al. 1990). (Furness 1990). The direct impacts of fisheries im- Similarly, congruent shifts in sandeel abundance pacts on prey availability can easily be overlooked in the western and eastern North Atlantic ecosys- as sea birds are able to compensate for food short- tems were also explained by predator removal by ages by working harder, allocating more time to fisheries (Sherman et al. 1981). Although the in- foraging, switching to alternative prey types, or crease in the amount of small fish intuitively must abandoning nesting attempts. Ultimately the im- have resulted in a larger potential resource for pis- pacts of significant reductions in prey popula- civorous sea birds, the availability of prey may not tions become apparent as sea birds starve to death. have changed quite that dramatically. Shifts in the

During the 1980s, the breeding success of sev- localized availability of small fish may negatively eral sea bird species in the Shetland Islands de- affect sea birds even when the wider fish stock is clined markedly, coincident with a marked decline relatively unaffected. For example, the reproduc- in landings of sandeels from an industrial fishery tive output of puffins (Fratercula arctica) that nest that operated close to the Shetland coast (Mon- on the Lofoten islands off Norway was reduced aghan et al. 1992). The sea bird species affected over a 20-year period following the collapse of local most severely were surface feeders such as Arctic herring stocks, even though, elsewhere, herring terns (Sterna paradisaea), black-legged kittiwakes stocks remained at relatively high levels (Anker- (Rissa tridactyla) and great skuas (Catharacta Nilssen 1992).

skua ) that fed predominantly on O-group sandeels.

349 The kleptoparasitic Arctic skuas (Stercorarius gregations of salmon are important sources of food

Ecosystem Effects of Fishing

parasiticus ) were also affected, as was the puffin. for many terrestrial animals and bird species that The diet of gannets changed from predominantly inhabit the relevant watersheds. Studies that have

sandeels to clupeoid and gadoid fish. After a few examined 15 N/ 14 N ratios in food webs of systems years of poor breeding success, significant declines with anadromous salmon and those without indi- occurred in breeding populations of Arctic terns, cate that the decomposing carcasses of salmon black-legged kittiwakes and common guillemots. are remineralized by primary producers that are The numbers of Arctic terns increased dramatical- consumed by fish predators (Kline et al. 1993; ly again in 1991, coincident with the appearance of Cederholm et al. 1999).

a large year class in the sandeel population. Salmon carcasses can supply a critical source These population shifts led to suggestions that of energy for some terrestrial vertebrates and pro- the industrial fishery competed for the same re- vide nutrients for riparian vegetation along some source as the sea birds and was responsible for the spawning streams. They are now viewed by some decline in their food supply. However, other stud- as keystone species in certain terrestrial vertebrate ies indicated that the decline in sandeel abundance communities (Willson and Halupka 1995). The was the result of poor recruitment to the Shetland reproduction cycle and seasonal distribution of stock. Hence, natural fluctuations in sandeel sur- species such as bald eagles (Haliaeetus leuco- vival and recruitment may have been the main cephalus ) is tied directly to a spawning run of cause of the decline in prey availability for sea salmon (McClelland et al. 1982). Salmon stocks birds breeding on the Shetland Islands (Anon. are, in general, overexploited (Nehlsen et al. 1991). 1994).

Hence the present-day inputs of marine nutrients from salmon carcasses are probably much lower than they were prior to overexploitation. While it

16.4 HOW FISHERIES

is not possible to attribute differences in past and

AFFECT ENERGY SUBSIDIES

present-day community structure to decreases in the amount of energy and nutrient subsidies pass-