the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.while the smaller petrels consume mainly planktonic crustaceans (e.g., Euphausia mucronata).Although available information shows that incidental mortality in the artisanal Patagonian toothfish(Dissostichus eleginoides) fisheries in southern Chile is low (0.047 birds per 1000 hooks; C.A. Morenoet al. 2006), incidental mortality of petrels and albatrosses in industrial long-line fisheries has not yetbeen assessed for northern Chile, where most of the Chilean swordfish vessels are based.Marine mammalsSpecies of mammals listed in Table 2 are those that mainly occur over the continental shelf in theHCS. Although there are no endemic species, the total species richness reaches at least 22 species,most of them being cetaceans. The number of species and abundance of individual sightings inTable 2 suggest that the upwelling regions between 18°S and 30°S are important feeding stationson the migration routes of whales (see also Rendell et al. 2004).For the sea otter (Lontra felina), available information shows that this species feeds mainly inareas dominated by macroalgae (Ebensperger & Castilla 1992, Ebensperger & Botto-Mahan 1997,Villegas 2002), where refuges on land are available (Sielfeld & Castilla 1999). The diet of thisspecies is principally composed of coastal fishes, crabs and seabirds (Sielfeld & Castilla 1999,Mattern et al. 2002) and foraging success is higher at wave-protected sites than at exposed sites(Villegas et al. 2007).The biology and ecology of otariids at both sea and land has been investigated in Peru (Majlufet al. 2002) and Ecuador (e.g., Dellinger & Trillmich 1999), but studies from northern-central Chilemostly focus on population size or demography (e.g., Guerra et al. 1987). Interaction between sealions (Otaria flavescens) and fisheries is an important issue in Chile (Hückstädt & Krautz 2003).Their traditional foraging behaviour (search and pursuit of prey) has changed to one of ‘sit andwait’ for food captured by fishing vessels (Hückstädt & Antezana 2003), a strategy that appears tobe exploited by killer whales (Orcinus orca) preying on sea lions near fishing vessels (Hückstädt& Antezana 2004). While it is well known that sea lions destroy nets and consume pelagic fish,the extent of this problem has not yet been systematically examined in northern Chile.Although there is a lack of information, it appears that there exists an inverse relationship betweenthe breeding distribution of sea lions and endemic seabirds in northern-central Chile (Table 2). Sealions are most abundant between 18°S and 26°S, while seabirds seem to be more abundant between27°S and 33°S. The reasons for this spatial separation are not well known, but one possibility mightbe that sea lions are common in northern Chile where they can successfully breed on small rockyoutcrops, while seabirds may be most abundant between 27°S and 33°S, where they form hugebreeding colonies on undisturbed islands. Additionally, maternal care might impose higher restrictions(duration and distance of foraging trips) on female sea lions (e.g., Trillmich 1990) than onfemale seabirds, which share parental care with their male partners and thus can afford longer absencefrom the nest and farther foraging trips during breeding. The apparent lower productivity of pelagicfish between 27°S and 33°S (see also Pelagic fisheries and fisheries management 1980–2005, p. 288ff.)might affect female sea lions (which usually forage close to the breeding sites; e.g., Trillmich 1986)to a higher degree than seabirds (which may travel >30 km from breeding colonies during individualforaging trips, as observed in Humboldt penguins; Luna-Jorquera & Culik 1999). Future studiesshould examine the causes for the inverse abundance patterns of sea lions and seabirds along thecoast of northern Chile.Effects of ENSO on seabirdsIn the upwelling area of Coquimbo (Region IV), the species composition of seabirds at sea fluctuatesamong different sites, and distribution patterns are linked to variation in SST and chlorophyll224
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEconcentration. Dense swarms of pelagic fishes attract high numbers of piscivorous birds, which isthe most important factor to explain their distribution patterns. Usually, only 3% of all birds recordedin a given area are seen directly attending fishing vessels (Weichler et al. 2004). Life-history theorypredicts that seabirds will respond to reduction in food availability by changing their behaviourand/or breeding effort, thus buffering adult survival. Data of Peruvian boobies (Sula variegata) arein agreement with this prediction (G. Luna-Jorquera unpublished data). In January 2002, about3000 breeding pairs of this species were registered in a colony off Coquimbo, but 1 month laterthere were only 10 pairs, coinciding with a positive SST anomaly (Figure 8). Counts of birds atsea conducted in ~420 km 2 around the colony showed that the numbers of boobies were lower thanin a ‘normal’ year, indicating that boobies left the area probably due to a reduction in foodavailability. This caused both a reduction in the total numbers of all seabirds foraging in feedingflocks and a simultaneous increase of seabirds attending fishing vessels. During cooler LN years,large concentrations of seabirds were seen in feeding flocks and few birds followed fishing boats(Figure 8). The apparent change in food availability is also reflected in the high number of breedingpairs of boobies during LN years (Figure 8).Endemic seabirds have evolved in the upwelling area of the HCS, where historically theirpopulations are subjected to large fluctuations, ENSO being an important selective force for theevolution of their breeding biology and life-history patterns. During EN, some species abruptlyleave their eggs and young to undertake eruptive movements away from the usual breeding andfeeding sites in apparent search for food; total or partial breeding failure then often occurs (Simeoneet al. 2002). Seabirds are generally characterised by longevity, high age of first breeding, slowreproductive rate and intense offspring care and are thus typical K-strategists. However, it has beenhypothesised that, compared with related species, Peruvian boobies and Guanay cormorants(Leucocarbo bougainvilli) have larger clutches, may attempt to breed more than once within 1 yrand reach sexual maturity at an unusually early age (Luna-Jorquera et al. 2003). This enables themto build up populations rapidly in the years after EN events because even young, unexperiencedadults are able to raise large broods because the food supply per bird is much greater than it wouldbe in a population in equilibrium with the environment (Furness & Monaghan 1987). However,since the establishment of the anchovy fishery, the dynamics of the seabird populations have changed(Duffy et al. 1984). Instead of rapidly increasing populations by raising large broods at least onceper year, endemic seabirds failed to respond to the reduced competition brought about by theirreduction in numbers. It seems that the anchovy fishery has taken up the superabundance of food,which in the past permitted the seabirds from the HCS to cope with the recurring crashes inducedby oceanographic perturbations (Jahncke et al. 2004). In fact, data available for Peruvian seabirdcolonies (Tovar & Cabrera 1985) show that the EN 1957–1958 caused a mortality of 39% of theestimated populations of 28 million adult birds. Mortality increased to 57% (of 6.54 million adultbirds) due to EN 1972–1973, after 1970 when anchovy landings had reached 12 million t (metrictons). The extreme EN 1982–1983 caused a decrease from 6 million birds to only 0.3 million birdsin Peru. After that, the populations showed an increase in numbers, reaching ~6 million birds, butthe EN 1997–1998 reduced it again to less than 0.4 million birds. In 2000, the size of the Peruvianseabird population was estimated to be 1.93 million adult birds, showing a slight recovery (IMARPE2006). No comparable datasets are available for the coast of northern-central Chile, where onlyinfrequent surveys of breeding colonies have been conducted so far (e.g., Simeone et al. 2003).Food resources or breeding sites?Seabirds and sea lions feed on fish and population crashes during EN years have been explainedwith the disappearance of fish stocks. Which proportion of the fish stocks do seabirds and sealsconsume? And how are they affected by variable food supply? Bioenergetic modelling provides225
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the humboldt current system of northern and central chile - figema

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