the humboldt current system of northern and central chile - figema

THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEmackerel (Trachurus murphyi), hake (Merluccius gayi) and cephalopods (Neira & Arancibia 2004).Top predators in the HCS are large pelagic fish such as tuna (Thunnus orientalis) and swordfish(Xiphias gladius), southern sea lions (Otaria flavescens) and seabirds.Most fish predators in the HCS appear to be non-specialists, which feed opportunistically ona wide range of different prey items. Sardines and anchovy consume small food particles (mainlyphytoplankton and copepods), with the anchovy able to consume larger food items than sardines(Balbontín et al. 1979). Anchovy are more specialised on large zooplankton, while sardines consumea wide range of food items from phytoplankton to small zooplankton (Alheit & Niquen 2004). Jackmackerel prey on copepods, euphausids, sardines and anchovies and benthic resources (Medina &Arancibia 2002). Jumbo squids (Dosidicus gigas) feed cannibalistically and on fish, including jackmackerel and anchovy (Chong et al. 2005). Some of the main fish consumers themselves are preyto larger predators, for example, swordfish (Ibáñez et al. 2004) and sea lions (Sielfeld 1999). Ageneralised scheme of the pelagic food web off central Chile shows the short trophic pathways andthe predominance of relatively few main fish consumers (Figure 7A). While the food spectra ofmost pelagic consumers are relatively well known, information about consumption rates, intra- andinterspecific competition or intraguild predation is limited. Similarly, relatively little is known aboutprey preferences and feeding strategies of the pelagic fish consumers in the HCS. This knowledgeis particularly important in light of variable oceanographic conditions, which may modify availabilityof preferred food items or produce a spatial segregation of predators and prey (e.g., Bakun2001, Alheit & Niquen 2004).A review by Cury et al. (2000) suggested that the availability of small pelagic fish in the HCSoff Peru determines the population size of higher trophic levels. Bakun (2001) pointed out thatthe interaction between small pelagic fish and their predators is highly dynamic, depending onmultiple environmental (oceanic fronts, climate-driven changes in oceanography), biological (reproductivestrategies) and behavioural (schooling behaviour) factors. Small fish consumers may temporarilyescape from predation in this dynamic system, and if their reproductive and behaviouralstrategies permit them to find a refuge from predation these species may build up and maintainhuge populations (Bakun 2001). Once this dynamic balance is interrupted (e.g., by climatic factors),regime shifts may occur (Alheit & Niquen 2004).It has been proposed that the regime shift from a sardine-dominated system to an anchovydominatedsystem (or vice-versa), which is related to long-term variations in oceanographic conditions(Chavez et al. 2003), may ultimately be mediated by trophic feedbacks (Alheit & Niquen 2004).During warm periods, the preferred prey items of anchovy (large copepods and euphausids) becomeless available (see also Zooplankton consumers, p. 214ff.) while predation pressure on adult anchoviesincreases, due to invasions of jack mackerel into coastal waters (Alheit & Niquen 2004). Simultaneously,sardines, which are also important predators on anchovy eggs (Alheit 1987), may be favouredbecause they have a wide prey spectrum including phytoplankton. The consequences of these regimeshifts, which have been analysed for Peru and northern Chile (Alheit & Niquen 2004), also extendto central Chile. Future studies need to examine to which degree bottom-up or top-down mechanismsare involved, and whether EN impacts and fisheries may accelerate these regime shifts.Both prey and predator behaviour also affects trophic interactions in the HCS. In a recent study,Bertrand et al. (2006) suggested that the feeding behaviour of jack mackerel is closely linked tothe OMZ and DVM of their prey. These authors discussed that during the day, when prey are hidingin deeper waters, jack mackerel rest near the upper limit of the OMZ. At night, when prey migrateinto upper oxygenated waters, jack mackerel become active (Figure 7B). If this model is confirmedin future studies, interannual variation in oceanographic conditions (e.g., the intensity and depthof the OMZ) may also affect the trophic efficiency of jack mackerel. This hypothetical scenariounderscores the importance of better knowledge of predator–prey behaviour and interactions inorder to better understand the pelagic food webs in the HCS.219