Abstract
Penguins are major consumers in the southern oceans although quantification of this has been problematic. One suggestion proposes the use of points of inflection in diving profiles (‘wiggles’) for this, a method that has been validated for the estimation of prey consumption by Magellanic penguins (Spheniscus magellanicus) by Simeone and Wilson (2003). Following them, we used wiggles from 31 depth logger-equipped Magellanic penguins foraging from four Patagonian colonies; Punta Norte (PN), Bahía Bustamente (BB), Puerto Deseado (PD) and Puerto San Julián (PSJ), all located in Argentina between 42–49° S, to estimate the prey captured and calculate the catch per unit time (CPUT) for birds foraging during the early chick-rearing period. Numbers of prey caught and CPUT were significantly different between colonies. Birds from PD caught the highest number of prey per foraging trip, with CPUT values of 68±19 prey per hour underwater (almost two times greater than for the three remaining colonies). We modeled consumption from these data and calculate that the world Magellanic penguin population consumes about 2 million tons of prey per year. Possible errors in this calculation are discussed. Despite this, the analysis of wiggles seems a powerful and simple tool to begin to quantify prey consumption by Magellanic penguins, allowing comparison between different breeding sites. The total number of wiggles and/or CPUT do not reflect, by themselves, the availability of food for each colony, as the number of prey consumed by foraging trip is strongly associated with the energy content and wet mass of each colony-specific ‘prey type’. Individuals consuming more profitable prey could be optimizing the time spent underwater, thereby optimizing the energy expenditure associated with the dives.
Highlights
Birds are major consumers in the marine environment, with estimations of their consumption amounting to between 55.6 and 83.7 million tonnes per year [1] and, as such, are assumed to play an important role in modulating marine food web structure [e.g. 2–6]
The wiggle classification adopted for the data obtained from the two different device types showed that the number of wiggles per dive during the foraging phase of trips did not differ, at any site, between the different recording systems used, and their associated recording frequencies (GLMMs; Punta Norte: X2 = 0.52, P = 0.47, N(dives) = 4025, N(ID) = 10; Bahıa Bustamante: X2 = 0.13, P = 0.72, N(dives) = 1934, N(ID) = 6; Puerto San Julian: X2 = 1.40, P = 0.24, N(dives) = 8034, N(ID) = 11)
General Foraging Patterns and Inter-colony Comparisons Some authors have shown that Magellanic penguins from different colonies adjust their behaviour at-sea to accord with local conditions [e.g. 29] and prey type [18,20,29]
Summary
Birds are major consumers in the marine environment, with estimations of their consumption amounting to between 55.6 and 83.7 million tonnes per year [1] and, as such, are assumed to play an important role in modulating marine food web structure [e.g. 2–6]. This highlights the critical need for determination of precisely how much birds consume even though our methods for doing this are rather crude; while prey types can be elucidated using stomach contents, guano and/or pellet analysis [e.g. 7,8,9], determination of actual rates of prey consumption by seabirds is not trivial. The best examples are beak-opening angles [16] and oesophageal temperature [3,17] because seabirds cannot ingest prey without opening their beaks and incurring an oesophageal temperature drop small prey may not always be registered by oesophageal drops (see [17])
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