In the Southern Ocean, Antarctic krill Euphausia superba are the dominant prey item for many predators, and a changing climate may affect the biomass of krill available to both predators and the krill fishery. We projected growth trajectories for individual krill within cohorts and estimated how total biomass in an area available to both predators and the fishery may vary from year to year simply due to fluctuations in temperature. We used an existing temperature-dependent growth model and a time series of temperature data (1970 to 2004) for 2 regions in the Southern Ocean: (1) around the Antarctic Peninsula, and (2) around the island of South Georgia. The growth model predicted increasing individual size within a cohort (in terms of length and weight) with increasing temperature in the cooler Antarctic Peninsula region and decreasing individual size with increasing temperature in the warmer South Georgia region. Years with many cohorts of small individuals in the population resulted in biomass well below average, whereas years with many cohorts of large individuals resulted in biomass well above the average, suggesting that temporal changes in Southern Ocean temperatures may have profound effects on the total biomass in an area that is available to both predators and the fishery. Moreover, the effects of a potentially warming Southern Ocean on krill biomass will likely be more pronounced in the warmer regions occupied by krill.
The method of collecting and analyzing boluses to characterize the cephalopod diet of albatrosses has been used in many diet studies. However, no study has validated this method. We compared boluses and stomach samples from Gray-headed Albatrosses (Thalassarche chrysostoma) and Black-browed Albatrosses (T. melanophris) to (1) study the consumption and diversity of cephalopods in these species, (2) investigate biases associated with each sampling method, and (3) estimate the number of samples needed to characterize these albatross’s cephalopod diet. We found that collection and analysis of boluses is a simple, efficient, and noninvasive method for assessing the cephalopod diet of these albatross species, but it is inadequate for characterizing the more easily digestible dietary components, such as fish and crustaceans. Both boluses and stomach samples showed that the two albatross species fed on cephalopods of similar sizes and from the same families (Ommastrephidae, Onychoteuthidae, and Cranchiidae). Furthermore, the main prey species (Martialia hyadesi, Kondakovia longimana, and Galiteuthis glacialis) and the total number of cephalopod species consumed (18–24 species) were the same for both albatrosses. To include all cephalopod species, using a sample-randomization technique, a minimum of 61 and 43 boluses were needed for Gray-headed and Black- browed albatrosses, respectively; but to adequately describe the diversity and size frequency of the main prey species, 82 and 371 boluses would be needed.Les Pelotes de Réjection: Une Méthode Efficace pour Évaluer la Proportion de Céphalopodes dans le Régime Alimentaire chez les Albatros
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