Estudio comparado de la alimentación entre algunos predadores de alto nivel trófico de la comunidad marina del norte y centro de patagonia
The comparative analysis of the food habits between top predator allows the evaluation of how these species use the trophic resources. They also make possible the study of potentially competitive relationships and the role of these predators in the regulation and structure of the community. The marine community of northern and central Patagonia is structured on the tri-trophic system composed by the Argentine anchovy (Engraulis anchoita), the Argentine shortfin squid (Illex argentinus), and Argentine hake (Merluccius hubbsi). Several marine mammals and elasmobranchs use these species as food resources. For this reason, the objective of this work was to analyze comparatively the diet of the South American sea lion (Otaria flavescens), the dusky dolphin (Lagenorhynchus obscurus), the Commerson’s dolphin (Cephalorhynchus commersonii), the beaked skate (Raja flavirostris), the spiny dogfish (Squalus acanthias) and the school shark (Galeorhinus galeus). The diet of these predators was studied by stomach content analysis. The prey remains found in the contents (fish bones and otoliths, cephalopod beaks, crustacean exoskeletons, etc.) were used to identify and quantify the preys. The length and weight of prey at the moment of the ingestion were estimated by alometric regressions. The relative importance of prey species was evaluated by the relative importance index IRI=(PH%+N%)FO%, where PH% is the percent in wet weight, N% is the percent in number, and FO% is the frequency of occurrence of the prey species in the diet. This index was relativized to 100% (IRI%). Intraspecific differences in diet allow the determination of trophospecies within a biological species. These differences were evaluated using the general overlap (GO) and the specific overlap (SO) index. These index were calculated using the occurrences of those prey species which had IRI% values greater than 1% in the total sample of the biological species. Differences in the sizes consumed by the trophospecies were evaluated using the Mann-Whitney test or the Kruskal-Wallis non-parametric one way ANOVA and paired comparisons. The diet of the South American sea lion was studied using samples obtained from animals found dead on the beach and incidentally caught by the fishery. Forty-one prey species were identified from the 59 stomachs analyzed. The most important prey were the Argentine hake, the red octopus (Enteroctopus megalocyathus), the raneya (Raneya brasiliensis), the Patagonian squid (Loligo gahi), and the Argentine anchovy. The differences in the diet associated with the source of sampling (dead on beach or entangled) and sex were evaluated using the GO and SO. Only differences between sexes were found. The dusky dolphin diet was studied from a sample of 25 dolphins. Most of this sample was obtained from incidental catches in nocturnal midwater hauls directed to the capture of Argentine red shrimp (Pleoticus muelleri). Intraspecific differences in the diet were not analyzed due the small sample size. The diet of the beaked skate was studied from a sample of 274 individuals obtained from the incidental catches of the hake fishery. The most important prey were the Argentine hake, the Southern cod (Patagonotothen ramsayi), the Argentine shortfin squid, the isopod (Serolis schythei), the raneya, and the Argentine anchovy. A total of 45 prey species were identified. The overlap analysis indicated no differences in the diet between sexes, but significant differences in the diet between immature and mature individuals. The diet of the spiny dogfish was studied from a sample of 132 individuals obtained from the incidental catches of the hake fishery between 1996 and 1998. The food habits were analyzed taking into account the potential differences in diet related to sex and maturity condition. Using the GO and SO index, differences in diet were found between immature individuals (SAinm), mature males (SAmm) and mature females (SAhm). The diet of the Commerson’s dolphin was studied from 9 stomach contents obtained from incidentally caught individuals. Even that this sample is small and only allows a preliminary analysis, the Argentine hake and the Argentine shortfin squid were the most important prey species. The sizes of prey indicate that the Commerson’s dolphin fed on primary juveniles of hake and small juveniles of squid. Then, it is reasonable to consider that this species, in the north and central Patagonia, is a pelagic predator which feed on pelagic species or pelagic stages of species which exhibit schooling behavior. The diet of the school shark was analyzed from 23 stomach contents obtained from individuals incidentally caught in commercial hauls. The most important prey were the Argentine hake, the Argentine shortfin squid and the red octopus, followed by the Southern cod and the Patagonian squid. Considering the ecological groups of the prey, the school shark fed mostly on demersal-pelagic and benthic species, suggesting that this predator feeds associated to the bottom. The sizes of the prey suggest that the school shark consume its prey according to their environmental availability. The inter-specific comparative diet analysis was made using the IRI%, calculated for prey species and ecological groups of prey. Considering the previous results and the sample sizes, the predator species were divided in trophospecies. These trophospecies were the males (OFm) and females (OFh) of the South American sea lion, the dusky dolphin (LO), the immature (RFinm) and mature (RFmad) individuals of the beaked skate, the immature (SAinm), mature males (SAmm) and mature females (SAhm) of the spiny dogfish, the Commerson’s dolphin (CC) and the school shark (GG). In order to determine if the sample size of each trophospecies was representative of the putative population, the Pielou’s pooled quadrat method and the Brillouin’s diversity index were employed. Also, this procedure was used to estimate the population trophic diversity (Hp) for each trophospecies. The Hp were compared using the Kruskal-Wallis test and paired comparisons. Additionally, two cluster analysis were performed using the IRI% for the prey species and the ecological groups of prey. The results indicate that all the sample sizes were representative of the populations, with the exception of the CC sample. For this reason, this species was excluded from the analysis. A gradient of Hp was detected, where OFh and LO were the trophospecies which showed the maximum and minimum values for Hp respectively. This gradient was positively correlated with the IRI% of the benthic and demersal-benthic prey. It suggests that the trophic diversity is reflecting the biological diversity of the environments where the trophospecies are feeding. Moreover, this result indicates that all these predators could be considered as generalist and opportunistic in the context of the environmental patch that they use as trophic habitat. Considering the cluster analysis based on the IRI% of the ecological groups of prey, three groups were identified. These groups were related to the consumption of demersal-pelagic preys (OFm, RFmad, SAmm, SAhm and GG), the high importance of pelagic preys (SAinm and LO), and the high importance of demersal-benthic preys (OFh and RFinm). Considering the IRI% of the prey species, the cluster analysis allowed the identification of 6 groups of predators. The “OFm and RFmad” group was characterized by the high importance of Argentine hake in the diet. The “GG” group was characterized by a relatively even distribution of the importance of Argentine hake, Argentine shortfin squid, and red octopus. The “RFinm” group presented high importance of Southern cod, followed by Argentine hake and isopods. The “SAinm, SAmm and SAhm” was related to the high importance of Argentine shortfin squid and the presence of ctenophores. The “LO” group was defined by the high importance of Argentine anchovy, followed by Argentine shortfin squid and Patagonian squid. The “OFh” group was defined by the high importance of red octopus in the diet. The overlap analysis between trophospecies indicated that the overlaps were low, and always significantly different to the complete overlap. Then, these top predators will used the same, abundant and frequent, trophic resources, but in different ways according to their trophic habitats. These differential uses of the food resources lead to a separation of their trophic niches and to reduce the possibility of ecological competition between them.