Marcelo F. Alonso

Body shape variation of three species of Percichthys in relation to their coexistence in the Limay River basin, in northern Patagonia

We describe morphological variation within and among three species of Percichthys (Perciformes, Percichthyidae) inhabiting two reservoirs within a single river drainage (Limay River) in Argentina. Two small-mouthed percas’, P. vinciguerrai and P. trucha were found in both reservoirs (Alicura and Piedra del Aguila), and a third species, the large-mouthed perca, P. colhuapiensis, was found in Piedra del Aguila. There was some specialization in diet where the three species coexisted. The large-mouthed perca ate a higher proportion of large prey (>25 mm) and a smaller proportion of small prey (< 15 mm) than did the small-mouthed species, suggesting some diet specialization based on prey size. Percichthys trucha and P. vinciguerrai had smaller gapes, as indicated by shorter heads and jaws, and narrower mouths, in Piedra del Aguila where they coexisted with the large-mouthed species, than in Alicura where the competitor was virtually absent. Thus the direction of morphological changes in trophic-related characters in P. trucha and P. vinciguerrai is consistent with the hypothesis of character shift due to interspecific competition.

Quantifying Predation on Galaxiids and Other Native Organisms by Introduced Rainbow Trout in an Ultraoligotrophic Lake in Northern Patagonia, Argentina: A Bioenergetics Modeling Approach

Abstract Exotic rainbow trout Oncorhynchus mykiss support an economically valuable recreational fishery in Patagonia but also create concern for impacts on native organisms. These concerns are intensified by the possibility of hatchery release programs in this region. We estimated losses of different prey from predation by rainbow trout in Lake Moreno, Rio Negro Province, Argentina, using a bioenergetics model combined with input data from directed sampling on growth, seasonal diet, distribution, and thermal experience. The fish community was sampled seasonally using gill nets, hydroacoustics, and ichthyoplankton nets. Pelagic galaxiid larvae and benthic juvenile and adult small puyen Galaxias maculatus were the most important components of the diet. Bioenergetics simulations showed that over a 6-year life span in the lake (ages 1–7), rainbow trout attained a body mass of 2.3 kg and consumed 74.7 kg of food, of which 20% consisted of galaxiid larvae and 16% consisted of adult small puyen. Based on an esti...

Response of Rainbow Trout to Different Food Web Structures in Northern Patagonia: Implications for Growth, Bioenergetics, and Invasiveness

Abstract Rainbow trout Oncorhynchus mykiss have successfully colonized almost all available water bodies in Patagonia. Introduction and restocking practices have taken place since the beginning of the 20th century without prior consideration of differences between environments in terms of fish community composition and trophic food web structure. We assessed how observed growth, growth efficiency, production, and biomass of rainbow trout populations from four Patagonian lakes were influenced by food web structure and prey quality, and we evaluated the growth strategies adopted by this species in response to environmental differences. Fish communities vary in terms of species composition and dominance. Bioenergetics simulations showed that size and growth patterns of rainbow trout varied between lakes, mainly owing to dietary differences. Fewer and lower-quality food items led to low growth efficiency and stunted growth, while a more diverse and higher-energy diet base led to higher growth efficiency and l...

Growth and nutritional condition of the larvae of Odontesthes microlepidotus (Atherinidae): an experimental approach

SynopsisThe relation between the feeding and mortality of fish larvae is mediated not only by starvation but also by their growth. In this work, daily otolith ring deposition was analysed in laboratory-reared larvae of Odontesthes microlepidotus (Jenyns), and their growth was studied in relation to ring number and otolith diameter. The nutritional condition of these larvae was assessed by means of morphometric comparisons between fed and starved larvae in order to develop a tool to evaluate the nutritional status of larvae in breeding areas. Changes in body shape have been found as a consequence of starvation. No dependence was found between mean hepatocyte nuclear area and fasting.

Diet of the Neotropical Cormorant (Phalacrocorax brasilianus) in a Patagonian Freshwater Environment Invaded by Exotic Fish

Abstract. The diet of the Neotropical Cormorant (Phalacrocorax brasilianus) was evaluated in the Limay River basin, a freshwater system invaded by exotic salmonids. Analyses of 106 pellets showed that fish were the most important prey (79.9% by numerical frequency and 86.2% by frequency of occurrence), followed by two crustacean species. Among fish, the most common species were exotic salmonids, representing 84% by numerical frequency. Morphometric comparisons enabled differentiation of sagitta otoliths from Oncorhynchus mykiss and Salmo trutta and permitted determination of similar contributions of these species to the diet. Although a wide prey-size range was found, 85% of fish were smaller than 150 mm in length. The results suggest that the Neotropical Cormorant has adapted to changes in the fish community after the introduction of salmonids. Flexible feeding strategies of the Neotropical Cormorant and/or its capacity to exploit different environments probably make it less vulnerable to environmental changes produced by introduction of exotic fish.