Ensambles de peces en los arrecifes norpatagónicos: diversidad, abundancia y relaciones tróficas y con el hábitat
This study describes the structure and seasonal dynamics of the reef fish assemblages that inhabit the Nuevo and San José gulfs of the Argentine coast. It analyzes relationships between the structure of the assemblage and several habitat variables, with emphasis on shelter availability and refuge characteristics. The diets and feeding strategies of the most conspicuous fish species are described and their impact on the density of one of their preys is evaluated. The northern Patagonian gulfs are in a temperate region (41° – 43° Lat. S) and are characterized by large extensions of sandy bottoms scattered by patches of rocky outcrops that form reefs. The fish assemblage associated to these reefs is low in species richness and has components of the warm-temperate region (Argentine province) and of the coldtemperate region (Magallanic province), although warm-temperate species were dominant in both gulfs. Of the species recorded, eight were demersal fishes and associated to refuges, and three were schooling species. The most conspicuous and abundant species were the Argentine sandperch Pseudopercis semifasciata, the Argentine sea bass Acanthistius patachonicus, the Brazilian sandperch Pinguipes brasilianus and the Patagonian redfish Sebastes oculatus, all of them demersal fishes associated to refuges. Other demersal fishes were rare and registered in low abundance: Serranus auriga, the horsefish Congiopodus peruvianus, Bovichtus argentinus and one nototenid Patagonothoten sp. Among the schooling fishes, the hawkfish Nemadactylus bergi, the silver porgy Diplodus argenteus and the red porgy Pagrus pagrus, the hawkfish was the most abundant. The composition of the assemblage and the relative abundance of the species varied seasonally, most of the species showing highest abundances during the warm season. Only two species appeared to be limited by shelter availability: the Argentine sea bass, a generalist fish that occupied the most number of possible refuges, and the Patagonian redfish, an ambush predator that may be associated to refuges that maximize the effectiveness of its feeding strategy. The Argentine sandperch would require the presence of refuges with particular characteristics, but its abundance would be limited by the requirement of a territory. The diversity of fishes increased with increasing variability in the relief of the reef; this trend could be explained on the basis of the relationship between the type of relief and the swimming behavior of the fishes that occur in a reef. The type of relief, the shape of refuge and the percentage of gravel and soft substrate explained microhabitat associations of the most conspicuous species. The diet analysis showed that the Argentine sandperch, the Argentine sea bass, the Brazilian sandperch and the Patagonian redfish were predators of the fourth trophic level. The Brazilian sandperch, and the Patagonian redfish in lesser extent, depended on local reef prey populations. The Argentine sandperch and the Argentine sea bass were consumers subsidized by the adjacent environments. These predators would have an important impact on the abundance of the squat lobster, one of their main soft bottoms associated preys. This impact was limited to the first meters around the reefs and varied with the physical structure of the reef. Based on these results it is hypothesized that the physical structure of the reef could influence the importance of food as a limiting resource for the Argentine sandperch and the Argentine sea bass. The existence of a trophic linkage among reefs and their adjacent environments suggests possible cascade effects on reef fishes caused by management and exploitation actions over the species of the adjacent environments. The four most conspicuous reef fishes, the Argentine sea bass, the Brazilian sandperch, the Patagonian redfish and the Argentine sandperch, constitute a guild of demersal fishes associated to refuges that are within the fourth trophic level. Food and shelter limited the densities of these fishes, however they differed in their relationship with these resources. These results show the importance of not making generalizations about the mechanisms that might regulate the relative abundance of species that compose an assemblage, even in simple systems with few species, but considering the biology and ecology of the particular species.