Grazing responses of tropical freshwater fishes to different scales of variation in their food

Abstract

Grazing fishes in neotropical streams confront variation in their attached algal food that ranges in scale from differences in quality among algal cells to differences in the primary productivity of habitats available to the fishes. Fishes may respond to this variation on some scales but not others. For example, loricariid catfish in a Panamanian stream tracked variation in algal productivity among pool habitats very closely. In sunny pools where algae grew about seven times faster than in shaded pools, loricariids were six to seven times denser. Consequently, growth rates of pre-reproductiveAncistrus spinosus (the most common species in pools) were similar in pools of different canopies, corresponding to predictions from the ‘ideal free distribution’ hypothesis. But on a smaller scale, within pools, avoidance of avian and terrestrial predators outweighed foraging considerations. Larger species and size classes avoided water shallower than 20 cm, where (as a result) the only standing crops of attached algae large enough to be measurable by scraping occurred. During the dry season when food was. most limiting, loricariids overlapped more in their substrate use as different species sought cover in common refuges such as logs and root tangles in pools. Seasonal variation in growth rates of pool-dwelling loricariids reflect these constraints.