Understanding fish diet and feeding is essential for resolving aquatic food webs and is central to proper fisheries and ecosystem management. In Puerto Rico, the American Eel and Bigmouth Sleeper play key roles as apex consumers and native, freshwater sport fishes. The trophic relationships of these species remain virtually un-documented in Caribbean rivers. In this context migrating postlarval fishes and shrimp make up a potentially important component of the available prey base. Recent studies show that in Puerto Rico’s postlarval fishes undergo mass-upriver migration events during 3 days during the last-quarter moon phase. Our objectives were to characterize the diets of American Eel and Bigmouth Sleeper in the Río Grande de Arecibo, Puerto Rico, and to compare their diets before and during a mass-migration event of amphidromous postlarval fishes. Riverine predators were collected during daytime hours by boat electrofishing during two time periods. The first period was five days preceding a mass-migration of postlarval amphidromous fishes, and the second was three days during the mass-migration event. Predator diets were sampled with acrylic esophageal tubes and by stomach excision. Preserved stomach contents were identified to the lowest taxa feasible, and diet items were enumerated and weighed.
Prior to the migration event, 3.8% of American Eel diet samples contained prey items, while during the migration, 43.8% contained prey items. In the second sampling period, the most important American Eel diet items were postlarval shrimps (mean 32.8% of diet by weight) and postlarval Sirajo Gobies (mean 29.7% of diet by weight). Bigmouth Sleeper diets were dominated by the Puerto Rican Freshwater Crab and Adult Shrimps (means of 29.2% and 24.6% of diet, by weight, respectively) during the first sampling period. During the second sampling period Bigmouth Sleeper diet samples contained mostly postlarval Sirajo Gobies and postlarval shrimps (mean 34.2% and 38.4% of diet, by weight, respectively). Our results suggest that both predator species shifted foraging behaviors and prey during the mass migration event. American Eels may have shifted from low feeding rates or nocturnal feeding to higher diurnal feeding rates during the migration. Bigmouth Sleepers appear to have shifted their diet from larger prey items, such as adult shrimps and juvenile crabs prior to the migration, to smaller items such as postlarval fishes and shrimp during the migration. In conclusion, amphidromous fish and shrimp migrations appear to play a periodically important role in the trophic ecology of Caribbean riverine predators.