Rainbow trout effects on zooplankton in the reproductive area of the critically endangered hooded grebe

Abstract

1- Aquaculture in arid Patagonia is potentially affecting the hooded grebe (Podiceps gallardoi), a critically endangered endemic waterbird. Exotic rainbow trout (Oncorhynchus mykiss) were stocked from 1994 in naturally fishless lakes, the primary reproductive habitat of this grebe. 2- Trout and grebes are visual predators, whose diets overlap. Consequently, trout could reduce the abundance of prey of the hooded grebe. 3- This study compared the size distribution and abundance of the pelagic zooplankton fraction preyed upon by trout in four fishless lakes and three lakes stocked with trout, including vegetated and unvegetated lakes. 4- The mean size of Daphnia spp. was 45% and 35% larger in fishless lakes than in stocked lakes, for unvegetated and vegetated lakes, respectively. Boeckella spp. were larger in fishless than in stocked vegetated lakes. 5- Fishless and stocked lakes had highly contrasting biomasses of large pelagic crustaceans. Amphipods were absent from the water column of all stocked lakes analysed, and were abundant in fishless lakes. Parabroteas sarsi was absent from the two large unvegetated lakes, stocked with trout. 6- These shifts in the abundance and size spectrum of the zooplankton may reflect competition between trout and hooded grebe, affecting the survival of the latter species. 7- The current conservation status of this rare aquatic bird demands the application of management tools to reduce the detrimental effects of aquaculture on their primary reproductive habitat.