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Estimates of Fueguian sprat consumption by humpback whales in the Magellan Strait feeding area as predicted by a bioenergetic model

Jorge Acevedo*, Jorge Urbán

*Corresponding author:

ABSTRACT: Knowing the biomass of preys consumed by a marine predator is a prerequisite for assessing the predator’s potential for competition with fisheries. Herein the biomass of Fueguian sprats to be consumed annually by a small subpopulation of humpback whales Megaptera novaeangliae in the Magellan Strait feeding area, was estimated based on a velocity-dependent bioenergetic model that integrates annual energy requirements by sex, age class, and reproductive status, proportion of Fueguian sprats Sprattus fueguensis in the diet, and annual population size of whales. The annual energy required in kcal per individual whale was estimated to be 18.88 × 107 for calves, 27.92 × 107 for adults, 30.71 × 107 for pregnant, and 42.59 × 107 for lactating females. These estimates result in an energy requirement of 19.32 × 109 and 23.41 × 109 for a seasonal abundance of 78 and 96 whales, respectively. Mean diet solutions predict that Fueguian sprats accounted between 27% and 33% of the diet of the whales. This implies that humpback whales remove between 2965 and 3896 tonnes of Fueguian sprats per yr during a feeding period of 120 d. However, if estimates are extended to an abundance of 204 humpback whales during the period 2004–2017, the consumption is elevated to 8167–8383 tonnes yr–1. The estimates provided here are useful to apply as input data of consumption by the humpback whales in the Fueguian sprat fishery management as well as for the conservation plans of this small and vulnerable feeding subpopulation of humpback whales.