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

Abstract

Knowing the biomass of prey consumed by a marine predator is a prerequisite for assessing the potential of the predator for competition with fisheries. Here, we estimated the biomass of Fuegian spratSprattus fueguensisconsumed annually by a small subpopulation of humpback whalesMegaptera novaeangliaein the Magellan Strait feeding area. We used a velocity-dependent bioenergetic model that integrates annual energy requirements by sex, age class, and reproductive status, proportion of Fuegian sprat in the diet, and annual population size of whales. The annual energy required in kcal per individual whale was estimated to be 18.88 × 107for calves, 27.92 × 107for adults, 30.71 × 107for pregnant females, and 42.59 × 107for lactating females. These estimates result in an energy requirement of 19.32 × 109and 23.41 × 109for a seasonal abundance of 78 and 96 whales, respectively. Bayesian dietary mixing models predict that Fuegian sprat represented between 27 and 33% of the diet of the whales. This implies that humpback whales remove between 2965 and 3896 t of Fuegian sprat per year 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 t yr-1. The estimates provided here are useful to apply as input data for consumption by humpback whales in the Fuegian sprat fishery management as well as for conservation plans of this small and vulnerable feeding subpopulation of humpback whales.