Abstract

The waters of the Magellan Strait shelter a persistent and small feeding subpopulation of humpback whales (HWs) which is growing at a slower rate than expected, making it highly vulnerable. Despite their low abundance, the HWs represent significant consumers in this austral ecosystem. To better understand the needs of this summering subpopulation in the Magellan Strait feeding area, we estimated the biomass of prey required by HWs to meet their annual energy requirements using a velocity-dependent bioenergetic model and scaled it to two estimates of annual abundance (2011 and 2012), as well as the fraction of primary productivity required (PPR) to support that biomass of prey using a simple trophic energy transfer model. Based on a range of abundance between 75 and 101 whales, the summering subpopulation was estimated to require between 4,007 and 12,673 tons of prey under different population abundances estimates, energy needs, and residence times. These findings imply a minimum and maximum estimate ranging from 12.1 to 62.6 g C/m(-2)(- |-) y(-1), or similar to 6.8 to 41.5 % of net primary productivity in the feeding area to sustain the prey species directly required by HWs. Even though our estimates of prey requirements and PPR may change as new and better data become available, the results show that this small summering subpopulation requires at least a significant portion of annual phytoplankton production.