Abstract

In 2003 a feeding aggregation of southeastern Pacific humpback whales (Megaptera novaeangliae) was reported in the Magellan Strait. While Chile established its first marine national park in the Strait to protect humpback whale habitat, fatal ship strikes remain a concern because of overlap with a busy shipping lane. To better understand population risk, we estimated abundance and survival for this population using Bayesian robust-design mark-recapture models fit to photographic data from 2004 to 2016. Overall, the model estimated a total of 204 whales (95% CI: 199-210) during the last 12 yr, and 93 (95% CI: 86-100) in the 2016/2017 austral summer. The population grew at 2.3% (CI: 2.1%-3.1%), an annual increase of two whales. Annual survival (including calves) was estimated at 0.892 (CI: 0.871-0.910). Our results corroborate a persistent feeding population, but one that is increasing relatively slowly. Owing to its vulnerability stemming from its small size, coupled with significant overlap with a busy shipping lane, we argue this subpopulation is at significant risk from ship strikes and may be one of the few populations where anthropogenic mortalities could regulate population dynamics. We therefore encourage continued monitoring via photographic mark-resighting surveys, and analyses explicitly investigating potential population-level ship strike effects.