Site-specific female-biased sex ratio of humpback whales (Megaptera novaeangliae) during a stopover early in the southern migration

Abstract

Previous research on humpback whales (Megaptera novaeangliae (Borowski, 1781)), in both the Northern and Southern hemispheres, has reported site-specific male-biased sex ratios in breeding grounds and along migratory corridors. However, one recent Southern Hemisphere study reported a female-biased sex ratio in a feeding area within a coastal migratory corridor, indicating that females may preferentially occupy some habitats. We investigated the classes and relative seasonal timing of humpback whales using Hervey Bay (Queensland, Australia) as a stopover early in the southern migration. Modeling and analyzes were undertaken using data from resighting histories of 361 individually identified whales between 1992 and 2009. The data consisted of 2131 sightings categorized by either sex, age, reproductive status, or maturational status. A female-biased sex ratio of 2.94:1 indicates that Hervey Bay is a preferential stopover for females. The data revealed that the bay is important for mature females who co-occur with immature males and females during August. During September and October, mothers with calves accompanied by a few escorts dominate the Bay. Immature males and females socialize with mature females during August and September. The data support the hypothesis that habitat preferences and differential migration of females and males provide a plausible explanation for site-specific sex-bias in breeding grounds, migratory stopovers, and along migratory corridors.