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Southern hemisphere humpback whales have evolved energetically demanding capital breeding and migratory life-history behaviours. It has been hypothesised that not all individuals of a population participate in the seasonal migration each year, or only undertake partial migrations.Given the cost of migration and reproduction, we explored the possibility that specifically, not all mature females participate in the seasonal migration every year, or significantly delay or shorten their migration, in response to poor feeding conditions. That is, females must attain a minimum threshold of accumulated energy reserves to commit to a reproductive event that likely occurs as a product of mating during migration. With a 1:1 male to female birth ratio, yet a male bias observed along the main migratory corridor; this study utilised inter-annual migratory cohort sex ratios to explore their potential to serve as measures of population fecundity, as a function of ecosystem health. The sex ratios of randomly biopsied adult humpback whales, sampled at a defined location and set time-points along the main migratory corridor from 2008 to 2016 were investigated. Northward migration sex ratios in 2009, 2014 and 2016 revealed a lower male bias suggesting good female participation in the migration and therefore apparent optimal provisioning during the two preceding summers. By contrast, the 2011 southward migration, revealed the highest male bias recorded of 5.75:1. Southward migration sex ratios were found to oscillate closely with measures of population adiposity, a sentinel parameter employed for longterm surveillance of the Antarctic sea-ice ecosystems under the Southern Ocean Observing System-endorsed Humpback Whale Sentinel Program. Anomalously poor humpback whale body condition recorded in 2011 was attributed to poor Antarctic feeding conditions during the extreme La Niña event of 2010/11. These findings lend support for the application of migratory cohort sex ratios, standardised by time and location, as a measure of relative inter-annual

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