Abstract
Humpback whalesMegaptera novaeangliaewere severely depleted by commercial whaling. Understanding key factors in their recovery is a crucial step for their conservation worldwide. In Oceania, the Chesterfield-Bellona archipelago was a primary whaling site in the 19thcentury, yet has been left almost unaffected by anthropogenic activities since. We present the results of the first multidisciplinary dedicated surveys in the archipelago assessing humpback whale populations 2 centuries post-whaling. We encountered 57 groups during 24 survey days (2016-2017), among which 35 whales were identified using photographs of natural markings (photo-ID), 38 using genotyping and 22 using both. Humpback whales were sparsely distributed (0.041 whales km-1): most sightings concentrated in shallow inner-reef waters and neighbouring offshore shallow banks. The recently created marine protected area covers most of the areas of high predicted habitat suitability and high residence time from satellite-tracked whales. Surprisingly for a breeding area, sex ratios skewed towards females (1:2.4), and 45% of females were with calf. Connectivity was established with the New Caledonia breeding area to the east (mtDNAFST= 0.001, p > 0.05, 12 photo-ID and 10 genotype matches) and with the Australian Great Barrier Reef breeding area to the west (mtDNAFST= 0.006, p > 0.05). Movement of satellite-tracked whales and photo-ID matches also suggest connections with the east Australian migratory corridor. This study confirms that humpback whales still inhabit the Chesterfield-Bellona archipelago 2 centuries post whaling, and that this pristine area potentially plays a role in facilitating migratory interchange among breeding grounds of the western South Pacific.
Highlights
Large whales were severely depleted by commercial whaling in the 19th and 20th centuries (Clapham 2016)
Due to a historical lack of data, humpback whales migrating along the east Australian coast were considered to be a proxy for BSE1 and most of the literature referring to BSE1 were from data collected on this migratory corridor which could potentially be used by multiple stocks
We acknowledge that the number of genetic samples collected in the Chesterfield-Bellona archipelago is relatively small, our results suggest that the humpback whales currently visiting this area are not significantly different from the breeding substocks BSE1 (Great Barrier Reef) and BSE2 (New Caledonia), as indicated by indices of differentiation based on mtDNA data
Summary
Large whales were severely depleted by commercial whaling in the 19th and 20th centuries (Clapham 2016). The humpback whale breeding population of Oceania was estimated to be the least abundant in the Southern Hemisphere by Constantine et al (2012). Due to a historical lack of data, humpback whales migrating along the east Australian coast were considered to be a proxy for BSE1 and most of the literature referring to BSE1 were from data collected on this migratory corridor which could potentially be used by multiple stocks. The Chesterfield-Bellona archipelago (18.9° to 21.9° S), located in the Coral Sea halfway between the east Australian coast and New Caledonia (see Fig. 1a), was considered as a potential breeding ground for humpback whales passing by the east Australian coast (Dawbin & Falla 1949) during their northern migration from feeding Area V in the Antarctic
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