Local abundance, apparent survival and site fidelity of Bryde’s whales in the Hauraki Gulf (New Zealand) inferred from long-term photo-identification

Abstract

Bryde’s whales Balaenoptera edeni in New Zealand are classified as ‘nationally critical’ according to the New Zealand Threat Classification System. In the Hauraki Gulf, Bryde’s whales occur year-round and are subject to ship-strike mortality events. Photo-identification surveys were conducted to estimate local abundance, apparent survival and site fidelity during 2 periods from 2004 to 2006 (261 daily surveys) and from 2011 to 2013 (382 daily surveys). The photo-identification database contained a total of 364 sighting records of 72 Bryde’s whales. Overall, 20 whales were sighted across the 2 survey periods, indicating long-term site fidelity. Local abundance was estimated using the robust design (RD) and POPAN mark-recapture approaches for each period, including upward adjustment for the proportion of unmarked whales. RD seasonal abundance estimates varied from 17 to 43 whales between 2004 and 2006, and from 13 to 31 whales between 2011 and 2013. Temporary emigration followed a random pattern (γ’ = γ’) and was estimated at 0.557 between 2004 and 2006, and 0.610 between 2011 and 2013. POPAN seasonal abundance ranged from 38 to 74 whales for the 2004 to 2006 period and from 42 to 68 whales for 2011 to 2013. Apparent survival was estimated across periods at 0.878 (95% CI = 0.811-0.923). From the ‘super population’ estimate of the 2011 to 2013 survey period (Nsuper = 135 whales, CI = 100-183), we calculated a potential biological removal (PBR) of 1 whale yr-1. Given the impact of ship strikes on this local unit, it is important to continue long-term photo-identification of Bryde’s whales. This technique provides valuable demographic information for a poorly known species.