Estimating seasonal abundance trends and survival probabilities of humpback whales in Hervey Bay (east coast Australia)

Abstract

The abundance of east Australian Group V substock (EAGVS) humpback whales resident during winter in Hervey Bay was estimated from a 10 yr mark-resight study using photo-identification of 969 individual humpbacks sighted between 1987 and 1996. Hervey Bay is on the east coast of Australia and is the major southbound stop-over site for humpbacks returning to Antarctic waters from overwintering in Great Barrier Reef (GBR) waters. Seasonal abundance estimates were derived from mark-resight profiles using a reduced form Cormack-Jolly-Seber (CJS) model (constant survival, time-varying resight likelihood) that fitted the data well. The bootstrap mean CJS abundance estimate over the 9 yr period from 1988 to 1996 was 855 (95% CI: 750 to 936). Estimated humpback abundance m Hervey Bay showed significant temporal variability superimposed on an increasing linear trend estimated using times series regression model bootstrapping at 6.3 % yr (95 % CI: 2 to 11%). The seasonal Hervey Bay population comprised 30 to 50 % of the EAGVS southbound to Antarctic feeding grounds. Estimated abundance increased from 554 post-yearling humpbacks in 1988 to a peak of 1040 in 1991 before declining to 921 by the mid-1990s. Standard errors of abundance estimates suggested good precision and were derived using a variance components approach that separated sampling error from ecologically relevant variation. The trends in temporal variability and annual rate of humpback abundance increase were consistent with findings from an aerial surveillance study (1982 to 1996) of monthly sightings of the EAGVS overwintering in southern GBR waters. The concurrence of findings from an independent method of abundance estimation provides confidence in the CJS model used in this study to estimate abundance. Post-yearling survivorship was estimated from a 4 yr (1993 to 1996) photo-identification study of 517 individual humpbacks sighted at 2 seasonally sequential overwintering sites (Hervey Bay, Whitsundays) using a robust design CJS modelling approach with estimators that account for bias due to temporary emigration. A reduced form CJS model (constant survival, time-varying resight likelihood) also fitted the data well with the mean annual survival rate for the EAGVS humpbacks estimated at 0.966 (95% CI: 0.87 to 1.00). The good fit of the robust design survival rate model provides further confidence in the Hervey Bay abundance model, which suggests that the EAGVS has been recovering but at a slow and variable rate.