Indo-Pacific humpback dolphins (Sousa chinensis) in Hong Kong: Modelling demographic parameters with mark-recapture techniques

Abstract

Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting Hong Kong waters are thought to be among the world's most anthropogenically impacted coastal delphinids. We have conducted a 5-year (2010–2014) photo-ID study and performed the first in this region comprehensive mark-recapture analysis applying a suite of open population models and robust design models. Cormack-Jolly-Seber (CJS) models suggested a significant transient effect and seasonal variation in apparent survival probabilities as result of a fluid movement beyond the study area. Given the spatial restrictions of our study, limited by an administrative border, if emigration was to be considered negligible the estimated survival rate of adults was 0.980. Super-population estimates indicated that at least 368 dolphins used Hong Kong waters as part of their range. Closed robust design models suggested an influx of dolphins from winter to summer and increased site fidelity in summer; and outflux, although less prominent, during summer-winter intervals. Abundance estimates in summer (N = 144–231) were higher than that in winter (N = 87–111), corresponding to the availability of prey resources which in Hong Kong waters peaks during summer months. We point out that the current population monitoring strategy used by the Hong Kong authorities is ill-suited for a timely detection of a population change and should be revised.

Threshold of long-term survival of a coastal delphinid in anthropogenically degraded environment: Indo-Pacific humpback dolphins in Pearl River Delta

“…3 ) can be applied in the monitoring of population status as part of integrated management program. Estimates of population survival rates can be obtained through either the construction of life tables 8 57 , or by means of photo-ID mark-recapture studies 7 22 58 . The former requires large sample sizes of recovered carcasses, which in the case of cetaceans may take many years to collect.…”

“…During that time, a substantial part of the population would have been already lost. A revision of the current monitoring strategy is therefore highly advisable and should be considered as a matter of urgency (see Chan and Karczmarski 58 ). Furthermore, we postulate that the approach recommended here (periodic re-assessment of adult survival rate) can be effectively applied to other coastal species elsewhere and prove useful in many cetacean conservation projects.…”

Abundance and residency dynamics of the Indo‐Pacific humpback dolphin, Sousa chinensis, in the Dafengjiang River Estuary, China

“…The model with the lowest AIC value and estimable SE of parameters was selected as the best‐fitted model. Models with ΔAIC < 5 were selected as the plausible models (Chan & Karczmarski, ).…”

A mark–recapture approach for estimating population size of the endangered ringed seal (Phoca hispida saimensis)

“…As a statistically significant transient effect was observed, this should be taken into account in future population models when surveying the whole Lake Saimaa basin since it has been pointed out that sub-adult seals may move to the edges of the breeding areas, which would support this transient effect [ 69 ]. This effect could be introduced into the Cormack–Jolly–Seber models and the estimation of survival in future studies, but it is not applicable to the POPAN model as used here (see [ 30 ]). In addition, the survey effort can be incorporated into the models as a covariate parameter.…”

“…Photo-identification (photo-ID)-based mark-recapture studies of estimating population have increasingly been used in studies of marine mammals, mostly cetaceans (e.g., [ 25 – 30 ]), and these techniques has also been utilized recently for estimating pinniped populations [ 31 – 35 ]. Since the permanent pelage pattern of ringed seals is individual and lifelong, photo-ID has been shown to be a suitable approach for monitoring the Saimaa ringed seal [ 36 ].…”