Abstract
This report provides a summary of the photo-identification surveys conducted by the Sea Mammal Research Unit (SMRU) in the summers of 2017 to 2019 in the Tay estuary and adjacent waters, supported by the European Offshore Wind Development Centre. The survey data were used to estimate the abundance of animals using this area since 2009, and compared to the total east coast of Scotland population, estimated using part of the long-term photo-identification dataset collected by the University of Aberdeen and SMRU (2009-2019). In addition, this collaborative long-term dataset (1989 - 2019) was used to provide estimates of survival and fecundity rate, as well as analyse the movement of animals between the Tay estuary and adjacent waters and the Moray Firth Special Area of Conservation (SAC).
A total of 63 boat-based photo-identification surveys were conducted in the Tay estuary and adjacent waters across the three-year study period between May and September each year. These resulted in 54 encounters with bottlenose dolphin groups, and a total of 154 different individuals from all age classes identified from high quality photographs.
The estimated abundance of animals in the Tay estuary and adjacent waters ranged between 84 dolphins (95% CI 77 - 93) in 2011 to 138 dolphins (95% CI 110 - 173) in 2016. On average, the number of animals using this area between 2009 and 2019 represented 53.8% of the estimated total population using the main range between the Moray Firth SAC and the Firth of Forth.
A total of 230 identified juvenile or adult bottlenose dolphins were included in the analysis to estimate survival rate between 1989 and 2019. A total of 105 females gave birth at least once during that time period and were used to estimate birth rate, defined here as the annual probability of a reproductive female having a calf. The estimated apparent survival probability for juveniles/adults was 0.944 (95% CI 0.933 - 0.953) based on the most supported model. The expected inter-birth interval for the population was estimated at 3.95 years (95% CI 3.63 - 4.20), resulting in an estimated birth rate of 0.253 (95% CI 0.238 - 0.275).
A continuous time hidden Markov model was used to assess movement patterns of male and female bottlenose dolphins between the Moray Firth SAC and the Tay estuary and adjacent waters. Between 2017 and 2019, 112 individuals were only seen in the Moray Firth SAC, 103 were only seen in the Tay estuary and adjacent waters and 51 were seen in both areas. Of the 51 seen in both areas, 40 were seen in both areas within the same year. Model results for the period 1990 - 2019 indicated that movement between the two sites is infrequent but that, despite the clear individual heterogeneity, there is a seasonal movement pattern that is directional and consistent over years. The transition intensities (movement rates) were highest from the Tay estuary and adjacent waters towards the Moray Firth SAC in early summer and from the Moray Firth SAC to the Tay estuary and adjacent waters in late summer. This pattern was consistent across individuals of both sexes, but male dolphins had higher transition intensities than females leading to differences in estimated mean sojourn times spent in one area or the other. It is unclear what drives different individuals to move between these two locations.