East coast of Scotland bottlenose dolphins: estimate of population size 2015-2019
Published: 2021
Authors:
Hammond, P.S., Arso Civil, M. Sea Mammal Research Unit, Scottish Oceans Institute, University of St Andrews.
Cheney, B. & Thompson P.M. Lighthouse Field Station, Institute of Biological and Environmental Sciences, University of Aberdeen.
The number of bottlenose dolphins along the east coast of Scotland was calculated using a weighted mean of annual estimates for 2015-2019 from Arso Civil et al. (2021).
The input estimates are provided in Table 1.
|
Year |
Estimate |
CV |
|---|---|---|
|
2015 |
214 |
0.048 |
|
2016 |
244 |
0.068 |
|
2017 |
230 |
0.036 |
|
2018 |
213 |
0.059 |
|
2019 |
213 |
0.071 |
The weighted mean was calculated using inverse-CV2 as weights, which are more appropriate than inverse variance weights for mark-recapture estimates of population size because in these estimates the variance is not independent of the estimate. In practice, however, this makes almost no difference for these estimates.
The weighted mean was calculated as:
Equation for weighted mean population size
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The weighted mean population size, represented as N̂w, is given as the sum Σ of the weighting applied to each year wy multiplied by the estimate for each year N̂y, where y is year (2015 … 2019).
with variance:
Equation for variance of the weighted mean
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Given as the sum Σ of the weighting squared applied to each year y (2015 … 2019) multiplied by the variance of the estimate of population size in year y, represented as N̂y.
where N̂y = estimate of population size in year y (2015 … 2019), and
Equation for the weighting applied to each year y (2015 … 2019)
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The weighting applied to each year, wy, is given as the inverse-CV2 for the estimate of the population size in year y (2015 … 2019), divided by the sum of inverse-CV2 for all years. CV is the coefficient of variation.
The weighted mean population size, N̂w = 224 (CV = 0.023) with lognormal 95% confidence interval = 214-234.
Reference
Arso Civil, M., Quick, N., Mews, S., Hague, E., Cheney, B.J., Thompson, P.M. & Hammond, P.S. 2021. Improving understanding of bottlenose dolphin movements along the east coast of Scotland. Final report. Report number SMRUC-VAT-2020-10 provided to European Offshore Wind Deployment Centre (EOWDC), March 2021 (unpublished).
