Pup mortality in a rapidly declining harbour seal (Phoca vitulina) population

Abstract

The harbour seal population in Orkney, off the north coast of Scotland, has reduced by 65% between 2001 and 2010. The cause(s) of this decline are unknown but must affect the demographic parameters of the population. Here, satellite telemetry data were used to test the hypothesis that increased pup mortality could be a primary driver of the decline in Orkney. Pup mortality and tag failure parameters were estimated from the duration of operation of satellite tags deployed on harbour seal pups from the Orkney population (n = 24) and from another population on the west coast of Scotland (n = 24) where abundance was stable. Survival probabilities from both populations were best represented by a common gamma distribution and were not different from one another, suggesting that increased pup mortality is unlikely to be the primary agent in the Orkney population decline. The estimated probability of surviving to 6 months was 0.390 (95% CI 0.297 - 0.648) and tag failure was represented by a Gaussian distribution, with estimated mean 270 (95% CI = 198 - 288) and s.d. 21 (95% CI = 1 - 66) days. These results suggest that adult survival is the most likely proximate cause of the decline. They also demonstrate a novel technique for attaining age-specific mortality rates from telemetry data.

Figure 1. Harbour seal pup mortality and hazard rates.

(A) The probability density function of mortality events (f(t)) and (B) the hazard rate (h(t)) plotted against age for exponential (dotted lines), gamma (solid lines) and lognormal (broken lines) distributions. Each curve shown uses the parameters that best fit the tagging data and demonstrates the different characteristic shapes of the functions. The exponential distribution gives a constant hazard rate; the gamma distribution shows an increasing hazard rate over time, and the lognormal hazard rate increases to a peak then declines.

Figure 2. Habour seal pup tag durations.

Pups were tagged on the west coast of Scotland at Lismore (red) in late June and in the Orkney Islands (blue) in early July. The date of last transmission from a live animal was determined by examining the pattern of movement from location data.

Figure 3. Survival of harbour seal pups and tags.

Open squares are the dates of last ‘live’ transmission from tags attached to harbour seal pups at Lismore and solid circles are those deployed at Orkney. Solid lines are the estimates of pup (blue; assumed to have been born on the 20^th June) and tag (green; assumed to have been attached on 23^rd June) survival from the best of the fitted models, which had a common gamma distribution for survival for both Orkney and Lismore and a common birth date. Shaded regions show 95% confidence limits generated from 500 bootstrap re-samples of the data.

Figure 4. Survival trajectories of simulated tagged pups.

Each line connects the last ‘live’ transmission day for a set of 24 simulated tagged harbour seal pups (100 simulations). Pup mortalities were drawn from a gamma distribution. Animals were considered to be tagged at birth and tag failure to be normally distributed. All parameter values were taken from Table 2. The Orkney (solid circles) and Lismore (open squares) data also are shown.