Using sparse survey data to investigate the declining abundance of British harbour seals

Abstract

This study presents an analysis of changes in the regional abundance of harbour seals Phoca vitulina based on repeated aerial surveys of haulouts, and demonstrates the use of sparse data to deliver advice about population status and management. Generalized linear models with negative binomial errors were used to represent these overdispersed data. The shape parameter of the negative binomial distribution was directly estimated from the data where this was possible. Information from time-series of counts where there were few gaps in the data was used to improve the estimation of this parameter in areas where fewer surveys had been carried out. The results show that the number of harbour seals in eastern England has not increased since the end of the 2002 phocine distemper epidemic. There is also evidence of a general decline in most of the large harbour seal colonies around Britain. The populations in the Inner Hebrides were an exception, with numbers appearing to be stable or increasing. Between 2001 and 2006, the population in Orkney and Shetland declined by 40% (95% confidence interval: 30-50%), indicating harbour seals in these areas experienced substantially increased mortality or very low recruitment over this period. The widespread declines, ranging from Shetland to The Wash, suggest that the causes may have been present over a large part of the North Sea.

The status of harbour seals (<i>Phoca vitulina</i>) in the United Kingdom

“…A population model was developed that independently estimates two types of uncertainty: the inter-annual environmental variability that modifies the growth rate and therefore has persistent effects on the population; and observation error, the shorter-term changes that influence individual surveys separately (Lonergan et al 2007). Table 1.…”

“…Scotland Different components of the Scottish harbour seal population have shown different dynamics over the last 10 to 15 years (Lonergan et al 2007). The population on the mainland coast of north-west Scotland and in the northern Inner Hebrides are apparently stable.…”

“…However Orkney-wide surveys indicated that this could be explained by local redistribution of individuals within the archipelago (Lonergan et al 2007). …”

The status of the harbour seal (<i>Phoca vitulina</i>) in Ireland

“…However declines in abundance have also been observed in many areas and have been attributed to recruitment failure, competition for resources, disturbance and disease (Frost et al 1999, Thompson et al 2001, Matthews and Pendleton 2006, Lonergan et al 2007. Predicting the potential long-term effects of disease such as Phocine Distemper Virus (PDV) on harbour seal populations requires information on pre-epizootic population trajectories (Harding et al 2002, Lonergan andHarwood 2003).…”

“…In light of a recent suspected outbreak of PDV amongst harbour seal populations in the Baltic Sea (CWSS, 2007) and the fact that it has been over 5 years since the last national harbour seal survey, urgency exists to establish the current population estimate. Moreover, there is recent evidence of a general decline in most of the large harbour seal colonies around Britain, between 2001 and 2006, the population in Orkney and Shetland declined by 40% indicating substantially increased mortality or very low recruitment over this period (Lonergan et al 2007). These declines are more than 4 times the current threshold for possible corrective action defined under the OSPAR international convention.…”