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.
ABSTRACT1. Compliance with conservation legislation requires knowledge on the behaviour, abundance and distribution of protected species. Seal life history is characterized by a combination of marine foraging and a requirement to haul out on a solid substrate for reproduction and moulting. Thus understanding the use of haul out sites, where seals are counted, as well as their at-sea movements is crucial for designing effective monitoring and management plans.2. This study used satellite transmitters deployed on 24 harbour seals in western Scotland to examine movements and haul-out patterns.3. The proportion of time harbour seals spent hauled out (daily means of between 11 and 27%) varied spatially, temporally and according to sex. The mean haul-out duration was 5 h, with a maximum of over 24 h.4. Patterns of movement were observed at two geographical scales; while some seals travelled over 100 km, 50% of trips were within 25 km of a haul-out site. These patterns are important for the identification of a marine component to designated protected areas for the species.5. On average seals returned to the haul-out sites they last used during 40% of trips, indicating a degree of site fidelity, though there was wide variation between different haul-out sites (range 0% to >75%).6. Low fidelity haul-out sites could form a network of land-based protected areas, while high fidelity sites might form appropriate management units.
Conservation policies require the status of protected species to be monitored. The choice of monitoring methods may be constrained in situations in which there is concern about disturbance or in which sighting individuals is difficult. This study investigated the potential of using a computer-assisted photo-identification method to measure population size in adult harbour seals (Phoca vitulina). Pattern cells or combinations of pattern cells from photographs (i.e., ventral, flank, shoulder, and head) were used for computerized selection of potential matching pairs, and the pelage patterns of those pairs were then checked visually. There was monthly variation in capturerecapture population estimates, with the highest number of adult harbour seals in May (117, CV = 7.2). Around three times more individuals used the sampling area in northwest Scotland between April and October (268, CV = 0.04) than were estimated per month (mean = 86, CV = 0.07). Using computer-assisted photo-identification and capture-recapture methods may be the only practical way of obtaining a measurement of how many seals use a site. This approach has important implications for determining the effectiveness of designated conservation areas for protecting seals and will influence management decisions, including the size of management units.
Aerial surveys of harbour seals (Phoca vitulina) are usually carried out to provide an index of population size. This can be normalized, either by design or by post-hoc analysis to reduce the effects that date, time of day, tide and weather might have on the number of seals counted. In order for long-term trends to be determined from these counts it is assumed that the mean number of seals at a particular site does not vary during the survey period, and that the start and duration of the survey window does not vary with location or between years. This study used a combination of repeat land-based and aerial surveys to test the assumption for constancy of counts during the survey period. The study focused on harbour seal abundance at haul-out sites around the Isle of Skye in north-west Scotland. The coefficient of variation in these counts was estimated to be 15%, based on repeat aerial surveys using thermal imaging. Land-based counts were used to examine the effect of covariates on seal numbers using generalized additive modelling. This site-specific model predicted that the current aerial survey window for harbour seals in the UK, which is a three-week period during the moult, is about a week too early and that count variation could be reduced by surveying 1 1/2 hours earlier in the tidal cycle. Furthermore, the pupping period showed even higher (though more variable) abundance of hauled out seals than during the moult.
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