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Harbour seal haulout transition rates in and around the Sound of Islay

Research output: Book/ReportCommissioned report

Abstract

The aim of this study was to predict the changes in the number of seals hauled at the South-East Islay Skerries Special Area of Conservation (EIS SAC) in response to disturbance at other haulout sites.

Telemetry data from 25 harbour seals, tagged between 2011 and 2014 at capture sites close to the Sound of Islay, were used to populate a movement model based on individual haulout transition matrices. This model generalised the matrices in order to represent population movements. Disturbance was modelled as the serial permanent closure of one of the 35 haulouts used by the tagged seals. The model excluded movement during the breeding season. The modelled response was the change in numbers hauled out at the Ardmore haulout site within in EIS SAC. The varying effect of disturbing different haulout sites reflected the complexity of the haulout network.

Most disturbances had a positive effect of the number of seals at Ardmore (range: -0.5% to +21%). Haulout sites with the largest effects were within 50 km of Ardmore and there was little or no effect when the disturbed site was more than 150 km away. However the response was variable. Within 50km however, distance did not predict which disturbed haulouts affected Ardmore, as many sites within 50km had little or no effect. Thus the power to infer the effect of remote haulout disturbance by distance alone was limited, other than to say that the effect is greatest within 50 km of the haulout of interest.

However, within a range of 50km, the shortest network path between the disturbed haulout site and Ardmore provided more information about which sites had an effect. Haulout sites network adjacent to Ardmore (such as Machrihanish and Eilean nan Coinein) had a larger influence. There was no significant effect when a disturbed haulout site was more than two transition jumps (connections) from Ardmore. Such network path information can be efficiently obtained in other areas with a simplified and cheaper telemetry system

The effect of disturbance on the entire EIS SAC depended on the representativeness of the 25 tagged seals’ usage within the EIS SAC. The distribution of haulouts in the August moult survey differed from the haulout usage of the tagged seals in this study. However this may be due in part to redistribution during the breeding season. If the tagged seals were representative, the proportional effect of a disturbance to the EIS SAC would be similar. If, however, seals that used other haulout sites in the EIS SAC were part of a completely different network of haulout site then the effect reported here would be reduced.

Whilst useful in this study, the model that was developed was essentially mechanistic. The limitations of this approach are reviewed and recommendations about future work on Individual Based Models are made.
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Details

Original languageEnglish
PublisherScottish Government
Commissioning bodyScottish Government
Number of pages22
StatePublished - 2015

    Research areas

  • Harbour seals (Phoca vitulina), Conservation management, Haulout pattern, Movement ecology, Simulation model, Disturbances

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ID: 224166885