Harbour Seal Decline – vital rates and drivers. Report to Scottish Government HSD2

Numbers of harbour seals (Phoca vitulina) have dramatically declined in several regions of the north and east of Scotland, while numbers have remained stable or have increased in regions on the west coast. For any management and mitigation plans to address this situation, the relative contribution of various factors in the decline of harbour seals in Scotland needs to be identified, understood and assessed. Potential drivers of the decline include changes in prey quality and/or availability, increasing grey seal population size which may be influencing harbour seal populations through direct predation or competition for prey resources, and the occurrence and exposure of seals to toxins from harmful algae (domoic acid and saxitoxins).

Population model: Work continued to develop an integrated harbour seal population model. The model-fitting process was built upon, using a decision-support simulation tool to fit an age-structured population model to harbour seal count data, investigating the effect of ‘reducing’ the data by only including moult counts (excluding pup counts) and thinning the number of available data points. A visualisation tool was developed to support discussions about the relative impacts of effects that might be important during the different phases of harbour seal life-history. Based on simulated data, a number of scenarios were explored in which additional mortality, fecundity, and adult and pup survival were allowed to vary within plausible limits. The resulting effect on the predicted (simulated) population growth was visualised by means of a surface plot.

Photo-identification mark-recapture to estimate fecundity and survival: Photo-identification data were collected at selected harbour seal haulout sites in Orkney, Kintyre and Loch Dunvegan (Isle of Skye) during the pupping season in 2016 and 2017, primarily during the months of June and July. All photographs were graded for quality and individual seals identified from the unique patterns in their pelage. Photo-identification data collected in 2017 is currently being processed. For 2016, a summary of all catalogued seals by area with details on approximate age class and reproductive history has been made. Loch Dunvegan produced the highest number of catalogued seals. One of the monitored haulout sites in Kintyre was male-dominated, while mum-pup pairs were found in other sites.

Live capture-release studies: Live capture-release studies were conducted in Isle of Skye in March and Orkney in April and May 2017 in accordance with the SMRU Animal (Scientific Procedures) Act, 1986, (Home Office Licence No. 192CBD9F). Adult and juvenile harbour seals were captured, individual covariate data were collected from each seal, and telemetry tags (GSM/GPS and LO tags) were deployed primarily on adult females. Pregnancy status was determined from progesterone concentrations in the plasma and in blubber. The proportion of the live-captured adult females that were pregnant was 100% (95% CI 95% – 100%) in Isle of Skye and 67% (95% CI 39% – 95%) in Orkney, but the proportions were not statistically significantly different. Given the small sample size, further investigations must be carried out before any conclusions can be drawn. Domoic acid concentrations in the urine and faecal samples collected from the live capture-release animals were determined. Domoic acid concentrations were lognormally distributed, with some individuals having very high levels but in most animals concentrations were low. There was no difference in the median concentrations by region, with the Skye animals also being exposed to domoic acid.

Prey samples: Two fishing trips to collect prey samples were undertaken in July and November 2017 in the waters of Scapa Flow. Additionally, opportunistic fish samples were collected in North Ronaldsay. All fish viscera were analysed for domoic acid content, using the same method as for the seal samples. All samples were above the limit of detection, with the bullrout, and mackerel caught in the summer showing the highest concentrations. Fish guts sampled in Orkney in July 2017 and in Sinclair Bay (Caithness) in June 2017 were analysed for PSP toxins, but none of the samples contained any detectable level of saxitoxin.

Counts of harbour seals during the moult

Aerial surveys of harbour seals numbers hauled out during the moult were conducted in the study sites of Kintyre, Scapa Flow (Orkney) and Loch Dunvegan (Isle of Skye) in August 2015, 2016 and 2017, respectively, as part of the annual surveys conducted by SMRU (funded by Scottish Natural Heritage (SNH) and Natural Environment Research Council (NERC)). Results on the number of harbour and grey seals counted within the defined study areas are presented.

Stranded seals: A summary of all seal carcasses reported to Scottish Marine Animal Stranding Scheme (SMASS) within and nearby the study sites between March 2017 and February 2018 is provided, with details on species, age class and proximate cause of death when available.