Dublin Bay Seal Census 2018
Due to the varied nature of the shoreline and the presence of offshore islands, the survey was carried out both from land-based vantage points and from boats and kayaks where necessary. No aerial survey techniques were used. The aim being to achieve at least 1 high quality, co-ordinated, count of the full survey area during the survey period. Additional counts were then used to identify gaps, issues and to provide back up for poor count conditions where necessary.
Field methods were adapted from Ó Cadhla, (2017) Harbour Seal Survey 2017 Guidelines (NPWS, unpublished). These were felt to provide the best methods available for surveying both species of seals present and for the range of conditions likely to be encountered. Given the potential for identification challenges of seals at distance, assistance with identification was also provided in the form of an ID chart. Field maps were provided of each count section on which surveyors could mark haul out sites and other pertinent details.
The survey was carried out over the weekend of 22nd and 23rd September 2018. The weather during the survey was largely favourable with winds only increasing to moderate on the second day and with a benign day of light winds on day 1. Count quality was highest on day 1 (22/09). The survey was timed to coincide the window of 2 hours before and after low tide. This provided the best opportunity for seals to be hauled out. The autumn dates meant that it was best timed for Grey Seals when their attendance at the colonies and haul outs would be close to its highest. While this was less optimal for Harbour Seal. Results were returned quickly to the survey co-ordinator either on paper forms provided or electronically.
The survey area extended from Irelands Eye in the north to Dalkey Island in the south and was split into 9 count sectors. Each was covered by a volunteer or volunteer team with both Irelands Eye and Dalkey Island requiring boat-based surveyors to cover offshore areas. Figure 1. illustrates the survey area and count sectors.
Full and well-coordinated coverage was achieved on 2 out of the 6 counts. These were the optimum count times falling at or about the peak of low tide. These counts form the basis for calculating the population size encountered. With the peak count of the adult population occurring on the 22nd September and the highest number of detectable pups on 23rd September.
It was not possible to accurately assess the age distribution given the variable range of expertise of the surveyors and as a result all well grown adults and 1st year + individuals were assigned as “adult/immature”, with only obvious pups being classed separately.
The minimum population size estimated during the survey is 92 Grey Seals and 5 Harbour Seals (see Table 1). This is derived from the highest count total from single co-ordinated “full” counts with additional information from partial counts used where clearly identifiable separate/additional individuals can determined. In the case of Grey Seals, this was limited to pups and a peak pup count of 4 animals was detected on 23rd September.