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Increasing numbers of harbour seals and grey seals in the Solent

Marley, Sarah; Castles, Robyne; Woods, Fiona; Hughes, Peter; Arnott, John; MacCallum, Louise

Harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus) both occur within the UK, but display regional contrasting population trends. Whilst grey seals are typically increasing in number, harbour seals have shown varying trends in recent decades following repeated pandemics. There is a need for monitoring of regional and local populations to understand overall trends. This study utilised a 20-year dataset of seal counts from two neighbouring harbours in the Solent region of south England. Generalised additive models showed a significant increase in the numbers of harbour (mean 5.3 to 30.5) and grey seals (mean 0 to 12.0) utilising Chichester Harbour. Conversely, in Langstone Harbour there has been a slight decrease in the number of harbour seals (mean 5.3 to 4.0). Accompanying photographic data from 2016-18 supports the increase in seal numbers within Chichester Harbour, with a total of 68 harbour and 8 grey seals identified. These data also show evidence of site fidelity of harbour seals in this area, with almost a quarter of animals resighted within the past three years. Overall, this long-term study indicates an increasing number of both harbour and grey seals within the Solent. However, more research is required to identify the drivers of this trend.

"AllSealCounts.csv" contains counts of harbour and grey seals collected during visual surveys in Chichester and Langstone Harbours, UK.

"SolentSeals.html" presents the data exploration and analysis, with an extended example of applying GAMs to harbour seal counts in Chichester Harbour.

"SolenSeals.Rmd" presents the RMarkdown code used to prepare SolentSeals.html

"SolentSealCatalogue.pdf" contains photographs of all the individual seals recorded in this study, along with their sex (if known) and first sighting date.

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