title: State-space modelling reveals proximate causes of harbour seal population declines.
abstract: Declines in large vertebrate populations are widespread but difficult to detect from monitoring data and hard to understand due to a multiplicity of plausible biological explanations. In parts of Scotland, harbour seals (Phoca vitulina) have been in decline for 10 years. To evaluate the contributions of different proximate causes (survival, fecundity, observation artefacts) to this decline, we collated behavioural, demographic and population data from one intensively studied population in part of the Moray Firth (north-east Scotland). To these, we fit a state-space model comprising age-structured dynamics and a detailed account of observation errors. After accounting for culling (estimated by our model as 14% of total mortality), the main driver of the historical population decline was a decreasing trend in survival of young individuals combined with (previously unrecognised) low levels of pupping success. In more recent years, the model provides evidence for considerable increases in breeding success and consistently high levels of adult survival. However, breeding success remains the most volatile demographic component of the population. Forecasts from the model indicate a slow population recovery, providing cautious support for recent management measures. Such investigations of the proximate causes of population change (survival, fecundity and observation errors) provide valuable short-term support for the management of population declines, helping to focus future data collection on those ultimate causal mechanisms that are not excluded by the demographic evidence. The contribution of specific ultimate drivers (e.g. shooting mortality or competitors) can also be quantified by including them as covariates to survival or fecundity.
authors.affiliation: Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medicine, Veterinary and Life Sciences, University of Glasgow, Graham Kerr Building, Glasgow, G12 8QQK, UK, jason.matthiopoulos@glasgow.ac.uk.
authors.forename: Jason
authors.initials: J
authors.lastname: Matthiopoulos
authors.forename: Line
authors.initials: L
authors.lastname: Cordes
authors.forename: Beth
authors.initials: B
authors.lastname: Mackey
authors.forename: David
authors.initials: D
authors.lastname: Thompson
authors.forename: Callan
authors.initials: C
authors.lastname: Duck
authors.forename: Sophie
authors.initials: S
authors.lastname: Smout
authors.forename: Marjolaine
authors.initials: M
authors.lastname: Caillat
authors.forename: Paul
authors.initials: P
authors.lastname: Thompson
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D001947:Breeding; D003710:Demography; D005260:Female; D005298:Fertility; D008297:Male; D008954:Models, Biological; D046049:Phoca; D011156:Population Density; D011157:Population Dynamics; D012606:Scotland
references.citation: Trends Ecol Evol. 2005 Nov;20(11):579-81
references.pmid: 16701438
references.citation: Ecol Appl. 2008 Dec;18(8):1932-55
references.pmid: 19263889
references.citation: Conserv Biol. 2007 Aug;21(4):945-55
references.pmid: 17650245
references.citation: Trends Ecol Evol. 2010 Jun;25(6):345-53
references.pmid: 20188434
references.citation: Proc Biol Sci. 2013 Jun 19;280(1764):20130847
references.pmid: 23782881
references.citation: Acta Biotheor. 1982;31(3):145-64
references.pmid: 6815944
references.citation: Biometrics. 2009 Jun;65(2):572-83
references.pmid: 18565166