The use of blubber fatty acid profiles to detect inter-annual variations in the diet of grey seals from two breeding colonies

Diet, and how it varies in place and time, are important factors in understanding the interactions between seals and fisheries, The grey seal eats a wide variety of prey species and variations in availability of prey can lead to changes in both foraging behaviour and diet that could affect the ecological impact of the seals. Since it is difficult to observe the feeding behaviour of seals directly, we have used changes in the fatty acid profiles of blubber to indicate changes in their diet. We studied inter-colony and inter-annual variations in the fatty acid profiles of female seals from 2 Scottish breeding colonies, North Rona (RON) and the Isle of May, over 3 consecutive years from 1996 to 1998. The fatty acid profiles obtained were compared and tested using several multivariate statistical methods, including a new inter-population measure Dfap (a distance measure based on fatty acid profiles), There were significant inter-colony differences in blubber fatty acids in all years,of the study. There were significant within-colony variations among years for the seals at the Isle of May but not at RON. Individual seals sampled in more than 1 yr tended to change in a similar manner at the Isle of May but not at RON. Discriminant analysis of the fatty acid profiles provided classifications of the Isle of May seals according to year with 84 to 97 % accuracy, but was less successful with the RON seals with 56 to 74 % accuracy. Only 4 out of 166 seals were 'mis-classified' into the wrong Isle of May or RON grouping. This analysis suggested a shift in diet at the Isle of May that was supported by evidence from a separate study on the analysis of otoliths in seal faeces.