Dear colleagues,

My co-authors and I are pleased to share our new article looking at competition between grey and harbour seals in Scotland through temporal changes in their dietary niche.

Langley, I., Hammond, P. S., Thompson, P. M., Pierce, G. J., Wilson, L. J., Hall, A. J., Hastie, G., D., & Russell, D. J. F. (2026). Temporal changes in the dietary niche of sympatric seals provides insight into the role of competition in population declines. Oikos, e11875.

Abstract:

Competition theory suggests that interspecific prey competition can result in changes to the dietary niche, but obtaining timeseries of data from sympatric species experiencing temporal variation in competition is challenging. Scotland is an important area for two species of seals, but over the past 20 years, populations of harbour seals Phoca vitulina vitulina in some areas have declined, and competition with stable/increasing numbers of grey seals Halichoerus grypus is a potential factor. Here, we standardised disparate seal diet datasets (based on analysis of prey hard parts in faeces) to investigate summer dietary metrics in two regions spanning a 24-year period. We estimated dietary niche breadth (Simpson's diversity, D) modelled as a function of sample size, proportional biomass diet composition corrected for digestion, and interspecific dietary niche overlap (Pianka index, O). Dietary niche overlap was high across time and space (O >0.7). In the Moray Firth, the harbour seal dietary niche was narrow when the population was high and stable (1987–1995, mean ± SE, D = 2.9 ± 0.3) but was significantly broader when the population depleted (2010, D = 7.2 ± 0.5). Prey composition shifted from energy-rich sandeels and clupeids to a more diverse diet of gadids and flatfish. Conversely, the dietary niche of the stable/increasing grey seal population was consistently narrow, dominated by sandeels (e.g. 2010, D = 2.3 ± 0.4). In southeast Scotland, where grey seal numbers were stable and harbour seals declined, there was no trend in the dietary niche of either species, though sandeels were less prevalent in later years. We propose that grey seals outcompeted harbour seals in the Moray Firth, and that the availability of sandeels to both seal species decreased in southeast Scotland. Changes in prey availability have therefore likely been a factor in regional harbour seal declines and could be preventing recovery.

https://nsojournals.onlinelibrary.wiley.com/doi/10.1002/oik.11875

Cheers,

Izzy

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Dr Izzy Langley

Sea Mammal Research Unit (SMRU)

Scottish Oceans Institute

University of St Andrews

St Andrews, Fife, KY16 8LB

Pronouns: She/her

Social media: @izzylangley.bsky.social @LangleyIzzy

For information about grey seal predation, see our project webpage here or email SMRUsealreports@st-andrews.ac.uk

The University of St Andrews is a charity registered in Scotland: No SC013532