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.
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