The role of salmonids in the diet of grey and ringed seals in the Bothnian Bay, northern Baltic Sea

We examined the digestive tract contents of 63 Baltic grey seals (Halichoerus grypus) and 37 Baltic ringed seals (Phoca hispida botnica) collected during May to November in 2008 and 2009 in the northern part of the Bothnian Bay to assess the role of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) in the diet of seals. For grey seals the three most common prey species in numbers were vendace (Coregonus albula), Baltic herring (Clupea harengus) and common whitefish (Coregonus lavaretus). Thirteen grey seals contained remnants of a total of 93 salmonids (Atlantic salmon and sea trout). Salmon ingested were, in general, older and larger than the ingested sea trout. Six grey seals had in their digestive tract Carlin-tags which are used to mark stocked salmonid smolts. Stocked sea trout appear particularly vulnerable to predation during the first months after the stocking. Our study suggests that salmonids may play a marked role in the diet of grey seals during the season when salmonids aggregate in coastal waters in the Bothnian Bay. Three-spined stickleback (Gasterosteus aculeatus), Baltic herring, smelt (Osmerus eperlanus) and vendace dominated in the ringed seal's diet. No salmonids were found in the dietary tracts of ringed seals.

► Salmonids play a marked role in the diet of Baltic grey seal in the Bothnian Bay.
► Atlantic salmon ingested by grey seal were older than the ingested sea trout.
► Stocked sea trout smolts appeared particularly vulnerable to grey seal predation.
► No salmonids were found in the dietary tracts of Baltic ringed seals.