Harbour porpoises are piscivorous predators, but also prey themselves, of grey seals. Porpoise diet is shaped by their environment: the prey locally on offer and e.g. seasonal variation in the prey base, but also by individual characteristics of the predator. Young porpoises mostly feed on small fish, such as gobies but as they grow older and larger, they gradually switch to eating larger prey, and prey of a higher energy density. Porpoises that are in a very good body condition eat relatively much fish with a high fat content, while emaciated porpoises had a much leaner diet, shortly before death.
Diet reconstructions were made using species-specific prey hard parts that were still present in the stomachs of porpoises that had stranded dead on Dutch beaches. Necropsies, performed on the same animals by veterinary pathologists, provided the means to link diet with individual characteristics of these animals (such as size and body condition), but also with presumed cause of death.
Two particular causes of death were further studied: predation by grey seals (as evidenced by gross mutilations on the porpoise carcasses and by DNA of grey seals found in the wounds inflicted) and drowning in fishing gear. The combination of specific stomach contents and pathology showed that grey seals can attack porpoises both near the sea floor and in mid-water.
Likewise, stomach contents of porpoises known to have drowned in bottom-set gill nets were dominated by demersal prey, which helped correctly identify other victims of such nets in which the pathological evidence was less clear. Finally, a regional study showed that porpoises that are currently returning to the river Western Scheldt, are feeding on diadromous fishes that are also returning to this recovering ecosystem.