The harbour porpoise (Phocoena phocoena) is probably the most abundant small cetacean in the northeast Atlantic and as such is an important top predator. It is also one of the most threatened species, particularly as a consequence of fishery by-catch.
Porpoises feed mainly on small shoaling fishes from both demersal and pelagic habitats. Many prey items are probably taken on, or very close to, the sea bed. Even though a wide range of species has been recorded in the diet, porpoises in any one area tend to feed primarily on two to four main species (e.g. whiting (Merlangius merlangus) and sandeels (Ammodytidae) in Scottish waters).
Evidence for selective predation is equivocal. Many studies provide evidence of geographic, seasonal, interannual, ontogenetic or sexual differences in prey types or prey sizes, and such differences are often (speculatively) interpreted in terms of prey availability. A few studies demonstrate trends in diet selection that are consistent with changes in prey abundance. However, lack of availability of prey abundance data at an appropriate spatial and temporal scale is often a problem.
Porpoise diets overlap extensively with diets of other piscivorous marine predators (notably seals). Many of the main prey species are also taken by commercial fisheries, although porpoises tend to take smaller fishes than those targeted by fisheries. Given their high abundance, porpoises clearly remove substantial quantities of fish.
The literature on porpoise diets in the northeast Atlantic suggests that there has been a long-term shift from predation on clupeid fish (mainly herring Clupea harengus) to predation on sandeels and gadoid fish, possibly related to the decline in herring stocks since the mid-1960s. Evidence from studies on seals suggests that such a shift could have adverse health consequences.
Food consumption brings porpoises into contact with two important threats – persistent organic contaminants and fishing nets, both of which have potentially serious impacts.