Dietary overlap and niche partitioning of sympatric harbour porpoises and Dall’s porpoises in the Salish Sea

Ecological theory regarding the coexistence of similar species predicts resource partitioning will arise through competition and lead to different ecological niches. The diets of harbour porpoises (Phocoena phocoena) and Dall’s porpoises (Phocoenoides dalli) in the Salish Sea were investigated for evidence of resource partitioning between these ecologically similar species. Stomach contents of 36 harbour porpoises and 11 Dall’s porpoises were analysed and ten and six fish taxa were identified in each, respectively. Pacific herring (Clupea pallasi) was important in the diet of both porpoise species and walleye pollock (Theragra chalcogramma) was second in importance in the Dall’s porpoises. Pacific herring was estimated to contribute 60% to the total caloric intake of harbour porpoises. In addition to herring, Pacific hake (Merluccius productus), walleye pollock and a species of Pyschrolutidae were present in the diet of both porpoise species. Pianka’s Index of niche overlap indicated substantial dietary overlap between the porpoise species based on measures of prey frequency of occurrence and prey percent numerical abundance. These results seem contrary to predictions from ecological theory. However, habitat and activity pattern differences between the two porpoise species exist and represent other dimensions of niche that likely contribute to resource partitioning in ways that were not strongly evident in stomach contents. Dall’s porpoises, which prefer deeper water habitat in the Salish Sea than harbour porpoises, may feed more on walleye pollock which spawn in deep water. Dall’s porpoises are also known to feed at night when lipid-rich mesopelagic prey such as Myctophidae and Bathylagidae, both present in the Dall’s porpoise samples, migrate upwards from depths in excess of 200m. In contrast shiner perch, present only in harbour porpoise samples, is a species associated with shallow nearshore habitats. Resource partitioning is also likely to occur in accordance with seasonal prey availability.