A comparison of behaviour and feeding ecology of saithe, Pollachius virens (L.), and pollack, Pollachius pollachius (L.), in a Scottish sea loch
Two closely related and morphologically similar gadoid predators, saithe, Pollachius virens (L.), and pollack, Pollachius pollachius (L.), coexist in close proximity on a submerged reef and in other similar inshore habitats in Loch Ewe, Scotland. The degree of overlap between the niches of these two gadoids in the wild was investigated by means of acoustic tracking, underwater television and an examination of stomach contents. Their density and distribution in Loch Ewe was investigated by a tag-and-release sampling program. Simultaneous tracking of individuals of both species in proximity of the reef revealed that pollack generally swim more slowly than saithe, limiting much of their movements to restricted areas of the submerged reef. Saithe ranged more widely around the reef as part of a school during the day, moving onto the reef at night. Individual saithe were observed abandoning the reef, either temporarily or permanently. This is thought to be part of a gradual shift of the population towards the open waters. A sampling program in Loch Ewe provided further evidence for a seasonal migration of large immature saithe (2+) from inshore to offshore waters. Although the dietary overlap was considerable, saithe consumed a wider range of prey than pollack, including numerous species from the fauna encrusting the kelp. Both species were highly piscivorous and fed on the same species of fish, but their feeding behaviour appeared largely different. Video recordings showed that saithe swam actively and foraged in small groups, taking prey items from the kelp. They appeared to be active, 'opportunistic' foragers. Pollack were generally solitary and used the kelp forest exclusively for cover, behaving as 'sit-and-wait' predators and ambushing their potential prey beneath the kelp canopy. Pollack were often observed engaging in aggressive interactions with conspecifics, probably to defend feeding territories. The relationship between different aspects of behaviour and feeding ecology of saithe and pollack are discussed in the wider context of recruitment of juveniles to the commercial stocks.