The transition of adult Atlantic salmon (Salmo salar L.) from saltwater to freshwater
Aspects of the behaviour of adult Atlantic salmon in estuaries, while on their spawning migration has been investigated using a salinity telemetric system. Eighteen fish were tracked in total in different Scottish estuaries. Studies involving the displacement of salmon from freshwater back into higher salinities on the River North Esk, Montrose showed fast movement through the estuary back into freshwater (5h), movement being initiated by the sensing of low salinities or the start of the flood tide. Tracking in the Dornoch Firth at Bonar Bridge (a channel type estuary) showed fish maintaining one position against both flood and ebb tides, experiencing salinities fluctuating widely about the isosmotic point. The qualitative assessment of salmon on passage through the estuary of the freshwater discharge of a non-natal river was also demonstrated, repeated entries of a very brief nature (21s) being recorded. The position held by salmon whilst on suspended migration in an estuary was recorded, no sampling of the freshwater discharge being undertaken on a tidal basis. The resting metabolic rate of farmed salmon of different stages of sexual development was recorded in saltwater, and on transfer to freshwater. Maturing fish exhibited a raised metabolic rate in saltwater, suggesting increased osmoregulatory costs. This was in contrast to non-maturing fish. The cannulation of wild salmon subjected to fluctuating salinities was undertaken. Plasma electrolyte levels and osmolalities of fish subjected to cyclic salinities were intermediate between salt-adapted and freshwater-adapted values, showing only small fluctuations. An inability to osmoregulate successfully in high salinities following 'stress' events was recorded in maturing farmed and coastally caught wild salmon, suggesting a maladaption to that environment. By contrast, non-maturing farmed salmon exhibited a greater degree of euryhalinity. Examination by electron scanning microscopy of apical pit structure of gill chloride cells supports the idea that increasing maturity or freshwater exposure reduces tolerance to saltwater. It is concluded that Atlantic salmon are euryhaline during at least part of their oceanic feeding phase, and on arrival in coastal waters on the spawning migration require no period of acclimitisation to freshwater. A reduction in tolerance to high salinities linked to advancing maturation/ freshwater exposure would thus serve to ensure river entry.