CALCIUM-METABOLISM AND OSMOREGULATION DURING SEXUAL-MATURATION OF RIVER RUNNING ATLANTIC SALMON

As Atlantic salmon return from the ocean to undertake the anadromous s pawning migration up the river of origin, profound changes in calcium metabolism and osmoregulation take place. Using tartrate resistant aci d phosphatase as a marker, scale osteoclast activity was found to incr ease throughout sexual maturation and spawning migration. Thus, the pa rticipation of osteoclasts in the elevated scale resorption observed d uring this phase is established. As calcium was simultaneously accumul ated in the female gonads, it is proposed that the scales are resorbed in order to provide calcium for the growing ovaries, Plasma oestradio l-17 beta levels were elevated in females during sexual maturation, an d had decreased at the time of spawning. Plasma testosterone levels we re similar in males and females during the first part of the upriver m igration, but had increased in males and decreased in females at spawn ing. In addition to the role of these sex steroids in the gonadal grow th, their possible involvement in the increased scale resorption durin g this phase is discussed, Plasma growth hormone and thyroxine levels were elevated in both sexes at spawning, with the triiodothyronine/thy roxine (T-3/T-4) ratio declining sharply, indicating possible roles fo r these hormones in the maturational process. The relatively low gill Na+,K+-ATPase activity of salmon caught in the estuary implies that th e fish had already adapted to a hypoosmotic environment. During the up river migration, the gill Na+,K+-ATPase activity decreased further, in dicating that the hypoosmoregulatory ability was suppressed further du ring sexual maturation and spawning migration. (C) 1998 The Fisheries Society of the British Isles.