IMPORTANCE OF BODY SIZE FOR ACCLIMATION OF UNDERYEARLING ATLANTIC SALMON PARR (SALMO-SALAR L) TO SEAWATER

Starting on 4 October, four size groups (fork length < 9.5 cm, 9.5-15.3 cm, 14.0-16.1 cm and 16.0-20.3 cm) of underyearling Atlantic salmon (Salmo salar L.) were subjected to a stepwise increase in salinity (5-10 ppt, every 4 days) up to full seawater (32 ppt). Fish < 9-5 cm commenced to die when salinity increased above 15 ppt, with a cumulative mortality of 82% at the end of the study on 21 November. By contrast, fish of 9.5-15.3 cm exhibited 12% mortality, all but one of which were sexually mature males, and the two larger size groups exhibited 0% mortality. Periodic 96 h, 37 ppt high salinity tolerance tests resulted in fish < 9.5 cm dying within 24 h, but in fish > 9.5 cm survival increased from 5% in freshwater to 75-90% after acclimation to full seawater. All groups exhibited significant increases in gill Na+K+-ATPase activity in association with the increase in salinity above 15 ppt, with mean levels 3- to 8-fold higher than initial freshwater values after 16 days in full seawater. Mean plasma osmolality in fish > 9.5 cm was relatively stable (300-320 mOsmol kg-1), but in fish < 9.5 cm it increased in association with salinities above 15 ppt, exceeding 400 mOsmol kg-1 by the end of the experiment. Muscle water content was maintained at 74-76% in fish > 9.5 cm, but in fish < 9.5 cm it decreased significantly to 70.1% when salinity was increased to 20 ppt, increasing significantly thereafter reaching 79.1% by the end of the study.