Seawater tolerance in captive high-Arctic Svalbard charr (Salvelinus alpinus): effect of photoperiod and body size

Anadromous Arctic charr (Salvelinus alpinus) returning after spending summe r at sea were captured in a fish trap in the Dieset River on Spitsbergen (7 9 degrees 10'N), Svalbard. Fish selected for breeding were transported to T rondheim in mainland Norway. Eggs obtained from the charr were fertilized a nd incubated in total darkness. First-fed alevins and resulting parr were k ept under continuous light until an age of 0 + and 1 + years, respectively. Some 1 + charr were kept as controls under a continuous short-day photoper iod (6L:18D) from autumn until the end of the experiment the following July . Charr aged 0 + and 1+ years old were exposed to a short-day photoperiod f rom October until January and a simulated natural photoperiod for 80 degree sN from January until the end of the experiments. Challenge tests demonstra ted a size-dependent seawater tolerance for charr with a body length less t han 18 cm. Fish smaller than 12 cm did not survive the 96-h test period. Th e larger charr kept under simulated natural photoperiod developed increased hypoosmoregulatory capacity. Charr kept under short-day treatment showed a slight, short-lived increase in seawater tolerance. A 7-days seawater chal lenge test at the end of the experiment (July) demonstrated that the antici patory seawater preparation in charr is influenced by photoperiod. We concl ude that offspring from anadromous high-Arctic charr must achieve a thresho ld body size ( > 25 cm) before they can respond to photoperiod signals whic h trigger the development of the hypoosmoregulatory capacity typical for sm oltifying salmonids.