FIRST ACCESS TO TERRITORIAL SPACE AND EXPOSURE TO STRONG PREDATION PRESSURE - A CONFLICT IN EARLY EMERGING ATLANTIC SALMON (SALMO-SALAR L) FRY

This work aimed to elucidate conflicting factors that may explain the narrow and synchronous emergence in salmonids. Fry are highly vulnerab le to predation and stand a better chance of surviving if they emerge synchronously. On the other hand, fry that leave the gravel first shou ld increase their chance of obtaining one of the limited number of fee ding territories. The risk involved in early emergence for Atlantic sa lmon fry was evaluated by exposing them to predatory Salmo trutta. Yol k sac alevins were incubated in an artificial redd in order to catch t hem by their time of emergence. Early-, peak- and late-emerging fry we re then successively marked and transferred to flume tanks in which th e predators were either present from the start or not introduced until all fry had been added. When the predators were initially present, th e predation pressure differed depending on the time of fry emergence, resulting in survival rates of 11.6, 44.9 and 51% in early-, peak- and late-emerging fry, respectively. By assuming that the predation rate of the three emergence groups was dependent both on time of emergence and fry density the survival rates were calculated to be 7.5, 41.1 and 53.5% in groups I, II and III, respectively. These figures correspond ed well to the observed rates. When the predators were added after com pleted emergence the resulting survival rates were 56.9, 39.7 and 25.2 % in early-, peak- and late-emerging fry, respectively. Thus, predatio n after complete emergence gave a survival probability that varied acr oss the three emergence-date groups, despite being exposed to a predat or during the same number of days. The presence of fish predators in c ombination with a limited territorial space seems to make both early a nd late emergence hazardous and favours synchronous swimming movements .