Population growth of Gyrodactylus salaris (Monogenea) on Norwegian and Baltic Atlantic salmon (Salmo salar) stocks

Reproductive success of Gyrodactylus salaris from River Lierelva was compar ed experimentally on 3 stocks of salmon (12.5 degreesC+/-0.2). Isolated fis h from 2 susceptible Norwegian stocks (Rivers Lier and Alta) and 1 resistan t Baltic stock of salmon (River Neva) were infected with a single gravid no rm in order to record the temporal sequence of births and age at death of i ndividual parasites. Establishment success (proportion of worms surviving t o give birth) was generally low and mortality high, but significantly fewer worms survived on Neva (45 %, with mean survival of 3.5 days) compared to Alta and Lier fish (60 %, mean survival 7.9 and 5.2 days, respectively). Th ere was a dramatic difference in parasite fecundity between the host stocks : only 2 births occurred on Neva fish compared to third and fourth births o n both Alta and Lier hosts. The timing of the first birth was more variable on Neva hosts and was significantly extended (mean 2.3 days) relative to t hat on Alta and Neva fish (18 days). However, timing of the second birth di d not vary on any of the 3 salmon stocks. Age-specific mortality and fecund ity data are consistent with exponential population growth of G. salaris on Alta and Lier fish but eventual extinction on Neva hosts. This is the firs t demonstration that gyrodactylids maintained on different host stocks exhi bit variations in fecundity, development and mortality, which may in turn a ccount for the variable virulence noted when G. salaris infects different s almonid hosts.