Variation in susceptibility and body growth between hosts identified by their infection history

Six hundred Atlantic cod (Gadus morhua L.) were individually marked and caged for 19 months. During this period each cod was nspected for Lernaeocera branchialis (L.) at several intervals. During the caging 79 % of the cod remained uninfected, 8.5 % we re infected, but lost the parasite, 8 % were infected with one parasite, and 4.5 % were infected with more than one parasite. One group of hosts developed significantly higher infection rates than the others. These were fish which had been infected prev iously, but had lost their infection. Furthermore, growth rates in the four groups of cod, identified by their infective his ory, were compared. The highest rate of increase, both in body mass and body length, was recorded in the group of male fish infe cted with one parasite throughout the experimental period. Conversely, the males which were free from infection, showed sign ficantly lower growth rates. The observed differences in growth rates could not be explained by changes in parameters related to life history strategies. The alternative explanation to these results is that resistance to L. branchialis was associated w th costs in terms of reduced growth rates of body mass and length.