The present study examines the changes in ultraviolet (UV) photosensitivity that occur during the growth of rainbow trout (Salmo gairdneri). A comparison of the ocular media transmission of small (n = 3) and large (n = 3) trout eyes did not reveal large changes in the transmission of UV radiation through the eye. We used the heart-rate conditioning technique to measure spectral sensitivity in immobilized trout. Four trout, each weighing less than 30 g, exhibited a UV-sensitivity peak at 360 nm while four additional trout weighing more than 60 g each exhibited no evidence of UV sensitivity. Spectral-sensitivity measurements of two trout weighing 44 g and 60 g revealed UV sensitivity, but when measured one month later (after a 25% increase in body weight) both fish exhibited no UV-sensitivity peak. At this time their sensitivity appeared to conform to the known blue-sensitive cone mechanism.

Rainbow trout were infected simultaneously with cystacanths of the two acanthocephalan species Pomphorhynchus laevis and Acanthocephalus anguillae in the laboratory. Infections consisted of varying proportions of the two species (from 0 to 100%) at low (20 cystacanths) and high (60 cystacanths) levels. Fish were sacrificed at 7, 56 and 112 days post-infection (p.i.) and the parasites recovered. At low-level infections there was a high degree of overlap in the range of intestine occupied by the two species (44·12 %) and no evidence for interspecific competition affecting either species. At high-level infections the establishment of both species was unaffected by the presence or number of individuals of the other species. The survivorship and the range of intestine occupied by A. anguillae were found to be reduced in the presence of P. laevis. The interaction was one-sided, as P. laevis remained unaffected by A. anguillae, and so indicated interference competition. The possibility that both intraspecific and interspecific competition are implicated in explaining the distribution of the two species in the British Isles is discussed.