ALBERT

Notes: Infections of brown trout Salmo trutta L. by larvae of the nematode Eustrongylides sp. were studied over a period of two years in Fernworthy Reservoir, Devon. The parasite, here as elsewhere in Britain, is acquired only in the lake itself and not in the streams feeding it. Both native and introduced brown trout were infected but not rainbow trout which did not survive sufficiently long in the reservoir. The parasite showed a preference for female fish and for fish over 20 cm in length. Since these latter were approaching the end of their natural life span in the lake, there was insufficient time for the parasites to accumulate in older fish. No seasonal changes in infection levels or size composition of the parasite population were recorded. The larvae occurred in capsules, the majority of which were attached to the stomach wall. No local pathological effects associated with the capsules were observed and infested fish were of similar length, weight and condition to uninfected ones. The absence of harmful effects upon the trout in Fernworthy Reservoir is compared and contrasted with the effects of the parasite in other localities. It is concluded that although Eustrongylides can occasionally cause damage to some species of fish, there is no conclusive evidence that it does so to trout. but its unsightly appearance and harmful effects on birds renders it an undesirable parasite in any fishery.

Notes: Samples of Eubothrium parvum were obtained from capelin Mallotus villosus at 55 stations throughout the Barents Sea and from Balsfjord, North Norway. The parasite is distributed widely throughout the Barents Sea, but both incidence and intensity of infection are higher in the regions off Murmansk and the Kola peninsula, and Spitsbergen. E. parvum exhibits a seasonal peak in maturation and probably also in acquisition of new infections. The incidence of infection is greatest in 1 + fish, whereas the intensity is more independent of host age. It is suggested that the parasite requires only a single intermediate host, a plank-tonic copepod, and its distribution in relation to age of host is a reflection of the dietary preference shown by young capelin for copepods. The frequency distribution of E. parvum in capelin was over-dispersed in Balsfjord, where infection levels of between 1 and 28 parasites per fish were encountered in all samples, but under-dispersed in the Barents Sea, where infections of more than four parasites per fish were never found and even infections with three and four parasites were very local. It is suggested that the underdispersion is due to a very low probability of infection in the open waters of the sea. Although the presence of E. parvum cannot be used as a biological tag for capelin, its abundance and frequency distribution can. The difference in frequency distribution and the failure to find any heavily infected fish in the Barents Sea confirm the suggestion that the capelin of Balsfjord form a local isolated population, which does not migrate into the Barents Sea. The differences in infection levels within the Barents Sea suggest the further possibility that there are at least two stocks of capelin there, but this requires further investigation and confirmation.

Notes: Marked changes in the relative numbers of roach Rutilus rulilus (L.), rudd Scardinius erylhroplhalmus (L.) and perch Perca fIuviatilis L., in Slapton Ley, Devon, have been observed over the period 1967–1978. Historically, the lake had been dominated by rudd and perch; significant numbers of roach were not thought to be present until 1967. From then onwards the roach population exhibited a considerable expansion, apparently replacing the rudd, which had virtually disappeared by 1974. In 1975 extensive mortalities of roach were caused by the pseudophyllidean cestode Ligula intestinalis L. During 1976 and 1977 larger number of rudd, belonging to the 1975 and 1976 year classes, were present, suggesting that the roach mortalities might be giving the rudd a chance to recover. However, very few rudd were present during 1978, possibly because lower incidences of ligulosis during 1976–1978 had allowed the roach to recover from the main outbreak of the disease in 1975, and to replace the rudd for the second time. Evidence from trap catches indicated that the perch population had shown a consistent decline between 1970 and 1977. The major phase of this decrease occurred between 1971 and 1973, when the roach population was increasing the most rapidly. Comparisons were made with changes in fish populations in other localities. The major reason for the changes observed was thought to be a competitive relationship between the plankton feeding younger stages, with roach being the dominant species, although other factors may also have been involved.

Notes: Samples of resident freshwater char, Salvelinus alpinus were obtained from three lakes on Bjørnøya and their parasite faunas examined. Comparison of the species composition, number, diversity and equitability of the parasite faunas of Bjørnøya with those of other Arctic islands, an inshore island and lakes on the Norwegian mainland indicated that the communities on Bjørnøya formed a distinct unit with a high degree of similarity between the three lakes. Small differences could be related to differences in the ecology of the lakes. The parasite community of char on Spitsbergen showed the greatest similarity to that on Bjørnøya, and the communities of char in mainland lakes the least. Species number and diversity of parasites were often higher on the Arctic islands than on the mainland, and did not correlate with island size or distance from the mainland. The island parasite communities were often dominated by a single species, but a similar situation was also observed in the mainland lakes. It is concluded that the parasite fauna of char on offshore Arctic islands does not agree well with the predictions of island biogeographical theory.

Notes: The literature pertaining to the distribution, habitat, specificity, life cycles and population biology of all the species of Eubothrium is reviewed. Records in which scolex morphology is used to identify species cannot be used as this character is too variable to be of value in specific recognition. Study of the valid records suggests that E. crassum comprises three races. There is a freshwater race, whose preferred host is non-migratory Salmo trutta fario, but which also infects parr, smolts and landlocked adult S. salar and parr of migratory S. trutta trutta. Its distribution is confined to Europe and Eurasia and is co-extensive with that of S. trutta: it does not occur in N. America or E. Asia. It may be carried to sea in migratory salmonids, where it dies out in S. salar but may survive in S. trutta. Its life cycle only takes place in fresh water, and requires a copepod as the only intermediate host. Other fish species may serve as paratenic or accidental hosts. The preferred host of the marine Atlantic race of E. crassum is S. salar, but S. trutta is also infected. Its distribution is co-extensive with that of S. salar on both sides of the Atlantic Ocean. It may be carried into fresh water by both hosts, when its numbers decline in S. salar but not in S. trutta. The life cycle is unknown, but infection probably takes place in marine coastal waters. The preferred hosts of the marine Pacific race are all species of Oncorhynchus, with which its distribttion is co-extensive on both sides of the Pacific Ocean. It may also be carried into fresh water, and its life cycle is also unknown. E. salvelini comprises two races. The European freshwater race is specific to Salvelinus alpinus, and does not infect Salmo spp. Its life cycle takes place in fresh water and involves only one intermediate host, a copepod. It may be carried to sea in migratory fish, and survive to return to fresh water. The American race, found throughout North America and East Asia, infects species of Salmo, Salvelinus, Oncorhynchus, Cristivomer and other genera. Its biology is otherwise similar to that of the European race, although its return from the sea has not yet been confirmed in America. The other species of Eubothrium are more local in their distribution, some being endemic; do not comprise different races, and infect marine and anadromous fish. Nothing is known of their biology or life cycle. Eubothrium is recognised as being a marine genus that has invaded fresh water with anadromous fish. The inability to distinguish the races of E. crassum and E. salvelini on morphological characters, their low rate of survival in the other medium and their wide distribution renders them unsuitable as biological tags.

Notes: Two sudden and pronounced changes in the growth rate of roach, Rutilus rutilus (L.), in Slapton Ley, Devon, have been observed between 1972 and 1976. At first the growth rate declined to the point at which the population became stunted, then subsequently it improved again. These changes were correlated with population density; the numbers of roach were increasing during the late 1960's/early 1970's, and this increase was substantially reinforced by very strong year classes in 1972, and, to a lesser extent, in 1973. The increase in population density resulting from the strong 1972 year class was apparently adequate to accelerate the onset of the stunting observed. The improvement in growth rate in 1976 followed immediately after extensive mortalities amongst the roach during 1975, caused by an epidemic of the parasite Ligula intestinal is (L.). Some improvement in the growth of O group roach had been evident during 1975, but this was not observed in the population. Ford-Walford plots showed a decline in the value of L after the onset of stunting in 1972.Differences in the reproductive biology of the fish were found following the alleviation of stunting; the gonads of both sexes achieved a greater relative weight, and correspondingly the fecundity of the females was higher, although this latter effect became progressively less marked with increasing size. It was not apparent amongst the largest individuals. There was also some evidence of a decline in the proportion of fish maturing at minimum size. The changes in the reproductive biology were associated with the improvement in conditions for individual fish following the decline in population density.

Notes: Samples of Gammarus pulex and dace, Leuciscus leuciscus, taken from the River Avon, Hampshire, at intervals over a period of 9 years were examined for the presence of the acanthocephalan. Pomphorhynchus laevis. Changes in the incidence and intensity of infection of P. laevis in dace and in G. pulex and in the frequency distribution of P. laevis in dace were used as indicators of the population size of the parasite. Throughout the period of observation, the parasite showed a similar pattern of distribution along the length of the river, although at some sites its abundance varied from year to year. Levels of both incidence and intensity of infection of P. laevis in both hosts at one specific locality remained fairly constant over the 9-year period, and the annual variations fell within the natural range of monthly variation over any 12-month period. The frequency distribution of P. laevis in dace also exhibited a similar pattern throughout the period. It was thus concluded that the population size of P. laevis had not changed to any great extent over the 9 years. The factors responsible for this constancy are discussed in the light of conclusions drawn from earlier work.

Notes: Incidence, intensity and frequency distributions of infections in perch, Perca fluviatilis, with the eyeflukes Tylodelphys clavata and Diplostomum gasterostei were studied in a lake at monthly intervals over a period of two years. T. clavata had appeared in the lake immediately prior to the study, and its levels of infection continued to rise throughout the period of investigation whereas those of D. gasterostei remained relatively steady. The main period of infection of fish by both species of parasites was late summer, but a second and minor infection took place in spring. In the first year after its introduction T. clavata exhibited a seasonal cycle in incidence and intensity of infection and disappeared from fish in summer, but in the second year this cycle was masked by increased variation in development times and lack of synchrony throughout the parasite population. It is concluded that T. clavata has a life span of one year or less and that the parasites die within the fish, mainly in summer. D. gasterostei in contrast has a life span of at least one year, and disappearance of the parasite from the population is probably due to death of some heavily infected fish, although it was not possible to determine if this was due directly to the presence of the parasite.