The effects of a change in the growth rate of roach, Rutilus rutilus (L.), on the biology of the fish tapeworm Ligula intestinalis (L.)

Investigations into the biology of the roach and the pathogenic tapeworm Ligula intestinalis (L.) populations at Slapton Ley, Devon were carried out between October 1982 and December 1984. Data collected from the lake since 1977 have also been re-analysed to determine how the dramatic improvement in the individual growth rate of the roach over this period may have affected the growth, maturation and life-cycle of Ligula. Only the very young roach at this site become infected, so it was possible to follow cohorts of plerocercoids of similar age through each roach year class. Results for the 1978 and 1983 year classes are presented in detail. The roach grew extremely rapidly from May to August in each year, resulting in a pronounced cyclical pattern of changes in the condition of the roach, with the lowest condition occurring in late winter and spring. The yearly increase in the roach growth rate was accompanied by an increase in plerocercoid growth rate in the 0 + roach, but not in the 1 + roach. The growth rate of the plerocercoids was very high compared to that at other sites. It is usual for the parasite index (PI) of Ligula-infected fish to be high and to increase throughout their first few years of life. At Slapton, however, a lack of multiple infections has prevented high PIs from occurring, and in recent years the growth rate of the roach has been so high that the plerocercoids were unable to maintain a high weight relative to the fish, and the highest PIs occurred in the 0 + roach throughout late winter and spring. In recent year classes, therefore, the maximum PIs and highest pathogenicities coincided with the period of lowest condition in the 0+ roach. Observations of both caged and natural populations of 0+ roach over winter showed that a significant loss of roach containing the larger plerocercoids occurred from the population. In vitro cultivation of Ligula plerocercoids showed that they were capable of maturation at weights of 0.5 g, and only 6 months after having infected the roach. The increase of the growth rate of the plerocercoids in the 0+ roach has therefore resulted in a greater proportion of these plerocercoids being capable of infecting the definitive host. As a result of the increase in individual growth rate of the roach at Slapton, the potential for Ligula transmission, as measured in terms of both their pathogenicity and maturity, has shifted from the 1 + to the 0+ roach.