Ergasilus sieboldi has been reported from a number of trout fisheries in England and Wales. The population dynamics of this parasitic copepod in Rutland Water, a large reservoir in Central England was studied from 2003 to 2005. A combination of angler and net caught fish were examined to record numbers of adult females and egg production throughout each year. The parasite overwintered in large numbers on trout and commenced egg production in April which then continued until October/November. The prevalence of infection and the abundance of the parasite were very high in overwintered rainbow trout but these parameters then decreased in March as large numbers of uninfected fish were stocked into the reservoir. The parasite population then increased until October. Infection levels in 2004 and 2005 were significantly lower than in 2003. Infections of cage-held rainbow trout showed that E.sieboldi could become ovigerous within two weeks of attachment to trout in July and August. New infections occurred from June until November. Several species of coarse fish examined were also shown to be infected by the parasite. Cage trials showed that triploid rainbow trout were infected by significantly higher numbers of the parasite than diploid rainbow, brown trout or “blue” rainbow trout.
Observations of infected fish in experimental tanks showed that overwintering parasites were stimulated to commence oviposition by increasing water temperatures. Photoperiod had no noticeable effect on the parasite. Egg viability and rate of development was studied using tank held infected fish and in vitro incubation techniques. Viability of eggs in sacs detached from the adult parasite was greater than those remaining attached. The rate of egg development was modelled and was shown to be predicted by temperature. Development of eggs was estimated to commence at 3.6ºC. Eggs developed more rapidly at higher temperatures and at peak production, inter-clutch interval was between 0 and 0.5 days. Egg production models estimated that an overwintered parasite could produce up to 19 clutches of eggs between April and October under normal temperature regimes measured at the reservoir. Ovarian development during the winter was confirmed using classifications of ovary size and shape based on parameters measured using image analysis techniques. The life span of E.sieboldi was estimated at 10-12 months.
Nauplii culturing techniques were compared, and nauplii to stage V were successfully developed. Nauplii hatched from the eggs of adult parasites occurring in the spring were larger and conditioned to develop at lower temperatures than those hatched later in the year. Nauplii were fed on 4 different types of algae held in monocultures but development occurred only in algal polycultures. A comparison was made of nauplii feeding preferences and development with algae recorded in Rutland Water in 2003 and 2005 but no correlations were found.
Fish stock assessment was carried out using models of angler catch, effort and stocking figures from the fishery. Parasite numbers on the overwintered fish were estimated at 12 million parasites in April 2003, 8.3 million in April 2004 and 1.2 million in April 2005. Stock assessments suggested a reduction in number of overwintering trout and effects of stocking policy to be at least partially responsible for the decline in the parasite population. The results of this study formed a management strategy for the operation of the trout fishery
