THE STRUCTURE OF A NATURAL-POPULATION OF THE MEDICINAL LEECH, HIRUDO-MEDICINALIS, AT DUNGENESS, KENT

1. A hot branding method was used to mark medicinal leeches (Hirudo medicinalis L.) from five samples (four in 1984, one in 1985) at a gravel pit at Dungeness in Kent. Analysis of capture-recapture data by the Fisher-Ford and Jolly methods gave similar results and indicated a population in excess of 10 000 (0.112 leeches per m2) in 1984 and 1985. Successive estimates were each higher than the last suggesting a thriving community. 2. The population structure was skewed towards the lower weight classes. This may indicate good recruitment of hatchlings, as also indicated by the presence of cocoons, or the failure to capture heavier satiated or gravid leeches because of their relative unresponsiveness to the sampling method involving water disturbance, or their complete absence from the water during cocoon laying. Gravid leeches were always above 2 g in weight. Recaptured marked leeches revealed that they migrated throughout the site, indicating active foraging for hosts.