Biocide treatment of ponds in Scotland to eradicate signal crayfish

Abstract

This trial aimed to eradicate illegally introduced signal crayfish Pacifastacus leniusculus in the North Esk catchment, Scotland. Sites treated were (1) an isolated gravel-pit (c. 9,000 m3), with crayfish present for 6 years; (2) three dammed ponds, (c. 5,000 m3) and (3) a leaking, offline pond (c. 6,000 m3), with crayfish for two years. Preliminary toxicity tests with substrate present indicated doses. Treatment at sites (1) and (2) (in October 2004, water temperature 13°C) comprised deoxygenation with sodium sulphite to stimulate emergence, then application of natural pyrethrum (Pyblast). Exposed margins were sprayed with Pyblast to prevent escapes. Crayfish mortality was high, but one survivor was seen after 5 days. Pyblast was applied from a tank with Na2SO3 residue, which subsequent investigation indicated reduced Pyblast below the target 0.1 mg I-1. Site (1) was re-treated (end October, target 0.15 mg I -1 Pyblast, no Na2SO3, 9°C). Mortality was confirmed using caged crayfish. Prior to treatment at site (2), throughflow was stopped and fish removed. Biomonitoring was carried out with freshwater shrimps Gammarus in the adjacent watercourse. Treatment of site (3) (December, target 0.2 mg I-1, 4°C) necessitated continuous back-pumping of leakage for a 2-week recovery period to avoid contamination of the river downstream. Caged crayfish took up to 5 days for 100% mortality. No crayfish were found in a summer survey after the treatments with Pyblast alone, but were caught in the ponds with Na2SO3 pre-treatment. Monitoring is required for 2-5 years.