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Collection of larval lampreys (Entosphenus tridentatus and Lampetra spp.) using a portable suction dredge—A pilot test

January 20, 2022

A portable suction-dredge and sluice-box system were used to collect larval lampreys (Entosphenus tridentatus and Lampetra spp.) from fine and coarse sediment in field and laboratory tests. We evaluated the injury rate, survival, and burrowing capability of lamprey following passage through the dredge system and used collection of lamprey from water without sediment as a control. The system used a hydraulic eductor (also known as a Venturi valve) to create suction so that sediment and lamprey avoided passage through the pump impeller. For the field test, lamprey were tagged with visible elastomer implants based on small (89 millimeter [mm] or less) and large (92 mm or more) size categories and stocked into mesh enclosures over fine or coarse sediment. The dredge was used inside each enclosure to collect lamprey and they were transported to the laboratory for evaluation and holding. The mean time to burrow was recorded for each study group (3 fine, 3 coarse, 3 controls) on the day of the field test; injury was evaluated at 24 hours; and survival was evaluated at 24 hours, and at 7 and 14 days after the test. The suction dredge collected 32 lamprey in fine sediment, 21 lamprey in coarse sediment, and 28 lamprey in the control group, including 30 lamprey that were not initially stocked. One lamprey died the day of the test (fine sediment) and 24 hours later, three lamprey were found to be injured (2 in fine and 1 in coarse sediment). No injuries or mortalities occurred in the control group. Lamprey burrowing performance was similar across the two treatment groups and the controls. The mean time for all fish in a group to burrow was highly variable. For all groups in a treatment combined, the mean burrow times were fastest for the fine treatment (9.8 minutes), followed by the controls (11.4 minutes) and the coarse treatment (11.6 minutes). The mean times to burrow for the main group of fish in each treatment group (those that burrowed in quick succession) were similar: 4.3 minutes for the fine group, 4.4 minutes for the coarse group, and 4.5 minutes for the controls. The laboratory test collected 147 lamprey (73 small and 74 large size category) from coarse sediment using the same procedures as the field test. One fish (small) was killed the day of the test, and six lamprey (3 small and 3 large) were found with injuries during the 24-hour exams. No mortalities were recorded 7 days after the test, when monitoring was terminated. The overall injury rate for the laboratory test was 4.1 percent and the mortality rate was 0.7 percent. Injuries in the field and laboratory tests were localized minor hemorrhages or red, irritated areas. The suction- dredge system appears to be a safe option to collect larval lamprey from sediment and will be a useful addition to lamprey assessment and salvage tools.