Defining sediment handling practices to limit negative impacts to larval lampreys

Sediment manipulation activities such as dredging and restoration efforts (e.g., culvert install/repair) may disturb habitats where larval lampreys live burrowed in stream sediments. Sediment added on top of larval lamprey burrowing areas results in ‘overburden’, which poses risks of mortality from respiratory distress, reduced movement, and crushing injuries or may have sublethal effects such as changes in growth or burrowing activity. To avoid negative outcomes, larvae must be able to vertically migrate through the overburden to reach areas near the new (elevated) sediment-water interface. Current guidance on how to limit negative impacts to lampreys during sediment handling activities is limited by a lack of information on how lampreys respond to overburden. The goal of this study was to evaluate larval lamprey responses to variable overburden depths and exposure durations using two sediment types: fine (<0.250 mm) and sand (0.250 - 0.500 mm). We assessed lamprey (mix of Pacific Lamprey Entosphenus tridentatus and Lampetra spp.) survival and injury and evaluated burrowing performance as an indicator of sublethal impacts of overburden stress.

We designed an experimental system using 15-cm diameter plastic tubes partitioned into sections that allowed us to describe the approximate vertical position of larval lampreys within the overburden so we could assess vertical migration ability. The tube sections were stacked in a column and secured using straps. The bottom tube section was 10 cm tall and was attached to a flat, rectangular base plate. This section was filled with sediment and lampreys at the start of a test. The middle tube sections were each 10 cm tall, with the number of sections adapted to the depth of the overburden. The top tube section was 20 cm tall to allow room for 10 cm of water over the surface of the overburden. Sampling involved removing the tubes from a holding tank and quickly separating the sections to locate lampreys and assign their position to the tube section where they were recovered.

Our primary objectives for the study were to better understand how depth of overburden and exposure duration affect larval lampreys, but there was no previously published information to inform our selection of test parameters. Thus, we used a two phased approach where Phase 1 was a pre-test to help us refine appropriate treatments for Phase 2.

In Phase 1 of the study, we used a constant overburden depth (50 cm) and exposure duration (24 h) to compare four sediment treatments that varied the initial burrowing sediment and the overburden sediment: fine-fine, fine-sand, sand-fine, sand-sand. In all four treatments, most lampreys vertically migrated through the overburden and were recovered in the uppermost tube section 24 h after overburden was added. There were no lamprey mortalities, few injuries, and little evidence of sublethal effects of overburden stress based on burrowing performance tests. Lampreys moved the most in the fine-sand and fine-fine treatments, so they were used for continued testing.

In Phase 2 we doubled the overburden depth to 100 cm and tested extended exposure durations (1 to 4 d), and based on those results, added tests of shorter durations (1 to 8 h). Since both treatments used fine sediment for initial borrowing, these tests effectively compared fine and sand sediments as overburden.

Overall for Phase 2, there were no lamprey mortalities, little evidence of sublethal effects, and 6.4% of the treatment fish were injured. Most of the injuries were mild abrasions, but some fish had serious damage such as a large gash or nearly complete bisection. We estimate that the serious injuries occurred as tube sections were separated during sampling, but the cause of the minor injuries could not be clearly ascribed to overburden stress or sampling activity.

Lampreys moved to the upper sections of the tubes through 100 cm of fine and sand overburden within 1 d. Lamprey position was comparable among the 1-4 d exposures, although position was more variable at 1 d compared to 4 d. Tests of exposures less than 24 h showed that fewer than 40% of lampreys completed vertical migrations to the top of the tube after 4 h, and after 8 h the uppermost tube section contained 33% of lampreys in the sand treatment and 44% of lampreys in the fine treatment. We concluded that the minimum time needed for most lampreys to complete vertical migrations through 100 cm of overburden lies somewhere between 8 and 24 h.

Lamprey size influenced recovery position in 4 and 8 h exposures to sand overburden, but not for other tests. The mean size of fish near the bottom of the tubes was significantly larger than the size of fish near the top of the tubes.

To our knowledge this was the first formal study to evaluate lamprey responses to overburden. Although our test systems were imperfect simulations of natural conditions, and several questions would benefit from additional research, our findings can inform guidelines on sediment handling practices to limit negative impacts to larval lampreys.