Larval lamprey habitats are disturbed by a range of sediment manipulation procedures including dredging, suction-dredge mining, restoration actions (e.g., culvert install/repair) and fishway/diversion maintenance. Sediment added on top of larval burrowing areas results in ‘overburden’, posing risks of mortality from respiratory distress, reduced movement, and lethal crushinginjuries (1). Sublethal effects may include injury and changes in feeding, growth, and burrowing activity (1,2). Lethal and sublethal effects can cause local population declines because larvae congregate in high densities, representing multiple age classes. Therefore, localized sediment disturbances can affect many larvae and multiple generations. Negative outcomes are likely if larvae are unable to vertically migrate through the overburden to reach the new sediment-water interface, which is critical for respiration and feeding (1).  Studies with a range of benthic organisms suggest that survival is variable across species (3) and influenced by the depth of sediment added, exposure duration, and behavioral factors including mobility and residence depth (1). Threats of stream/floodplain degradation, dewatering -flow management and water quality due to sediment manipulation exist in multiple RMUs, yet information on larval lamprey responses to overburden is severely lacking (4).  The goal of this research is to determine the threshold overburden depths that cause mortality for different size classes of larvae. Study findings will be used to develop and distribute guidance for sediment handling practices to limit negative impacts to larval lampreys in an updated Best Management Guidelines (BMGs) for in-water work (5) and expand on information in the LTWG guidance on dredging (6).

Variables that may influence larval response include lamprey size, sediment type, overburden depth, and exposure duration. Overburden stress can cause lamprey injury/death, varying with the depth and bulk density of the sediment added (1). Fine sediment (fines; silt/mud) and sand have different bulk densitiesand compressibility and therefore exert different overburden stress at the same sediment depth. To understand these effects, we plan two test phases: Phase 1 (P1) comparing sediment types with a consistent overburden depth and Phase 2 (P2) using P1 findings to compare effects of overburden depths and exposure durations.  All laboratory testing will
be conducted at 10°C water temperature and ambient daylight conditions.

P1: Sediment Type
The test system will be a 15-cm diameter tube, partitioned into sections that are stacked in a column and secured using neoprene bands (Figure 1A).  Tubes will be 30-cm tall, divided into three 10-cm sections (Figure 1B).  Tubes will be placed inside 1.5-m diameter tanks filled with water. Six lampreys (range of sizes) will initially be placed in 10cm of sediment in the bottom of each tube. This approach limits/controls initial burrowing depth which we anticipate could influence risk of injury. We will add 15cm of sediment to the tubes while they are immersed. Tubes will be removed from the tank 24-hours post-test and separated into sections (Figure 1A) to determine larval location and fate (alive, dead, injured). Test groups include: 1) 4 groups (6 replicates) using fines and sand both for initial burrowing and added sediment/overburden (Table 1A), and 2) controls (3 replicates) with fines and sand for initial burrowing without addition of overburden. The two groups with the highest proportions of larvae found above the bottom tube section will advance to P2.

P2: Overburden Depth/Exposure Duration
We will use two sediment groups to evaluate lamprey responses to overburden depth and exposure duration.  Four overburden depths will be tested for each sediment type (Table 1B). We propose to test 20, 30, 60 and 100 cm, but ultimately test depths will be determined adaptively, based on findings (Figure 2). Each test (row in Table 1B) will follow P1 procedures except sampling will occur 1, 2, 3 and 4 days after sediment addition (Table 1B). Eight tubes (6 lamprey/tube) will be placed into a tank and 2 tubes will be removed on each sample day. Location and fate of larvae will be recorded, and controls (per P1) will accompany each test (see details in Table 1B header). Tubes will be like P1, with as many 10-cm tube sections as required to accommodate the test overburden depth. 

Data analyses will explore the influence of lamprey size, sediment type, overburden depth and exposure duration on larval injury and mortality rates. Recommendations for sediment handling will be developed (e.g., overburden should not exceed 10 cm over a 24 h period), incorporated into updated LTWG products such as the BMGs for in-water work (5) and the dredging guidance document (6), and disseminated through presentations, reports, and publications shared using the PLCI partner network.