U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2020–1026

Prepared in cooperation with U.S. Fish and Wildlife Service, Fish and Wildlife Office, Portland, 

Oregon; and Columbia River Fish and Wildlife Conservation Office, Vancouver, Washington

Evaluating Dewatering Approaches to Protect 

Larval Pacific Lamprey
Cover photographs: 

Background: Electrofishing for larval lamprey, Wind River, Washington. 

Inset: Small and large Pacific lamprey larvae shown on hand. 

Photographs by Lisa Weiland, U.S. Geological Survey, 2019.
Evaluating Dewatering Approaches to Protect Larval 

Pacific Lamprey

By Theresa L. Liedtke, Lisa K. Weiland, Joseph J. Skalicky, and Ann E. Gray

Prepared in cooperation with U.S. Fish and Wildlife Service, Fish and Wildlife Office, 

Portland, Oregon; and Columbia River Fish and Wildlife Conservation Office, 

Vancouver, Washington 

Open File Report 2020–1026

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020 

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Suggested citation: 

Liedtke, T.L., Weiland, L.K., Skalicky. J.J., and Gray, A.E., 2020, Evaluating dewatering approaches 

to protect larval Pacific lamprey: U.S. Geological Survey Open-File Report 2020–1026, 32 p., 

https://doi.org/10.3133/ofr20201026. 

ISSN 2331-1258 (online)
iii

Contents

Executive Summary....................................................................................................................................... 1

Introduction.................................................................................................................................................... 2

Methods......................................................................................................................................................... 4

Fish Source, Size, and Maintenance.......................................................................................................... 4

Movement Trials......................................................................................................................................... 4

Movement Rates.................................................................................................................................... 6

Repeated Dewatering and Rewatering Trials............................................................................................. 7

Individual Fish Response....................................................................................................................... 7

Vertical Distribution................................................................................................................................ 8

Data Analysis ............................................................................................................................................. 8

Results........................................................................................................................................................... 9

Movement Trials......................................................................................................................................... 9

Movement Rates.................................................................................................................................. 15

Repeated Dewatering and Rewatering Trials........................................................................................... 17

Individual Fish Response..................................................................................................................... 19

Availability for Salvage......................................................................................................................... 22

Vertical Distribution.............................................................................................................................. 23

Discussion ................................................................................................................................................... 25

Conclusions ................................................................................................................................................. 29

Acknowledgments ....................................................................................................................................... 31

References Cited......................................................................................................................................... 31

Figures

Figure 1. Schematic diagram of the tank constructed to evaluate larval lamprey movements in 

response to dewatering ................................................................................................................................. 5

Figure 2. Distribution of larval lamprey total length in movement trials for dark and light conditions at 

each of five dewatering rates and for controls ............................................................................................. 10

Figure 3. Graph showing percentage of larval lamprey that were stranded or safe at the end of 

dewatering trials, by group........................................................................................................................... 11

Figure 4. Graphs showing percentage of larval lamprey in each tank section at the end of movement 

trials conducted with five dewatering rates under dark and light conditions................................................. 12

Figure 5. Graphs showing summary of percent emergence and total length for larval lamprey during 

dewatering trials at five dewatering rates under dark and light conditions ................................................... 14

Figure 6. Graph showing median and minimum and maximum time to emerge from sediment for larval 

lamprey exposed to dewatering at four dewatering rates and dark and light conditions............................... 15

Figure 7. Graph showing movement rate for larval lamprey by size category and movement number....... 17

Figure 8. Graphs showing the summary of the repeated dewatering and rewatering trials for two 

dewatering rates and dark and light conditions, by time .............................................................................. 18

Figure 9. Graphs showing individual fish responses and percent emergence for larval lamprey exposed 

to repeated dewatering and rewatering trials with two dewatering rates and dark and light conditions........ 21
iv

Tables 

Table 1. Summary of dewatering trials. ........................................................................................................ 9

Table 2. Mean time required to reach the test water level in movement trials. ........................................... 10

Table 3. Summary of larval lamprey in movement trials ............................................................................. 13

Table 4. Summary of small and large larval lamprey size categories ......................................................... 19

Table 5. Vertical distribution of larval lamprey by group and depth strata. ................................................. 23

Table 6. Vertical distribution of larval lamprey by control and treatment group........................................... 24

Table 7. Vertical distribution of larval lamprey ............................................................................................ 24

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

International System of Units to U.S. customary units 

Multiply By To obtain

Length

centimeter (cm.) 2.54 inch (in.)
Evaluating Dewatering Approaches to Protect Larval 

Pacific Lamprey

By Theresa L. Liedtke1, Lisa K. Weiland1, Joseph J. Skalicky2, and Ann E. Gray2

Executive Summary

Larval Pacific lamprey live for several years burrowed in nearshore sediments where they 

filter feed on detritus and organic matter. Dewatering of larval habitat can occur as a result of

flow-management practices, construction projects, or seasonal closures of irrigation diversions. 

Effective management of dewatering events requires guidance on approaches to protect lamprey, 

such as dewatering rates and light conditions (day or night) that allow lamprey the best 

opportunity to relocate water and avoid being stranded. We conducted controlled laboratory 

experiments comparing five dewatering rates (1, 1.8, 4, 8, and 16 inches per hour [in/h]) and two 

light conditions (light and dark) to evaluate their effectiveness in protecting larval lamprey. We 

used a tank with a simulated shoreline at a 10-percent slope filled with river sediment and 

manipulated the outflow to control the rate of dewatering until water was covering only the 

sediment in the lowest tank section, at the bottom of the slope. Following dewatering, larvae 

were classified as either stranded (in or on the substrate outside the watered area) or safe 

(relocated to the wetted area at the lower end of the tank). All study groups experienced high 

rates of stranding. The lowest stranding rates were for 1 in/h, in both light (77 percent) and dark 

(80 percent). Faster dewatering rates generally produced higher percentages of stranded fish, and 

both the dark and light trials at 16 in/h stranded all larvae. At each of the five dewatering rates, 

trials conducted in the dark stranded the same or higher proportions of fish than the 

corresponding trial conducted in the light, so there was no clear advantage to dewatering during 

dark conditions. The largest contribution to stranding rates for all study groups was the high 

number of larvae (50–80 percent) that did not initiate movement in response to dewatering and 

remained in the uppermost tank section where they were stocked at the start of the trials. The

proportion of larvae that emerged from the sediment during dewatering trials was approximately 

30 percent, and fish that emerged were consistently smaller than those that remained burrowed. 

Combining all dewatering rates, emergence was 31.3 percent for groups under dark conditions 

and 30.7 percent for groups under light conditions. We recorded the timing of emergence for 58 

larvae and their median time to emerge (after the surface of the sediment in the uppermost tank 

section was dewatered) was 0.62 hour (h) (range 0–4.5 h). We measured larval movement rates 

and found that large fish moved faster than small fish. Differences in larval movement rate based 

on light condition were significant only for large fish, which had a significantly faster rate during 

light conditions. Larval lamprey moved, over short distances, at rates that exceeded the fastest 

dewatering rate we tested. The mean movement rates for groups ranged from 19.0 to 44.4 

1

 U.S. Geological Survey 

2 U.S. Fish and Wildlife Service
2 

centimeters per minute [cm/min]) and the fastest dewatering rate (16 in/h) is equivalent to less 

than 6.8 cm/min. Only the slowest movement rate measured, 6.6 cm/min for one individual 

lamprey, was slower than the fastest dewatering rate.

We also investigated lamprey responses to a series of dewatering and rewatering events.

Individual larvae were held in cylinders and exposed to four cycles of dewatering and rewatering 

using dewatering rates of 1 and 16 in/h and a rewatering rate of 2 in/h. Each dewatering rate was 

tested under both dark and light conditions. The location of fish, either on the surface of the 

sediment or burrowed, was recorded after each dewatering event for four rounds. The most 

common individual fish response for all study groups was to remain burrowed through all four 

rounds, and there were large differences in response between small and large larvae. Overall for 

small larvae, combining all groups, 14 of 28 fish emerged, and of those, 8 died and 1 was 

lethargic. The 1-in/h rate had 7 of the 8 mortalities, split about equally between the dark (3 fish) 

and light (4 fish) trials. All but one fish that died emerged from the sediment at some point 

during the four rounds of dewatering. Large larvae predominantly remained burrowed in all four 

rounds and did not experience any mortality. None of the large fish emerged for more than a 

single round, and emergence occurred only in the first and second rounds. Larvae emerged more 

quickly as the number of dewatering events increased. The mean time to emerge after the surface 

of the sediment in the tube was dewatered, combing all four groups, was 42 minutes (min) in 

round 1 (14 fish), 16 min in round 2 (5 fish), 11 min in round 3 (3 fish), and 8 minutes in round 4 

(3 fish). When all groups and rounds of dewatering were combined, the overall mean time to

emerge was 29 min (25 fish) and ranged from 1 min to 2 hours after the surface of the sediment 

was dewatered. Larvae burrowed deeper during the 1-in/h trials than the 16-in/h trials, and few

fish were deeper than about 23 centimeters (cm). Large larvae burrowed deeper than small 

larvae. Small larvae were most concentrated from 0 to 7.6 cm (83.7 percent), and large fish were 

concentrated from 15.2 to 22.8 cm (43.3 percent). The second dewatering event resulted in 

greater mean burrowing depth than the first event, but trends after the second event were less 

clear.

Larval size played a role in lamprey responses to dewatering, having a significant effect

on emergence, movement rate, and vertical distribution. The sediment used for laboratory testing 

or occupied by lamprey in the field appears to affect lamprey response to dewatering and 

deserves greater attention in future studies. Larvae were more active in the dark, but darkness did 

not consistently provide better outcomes (e.g., more emergence or reduced stranding) compared 

to daylight. An improved understanding of the cues that prompt larvae to emerge from the 

sediment, combined with the ability to manage dewatering rates, would be useful to guide future 

dewatering events to minimize negative effects to lamprey.

Introduction

Pacific lamprey (Entosphenus tridentatus) numbers in the Columbia River Basin have 

decreased from historical levels (Close and others, 2002; Wang and Schaller, 2015; Clemens and 

others, 2017), which has raised concerns from managers of Federal, State, and Tribal entities.

Recovery planning is hindered by a lack of information on the basic biology and ecology of 

lampreys, including limiting factors such as anthropomorphic effects on habitat. Larval lamprey

spend 3 to 10 years of their lives burrowed in river sediments, where they filter feed on detritus

and organic matter, making them vulnerable to water-level fluctuations that can dewater their

habitat. Dewatering events occur regularly in the Columbia River Basin for several reasons,

including operation and management of hydropower facilities, seasonal or maintenance closures 
3 

of irrigation diversions, and in-water construction projects for habitat restoration, including 

culvert or bridge repair or replacement. Hundreds to thousands of dewatered larval lamprey have 

been documented during annual seasonal closures of irrigation diversions, and salvage efforts are 

sometimes used to capture and relocate these fish (Beals and Lampman, 2018; Skalicky and 

others, 2019). Guidance on best practices to limit adverse effects on lamprey during planned 

dewatering events like seasonal closures of an irrigation diversion or in-water construction 

projects is lacking. Few studies have evaluated dewatering practices, although field observations 

during salvage operations indicate that slower dewatering rates may be more protective for 

lamprey than faster rates (Beals and Lampman, 2018). Liedtke and others (2015) compared two 

dewatering rates in a controlled laboratory setting and reported that fewer larval lamprey were 

stranded at 3 inches per hour (in/h) than at 20 in/h. Additional studies addressing dewatering 

rates and light conditions (day or night) are needed to provide effective guidance for planned 

dewatering events that allow lamprey the best opportunity to relocate water successfully and 

avoid being stranded. 

The current study was designed to complement evaluations conducted by Liedtke and 

others (2015) by expanding the range of dewatering rates and including variable light conditions. 

Our initial evaluation of lamprey responses to dewatering (Liedtke and others, 2015) compared 

rates of 3 and 20 in/h during daylight conditions. To help determine dewatering rates that might 

be included in the current study, we met with State and Tribal partners who often work to 

salvage lamprey after dewatering events. Through discussion, we selected five dewatering rates 

ranging from 1 to 16 in/h that would be tested during both dark and light conditions. Lamprey 

have increased activity at night (Moursund and others, 2000; Goodman and others, 2015; Moser 

and others, 2015), which might encourage initiation of movement in response to dewatering 

more quickly at night compared to during the day. Lamprey might also have variable movement 

rates or other behaviors that could result in fewer fish being stranded on dewatered substrate

during darkness. 

A period of rewatering following a dewatering event may result in improved salvage 

opportunity if additional larvae emerge from the sediment where they can be collected and 

relocated. During a daytime dewatering event that occurred at the Fivemile-Bell Restoration 

Project on the Oregon coast, salvage was conducted by locating and collecting larvae that 

emerged from the sediment and relocating them to watered habitat. Overnight, dewatering was 

halted (pumps were turned off) and seepage caused the water level to rise, rewatering larval 

habitat that had previously been dewatered. When dewatering continued the following day, 

additional larvae emerged from areas where salvage had previously been conducted (Paul Burns, 

U.S. Forest Service, oral commun., 2019). Evaluations in the laboratory (Liedtke and others, 

2015) and field (Skalicky and others, 2019) have shown that approximately 50 percent of larvae 

emerge from the sediment following dewatering and could potentially be removed from the 

surface of the sediment and salvaged. If repeated dewatering and rewatering could encourage 

approximately 50 percent of larvae to emerge each cycle, then most larvae (about 93 percent) in 

an area could potentially be salvaged after four cycles. To test this theory, we tested a repeated 

dewatering and rewatering approach using 1- and 16-in/h dewatering rates, a 2-in/h rewatering 

rate, and dark and light conditions. 
4 

This study evaluated the effects of several dewatering approaches on larval Pacific 

lamprey. The objectives of this controlled laboratory study were to document the response of 

larval lamprey to dewatering of their habitat under dark and light conditions, specifically—(1) 

their movements relative to dewatering rate and fish size, resulting in their either being stranded 

or regaining access to water, and (2) their emergence behavior during repeated dewatering and 

rewatering relative to dewatering rate, light condition, and fish size. 

Methods

Fish Source, Size, and Maintenance

Larval lamprey were collected using lamprey-specific electrofishing settings and 

sediment grab samples from the Wind River, Washington. Larvae longer than 60 millimeters 

(mm) can be identified to species, allowing separation of Pacific lamprey from sympatric 

Western Brook lamprey (Lampetra richardsoni). We wanted to include larvae less than 60 mm 

in our tests to increase the overall size range but could not confirm they were Pacific lamprey. Of 

the larvae identified to species, 88.3 percent were Pacific lamprey, so we assume that the 

proportion would be similar in the group of small lamprey that could not be identified. 

Fish were transported to the Columbia River Research Laboratory in Cook, Washington, 

and held in fiberglass tanks (51×43×27 cm) with beach sand for burrowing at a depth of about 5 

cm and supplied with heated (8.5–13.6 degrees Celsius [°C]), filtered water (1.5 liters per minute 

[L/min]) from the Little White Salmon River. Lamprey were fed a slurry of active yeast and 

commercial fry food (Gemma Wean 0.1; Skretting, Vancouver, British Columbia, Canada) using 

methods modified from Rose and Mesa (2012). 

During field collections of larvae we defined three size classes—fish less than 57 mm, 

58–84 mm, and greater than 85 mm total length (TL). For dewatering trials, larvae were selected 

from each size class in approximately equal numbers to include the full range of sizes available. 

Individual larvae were not reused for multiple experiments, so all trials used nonexposed or 

naïve fish. To best represent larvae in natural systems and minimize long-term laboratory 

holding, we acclimated fish for 2 weeks prior to any testing and completed all tests within 6 

weeks of the date of collection. We collected larvae on four occasions (June–August 2019) to 

ensure an adequate number of fish and sufficient time to execute all study trials. Tests of 

movement rate were an exception in that the larvae were not naïve and some were held for 

longer than 6 weeks as a result of fish shortages. 

Movement Trials

To test larval lamprey movements in response to dewatering, we built a wooden tank 

(239×30×91 cm) with a false bottom at a 10-percent slope (5.7°) and coated it with epoxy paint 

(fig. 1). A drain was placed at one end with a perforated divider positioned 29 cm from the end 

to hold the substrate in place but allow water to drain from the tank. The tank was filled to a 

depth of 22.9 cm with sediment, maintaining the 10-percent slope. The sediment used in the tank 

was collected from the mouth of White Salmon River, Washington, and had a median particle 

size of 0.445 mm (medium sand). The tank was portioned into four sections (fig. 1). Sections 1 

and 4 were 30.5 cm long, and sections 2 and 3 were 74.5 cm long. The tank received heated river 

water (3 L/min) whose temperature ranged from 9.2 to 9.6 °C. 
5 

Figure 1. Schematic diagram of the tank constructed to evaluate larval lamprey movements in response 

to dewatering. The tank was sloped at a 10-percent grade, filled with sediment, and divided into four 

sections. Dividers between tank sections 1 and 2 and between tank section 4 and the drain were physical 

dividers (solid lines) and dashed lines represent divisions used descriptively. The elevations of the starting 

and final dewatering water levels are shown by horizontal blue lines. (cm, centimeters; m, meters)

Five dewatering rates were tested, each under dark and light conditions. The dewatering 

rates were selected to further the work conducted by Liedtke and others (2015) using larval 

lamprey in the same test system and to be relevant to dewatering rates observed regionally. We 

tested dewatering rates of 1, 1.8, 4, 8, and 16 in/h. Liedtke and others (2015) compared results 

from rates of 3 and 20 in/h. We selected 1.8 in/h as a test rate because it was used for a 

dewatering event in the field where larval lamprey response and salvage techniques were 

evaluated (see Skalicky and others, 2019). The tank was dewatered at one of the test rates, to the 

point where tank sections 1 to 3 had exposed substrate, and the only surface water available was 

in tank section 4, where water covered the full length of the substrate. We completed three 

replicate trials of each rate and light condition using 10 larvae per trial. The combination of 

dewatering rate and light condition resulted in 10 study groups: 1 in/h dark (1–D), 1 in/h light 

(1–L), 1.8 in/h dark (1.8–D), 1.8 in/h light (1.8–L), 4 in/h dark (4–D), 4 in/h light (4–L), 8 in/h 

dark (8–D), 8 in/h light (8–L), 16 in/h dark (16–D), and 16 in/h light (16–L). Additionally, three 

control trials were completed under both dark (C–D) and light (C–L) conditions, where the tank 

was not dewatered. Twelve study groups with three replicates per group resulted in 36 

dewatering trials. All trials used a 1-h waiting period after trial conditions were established in the 

tank, after which fish were located and removed. 

To stock fish, the tank was filled with water and a metal divider was pushed into the 

substrate to the bottom of the tank at the lower border of section 1 to prevent fish from moving 

out of section 1. Ten fish (2–5 fish from each size class) were haphazardly selected and placed in 

section 1. After all fish burrowed into the substrate, the inflow water was turned on and the fish 
6 

remained in section 1 a minimum of 12 h. To begin a trial, the inflow water was turned off, the 

drain pipe and the section 1 metal divider were removed, and the drain valve was set to the test 

dewatering rate. The tank was checked regularly, and adjustments were made as necessary to 

maintain the desired test rate. Once the final dewatering level was reached (water available only 

in tank section 4), the drain valve was closed, air and water temperature were recorded, and the 

1-h waiting period began. Larval movements were monitored closely to help determine final tank 

position and time of emergence from the sediment. At the completion of the waiting period the 

trial was terminated. We located each fish and recorded the tank section and whether they were 

on the surface of the substrate or burrowed into the substrate. To locate burrowed fish, sediment 

was removed with a scoop, placed on a sieve over a container, and gently sprayed with water. 

Following removal from the tank, larvae were anesthetized in a solution of tricaine 

methanesulfonate (100 milligrams per liter [mg/L]) and buffered with an equal amount of 

sodium bicarbonate to facilitate collection of weight and length data. Control trials followed the 

same procedures, except that the tank was not dewatered and the waiting period began as soon as 

the inflow was turned off and the divider for section 1 was removed. 

Dark and light trials were conducted using natural light conditions, and we used red light 

to monitor fish during dark trials. The 1-in/h dewatering rate required almost 13 h to reach the 

final dewatering level, and it was impossible to execute fully in darkness because the trials were 

conducted during the summer. The other dewatering rates required less than 8 h of a constant 

light condition and were more manageable. For the 1-in/h trials, we ensured that the targeted 

light condition was reached when the water level reached the surface of the substrate. 

Dewatering of the substrate was always executed under the target light condition. If periods of 

the nontarget light condition had to be included, we planned the trial so they occurred before the 

water level reached the substrate, when lamprey were less likely to be moving (on the basis of 

our observations from Liedtke and others [2015]). 

Movement Rates

We estimated larval movement rates in the test tank under dark and light conditions to aid 

in interpretation of stranding risk during movement trials. On the basis of Liedtke and others 

(2015), we anticipated the largest difference in movement rates between the smallest and largest 

larvae, so we did not measure movement rates for mid-sized fish. Initially we placed 10 larvae in 

tank section 1 while the tank was watered, allowed them to burrow and acclimate briefly, and 

then rapidly dewatered the tank. We planned to monitor larvae as they emerged from the 

sediment and moved down the tank slope. After several attempts in which few fish emerged or 

moved, we modified our approach. We selected fish and gently placed them on the surface of the 

sediment in tank section 1 shortly after the tank was dewatered. As fish began moving, we noted 

the time and their initial location. When a fish stopped moving, we noted the time and its final 

location. The total horizontal distance between the initial and final locations was measured and 

recorded as a movement. The total distance moved and total time were used to calculate a travel 

rate. Some larvae moved, stopped briefly, and then continued. If the period of inactivity was less 

than about 20 seconds (s), we continued monitoring the movement. If inactivity was more than 

about 20 s, we considered the first movement complete and monitored for initiation of additional 

movements. A second or third movement for the same individual was measured and reported 

unless inactivity exceeded about 2 min; then recording for that individual was terminated. 

Movement rates were reported for small and large fish under dark and light conditions. The 
7 

maximum horizontal distance larvae could move in the tank, from the lower margin of section 1 

to the upper margin of the water in section 4, was 149 cm. 

Larval lamprey move using a serpentine motion such that forward motion is typically

accompanied by lateral movements. To estimate the amount of lateral movement we observed as 

we measured movement rates, we divided the width of the tank into three lanes, each 10.2 cm 

wide. To help visualize the lanes when lamprey were moving in the tank, we suspended string at 

the lane boundaries, a few centimeters above the surface of the sediment. We recorded the lane 

where lamprey initiated and ended each movement. To be assigned to a lane, the lamprey’s 

entire body had to cross the lane boundary. 

Repeated Dewatering and Rewatering Trials

Repeated dewatering and rewatering trials were conducted using two dewatering rates (1 

and 16 in/h) and dark and light conditions. Larvae were held in sediment-filled (15.2-cmdiameter) plastic tubes positioned in tanks, and tanks were dewatered and rewatered four times 

for each trial. The dewatering rate was set to one of the two test rates, and the rewatering rate 

was 2 in/h for all trials. A similar test system was used in Liedtke and others (2015) to evaluate 

survival of larvae after dewatering. We had two objectives for these trials: (1) describe the 

response of individual larvae to four rounds of dewatering and rewatering, specifically which 

fish emerged from the sediment each round; and (2) describe the vertical distribution of larvae 

following each round of dewatering. Trials for both objectives were completed simultaneously, 

with slightly different test systems. Controls were configured like the vertical distribution test 

system but supported both objectives. 

Individual Fish Response

To describe individual fish response, we put one larval lamprey into a 15.2-cm-diameter 

plastic tube, 38.1 cm tall, filled with 30.5 cm of sediment. The sediment was the same material 

used in the movement trials. The tube bottoms were covered with nylon bolt cloth (aperture of 

425 micrometer [μm]) to allow water to drain. Seven treatment tubes were haphazardly placed 

on the bottom of a 1.22-meter-diameter fiberglass tank and two control tubes were placed in an 

adjacent, equivalent tank. Control tubes were modified slightly from treatment tubes, to match 

the vertical distribution treatment group (see below). Two replicates of each treatment were 

completed simultaneously, in separate tanks. For each trial, there were three tanks: a control tank 

with two tubes, and two treatment tanks, each with seven treatment tubes.

Prior to a trial, tanks were filled with water until the level was just above the sediment 

surface in the tubes to facilitate stocking of one test fish per tube. When all fish were stocked, the 

tops of the tubes were covered with mesh, the tanks were filled to a depth of 39 cm, and fish 

were allowed to acclimate for at least 14 h. Water temperatures in the tanks ranged from 8.8 to 

10.3 °C. At the start of a trial, the inflow to the treatment tanks was turned off and the tanks were 

drained with a siphon at the target dewatering rate. After the water level in the tanks was below 

the top of the tubes, the mesh covering was removed for improved visibility. Tubes were 

monitored and time of emergence was recorded if observed. When the tank was completely 

drained, the 1-h waiting period began. At the end of the waiting period, the location of the fish in 

each tube was recorded as either burrowed in the sediment or on the surface of the sediment, and 

the first round of dewatering was complete. The tank inflow was set to refill the tanks at 2 in/h to 

begin the second round of the trial. Four rounds of repeated dewatering and rewatering were 

completed for each of four trials: 1–D, 1–L, 16–D, and 16–L. 
8 

Vertical Distribution

To describe the vertical distribution of larvae, we modified the plastic tubes used for the 

individual response objective. The 15.2-cm-diameter plastic tubes, 38.1 cm tall, were partitioned 

into four sections. The three lower sections were 7.6 cm tall, and the top section was 15.2 cm 

tall. The four tube sections were stacked, secured in a column using neoprene sleeves, and filled 

with 30.5 cm of sediment. The top tube section had 7.6 cm of sediment and 7.6 cm of unfilled 

space to allow fish access to the surface of the substrate. Four vertical distribution tubes were 

haphazardly placed into each of the two treatment tanks, and two control tubes (with the same 

configuration) were placed in the adjacent, control tank. Tubes were stocked with two larvae 

(one small and one large) and acclimated with the individual fish response tubes. 

Two tubes (one from each treatment tank) were randomly assigned to each of the four 

dewatering rounds. Tubes were dewatered and rewatered for 1 to 4 rounds with the individual 

fish response tubes (they were in the same tanks). At the completion of each round (after the 1-h 

waiting period), the tubes preassigned to that round were removed from the tank. Fish on the 

surface were removed and counted. The top neoprene sleeve was removed, and the top tube 

section was slid quickly onto an adjacent tray. The next two sections were removed in the same 

manner. Sediment from each tube section was searched to determine the depth stratum where 

each fish was burrowed. Fish were counted, anesthetized, weighed, and measured. Control tubes 

were processed using the same procedure, except that the tank was not dewatered. 

We selected small and large larvae for these trials and report findings based on these size 

categories. Fish were classified as small if their total length (TL) was about 35 to 70 mm and as 

large if their TL was at least about 85 mm. Larvae between these size categories were not used to 

reduce the total number of larvae collected for the study and because we anticipated the greatest 

difference in response to result from the difference in TL.

Data Analysis 

In the movement trials, we pooled the three replicate trials for each of the 12 study 

groups and present mean findings for each group. At the end of each movement trial we recorded 

fish position in the tank by noting the tank section and whether fish was on the surface of the 

sediment or burrowed. For analysis, we categorized larvae as either safe or stranded. Fish were 

considered safe if they were in tank section 4, either on the surface or burrowed in the sediment. 

Larvae outside tank section 4 (on the surface or burrowed) were classified as stranded. The 

proportion of fish that emerged from the sediment was calculated by summing the number of fish 

observed to emerge in tank section 1 (but not leave section 1) and the number of fish that moved 

out of section 1. Fish that moved outside tank section 1 were included because we assumed, on 

the basis of Liedtke and others (2015), that movement out of section 1 required emerging from 

the sediment and traveling over the surface. Time of emergence was monitored and documented 

opportunistically, and we summarized the time to emerge as the amount of time after the surface 

sediment in tank section 1 was fully exposed. The surface of the sediment in tank section 1 was 

dewatered in 7.2 h for 1 in/h, 4.0 h for 1.8 in/h, 1.8 h for 4 in/h, 54 minutes for 8 in/h, and 27 

minutes for 16 in/h. 

In the repeated dewatering and rewatering trials, we pooled the replicates for each study 

group and present mean findings by group. For vertical distribution trials, the number and 

proportion of fish were summarized by the stratum (1–4) in the sample tube where fish were 

recovered. Mean depth stratum was calculated using the stratum for each individual fish in a 

group. Emergence timing was calculated as the movement trials were, beginning when the 
9 

surface of the sediment in the tubes was dewatered. The surface of the sediment was exposed 3.2 

h after dewatering started for the 1 in/h rate and 13 minutes after dewatering started for the 16 

in/h rate. 

Comparisons of fish size and movement rate were made using t-tests or analysis of 

variance (ANOVA) and Duncan’s Multiple Range Test at a significance level of 0.05. 

Results

Movement Trials

We completed 36 dewatering trials for the 12 study groups during June 3–September 11, 

2019, using 360 larvae (table 1). Water and air temperatures during trials were similar across all 

groups (table 1) and the mean TL of larvae for the groups were not significantly different 

(F=0.12, df=11, p=0.99; fig. 2). The amount of time required to reach the dewatering level 

ranged from 49 minutes for the 16-in/h rate to almost 13 h for the 1-in/h rate (table 2). 

Table 1. Summary of 36 dewatering trials conducted in 2019 to monitor larval lamprey movements, 

including dewatering group, trial dates, mean water temperature, and mean air temperature.

[Group names are a combination of dewatering rate (1, 1.8, 4, 8, or 16 inches per hour) and dark 

(D) or light (L) conditions; C, control; °C, degrees Celsius; SD, standard deviation] 

Group Trial dates Mean water temperature 

(°C) (SD)

Mean air temperature 

(°C) (SD)

1-D 12 June–23 July 9.3 (0.2) 19.6 (1.7)

1-L 10 June–15 July 9.5 (0.2) 19.1 (0.9)

1.8-D 6 June–18 July 9.3 (0.1) 18.0 (1.6)

1.8-L 3 June–8 July 9.3 (0.3) 19.3 (0.6)

4-D 5 June–16 July 9.3 (0.1) 18.9 (1.6)

4-L 4 June–12 July 9.3 (0.1) 18.8 (1.1)

8-D 27 June–11 September 9.3 (0.1) 18.1 (0.7)

8-L 26 June–22 July 9.2 (0.3) 19.6 (1.4)

16-D 16 June–20 August 9.6 (0.3) 19.2 (1.0)

16-L 11 June–13 August 9.2 (0.1) 18.9 (0.7)

C-D 22 June–21 August 9.4 (0.1) 18.8 (2.0)

C-L 14 June–14 July 9.4 (0.2) 19.6 (1.5)
10 

Table 2. Mean time required to reach the test water level in movement trials conducted under dark and 

light conditions for five dewatering rates. Each rate and light condition is a mean of three replicate trials.

[in/h, inches per hour; h, hours; SD, standard deviation] 

Rate (in/h) Dark (h) (SD) Light (h) (SD)

1 12.78 (0.14) 12.76 (0.12)

1.8 7.09 (0.02) 7.19 (0.03)

4 3.16 (0.06) 3.23 (0.03)

8 1.60 (0.06) 1.66 (0.05)

16 0.82 (0.01) 0.83 (0.01)

Figure 2. Distribution of larval lamprey total length in movement trials for dark and light conditions at 

each of five dewatering rates and for controls. Controls were not dewatered. On each bar, means are 

indicated with an X, and medians with a line; the ends of the bars represent the minimum and maximum 

values. (inches per hour, in/h; mm, millimeter)

Larvae in the control groups, which did not experience dewatering, stayed predominantly 

in tank section 1. In the C–L group, 93.3 percent of the fish remained in section 1 and two fish 

(6.7 percent) were recovered in the uppermost part of section 2, near the border with section 1. 

The C–D group showed more movement, with 86.7 percent of the larvae recovered in tank 

section 1 and 13.3 percent outside section 1. One of the four fish that moved outside section 1 

was in the uppermost part of section 2. The three other larvae were in section 3 (two fish) or the 

middle of section 2 (one fish) at the end of the trial. 

All study groups experienced high rates of stranding. The slower dewatering rates 

stranded fewer fish than the faster rates, and there was no clear reduction in stranding rate under 

dark conditions compared to light (fig. 3). The lowest stranding rates were for 1–L, with 76.7 
11 

percent stranded, and 1–D, with 80.0 percent stranded. All larvae (100 percent) were stranded in 

the 8–D, 16–D, and 16–L groups (fig. 3). At each of the five dewatering rates, trials conducted in 

the dark stranded the same or higher proportions of fish than the corresponding trial conducted in 

the light. In most cases, the differences in stranded proportions between the dark and light trials 

for a given rate were small. Two rates, 4 in/h and 16 in/h, had the same stranding rate for dark 

and light trials. The largest difference between light conditions occurred at the 1.8-in/h rate, with 

86.7 percent of larvae stranded in the light and 96.7 percent stranded in the dark. There was a 

difference of 3.3 percent between light conditions for the 1-in/h trials and a difference of 6.7 

percent for the 8-in/h trials. Statistical comparison of the mean TL of larvae that were stranded 

and safe was possible only for the 1-in/h rate, because only one to two fish were classified as 

safe for all other rates. There was no significant difference in mean TL of safe fish (60.2 mm, 6 

fish) and stranded fish (71.9 mm, 24 fish) (t=1.14, p=0.26) in 1–D. In the 1–L group, however, 

safe fish were significantly smaller (47.0 mm, 7 fish) than stranded fish (77.7 mm, 23 fish) 

(t=3.33, p=0.002). 

Figure 3. Graph showing percentage of larval lamprey that were stranded (in tank sections 1–3) or safe 

(in tank section 4) at the end of dewatering trials, by group. Group names are a combination of a 

dewatering rate (1, 1.8, 4, 8, or 16 inches per hour (in/h)) and dark or light condition (D or L). 

The largest contribution to stranding rates for all study groups was the high number of 

larvae that did not initiate movement in response to dewatering and remained in tank section 1. 

From 50.0 to 80.0 percent of larvae in the study groups were recovered in section 1 (fig. 4). 

Fewer fish were in section 1 at 1 in/h (50.0 percent for 1–D and 56.7 percent for 1–L) than other 

groups, and the highest proportions were for 4–D (80.0 percent), 1.8–D (76.7 percent), and 8–D 

(76.7 percent). The dark and light conditions had similar numbers of fish that remained in section 

1, differing only by one to two fish for each rate. Overall, three of the five dewatering rates had 

higher numbers of fish in section 1 under dark conditions than under light conditions. 
12 

Figure 4. Graphs showing percentage of larval lamprey in each tank section at the end of movement 

trials conducted with five dewatering rates under dark and light conditions. Fish in sections 1 to 3 were 

categorized as stranded and fish in section 4 were considered safe. Numbers at ends of bars indicate 

number of fish. (in/h, inches per hour)
13 

Larvae that initiated movement in response to dewatering but were ultimately classified 

as stranded were most commonly found in tank section 2. The 16-in/h dewatering rate had the 

highest proportion of fish stranded in section 2 or 3 (fig. 4), and of the 19 stranded fish, 12 (63.1 

percent) were in section 2 (table 3). Three of the five dewatering rates had more fish stranded in 

section 2 than in section 3. The exceptions were 4 in/h, at which the same number of fish were in 

sections 2 and 3, and 1 in/h, at which more fish were in section 3 than in section 2 (table 3). 

Overall, across the 10 study groups, 59.7 percent of fish that left section 1 but were ultimately 

stranded were recovered in section 2 and 40.3 percent were recovered in section 3, closer to the 

water at the bottom of the slope. 

Table 3. Summary of larval lamprey in movement trials that emerged from the sediment in tank section 1 

and initiated movement toward tank section 4 but were ultimately classified as stranded because they did 

not reach tank section 4 prior to the end of the trial. 

[Group names are a combination of dewatering rate (1, 1.8, 4, 8, or 16 inches per hour) and dark 

(D) or light (L) conditions] 

Group Number of fish in 

section 2

Number of fish in 

section 3

1-D 4 5 

1-L 2 4 

Total 6 9 

1.8-D 6 0 

1.8-L 3 1 

Total 9 1 

4-D 1 3 

4-L 4 2 

Total 5 5 

8-D 5 2 

8-L 3 3 

Total 8 5 

16-D 5 5 

16-L 7 2 

Total 12 7 

Total (percentage) 40 (59.7) 27 (40.3)

The overall proportion of larvae that emerged from the sediment during dewatering trials 

was approximately 30 percent, and fish that emerged were consistently smaller than those that 

remained burrowed. The highest proportion of emerging larvae, 50.0 percent, was observed for 

1–D; when all dewatering rates were combined, emergence was 31.3 percent for dark groups and 

30.7 percent for light groups (fig. 5). Comparing across dewatering rates with dark and light 
14 

conditions combined, the highest emergence was 45 percent for 1 in/h, followed by 32 percent 

for 1.8 in/h. The mean TL of fish that emerged was smaller than the mean TL of fish that 

remained burrowed in all study groups, and in 9 of the 10 groups, the size difference was 

statistically significant (fig. 5). Although this metric was not measured and recorded rigorously, 

observations made while digging in sediment to collect larvae indicate that smaller fish burrowed 

at shallower depths than larger fish. 

Figure 5. Graphs showing summary of percent emergence and total length for larval lamprey during 

dewatering trials at five dewatering rates under dark and light conditions. A, shows percent emergence for 

each dewatering rate and light condition and, B, shows the mean total length of larvae that emerged or did 

not emerge from the sediment in each study group. Error bars are the standard deviation. Asterisks indicate 

bars within a group that were statistically different as determined by t-tests. Group names are a 

combination of a dewatering rate (1, 1.8, 4, 8, or 16 inches per hour (in/h) through 16 in/h) and dark or light 

condition (D or L). 
15 

The time of emergence for 58 larvae (62.4 percent of the total number that emerged) and

their median time to emerge (after the surface of the sediment in tank section 1 was dewatered)

was 0.62 h (range 0–4.5 h). Because the dewatering period for the 1-in/h trials was long, we only 

observed three larvae emerge, so this rate was excluded from analyses for emergence timing. The 

dark and light groups for each dewatering rate generally had similar median emergence times 

(fig. 6). The 1.8–L and 8–L groups had slightly higher median or maximum times relative to

their corresponding dark groups (fig. 6). 

Figure 6. Graph showing median (filled circles) and minimum and maximum (at lower and upper extents 

of lines) time to emerge from sediment for larval lamprey exposed to dewatering at four dewatering rates 

and dark and light conditions. (h, hour; inch per hour, in/h)

Most larvae were recovered burrowed in the sediment at the end of a trial and those found 

on the surface of the sediment were predominantly in tank sections 2 and 3. Overall, 14.7 percent 

of larvae were on the surface of the sediment after the 1-h waiting period following dewatering. 

Of these, 88.6 percent were in tank sections 2 (28 of 44) and 3 (11 of 44). There were no large 

differences across the 10 study groups, with the exception that 1–L did not have any larvae on 

the surface. The 1-in/h rate had the lowest number of larvae on the surface (9.1 percent) and the 

16-in/h rate had the highest (34.1 percent), but this result was not consistent in the intermediate

rates. The 1.8-in/h rate had the second highest total number of fish on the surface (22.7 percent),

followed by 4 in/h (18.2 percent) and 8 in/h (15.9 percent). When rates were combined, the dark

and light conditions had similar numbers of larvae on the surface at the end of a trial: 54.5

percent for dark groups and 45.5 percent for light groups.

Movement Rates 

Movement rates were based on 62 larvae with one observed movement, 34 fish with two 

movements, and 7 fish with three movements. Larvae in the small size category ranged from 45 

to 68 mm TL and large larvae ranged from 87 to 115 mm TL. Larvae in the large size category 

were significantly bigger than fish in the small size category (F=172.9, df=3, p<0.001), but 
16 

within each size class the TLs in the dark and light groups were comparable. The mean TLs of 

small fish with dark (55.0+3.7 mm) and light (56.6+6.4 mm) movements were not significantly 

different. Similarly, for the large fish, the TLs were dark, 96.0 +9.0 mm, and light, 101.1+9.5 

mm. When all groups were combined, the mean distance traveled for the first movement was 

51.8 cm (SD, 30.1); distance was smaller for the second (20.7+ 13.4 cm) and third (15.7 +7.8 

cm) movements. 

Lamprey had limited lateral movement as they moved vertically down the slope in the 

tank, and distance decreased with subsequent movements. In the first movement, 59.7 percent of 

larvae ended their movement in the same lane as they started. Each of the three lanes that made 

up the total width of the tank was 10.2 cm wide, so this group of fish had less than 10.2 cm of 

lateral movement. A smaller group of fish (37.1 percent) ended their first movement in a 

different lane than the one in which they started, and 3.2 percent of the first movements involved 

traversing two lanes. Fish observed making a second or third movement showed less lateral 

movement than in their first movement. The majority (85.3 percent) of the second movements 

started and ended in the same lane, and 14.7 percent involved traversing an adjacent lane. Only 

seven fish made a third movement, with 71.4 percent staying in one lane and 28.6 percent 

moving laterally to an adjacent lane. 

Large fish moved faster than small fish, and differences in movement rate based on light 

condition were significant only for large fish. We compared rates for each movement among the 

four study groups: small fish during darkness (S-D), small fish during light (S-L), large fish 

during darkness (L-D), and large fish during light (L-L). Mean rates for the first movement were 

significantly different among the four groups (fig. 7; F=5.21, df=3, p<0.01), with the highest rate 

for L-L. There were no significant differences in mean rate among S-D, S-L, and L-D. For small 

fish, there was no significant difference between the movement rates under dark and light 

conditions (fig. 7). For large fish, the movement rate was significantly faster during light 

conditions. For the second movement, the comparison among groups showed significant 

differences (F=10.92, df=3, p<0.001) similar to those for the first movement (fig. 7). The mean 

rate for L-L was significantly higher than that for any other group, including L-D. For small fish, 

there was no significant difference between rates for dark and light conditions (fig. 7). Small fish 

had movement rates that were not significantly different from those of the large fish under dark 

conditions for both the first and second movements (fig. 7). As a result of small sample size 

(seven fish), no statistical comparisons were made for the third movements; however, as for the 

first and second movements, the L-L group had the highest rate (fig. 7).
17 

Figure 7. Graph showing movement rate for larval lamprey by size category and movement number. 

Error bars are the standard deviation and numbers are the sample sizes. Letters are Duncan’s Multiple 

Range Test groupings for movements 1 and 2, and bars without letters in common are statistically different 

at alpha = 0.05. (cm/min, centimeters per minute)

Larvae in all study groups maintained similar movement rates for their first and second 

movements, with no significant differences. For example, S-L had a mean rate of 19.0 cm/min 

for the first movement and 19.3 cm/min for the second (fig.7; t=0.12, p=0.91), and L-D had a 

mean rate of 26.3 cm/min for the first movement and 28.3 cm/min for the second (t= 0.48. 

p=0.64). The groups with the largest differences were S-D (difference of 9.4 cm/min; t=1.51, 

p=0.14) and L-L (difference of 9.9 cm/min, t=1.59, p=0.13). 

Repeated Dewatering and Rewatering Trials

Each trial of repeated dewatering and rewatering required 4 days to execute (fig. 8). The 

1-in/h trials were challenging to implement because the time to dewater (15.4 h) exceeded the 

available light (13.3 h) and dark (10.7 h) periods each day during our study period. On the basis 

of past experience with larvae in this test system (see Liedtke and others, 2015), fish tend to 

begin emerging from the sediment shortly after the surface of the sediment in the tube is 

dewatered. Trial timing was implemented so this period of potential emergence met the target 

light condition for the trial. On the basis of this priority, however, the 1-h waiting period at the 

end of each round of dewatering as well as the rewatering period occurred in the opposite 

(nontarget) light condition. For example, the 1-h waiting period for the 1-D trials started at 8:15 

a.m. and rewatering began at 9:15 a.m. (fig. 8). Rewatering was conducted at a rate of 2 in/h for 

all trials, requiring 7.7 h to refill the tanks. The 16-in/h trials were executed with the dewatering 

(58 min), 1-h wait, and rewatering events occurring within the target light condition (fig. 8). 

Between rounds of dewatering for the 16-in/h trials, the tanks were full of water for 

approximately 14.3 h, as we waited for the next target light period (fig. 8). 
18 

Figure 8. Graphs showing the summary of the repeated dewatering and rewatering trials for two 

dewatering rates and dark and light conditions, by time. Each trial was conducted over 4 days, with one 

dewatering event occurring each day. Blue lines indicate water level and shaded areas indicate periods of 

darkness. Dashed lines represent the level of substrate in the tubes. (in/h, inches per hour)
19 

Larval size was similar across groups in both the small and large size categories. Small 

fish ranged from 35 to 72 mm TL and the mean TLs were not significantly different among the 

four study groups and controls (F=1.20, df=4, p=0.33; table 4). Larvae in the large category 

ranged from 84 to 119 mm TL and were not significantly different among the study groups 

(F=1.60, df=4, p=0.20; table 4). 

Table 4. Summary of small and large larval lamprey size categories used in repeated dewatering and 

rewatering trials, including the number of fish and mean total length by group. 

[Group names are a combination of dewatering rate (1 or 16 inches per hour) and dark (D) or 

light (L) conditions; mm, millimeters; SD, standard deviation]

Small Large

Group Number of fish Mean total length 

(mm) (SD)

Number of fish Mean total length 

(mm) (SD)

1-D 7 57.8 (7.2) 7 100.1 (6.5)

1-L 7 51.9 (7.7) 7 95.5 (8.3)

16-D 7 57.9 (13.7) 7 103.1 (13.7)

16-L 7 49.6 (6.3) 7 97.3 (9.8)

Control 8 54.4 (6.6) 8 107.0 (9.7)

Individual Fish Response

The most common individual fish response to repeated dewatering and rewatering for all 

study groups was to remain burrowed through all four rounds, and there were differences in 

response between small and large larvae. Of the small larvae in both 1-D and 1-L, four of seven

fish (57.1 percent) remained burrowed for four rounds and three of seven fish (42.9 percent) 

emerged during at least one round of dewatering (fig. 9). The situation was reversed for small 

fish in the 16-D and 16-L trials; 42.9 percent (three of seven) remained burrowed and 57.1 

percent (four of seven) emerged for at least one round. The three small fish that emerged in both 

the 1-D and 1-L groups (six total fish) all died (fig. 9). There were two additional mortalities—

one fish in 1-L (fish 11) that remained burrowed for four rounds was dead at the end of round 4, 

and one fish in 16-L that emerged in rounds 1 and 2 was dead at the end of round 2. Another fish 

in 16-L (fish 28) emerged in round 1 and was lethargic, with mild hemorrhaging after it was 

recovered at the end of round 4. Overall for small larvae, when all groups were combined, 14 

fish emerged, and of those, 8 died and 1 was lethargic. The 1-in/h rate had seven of the eight 

mortalities, split about equally between 1-D (three fish) and 1-L (four fish). All but one fish that 

died emerged from the sediment at some point during the four rounds of dewatering. There were 

no mortalities after the first round, but at least one fish died after rounds 2 through 4 (fig. 9).

Within the small size category, the mean TL of fish that emerged (57.3+9.7 mm) was not 

significantly different from the mean TL of fish that remained burrowed (51.1+8.2 mm) (t=1.85,

p=0.08). Comparison of the mean TL of fish that emerged and died (n=7, 53.1+5.3 mm) and 

those that emerged and lived (n=7, 49.0+10.4 mm) showed no significant difference (t=0.94, 

p=0.37). Large larvae responded differently to repeated dewatering and rewatering. Large larvae 

predominantly remained burrowed in all four rounds and did not experience any mortality (fig. 

9). None of the large fish emerged for more than a single round, and emergence occurred only in 

rounds 1 and 2 (fig. 9). In control trials, all larvae (both small and large) remained burrowed and 
20 

there were no mortalities. The highest percent emergence for a single round of dewatering across 

study groups and fish size categories was 42.9 percent (fig. 9). For small larvae, the most 

common percent emergence was 28.6 percent (7 of 16 events) and for large larvae the most 

common percent emergence was 0 percent (12 of 16 events) (fig. 9). Overall, 50.0 percent of 

small larvae and 27.3 percent of large larvae emerged, for a combined group total emergence of 

20 fish or 35.7 percent. 
21 

Figure 9. Graphs showing individual fish responses and percent emergence for larval lamprey exposed 

to repeated dewatering and rewatering trials with two dewatering rates and dark and light conditions. Each 

trial included four rounds (R1–R4) of dewatering, each followed by a period of rewatering. Group names 

are a combination of dewatering rate (1 or 16 inches per hour) and light condition (dark or light, D or L). 

Small and large larvae are depicted separately. 
22 

Larvae emerged more quickly as the number of dewatering events increased. Emergence 

timing was not observed for all emergence events so sample sizes were generally low (1–5 fish 

per group and dewatering round). The mean time to emerge after the surface of the sediment in 

the tube was dewatered, when all four groups were combined, was 42 min in round 1 (14 fish), 

16 min in round 2 (5 fish), 11 min in round 3 (3 fish), and 8 min in round 4 (3 fish). Each 

individual group showed a similar pattern. Mean emergence time for the 1-L group gradually 

decreased from round 1 (51 min) to round 3 (17 min). In round 4, the one fish that emerged did 

so about 1 h prior to dewatering of the surface of the sediment. There were nine emergence 

events for 1-L, five of which involved larvae emerging and reburrowing during the active 

dewatering phase, prior to the 1-h waiting period. For 1-D, there were seven emergence events 

and in two of the events the fish reburrowed. Reburrowing was observed during all four rounds 

of dewatering (combining 1-D and 1-L), and all larvae observed to emerge and reburrow were in 

the small size category. One small fish in 1-L emerged during all four rounds of dewatering, but 

never reburrowed. Emergence timing for this fish decreased from 44 min after the surface was 

exposed in the first round to 17 min in the third round. Another small fish in 1-L emerged in 

rounds 1, 3, and 4 and reburrowed each time. Two small fish in 1-L emerged during the first 

round, reburrowed, and then did not emerge for any subsequent dewatering events. No fish were 

documented to emerge and reburrow in the 16-in/h groups. In 16-D, round 1 mean time to 

emerge was 35 min, compared to 8 min in round 4. Timing was similar for 16-L, for which mean 

time to emerge decreased from 38 min in round 1 to 10 min in round 4. One small fish in 16-D 

emerged during all four rounds, with similar emergence timing: 9, 8, 14, and 8 min (rounds 1–4, 

respectively) after the surface of the sediment in the tubes was exposed. When all groups and 

rounds of dewatering were combined, the overall mean time to emerge was 29 min (25 fish) and 

ranged from 1 min to 2 h after the surface of the sediment was dewatered. 

Availability for Salvage

The repeated dewatering and rewatering approach did not make many more fish available 

for salvage compared to a single dewatering event. If we assume that fish that emerged during 

the first round of dewatering were salvaged by removing them from the surface of the sediment, 

we can assess the potential for additional larval salvage during subsequent rounds of dewatering. 

In this way we compared the number of fish available for salvage during repeated dewatering 

and rewatering to the number available during a single dewatering event. There were 14 fish in 

each group, including 7 fish in each size category. In the 1-D trials, two fish could have been 

salvaged after round 1, and one additional fish could have been salvaged after round 3 (fig. 9). 

All of the salvaged fish would have been small because none of the large fish emerged. For 1-L, 

three larvae could have been salvaged after round 1, and one additional fish emerged after round 

2. A few more fish were available for salvage during the 16-in/h trials compared to the 1-in/h 

trials. In 16-D, three fish could have been salvaged after round 1, and three more were available 

in subsequent rounds. The highest potential salvage rate was in 16-L; six fish emerged after 

round 1, and one more after round 4, resulting in seven fish (50.0 percent) available for salvage.

Across all groups and rounds, 20 larvae emerged, 14 (70.0 percent) in round 1 and six (30.0 

percent) after round 1—10.0 percent in each of rounds 2 through 4 (fig. 9). 
23 

Vertical Distribution

Larvae burrowed deeper during the 1-in/h trials than the 16-in/h trials, and few fish were 

deeper than about 23 cm. The highest concentration of larvae was in stratum 3 for 1-D (50.0 

percent) and stratum 1 for 1-L (50.0 percent) (table 5). The mean depth stratum was 2.1 for 1-D 

and 2.0 for 1-L. In the 16-in/h trials, most larvae were in the uppermost stratum, 57.1 percent for 

16-D and 80.0 percent for 16-L (table 5). The mean depth stratum was 1.6 for 16-D and 1.3 for 

16-L. Overall, when all study groups were combined, the mean depth stratum was 1.8 and the 

highest concentration of larvae was in stratum 1 (55.7 percent). The controls, which did not 

experience dewatering, had a similar distribution to the overall treatment group, with 50.0 

percent of fish in the first stratum, no fish deeper than the third stratum, and a mean stratum of 

1.6 (table 5). Only two fish burrowed in stratum 4 (deeper than 22.8 cm), and both occurrences 

were during light conditions, one fish in 1-L and one fish in 16-L. 

Table 5. Vertical distribution of larval lamprey by group and depth strata. Control fish were not 

dewatered. For the overall total and each group individually, the stratum containing the highest percentage 

of fish is shown in bold. 

[Group names are a combination of dewatering rate (1 or 16 inches per hour) and dark (D) or light (L) conditions;

cm, centimeters] 

Strata number Depth strata Control 1-D 1-L 16-D 16-L Total

1 Fish 0-7.6 cm deep 

(percent) 50.0 37.5 50.0 57.1 80.0 55.7

2 Fish 7.6-15.2 cm deep 

(percent) 37.5 12.5 6.3 28.6 13.3 14.8

3 Fish 15.2-22.8 cm deep 

(percent) 12.5 50.0 37.5 14.3 0 26.2

4 Fish 22.8-30.5 cm deep 

(percent) 0 0 6.3 0 6.7 3.3

Mean depth 

stratum 1.6 2.1 2.0 1.6 1.3 1.8

Large larvae burrowed deeper than small larvae. Small larvae were most concentrated in 

the uppermost stratum (83.7 percent), with a mean stratum of 1.3 (table 6). Large fish were 

concentrated in stratum 3 (43.3 percent), with a mean stratum of 2.3. In control trials, small fish 

were most common in stratum 1 (62.5 percent) and large fish were equally common in strata 1 

and 2 (37.5 percent) (table 6). The mean depth stratum for small control fish (1.4) was similar to 

that for the small treatment group (1.3), but larger fish exposed to dewatering tended to be deeper 

(mean stratum=2.3) than those in the control group (mean stratum=1.9) (table 6). 
24 

Table 6. Vertical distribution of larval lamprey by control and treatment group, small and large larvae, and 

depth strata. The treatment columns represent two dewatering rates (1 and 16 inches per hour) and two 

light conditions (dark and light), pooled. Control fish were not dewatered. For each group category, the 

stratum containing the highest percentage of fish is shown in bold. 

[cm, centimeters] 

Stratum number Depth stratum

Control Treatment

Small Large Small Large

1 Fish 0-7.6 cm deep (percent) 62.5 37.5 83.7 26.7

2 Fish 7.6-15.2 cm deep (percent) 37.5 37.5 6.5 23.3

3 Fish 15.2-22.8 cm deep (percent) 0 25.0 9.7 43.3

4 Fish 22.8-30.5 cm deep (percent) 0 0 0 6.7

Mean depth stratum 1.4 1.9 1.3 2.3

Number of fish 8 8 31 30

The second dewatering event resulted in greater mean burrowing depth than the first 

event, but trends after the second event were less clear. The overall mean depth stratum 

increased from 1.6 after round 1 of dewatering to 1.9 after the second round (table 7). Mean 

depth was similar between round 2 and round 3 and became shallower after round 4 (table 7). 

The overall trend was affected mainly by the small fish because the mean depth for large fish 

was consistent across the four rounds of dewatering (table 7). 

Table 7. Vertical distribution of larval lamprey, expressed as mean depth strata, by small and large 

larvae and dewatering round during the repeated dewatering and rewatering trials. Depth stratum 1 was 0–

7.6 cm, stratum 2 was 7.6–15.2 cm, stratum 3 was 15.2–22.8 cm, and stratum 4 was 22.8–30.5 cm. 

[Numbers in parentheses are the number of fish tested in each round; cm, centimeters] 

Fish size Round 1 Round 2 Round 3 Round 4

Small 1.0 (8) 1.6 (8) 1.4 (7) 1.0 (8)

Large 2.3 (8) 2.3 (8) 2.3 (7) 2.4 (7)

Total 1.6 (16) 1.9 (16) 2.0 (14) 1.7 (15)

There were three mortalities in the vertical distribution tubes for the 16-in/h groups. 

There were no mortalities for the 1-in/h groups. One large fish (6.3 percent) died in 16-L in 

round 2, after emerging in round 1. Two fish (12.5 percent) died in 16-D, one large fish and one 

small fish. The small fish was on the surface in round 1 and dead at the end of round 2. The large 

fish was burrowed in round 1, on the surface for round 2, and dead at the end of round 3. 

Although mortality was also observed in the primary tubes for the repeated dewatering and 

rewatering trials (see individual fish response, above), all mortalities were small fish. In the tubes 

used to assess vertical distribution, however, two of the three mortality events were for large fish. 
25 

Discussion

None of the tested dewatering rates offered enough protection for larval lamprey to be 

considered an effective stand-alone conservation action. Our working hypothesis was that the 

slower dewatering rates, potentially combined with dark conditions, might allow most larvae to 

regain access to water, limiting stranding to perhaps 20 to 30 percent of the fish. Our tests found, 

instead, that all rates resulted in the stranding of high proportions of fish, and there was no 

evidence that dewatering during dark conditions was more protective than dewatering during 

light conditions. The slowest rate we tested had the lowest stranding rate, leaving about 77 to 80 

percent of the larvae in areas without access to water. Faster rates resulted in more fish being 

stranded. Reducing the number of lamprey stranded during a dewatering event would help 

protect larvae, but because the minimum stranding rate was 77 percent, the protective effect was 

minimal. On the basis of the range of rates we tested, it seems likely that even rates less than 1 

in/h may result in the stranding of high proportions of larvae. For example, the 0.8-in/h 

difference between the 1-in/h and 1.8-in/h dewatering rates resulted in differences in stranding 

rates of about 21 percent in the dark and 13 percent in the light. If we apply the same percent 

differences to a hypothetical dewatering rate of 0.2 in/h (0.8 in/h less than 1 in/h), we would 

predict stranding rates of 64 to 67 percent. Hypothetically, fewer larvae would be stranded, but 

the dewatering rate would not be protective for most of the fish. Previous reported observations 

of larval and juvenile lamprey in the laboratory and in the field indicate that they have increased 

activity at night (Moursund and others, 2000; Goodman and others, 2015; Moser and others, 

2015). We hypothesized on the basis of this increased activity that dewatering under dark 

conditions would result in fewer larvae being stranded. Our findings, however did not support 

this hypothesis. At each dewatering rate, the stranding rate for the dark group was the same as, or 

higher than, the rate for the light group. Although this study did not identify an “optimal” 

dewatering rate and light condition, it confirmed that lower dewatering rates are more protective 

than faster rates. This information is useful to guide the conditions under which future 

dewatering events are conducted, as dewatering locations are highly variable in terms of larval 

densities, shoreline slope, sediment type, and how long habitat may be dewatered. 

Most larvae were classified as stranded in the movement trials because they never 

emerged from the sediment in tank section 1, where they were initially stocked. That is, they 

showed no apparent response to dewatering. Larvae may have moved within the sediment, 

perhaps to deeper locations as dewatering progressed, but our test system did not allow us to 

monitor this behavior rigorously. Overall, about 20 to 50 percent of fish from each group 

responded by emerging from the sediment in tank section 1 and initiating movement downslope, 

attempting to regain access to water. Most groups (7 of 10) had no more than 30 percent of 

larvae emerge and initiate movement. The 1-in/h groups had the lowest stranding rates, in large 

part because they had the highest proportion of larvae that left section 1 (43–50 percent). When

all the evidence is taken together, it seems that dewatering rate is only one component of 

stranding. We need to better understand the factors that affect larval emergence during a 

dewatering event because emerging is the first action in the chain of events that could result in 

larvae being safe. After a fish emerges, it can either self-rescue by moving downslope and 

regaining access to the water or be recovered from the surface of the sediment during typical 

salvage operations. Once larvae emerge, the dewatering rate affects the likelihood that they can 

self-rescue, as slower rates allow them to traverse less exposed substrate and more time to regain 

access to the water as it retreats. An improved understanding of the cues that prompt larvae to 
26 

emerge, combined with the ability to manage dewatering rates, would be useful to guide future 

dewatering events to minimize adverse effects to lamprey. 

Larval lamprey moved, over short distances, at rates that exceeded the fastest dewatering 

rate we tested. We measured movement rates to better understand whether the risk of stranding 

was due to the inability of larvae to travel over the sediment fast enough to match the pace of the 

receding water. The fastest dewatering rate we tested, 16 in/h, resulted in water receding over the 

surface of the sediment at a rate of 6.8 cm/min. The mean movement rates for groups of lamprey 

ranged from 19.0 to 44.4 cm/min. Only the slowest movement rate we measured, 6.6 cm/min, for 

an individual lamprey, was slower than the fastest dewatering rate. This finding indicates that the 

high stranding rates we observed were not due simply to the inability of larvae to keep pace with 

the receding water but must also include factors such as the proportion of fish that emerged and 

initiated movement. In our test system, larvae would need to travel about 150 cm from the lower 

end of tank section 1 to the uppermost margin of the water at the bottom of the tank. Lamprey in 

our trials moved an average of about 52 cm, slightly more than one-third the total distance from 

where they were placed to where they could access water on the surface. It is unclear how long 

larvae may be able to sustain the movement rates we measured, but based on observations during 

these trials, as well as previous testing in the same tank (Liedtke and others, 2015), we would 

estimate that, depending on size, they may tire quickly. We did not measure movement rates in 

our previous work, but we found that small larvae would quickly become exhausted attempting 

to move over the surface of the sediment, especially if they encountered any depressions or 

ridges (Liedtke and others, 2015). It is important to note as well that on the basis of our 

procedures our reported rates may be biased high. Larvae were placed on the surface of the 

sediment in tank section 1 to begin tests of movement rate. Our goal was to monitor rates of 

larvae that were burrowed in the sediment, volitionally emerged, and then traveled down the 

slope toward the water. Early tests, however, forced us to change our approach, as very few fish 

emerged after they were burrowed. By placing fish on the surface of the sediment, larvae could 

begin moving immediately and did not expend any energy emerging from a burrow. In natural 

settings, lamprey burrowed deeply in the sediment or in sediment that compacts easily during 

dewatering would likely expend substantial energy to emerge. The comparisons of movement 

rate by light condition could also be affected by our procedure of placing larvae on the surface. 

We found the fastest movement rates during light conditions, possibly because the larvae

actively sought a location in which to burrow to avoid the light. Ideally, movement rates would 

be measured under variable light conditions following volitional emergence. 

Repeated dewatering and rewatering did not result in high proportions of larvae emerging 

from the sediment. The rationale behind testing this dewatering approach was based on field 

observations that additional larvae emerged following a dewatering event when it was rewatered 

overnight through seepage. Previous laboratory evaluations of larval responses to dewatering 

showed that about 50 percent of lamprey emerged shortly after the surface of the sediment was 

dewatered (Liedtke and others, 2015). The hypothesis was that about 50 percent of larvae would 

emerge in the first round of dewatering, making them accessible for salvage or self-rescue. Using 

this half-life approach for four rounds of dewatering, we would expect to have less than 10 

percent of the fish remaining in the dewatered habitat. In the trials, however, the highest percent 

emergence for a single round of dewatering was about 43 percent for small fish and about 14 

percent for large fish. The most common response, especially for large fish, was to remain 

burrowed for each of the four rounds of dewatering. Of the fish that emerged, 70 percent did so 

in the first round of dewatering. Each subsequent round added about 10 percent more fish. 
27 

Neither the rate (1 in/h compared to 16 in/h) nor the light condition had a large effect on the 

percentage of larvae that emerged. Using several rounds of dewatering with rewatering between 

them was not an effective approach to improve the opportunity for salvage or self-rescue. 

Few larvae burrowed deeper than 23 cm and vertical distribution was affected by fish 

size, dewatering rate, and the number of dewatering events. About 70 percent of larvae burrowed 

in the top two strata of our test system, which was less than about 15 cm. Adding one stratum, to 

a depth of about 23 cm, accounted for about 97 percent of the fish. Fish size affected vertical 

distribution, with large larvae consistently found deeper than small larvae, both following a 

dewatering event and when no dewatering occurred. Mean depth stratum was slightly greater

under dark conditions than light conditions, but the dewatering rate had a larger effect. Larvae 

were deeper during the 1-in/h trials than during the 16-in/h trials. It took about 15 h to 

completely dewater the tank holding the tubes using the slow rate, allowing larvae time to detect 

changing conditions and respond, whereas with the fast rate, the tank was totally dewatered in 

less than an hour (0.96 h). With conditions changing so quickly, it may be that larvae did not 

alter burrow depth because they did not have time to respond. During the repeated dewatering 

and rewatering trials, we found that small larvae burrowed deeper after the second round of 

dewatering than after the first round. After the second round, however, the trends were not 

consistent. Relative to our past work, larvae in these tests were burrowed deeper. Previously we 

observed that no larvae burrowed deeper than 15.2 cm, and the largest proportion of fish was 

shallower than about 8 cm (Liedtke and others, 2015). These trends applied to fish that were 

dewatered as well as controls that did not experience dewatering (Liedtke and others, 2015). 

Differences between the 2015 study and the current study include (1) the sediment used, (2) 

larval size, and (3) larval source. In our initial work in 2015, we used a commercial sand sieved 

for consistent size (median grain size 0.378 millimeters [mm], medium sand). It was selected 

because we used it for our lamprey culture and it had performed well in that capacity. We noted, 

however, that the sand quickly became compacted and hard following dewatering, basically 

eliminating the option for lamprey to reburrow (Liedtke and others, 2015). The sediment used in 

2019 was from a field setting where larval lamprey had been collected previously. It was slightly 

coarser (median grain size 0.445 mm, medium sand), with a smaller proportion of fine sand or 

smaller particles compared to the 2015 sediment, which could have affected burrowing behavior 

or vertical distribution of larvae. Fish size was the second difference between the 2015 and 2019 

studies that might have affected burrowing behavior. The mean TL of larvae in the 2015 study 

(69.3+22.8 mm) was smaller than the mean TL in 2019 (79.6+26.9 mm). Larger larvae burrow 

more deeply, and a 10-mm increase in length for the 2019 larvae likely affected the mean 

burrowing depth. Another potential factor affecting burrowing behavior was the source of larvae. 

In 2015, larvae used for testing had been maintained in our laboratory for several years. For the 

current work, larvae were collected from a field setting that experiences changing water levels

and held in the laboratory for no more than 6 weeks. For ease of culture and maintenance, our 

laboratory holding systems provide about 5 cm of sand for burrowing. Perhaps burrowing 

behavior, especially in response to dewatering, is altered through experience, and fish in field 

settings are exposed to a wider range of conditions relative to those in the laboratory. We found 

some support for this theory in our finding that small larvae were burrowed deeper after the 

second round of dewatering than after the first round. 

The proportion of fish that emerged from the sediment in response to dewatering was 

lower than that observed in previous laboratory work or in a field setting (Liedtke and others, 

2015; Skalicky and others, 2019). Our 2015 evaluation of larval responses to dewatering 
28 

reported that about 50 percent of larvae emerged, beginning shortly after the surface of the 

sediment was dewatered (Liedtke and others, 2015). During a field evaluation of dewatering, 

Skalicky and others (2019) reported the same finding, about 50-percent volitional emergence. In 

the present study we observed 30-percent emergence during the movement trials in the tank with 

a simulated shoreline at a 10-percent slope, and about 36-percent emergence during the repeated 

dewatering and rewatering trials conducted in tubes. In both test systems, small larvae emerged 

more often than large larvae. Comparisons between the 2015 and 2019 laboratory evaluations 

must consider the differences in sediment and fish source, as discussed previously. Emergence 

timing was broadly described in the 2015 study but was rigorously measured in 2019, as we 

thought it would be useful to guide future field investigations or salvage efforts. As for the 2015 

finding, we observed that larvae generally did not emerge for a period after the surface of the 

sediment was exposed. Overall, the delay in emergence was about 40 min and the maximum 

time to emerge after the sediment was dewatered was 4.5 h. 

Larval size played a role in lamprey responses to dewatering, having a significant effect

on emergence, movement rate, and vertical distribution. We carefully managed larval sizes for 

each trial and group to avoid any initial size bias. During the movement trials, larvae that 

emerged were significantly smaller than those that remained burrowed, a finding that matched 

our findings in 2015 (Liedtke and others, 2015). We could only make statistical comparisons of 

the size of safe as opposed to stranded larvae for the 1-in/h groups because the other dewatering 

rates had so few safe fish. The sizes of safe fish and stranded fish were not significantly different 

for 1-D, but safe larvae were significantly smaller than stranded larvae for 1-L. This difference is 

linked with the likelihood of emergence because most larvae classified as stranded never 

emerged from the sediment, which was the most common behavior for large fish, thereby 

increasing the mean size of stranded fish. 

Movement-rate testing showed that large larvae moved at faster rates than small larvae, 

especially during light conditions. Small larval movement rates were statistically 

indistinguishable under dark and light conditions and were not significantly different from the 

rates of large larvae under dark conditions. Large larvae moving under light conditions, however, 

were significantly faster than all other groups. This difference in movement rate could translate 

to improved survival during dewatering events in field settings where larvae must traverse 

exposed sediment to regain access to water. Larval behavior during the repeated dewatering and 

rewatering trials was affected by size. Small fish emerged at higher rates (50-percent emergence 

overall) and experienced several mortalities, typically after emerging during at least one round of 

dewatering. Large fish emerged at lower rates (27.3 percent overall), were more limited in the 

timing and duration of their emergence and did not experience any mortality. Comparing 

availability for salvage under the repeated dewatering and rewatering approach to a traditional, 

single round of dewatering, very few large fish would have been salvaged. 

The vertical distribution trials demonstrated that large fish were burrowed deeper than 

small fish, both in the control groups that did not experience dewatering and in dewatered 

groups. Taken together, these findings along with findings from the 2015 study highlight that 

small larvae are especially vulnerable to dewatering. They are more likely to emerge from the 

sediment, making them four times less likely to survive than larvae that remain burrowed 

(Liedtke and others, 2015), and they may struggle to move effectively over dewatered substrates 

to regain access to water. In addition, the 2015 study showed that the odds of surviving 

dewatering increased by a factor of 2 for every 10-mm increase in TL (Liedtke and others, 2015). 

It is not clear how larval size affects the choice to emerge and move along the surface toward
29 

water as opposed to remaining burrowed, or potentially burrowing deeper, during dewatering 

events in field settings. More information from field studies is needed to better understand how 

larvae of different sizes behave during dewatering, and how that behavior relates to survival. 

Combining field observations with the findings from laboratory studies will provide the best 

guidance for how dewatering can best be managed to reduce negative effects on larval lamprey. 

Larvae were more active in the dark, but darkness did not consistently provide better 

outcomes than daylight. Throughout all the experiments, there were indications that larval 

activity increased in the dark. For example, more larvae were detected on the surface during 

movement trials in the dark than in those in the light. A control movement trial (no dewatering) 

had a larva swimming throughout the tank until the 1-h waiting period was over and lights were 

turned on, at which point it immediately burrowed. During the repeated dewatering and 

rewatering trials, we saw lamprey actively swimming at night, but during the day they remained 

burrowed. The increased activity, however, did not result in fewer larvae stranded in movement 

trials or more fish available for salvage in repeated dewatering and rewatering trials. For 

example, the 1-in/h movement trials had more larvae emerge and move downslope in the dark 

than in the light, but more larvae were stranded in the dark trials. Light condition had no effect 

on movement rates for small larvae, and large larvae had significantly slower movement rates in 

the dark than in the light. As noted previously, however, movement-rate trials may not accurately 

depict larval behavior, and higher movement rates during the day may reflect a desire to find a 

new burrowing location and avoid the light. The vertical distribution of larvae was marginally 

affected by light condition, with fish found somewhat deeper in the dark. Overall, our findings 

are like those of other studies that report increased nighttime activity (Moursund and others, 

2000; Goodman and others, 2015; Moser and others, 2015). Our previous evaluation of larval 

responses to dewatering was conducted exclusively during daylight (Liedtke and others, 2015); 

the current work, to our knowledge, is the first laboratory evaluation of response to dewatering in 

which movement during dark and light conditions was compared. 

Conclusions 

Slower dewatering rates offered some opportunity for larval lamprey to escape stranding. 

The best outcome was for the slowest dewatering rate (1 in/h). Although this rate left about 77 

percent of the larvae stranded in areas without surface water, a slow dewatering rate used in a 

field setting could allow some fish to return to watered areas and prevent the 100-percent 

mortality that might occur with faster dewatering rates. Larval densities can be high in fluvial 

sediments and use of a slow dewatering rate in these areas can result in survival of large numbers 

of larvae. For example, Skalicky and others (2019) estimated more than 12,000 larvae in a single 

dewatered area (1,142 square meters) of a large reservoir, and multiple fine sediment deposits 

outside the study area were also dewatered. Thus, using a slow dewatering rate to allow a 

proportion of the larvae to escape stranding could potentially save thousands to millions of larvae 

in situations where larval densities are high and expansive areas are dewatered. 

Larvae were more active at night, but nighttime dewatering did not result in improved 

outcomes for lamprey compared to daytime dewatering. Like the findings of Liedtke and others 

(2015), the size of larvae had a significant effect on behavior and outcomes, with small fish 

consistently more vulnerable to negative effects. We expected to see about 50 percent of larvae 

emerge from the sediment on the basis of our previous laboratory observations (Liedtke and 

others, 2015) and reported field observations (Skalicky and others, 2019), but overall emergence 

was about 30 to 36 percent. Similarly, we found larvae to be burrowed in the sediment to a depth 
30 

of about 23 cm whereas our previous work documented that no larvae burrowed deeper than 

about 15 cm (Liedtke and others, 2015). New information collected during this study included 

emergence timing and movement rates. It has been shown that larvae generally emerge shortly 

after the surface of the sediment is dewatered (Liedtke and others, 2015; Skalicky and others, 

2019), but this study quantified that response. We found that emergence tended to occur within 

about 40 minutes after the surface of the sediment was exposed. The movement rates we 

measured for larvae indicate that they can, at least over short distances, move over the substrate 

and keep pace with fast dewatering rates (16 in/h), giving them the chance to regain access to 

water. These findings should be useful to provide insights into how planned dewatering events 

can best be managed to limit adverse effects on larval lamprey. 

The focus of this study was Pacific lamprey because of their declining status and cultural 

and ecological significance (Close and others, 2002; Wang and Schaller, 2015; Clemens and 

others, 2017). however, our findings can be considered for any species of lamprey, such as the 

sympatric Western Brook lamprey, that have strong associations with fluvial sediments and may 

be vulnerable to dewatering. Species-specific evaluations in the future may provide additional 

insights, but lacking other information, this study provides a defensible foundation. 

More evaluations of larval responses to dewatering in field settings would add to the 

information we learned in this study. There is growing interest and momentum in this arena, but 

to date (2020), few studies have been conducted (Beals and Lampman, 2018; Skalicky and 

others, 2019). Controlled laboratory studies certainly provide insights into larval responses, but 

conditions in field settings are substantially more complex. In the field, dewatered larvae may be 

exposed to environmental conditions that exacerbate desiccation, limit their ability to traverse 

dewatered substrate, and expose them to predation. The combination of laboratory and field 

studies is a powerful approach, leveraging the control of laboratory settings and the realism of 

field settings. Conducting future laboratory experiments with larvae collected from the field with 

a limited duration of laboratory holding will best represent larval behavior in the field. 

Sediment type appears to play a key role in larval behavior, and more research is needed 

to better understand its effect. Initially we conducted experiments using commercial sand and 

learned that it compacted after it was dewatered, potentially limiting larval behaviors like 

reborrowing (Liedtke and others, 2015). To remedy that concern, we conducted this study using 

sediment collected from a field location where lamprey had been collected previously. The 

natural sediment was an improvement, but still represents a simplified depiction of lamprey 

habitat in stream and river sediments. The proportion and size of the particles in a sediment 

(from gravel to silt and clay) are important descriptors of how sediments respond to dewatering, 

but the stratigraphy may also play a role. For example, a layer of silt or clay may resist 

dewatering, allowing water to be retained above the layer. Larger particles or large organic 

matter can create interstitial spaces where larvae may burrow more easily but may also desiccate 

more rapidly. If, like the commercial sand used in our early work, a sediment compacts quickly 

when dewatered, limiting the ability of larvae to reburrow, larvae that emerge may be more 

vulnerable to predation or desiccation if they are unable to access water. Conducting laboratory 

studies with sediments collected from field locations where lamprey have been observed and 

field studies that collect and describe the sediment components and stratigraphy at their study 

sites can help fill this information gap. 

Following a dewatering event, larval lamprey may emerge from the sediment and attempt 

to move to water, or remain burrowed, possibly increasing their depth, in anticipation that the 

substrate will be rewatered. There are tradeoffs to consider for both strategies and little is known 
31 

about conditions that affect this choice or which strategy grants the best survival advantage. As 

this study and others found that about 30 to 50 percent of larvae emerge, it seems that the 

dewatering rate and (or) light condition are not the only variables that must be considered. Future 

research ideally will focus on the cues that cause larvae to emerge from dewatered sediment. The 

microhabitat of each larva may affect emergence behavior. For example, if there is insufficient 

moisture to respire effectively, or the sediment becomes compacted and creates physical strain, 

larvae may emerge. We observed some larvae emerge and then, after a short period on the 

surface, reburrow. Were they comparing surface conditions to their burrowed microhabitat? Past 

experience may play a role; for example, in tidal-influenced habitats, larvae may not emerge as 

they anticipate rewatering. Improving our understanding of the drivers of these behaviors is 

critical to our ability to mitigate risks to larvae during dewatering. 

Acknowledgments

This work was funded through the U.S. Fish and Wildlife Service, Portland, Oregon, Fish 

and Wildlife Office, as part of ongoing collaborations with the Lamprey Conservation Initiative. 

Field collection of lampreys was made possible through assistance from U.S. Fish and Wildlife 

staff at the Columbia River Fish and Wildlife Conservation Office in Vancouver, Washington— 

specifically Tim Whitesel, who coordinated the collection permit, and Judith Barksdedt, Jennifer 

Poirier, Brook Silver, David Hines, and Jamie Sprando, who assisted with lamprey field 

collection efforts. U.S. Geological Survey colleagues, Ryan Tomka, Collin Smith, Joe Warren, 

and Morgan Andrews., at the Columbia River Research Laboratory, Cook, Washington are 

gratefully acknowledged for assisting with lamprey collection as well as experimental setup and 

execution. Reviews of this document by Laurie Porter and Jill Hardiman are greatly appreciated. 

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Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Western Fisheries Research Center

U.S. Geological Survey

6505 NE 65th Street

Seattle, Washington 98115-5016

https://www.usgs.gov/centers/wfrc
Liedtke and others—Evaluating Dewatering Approaches to Protect Larval Pacific Lamprey—Open-File Report 2020–1026

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20201026