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 de...

Anthropogenic dewatering of aquatic habitats can cause stranding and mortality of burrowed larval lampreys; however, the effects of dewatering have not been quantified. We assessed: (a) changes in spatial distribution, abundance, and emergence of larvae dewatered at Leaburg Reservoir (OR); (b) emergence and mortality of larvae dewatered in a laboratory; and (c) bias, precision, and interpretation of field results by simulation and modeling of laboratory results. In the field, we examined the distribution, abundance (by N-mixture model), and density of larvae by electrofishing at randomly selected sites before dewatering and after refill, and assessed the emergence rate by observation and excavation during dewatering. Due to dewatering in the field, about 42% of larvae emerged and spatial distribution changed toward sites dewatered less than 20 hours. Estimated average density decreased from 10.8 larvae/m 2 before dewatering to 2.3 larvae/m 2 after refilling, suggesting that abundance declined by 79%; simulation suggested this decline ranged 71-84% (interquartile range). In the laboratory, we examined the emergence and mortality rates of larvae dewatered 0-48 hrs. The emergence rate in the laboratory was similar to that in the field. Mortality rate increased with hours dewatered and was higher for emerged than burrowed larvae. Laboratory estimates of mortality rate predicted a 61% decline in abundance if only burrowed larvae survived and a 54% decline if both burrowed and emerged larvae survived. Abundance declines in the field could be from mortality (e.g., desiccation, predation) and relocation to watered habitat. Our results indicate dewatering can substantially affect spatial distribution and abundance of larval lampreys in freshwater ecosystems.

Priest Rapids Dam on the Columbia River produces large daily and hourly streamflow fluctuations throughout the Hanford Reach during the period when fall Chinook salmon Oncorhynchus tshawytscha are selecting spawning habitat, constructing redds, and actively engaged in spawning. Concern over the detrimental effects of these fluctuations prompted us to quantify the effects of variable flows on the amount and persistence of fall Chinook salmon spawning habitat in the Hanford Reach. Specifically, our goal was to develop a management tool capable of quantifying the effects of current and alternative hydrographs on predicted spawning habitat in a spatially explicit manner. Toward this goal, we modeled the water velocities and depths that fall Chinook salmon experienced during the 2004 spawning season, plus what they would probably have experienced under several alternative (i.e., synthetic) hydrographs, using both one‐ and two‐dimensional hydrodynamic models. To estimate spawning habitat under existing or alternative hydrographs, we used cell‐based modeling and logistic regression to construct and compare numerous spatial habitat models. We found that fall Chinook salmon were more likely to spawn at locations where velocities were persistently greater than 1 m/s and in areas where fluctuating water velocities were reduced. Simulations of alternative dam operations indicate that the quantity of spawning habitat is expected to increase as streamflow fluctuations are reduced during the spawning season. The spatial habitat models that we developed provide management agencies with a quantitative tool for predicting, in a spatially explicit manner, the effects of different flow regimes on fall Chinook salmon spawning habitat in the Hanford Reach. In addition to characterizing temporally varying habitat conditions, our research describes an analytical approach that could be applied in other highly variable aquatic systems.

Dewatering of fine sediments in rivers and streams can kill many thousands of larval lampreys (Order Petromyzontiformes) burrowed in these habitats. The larval life stage for lampreys lasts 3‐ 10 years, and because larvae often aggregate in large numbers, negative impacts from dewatering could potentially deplete local populations and affect multiple year classes. Larval lampreys have not traditionally been considered during in‐stream projects, but recent efforts to increase awareness of lamprey habitats have resulted in guidance on dewatering approaches to limit impacts to lampreys. Salvage efforts to rescue and relocate lampreys aim to mitigate losses, but a lack of understanding of lamprey responses limits optimization of dewatering and salvage procedures. We summarize the state of the science for nine factors that influence larval lamprey (Entosphenus and Lampetra spp.) responses to dewatering, including: burrowing depth, the prevalence and timing of emergence, movements, survival, and the influence of slope, dewatering rate, light, and lamprey size. Research suggests that shoreline slope influences movement capability; hot and sunny conditions increase the risk of mortality; salvage activities cause minimal direct mortality; and smaller larvae are especially vulnerable to negative impacts from dewatering because they are more likely to emerge and are less capable of movement. Critical uncertainties associated with dewatering include cues that drive emergence, influence of sediment composition and stratigraphy, vertical distribution of larvae in natural settings, use of the hyporheic zone, the scale of predation losses, and the effectiveness and impacts of salvage activities. Balancing investments in salvage efforts and lamprey exclusion efforts (e.g., screening) and developing field survey approaches to evaluate use of the hyporheic zone by lampreys are identified management implications and research needs. Addressing the critical uncertainties discussed here and providing updated, science‐based guidance on dewatering and salvage practices are suggested management actions to support lamprey conservation.

ObjectiveHuman‐induced dewatering of freshwater habitats causes mortality of larval lampreys (family Petromyzontidae). Salvage by electrofishing at dewatering events is assumed to reduce this mortality, but to our knowledge this assumption remains unassessed.MethodsWe estimated mortality of salvaged larval lampreys (Lampetra spp. and Pacific Lamprey Entosphenus tridentatus) within 24 h following collection at field dewatering events in July and October. To assess when salvage may reduce mortality, we compared mortality of salvaged individuals from field dewatering events to mortality of burrowed and emerged individuals in dewatered habitats in the laboratory. Salvage protocols included electrofishing and foot pressure from walking in test enclosures before and after dewatering. Electrofishing after dewatering (“dry shocking”) involves positioning probes on moist sediment to entice burrowed larval lampreys to emerge.ResultDuring the July salvage, air temperature averaged 36°C, bottom water temperature averaged 20°C, and many emerged larval lampreys were dead on the sediment surface. During two October events, air temperatures averaged 18°C and 11°C, bottom water temperatures averaged 12°C and 7°C, and only one dead emerged larval lamprey was observed. Estimated mortality of salvaged larval lampreys was 0.20 (90% credible interval = 0.09–0.37) in July and 0.00 (90% credible interval = 0.00–0.06) and 0.06 (90% credible interval = 0.01–0.18) in October. All larval lampreys that remained burrowed and were excavated from enclosures after salvage were dead in July but alive in October. Logistic regression suggested that mortality declined with increasing larval length. Mortality of salvaged 80‐mm larval lampreys in October was lower than that of 80‐mm individuals emerged for 1 h or burrowed for 8 h at similar water temperatures (8–10°C) in the laboratory.ConclusionIn this study, electrofishing for salvage caused minimal mortality of burrowed and emerged larval lampreys in dewatered habitats. Thus, salvage using electrofishing methods could aid conservation of native lampreys by reducing mortality associated with human‐induced dewatering events, especially when temperatures are elevated.