Date Published: February 6, 2019
Publisher: Public Library of Science
Author(s): Scott M. Miehls, Christopher M. Holbrook, J. Ellen Marsden, Michael Schubert.
Timing of activity, especially for juvenile anadromous fishes undertaking long migrations can be critical for survival. River-resident larval sea lamprey metamorphose into juveniles and migrate from their larval stream habitats in fall through spring, but diel timing of this migratory behavior is not well understood. Diel activity was determined for newly metamorphosed sea lamprey using day/night net sampling and passive integrated transponder (PIT) telemetry in two natural streams and PIT telemetry in an artificial stream. Downstream migration was primarily nocturnal in all studies. All but one of 372 sea lamprey were captured during night sampling in the day/night net collections and all detections (N = 56) for the in-stream PIT telemetry occurred within a few hours after sunset. Most (81% of 48) tagged lamprey moved downstream during the first night following release and moved at speeds consistent with observed water velocities. During long-term observation of behavior in the artificial stream most sea lamprey movement occurred during the night with limited occurrence of movement during daylight hours. Understanding seasonal and diel timing of downstream migration behavior may allow more effective management of sea lamprey for both conservation and control.
Animals use environmental cues to ensure that migrations occur when conditions are most favorable for survival, growth, or reproduction . For juvenile anadromous fishes, timing of downstream migration from tributary rearing habitats to marine or lacustrine feeding habitats may have evolved in response to predation risk, food availability, and abiotic environmental conditions. For example, nocturnal migration is thought to be a predator avoidance behavior common in many fish species , but the propensity to migrate at night can vary within and among species . Knowledge of migration timing has the potential to inform conservation of imperiled or threatened populations, but might also be used to improve assessment and control of pests. For example, appropriately timed spills of water and turbine shutdowns at hydropower facilities may limit mortality of downstream migrants such as salmonids  or eels .
We observed that nearly all downstream migratory activity by juvenile sea lamprey in the artificial and natural streams (net captures and PIT detections) occurred at night and began soon after dark. These results are consistent with observations of other lamprey species (Pacific lamprey, ; river lamprey, ; pouched lamprey, G. australis, reviewed in ). We are not aware of any empirical estimates of survival during downstream migration of juvenile sea lamprey, but it seems plausible that predation risk could be an important source of mortality during this stage because juvenile lampreys are relatively poor swimmers  and migratory habitats typically contain visual predators. In the Columbia River, migratory juvenile Pacific lamprey are the primary prey for northern pikeminnow (Ptychocheilus oregonesis) during peak lamprey migration (May and June) while terns and gulls have also been observed preying on downstream migrants . Similarly, nocturnal migration is common for Atlantic salmon smolts and is thought to be a response to avian predation .
Downstream movement of juvenile sea lamprey appears to be initiated shortly after dark and is predominately confined to nocturnal periods. However, individual variability was observed in the initiation of downstream migration, with some lamprey even moving downstream during daylight hours. Evidence exists for both passive drift during juvenile lamprey downstream migration as well as active behavioral components, such as diel behavioral patterns, maintaining station within desirable regions of the stream, or even potential for downstream swimming. Our current observations of downstream migration, paired with recent spatial distribution studies, may allow prediction of encounter probabilities, both temporally and spatially, within a stream, which could aid both conservation and control management efforts.