Synergistic and singular effects of river discharge and lunar illumination on dam passage of upstream migrant yellow-phase American eels

Abstract

Monitoring of dam passage can be useful for management and conservation assessments of American eel, particularly if passage counts can be examined over multiple years. During a 7-year study (2007–2013) of upstream migration of American eels within the lower Shenandoah River (Potomac River drainage), we counted and measured American eels at the Millville Dam eel pass, where annual study periods were determined by the timing of the eel pass installation during spring or summer and removal during fall. Daily American eel counts were analysed with negative binomial regression models, with and without a year (YR) effect, and with the following time-varying environmental covariates: river discharge of the Shenandoah River at Millville (RDM) and of the Potomac River at Point of Rocks, lunar illumination (LI), water temperature, and cloud cover. A total of 17 161 yellow-phase American eels used the pass during the seven annual periods, and length measurements were obtained from 9213 individuals (mean = 294 mm TL, s.e. = 0.49, range 183–594 mm). Data on passage counts of American eels supported an additive-effects model (YR + LI + RDM) where parameter estimates were positive for river discharge (β = 7.3, s.e. = 0.01) and negative for LI (β = −1.9, s.e. = 0.34). Interestingly, RDM and LI acted synergistically and singularly as correlates of upstream migration of American eels, but the highest daily counts and multiple-day passage events were associated with increased RDM. Annual installation of the eel pass during late spring or summer prevented an early spring assessment, a period with higher RDM relative to those values obtained during sampling periods. Because increases in river discharge are climatically controlled events, upstream migration events of American eels within the Potomac River drainage are likely linked to the influence of climate variability on flow regime.

Dynamics of upstream movements of the European eel Anguilla anguilla in an inland area of the River Meuse over the last 20 years

“…The initiation and cessation of annual upstream movements of eels in riverine systems is controlled by environmental variables such as water temperature and river discharge (Naismith and Knights 1988; Bark et al 2007;Nzau Matondo et al 2014). As a consequence, fish pass monitoring may play an important role by providing useful information on the upstream migration behavior of the riverine yellow eel (Welsh et al 2015) in the context described above.…”

Water temperature and precipitation stimulate small-sized Japanese eels to climb a low-height vertical weir

“…Our GAM results also revealed the positive effects of precipitation on the probability of eel climbing. It was thought that too much rainfall would impede eel climbing because the river discharge would be too strong [ 43 , 45 , 46 , 49 ]. A possible explanation for eels climbing the weir during increased river discharge is to avoid predation risk.…”

“…Our GAM results revealed that the lunar illumination was ineffectual regarding the probability of eel climbing. Previous studies reported that American eels exhibited high activity of upstream movement under low lunar illumination, that is, a new moon [ 46 , 49 ]. These previous and present studies used cloud cover, lunar phase, and/or lunar illumination as alternative indicators of illumination in a water column [ 46 , 49 ].…”

“…Previous studies reported that American eels exhibited high activity of upstream movement under low lunar illumination, that is, a new moon [ 46 , 49 ]. These previous and present studies used cloud cover, lunar phase, and/or lunar illumination as alternative indicators of illumination in a water column [ 46 , 49 ]. The alternative indices of illumination used in these studies may have been insufficient to explain the factors stimulating the upstream migration of eels because illumination in the water column is possibly affected by turbidity in the water column, water level/discharge, weather (i.e., cloudy), moon phase, moon illumination, and its interactions.…”

The drivers of anguillid eel movement in lentic water bodies: a systematic map

“…If light level is the true force driving anguillid eel movement, this may explain the variable results found in this review, as changing conditions such as cloud cover may alter lighting conditions, thereby minimising variance between different lunar phases. Lunar phase is linked to both upstream, downstream and vertical migration in several anguillid species (Hain 1975;Schabetsberger et al 2013;Chow et al 2015;Welsh et al 2015;Cresci 2020). Glass eels in lotic systems are sensitive to the lunar cycle and may use lunar cues for movement orientation during migration (Cresci et al 2021).…”