Diel periodicity and chronology of upstream migration in yellowphase American Eels (Anguilla rostrata)

This thesis examined 24-h diel periodicity of upstream migration of yellow-phase American Eels (Anguilla rostrata), and the chronology of upstream movements within diel periods (day, night, and twilight). Further, relationships were examined for total lengths of upstream migrants and diel movements (vespertine, nocturnal, matutinal, and diurnal), as well as for total lengths and season of year. The thesis is comprised of two chapters: (1) an introduction and literature review on American Eel life history, migration and movement, and population concerns, and (2) a research study of diel periodicity and movement chronology of upstream migrant yellow-phase American Eels at an eel ladder. Study objectives were to (1) examine diel periodicity of upstream migrants using time-series spectral analysis, (2) describe the distribution of passage counts during diel periods (day, twilight, and night) among seasons (spring, summer, and fall), and (3) examine size of upstream migrants relative to diel and seasonal periods. Data were collected at the Millville Dam eel ladder on the lower Shenandoah River, West Virginia, from 2011--2014. Six multi-day passage events with a high number of passage counts were selected for analysis and categorized by season (spring, summer, late summer/early fall, fall) and diel periods of movement (vespertine, nocturnal, matutinal, and diurnal). To examine diel periodicity of movements, I graphically-depicted passage count data as time-series histograms (10-min bins) and used time-series spectral analysis (Fast Fourier Transformation, FFT) to identify cyclical patterns and periodicity of upstream migration. I also pooled histogram data into 14-h periods (18:00--08:00 hours) using 10-min bins for each multi-day passage event (representing vespertine, nocturnal, and matutinal movements). Using pooled 14-h histograms, I examined patterns of movements for each passage event and described multiple peaks of passage counts for vespertine, nocturnal, and matutinal movements by fitting a normal model and eight normal mixture models (2--9 mixtures). The Bayesian information criterion (BIC) was used to select the best approximating model. A mixed-model methodology was used to examine relationships among total length (TL), diel period, and season. Periodicity of movements closely followed a 24-h cycle of activity with most movement being nocturnal. Based on mixture model analysis, multimodal models were supported by the data, but distribution patterns and timing of upstream migration were complex and variable across the six passage events. An additive-effects model of diel period + season was selected as the best approximating model for the mixed-model analysis of TL. Also, the mean TL of individuals using the eel ladder decreased as the night progressed (i.e., from vespertine to diurnal periods of movement) and was the highest during fall (330.3 mm +/- 1.9 SE, n = 472) relative to similar mean values of TL for spring (304.1 mm +/- 1.0 SE, n = 1700), summer (301.2 mm +/- 1.1 SE, n = 1548) and late summer/early fall (303.4 mm +/- 0.87 SE, n = 2269). This study increased our understanding of upstream migration ecology of yellow-phase American Eels and dam passage at the Millville Dam eel ladder

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.</jats:p