Season and Discharge Predict Downstream Emigration Rates for Reservoir Sport Fish Populations

Abstract

Given the importance of reservoir fisheries and the investments made by fisheries management agencies to create and maintain these fisheries, the loss of reservoir-stocked fish through downstream emigration warrants action to be taken to understand and prevent it. The factors driving emigration of reservoir fish and the measurable effects of emigration on fish population dynamics are not well understood. In this study, we sought to understand the seasonal emigration patterns of two popular sport fish, saugeye (female Walleye Sander vitreus × male Sauger Sander canadensis) and Muskellunge Esox masquinongy, and determine if their annual declines in abundance were correlated with predicted annual emigration rates. We modeled daily emigration probabilities of saugeye and Muskellunge from PIT tag data collected in the dam spillway of an Ohio reservoir. Using data on discharge from previous years, these models were used to estimate annual emigration rates, which were regressed against historical catch data to test whether emigration explained annual cohort declines. Our models showed that downstream emigration from the reservoir occurred primarily during spring for both species, especially during periods of high reservoir discharge. All saugeye and nearly all Muskellunge emigrants were adults, with 13% of tagged adult saugeye and 17% of tagged adult Muskellunge emigrating. Our models predicted historical annual emigration losses (across all age-classes) as high as 24% for saugeye and 8.6% for Muskellunge. Furthermore, historical annual emigration rates were negatively related to catch rates for both species. These results illustrate that, depending on flow regime and seasonal effects on fish movements, emigration can have measurable effects on reservoir sport fish populations. As such, it is important to understand the factors driving downstream migration and use this understanding to inform decisions on potential mitigation of emigration as well as fish stocking decisions.

Reservoir Fish Escapement in North America: A Historical Review and Future Directions

“…2013; Weber and Flammang 2019; Shane et al. 2021; Weber and Weber 2021a, 2021b). The current suite of mark–recapture models has been instrumental in providing inferences on fisheries, including fish survival, population size, movement, and angler catch and exploitation (Pine et al.…”

“…2015; Shane et al. 2021) or incorporating angler data into mark–recapture methods (e.g., LaConte 1996; Rischbieter 1998). When choosing methods to quantify escapement, it is vital to consider the benefits, drawbacks, and potential selectivity of each method and to critically evaluate how these factors may influence the ability to meet study objectives.…”

Effects of a minimum length limit on saugeye population and fishery characteristics in Ohio reservoirs

“…Experimental monofilament gill nets have been used by ODNR‐DOW in a standardized approach to assess saugeye populations in Ohio reservoirs (see Shane et al. 2021). During the period 2003–2015, the nets consisted of six 9.2‐ × 1.8‐m (length × width) panels with bar mesh sizes of 19, 25, 38, 51, 64, and 76 mm.…”

“…Because size structure metrics can be biased due to the size selectivity of gill nets (Hansen et al 1997;Shoup and Ryswyk 2016) and to account for the change in gill nets between the periods 2003-2015 and 2016-2021, we adjusted the gill-net catches for each 10-mm length category for each survey, following Shane et al (2021). During 2008, ODNR-DOW measured the total length of the saugeye that were captured by each mesh of the sixpanel gill net fished during the 2003-2015 standardized assessments, which included the four panels that were used in the 2016-2021 gill-net survey (ODNR-DOW, unpublished data).…”