Evaluating aquatic habitats is an important component of many ecological studies and natural resource assessments, but traditional habitat evaluations are time and labor intensive and do not provide continuous data. Side‐scan sonar (SSS) can provide a low‐cost method that collects continuous aquatic habitat data. We used SSS mapping to quantify suitable spawning substrate available to Walleye Sander vitreus during the 2015 spring spawning migration in a 10.8‐km reach of the Tamarac River, Minnesota. The SSS map had 78.0% agreement with reference points classified in the field, and the proportion of reference points predicted as suitable using the SSS map was not significantly different than the proportion of reference points observed to be suitable. Suitable substrate for Walleye spawning comprised 8.4% (26,392 m2) of the total area mapped. The estimated number of females that suitable substrate could support was lower than the number that likely migrate up the Tamarac River and suggests that access to spawning substrate may sometimes limit reproductive success. This study demonstrates that a relatively inexpensive SSS unit can be used to efficiently map aquatic habitat while acquiring quantitative and qualitative data. Received July 6, 2016; accepted November 5, 2016 Published online March 10, 2017
The effect of piscivorous birds on fisheries is a growing concern for fisheries managers, especially when native birds consume large quantities of fish. The Red Lakes, Minnesota, fishery is one such example, where congregations of American white pelicans (AWPEs) Pelecanus erythrorhynchos forage on spawning Walleyes Sander vitreus. We quantified AWPE consumption of Walleyes on the Tamarac River, a major tributary of the Red Lakes, by using empirical diet data collected from lethally sampled birds and separately by using a bioenergetics model. Furthermore, we evaluated the diet and foraging patterns of AWPEs on the river. Camera trap data revealed that AWPEs were foraging nearly completely nocturnally, likely in response to Walleye spawning migrations, with Walleyes accounting for 98% of AWPE diets. The empirical estimate of daily fish consumption from lethally sampled birds was not significantly different from the bioenergetics estimate. Monte Carlo simulations were used to provide estimates of uncertainty in annual Walleye consumption. Based on the simulations, all estimates of annual Walleye consumption between 2014 and 2016 represented < 1% of adult (age ≥ 3) Walleyes in the system and < 2.5% of adult Walleye natural mortality. This amount of Walleye consumption by AWPEs, at current population levels, does not pose a management concern.
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