Variable movements after catch-and-release angling and isotope niches of walleye (Sander vitreus) and sauger (S. canadensis)

Abstract

Walleye (Sander vitreus) and sauger (S. canadensis) are closely related freshwater species that are ecologically and economically important throughout North America. These two species are sympatric in many areas, and are often regulated as a single entity. However, the important similarities and differences that exist between these species in the context of various management issues remain uncertain. The first chapter of my thesis addresses the effects of catch-and-release fishing that is part of angling tournaments on walleye and sauger. Catch-and-release tournaments are popular in Saskatchewan and other areas, but frequently prompt management concerns regarding fish health and mortality. Potential sub-lethal effects on movement and habitat use are particularly poorly studied. I tracked both walleye and sauger after catch-and-release using telemetry following four 2015 tournaments. My objectives were to: (1) describe post-release movements; and (2) evaluate factors that may affect postrelease behaviour. The initial movements I observed were highly variable; for example, some fish moved less than 1 km, whereas others moved long distances, the maximum recorded being 31 km. All large movements I observed were made by walleye, while the farthest a sauger moved was 2 km. Fish exited the release area (500 m) faster (2.6±1.1 days versus 3.9±1.6 – 4.4±1.7 days) and moved farther in the first 48 hours (3,712.8±4,427.9 m versus 608.1±392.1 – 1065.3±687.8 m) at one tournament compared to the other three. Stress scores were significantly lower at this tournament (3.50±0.73 versus 4.60±1.17 – 4.75±1.48), and an important negative predictor of fish movement. However, there were few common predictors of movement among tournaments. I confirmed delayed mortality of tagged fish at only one tournament, which was high (45%) and mostly due to avian predation. I conclude that walleye and sauger behaviour after tournaments varies widely, likely due to varying tournament and environmental conditions. The understanding of factors that modify fish behaviour remains limited and warrants further study. The second chapter of my thesis assesses the ecological relationship between walleye and sauger when they co-exist. Limited information is available regarding potential niche partitioning across different time scales by these sympatric congeners, though I expected niche overlap to be high during early ages and lower later in life. I used multi-tissue (liver, muscle, and bone layers) isotopic niche analysis to quantify overlap by these species in two large reservoirs in Saskatchewan. Population niche size was larger for sauger (1.73 – 3.60 ‰2) than walleye (0.51 – 2.61 ‰2) in Lake Diefenbaker, but this trend was reversed in Tobin Lake (sauger = 0.54 – 2.57 ‰2; walleye = 1.30 – 2.94 ‰2). Inter-species niche overlap at the population level also varied by site (Diefenbaker = 3- 19%, Tobin = 9-31%), and niche partitioning was evident in both systems at all time scales. The niche size of individual fish and levels of intra- and inter-specific overlap were highly variable by site. Walleye and sauger appear to use different resources but the factors influencing variation in niche size and overlap are currently unclear. My findings indicate there is considerable variation in the response of walleye and sauger to catch-and-release angling, as well as in their resource use and niche overlap. The important differences I describe between these species suggest that a re-evaluation of regulating them as one entity may be warranted.