Trophic Ecology of Largemouth Bass and Northern Pike in Allopatric and Sympatric Assemblages of Voyageurs National Park, Minnesota
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- minnesota; largemouth bass; northern pike; voyageurs national park; ecology; Natural Resources and Conservation
This study focused on a comparative evaluation of general fish populations and lake characteristics, basic food habits, isotope signatures, and bioenergetics modeling of largemouth bass Micropten1s salmoides and northern pike Esox lucius from allopatric and sympatric assemblages in Voyageurs National Park (VNP), Minnesota. The study lakes were characterized as long and narrow with few coves or bays present. Littoral areas were limited in the study lakes, and steep drop-offs were common along the shorelines. Productivity was relatively low in the study lakes, and the abundance of zoobenthos and zooplankton also were low. The low productivity and limited littoral areas of the study lakes resulted in low percent aquatic vegetative cover (<2%). Angler use of study lakes was variable and likely depended on the distance required to portage to each lake. The mean gill-net catch per unit effort (CPUE) of stock-length (>35 cm) northern pike was similar among all lakes regardless of allopatry or sympatry. However, the CPUE of stock-length northern pike was generally higher during spring than summer or fall in all lakes, which was likely associated with spawning activity. Conversely, the mean angling CPUE of stock-length (>20 cm) largemouth bass was higher during summer and fall than during spring in all lakes, which was presumably due to sampling approximately two weeks after ice out when bass movement into littoral areas was limited. Furthermore, the CPUE of largemouth bass was lower in sympatric assemblages than in allopatric assemblages. The majority of northern pike captured during this study were within the stock to quality (S-Q, 35.0 - 52.9 cm) and quality to preferred (Q-P, 53.0 - 70.9 cm) length categories, which composed 62 to 82% of all pike collected. The proportional stock density (PSD) of northern pike populations sampled for this study (28 - 51), with the exception of Brown Lake (16), all were near or within the P SD objective range (30 - 60) for a balanced pike population. Although the PSD values for northern pike were within an acceptable range for northern pike, the lack oflarger (>71.0 cm) pike in the study lakes indicates the likelihood of stunting or the harvest of large fish. Growth of northern pike was similar to Minnesota pike mean growth rates for age 1 through age 3. However, after age 3, northern pike growth slowed, which may be due to the lack of larger prey in the study lakes. Furthermore, the relative weight (Wr) of northern pike ranged from 82 to 103 and was consistently lower during all seasons in the Q-P length category than the substock (SS-S), and the S-Q length categories, which further indicated predator/prey imbalances. The largemouth bass PSD values generally fell outside (24 - 95) the objective range (40 - 70) for balanced largemouth bass populations. The limited littoral zones of the study lakes could indicate that little suitable habitat for spawning and recruitment exists for largemouth bass. Growth of largemouth bass was similar to Minnesota bass mean growth rates. Growth of largemouth bass in sympatric assemblages was higher than allopatric assemblages, which likely was due to the low relative abundance of largemouth bass in sympatric assemblages. Similarly, the Wr values of largemouth bass were higher in sympatric assemblages than in allopatric assemblages, and ranged from 88 to 119. Stomach content analysis revealed that the percentage of empty stomachs varied among the study lakes for northern pike (34.7 to 66.7%) and largemouth bass (20.6 to 77.0%). Northern pike relied heavily on fishes, and especially yellow perch Perea jlavescens for food according to stomach content analysis, δ13C, and δ13N. Conversely, food habits analysis revealed that the largemouth bass diet primarily consisted of Odonata, cannibalized age-0 bass, and Decapoda. Northern pike consumed largemouth bass in sympatric assemblages; conversely, no northern pike were consumed by largemouth bass. No relationship was found between TL and the importance of any food type or δ13C values in the diets of northern pike or largemouth bass, which indicates opportunistic feeding habits. However, a significant relationship was found between total length and δ13 in northern pike and largemouth bass, which may be attributed to the ability of larger fish to ingest a variety of food items from multiple trophic levels.