LAUSR

First-year overwinter survival is a frequent bottleneck to the recruitment of Largemouth Bass Micropterus salmoides. Early ontogeny to piscivory provides increased overwinter survival through increased growth and the accumulation of lipids. This ontogeny is thought to be slowed by dense and complex vegetative habitats, but this hypothesis has not been directly tested. To address this question, we conducted enclosure experiments for 5 weeks during the typical time that juvenile Largemouth Bass would transition to piscivory (i.e., midsummer). Thirty-two enclosures were constructed across two 0.10-ha ponds and given one of four stem densities of simulated vegetation (0, 50, 250, and 500 stems/m). Three juvenile Largemouth Bass and 30 juvenile Bluegills Lepomis macrochirus were added to each enclosure. All fish were sampled twice per week. Largemouth Bass growth was measured on all sample dates, and stomach samples were collected to determine diets on one date each week. Bluegill sizes and densities were manipulated to maintain the number of fish at 30 fish that were 25–35% of Largemouth Bass TL after each sampling event. Largemouth Bass stomach contents (percent by weight) were initially dominated by insects, and the bass transitioned to mostly fish prey by the end of the experiment. The use of fish prey, as measured by the presence of surviving Bluegills in the enclosures, significantly increased at the beginning of the second week at all stem densities, but fewer fish prey were eaten by bass in the 250-stems/m treatment than in all other treatments throughout the experiment. Largemouth Bass also grew less in the 250-stems/m treatment. We concluded that vegetation density does affect the foraging rate of piscivorous juvenile Largemouth Bass, but not necessarily the timing of the ontogeny to piscivory. However, differing results between this experiment and other previously published studies suggests vegetation may have an interactive effect with available prey types. Largemouth Bass Micropterus salmoides undergo an ontogenetic shift in feeding habits from invertebrates to piscine prey during their first year of life (Werner and Gilliam 1984; Olson 1996; Ludsin and DeVries 1997; Mittelbach and Persson 1998). The switch to piscivory facilitates increased growth, as fish prey are a greater source of energy than invertebrate prey (Keast and Eadie 1985; Olson 1996; Mittelbach and Persson 1998). With this increased growth comes an ability to eat larger prey, thus available prey is increased, which further enhances growth (Mittelbach and Persson 1998). First-year growth is important as it facilitates an individual’s ability to avoid predation (Hambright 1991) and increases overwinter survival (Ludsin and DeVries 1997; Mittelbach and Persson 1998), both of which are considered major bottlenecks in Largemouth Bass recruitment (Houde 1987; Galarowicz and Wahl 2005; Parkos and Wahl 2010). Therefore, understanding the factors influencing the timing of ontogeny to piscivory is important for understanding recruitment dynamics of Largemouth Bass. Availability of fish prey that are small enough for juvenile Largemouth Bass to consume (approximately ≤ 50% of predator TL: Johnson and Post 1996) affects the timing of ontogeny to piscivory for this species (Mittelbach and *Corresponding author: [email protected] Present address: Utah Division of Wildlife Resources, Northern Regional Office, 515 East 5300 South, Ogden, Utah 84405, USA. Received July 10, 2017; accepted February 23, 2018 North American Journal of Fisheries Management 38:630–638, 2018 © 2018 American Fisheries Society ISSN: 0275-5947 print / 1548-8675 online DOI: 10.1002/nafm.10060