Transactions of the American Fisheries Society

The restoration of Lake Trout Salvelinus namaycush in the Great Lakes and Lake Champlain has been challenging due to the bottlenecks in recruitment that occur mostly during early life stages. Among possible sources of fry mortality (e.g., predation, starvation, and disease), the least is known about the diet and starvation risk of pre-emergent fry. The first feedings by fry are generally assumed to be delayed until close to the absorption of the yolk sac and the emergence of the fry. The stomach contents of 374 wild-caught Lake Trout fry from Lake Champlain were examined from hatching to the exogenous feeding stage to identify the earliest occurrence of feeding relative to yolk sac absorption and to describe the diet. Within 2 weeks of hatching, 19% of fry had food in their stomachs. At 4–6 weeks and after yolk sac absorption, 98% of fry began feeding. Diet was primarily comprised of Bosmina as well as calanoid and cyclopoid copepods, and fry contained up to 215 items per stomach. Our finding that fry began feeding within 2 weeks of hatching (prior to yolk sac absorption) is relevant to current concerns that Lake Trout fry mortality in the Great Lakes is caused by thiamine deficiency syndrome: wild Lake Trout fry may be able to mitigate thiamine deficiency with early feeding on thiamine-rich zooplankton. Restoration of Lake Trout Salvelinus namaycush in the Laurentian Great Lakes and Lake Champlain is challenging because native populations were largely extirpated during the late 19th and early 20th centuries. Despite intensive stocking efforts, only Lake Superior and local areas of Lake Huron have self-sustaining populations (Riley et al. 2007; Corradin et al. 2008). In Lake Champlain, pre-emergent wild Lake Trout fry are abundant at numerous locations lakewide (Ellrott and Marsden 2004), but no postemergent fry or wild juveniles have been found (Riley and Marsden 2009). The bottleneck in recruitment of wild Lake Trout occurs in their early life stages, after fry emerge from spawning reefs (Zimmerman and Krueger 2009). In Lake Champlain, egg deposition and hatching occur at high densities on several spawning reefs, but no postemergent Lake Trout have been collected, and the median annual incidence of age-3C wild Lake Trout since stocking began in 1972 is 3.4% (Ellrott and Marsden 2004; Vermont Department of Fish andWildlife, unpublished data). The factors that likely affect salmonid survival at early life stages are disease, predation, and starvation. Pathogens are readily and rapidly detected in the hatcheries that collect wild Lake Trout gametes for rearing and stocking fry, but no widespread threats to fry health have been identified. Predation on Lake Trout eggs and fry by interstitial and epibenthic predators can be high but is not sufficient to eliminate entire year-classes lakewide (Jonas et al. 2005; Riley andMarsden 2009). Predation by exotic species, including egg predation by Round Goby Neogobius melanostomus (Fitzsimons et al. 2006) and fry predation by Alewife Alosa pseudoharengus (Krueger et al. 1995, 2014), are of concern because they are new threats to the early survival of Lake Trout. Furthermore, *Corresponding author: ellen.marsden@uvm.edu Received April 2, 2015; accepted July 2, 2015 1 Transactions of the American Fisheries Society 145:1–6, 2016 American Fisheries Society 2016 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2015. 1073622 D ow nl oa de d by [ J. E lle n M ar sd en ] at 0 5: 09 2 2 D ec em be r 20 15 little is known about how starvation affects recruitment because the diets of pre-emergent Lake Trout fry have not been studied in the wild. For starvation to be significantly related to failures in population restoration, either Lake Trout food (plankton) or the spatiotemporal overlap of Lake Trout fry and their prey would have had to change since the extirpation of Lake Trout. Lake Trout spawning on nearshore, manmade structures (e.g., breakwalls; Marsden et al. 2005) and likely the warming of littoral waters from global climate change have altered both the location of fry and the phenology of zooplankton. Either factor or a combination of these factors may have shifted the predator–prey overlap. As a result, studies of fry diets are needed to evaluate the potential for starvation. Diet affects the health and survival of fish fry at a critical phase in their development. Exogenous feeding by salmonids is generally thought to coincide with the stage at which maternally allocated nutrients are depleted (Atkins 1905; Palmer et al. 1951). In hatcheries, manufactured food is presented to fry either at or slightly before yolk absorption. Prior to this stage, fry do not respond well to nonmoving prey (Hurley and Brannon 1969). In the wild, the timing of the first feeding of pre-emergent fry and their foraging behavior in the presence of live food are not well understood. The objectives of this study were to (1) identify the earliest stage at which Lake Trout begin exogenous feeding in the wild and (2) describe the composition of the diets of early-stage fry. The consumption by adult salmonids of thiaminase-containing prey, such as Alewives, has been widely linked to thiamine (vitamin B1) deficiency in salmonid eggs and is believed to contribute to Lake Trout recruitment failure (Fisher et al. 1996; Brown et al. 2005). Thiamine deficiency symptoms have been widely observed in hatchery fry which have hatched but which still have remaining yolk sac (e.g., Honeyfield et al. 2005; Jaroszewska et al. 2009). However, thiamine is later available to fry in the wild from a variety of environmental sources, including zooplankton (Hutchinson 1943; Meeren et al. 2008). These sources of thiamine could mitigate thiamine deficiency symptoms in wild fry.