Age, Growth, Survival, and Maturity of Lake Trout Morphotypes in Lake Mistassini, Quebec

Life history characteristics (age, growth, survival, and maturity) were compared between the deepwater “humper” and shallow-water “lean” forms of lake trout Salvelinus namaycush in Lake Mistassini, Quebec, to determine whether the two morphotypes may represent resource polymorphism. Lake trout were sampled using graded-mesh (range = 51–114 mm stretch) gill nets set in deep and shallow waters. Humpers were typically caught in deep waters (>50 m) and averaged 474 mm TL (range = 389–616 mm) and 852 g in weight (range = 470–1,710 g), whereas leans were typically caught in shallow waters (<50 m) and averaged 525 mm TL (range = 301–865 mm) and 1,210 g in weight (range = 200–6,500 g). Humpers and leans did not differ in weight–length relationships and grew slimmer with length over a common TL range (389–616 mm). Average age of humpers was 27 years (range = 13–49 years); average age of leans was 21 years (range = 6–42 years). The two forms did not differ in total annual mortality (A) of fish older than 17 years, the first age beyond which numbers declined with age for both morphs (A = 5.1%; 95% confidence interval = 2.4–7.8%). Humpers grew slower (annual growth rate ω = 53 mm/year) than leans (ω = 68 mm/year) and to a shorter mean asymptotic length (L∞ = 514 mm) than leans (L∞ = 605 mm). Mature humpers (mean TL = 475 mm, SE = 9.0; N = 58) were shorter on average than mature leans (mean TL = 539 mm, SE = 8.5; N = 65); mature humpers (mean age = 27 years, SE = 1.0; N = 56) were also older on average than mature leans (mean age = 23 years, SE = 0.98; N = 61). We conclude that lean and humper forms of lake trout in Lake Mistassini differed in age, growth, and maturity; this is consistent with the resource polymorphism that has been observed for other lake trout populations and other char species. Sympatric populations of fish may have multiple forms (morphotypes) that use different food or habitat resources. The presence of discrete intraspecific morphotypes that use different habitats and food is termed resource polymorphism (Smith and Skúlason 1996). Resource polymorphism is maintained by *Corresponding author: mhansen@uwsp.edu 1Present address: Washington Department of Fish and Wildlife, Wild Salmonid Production Evaluation Unit, Science Division, Fish Program, 1111 Washington Street Southeast, Olympia, Washington 98501, USA. Received June 24, 2011; accepted July 2, 2012 one or more mechanisms: (1) the morphotypes each belong to separate isolated gene pools, (2) the morphotypes belong to a single gene pool in which the polymorphism is genetically determined, and (3) the morphotypes are triggered through environmental variation acting on a single gene pool via conditional 1492 D ow nl oa de d by [ U SG S L ib ra ri es P ro gr am ] at 0 5: 18 0 9 O ct ob er 2 01 2 LAKE TROUT MORPHOTYPES IN LAKE MISTASSINI 1493 development (phenotypic plasticity; Hindar and Jonsson 1993). Among sympatric morphotypes, life history differences are often expressed as differences in growth rate, age structure, and size at maturity (Wood and Foote 1990; Jonsson and Skúlason 2000; Jonsson and Jonsson 2001). Examples of resource polymorphism are geographically diverse: rightand left-handed scale-eating cichlids Perissodus microlepis occur in Lake Tanganyika (Hori 1993), and four forms of Arctic char Salvelinus alpinus that differ in behavior, life history, and morphology are found in Lake Thingvallavatn, Iceland (Skúlason et al. 1989; Snorrason and Skúlason 2004). Sympatric morphotypes can also be genetically different (e.g., Power et al. 2009; Wood and Foote 1996). Morphotypes of lake trout Salvelinus namaycush have not previously been considered to represent resource polymorphism in Holarctic char (e.g., Snorrason and Skúlason 2004), although sympatric morphotypes of lake trout are known from several lakes in North America (Krueger and Ihssen 1995; Zimmerman et al. 2006). Multiple morphotypes of lake trout were recognized centuries ago in the Laurentian Great Lakes of North America and have recently been documented in other large North American lakes. A diversity of sympatric lake trout morphotypes using semi-isolated spawning habitats in the Great Lakes (i.e., from offshore shoals to rivers) were known long ago by aboriginal people (Agassiz 1850), early Jesuit missionaries (Goodier 1981), and French voyageurs (Roosevelt 1865). Three general forms are presently recognized as occurring in the Great Lakes (Krueger and Ihssen 1995): lean, siscowet, and humper lake trout. The lean lake trout (hereafter, “lean”) is a common form that typically uses shallow waters less than 50 m in depth, is streamlined in form, is low in fat content, and is often piscivorous when larger than 460 mm in length. The siscowet or fat lake trout (hereafter, “siscowet”) is deep bodied with a rounded snout, inhabits deep waters greater than 80 m in depth, is high in fat content, and often feeds by following the diel vertical movements of prey (opossum shrimp Mysis relicta and coregonines) in the water column (Hrabik et al. 2006). The humper lake trout (hereafter, “humper”) uses isolated offshore reefs surrounded by water deeper than 90 m or steep-sided sloping banks in Lake Superior. These three morphotypes are genetically differentiated, as is often observed in separate populations of the same species (Dehring et al. 1981; Krueger et al. 1989; Page et al. 2004). Physiological differences among forms are heritable for characteristics such as fat or oil content (Eschmeyer and Phillips 1965; Goetz et al. 2010), swim bladder gas retention (Ihssen and Tait 1974), and the developmental rates of fertilized eggs and fry (Horns 1985). Heritability of traits, especially those that affect depth regulation or buoyancy compensation (e.g., fat and swim bladder gas), indicates that the presence of sympatric morphotypes in the Great Lakes is not simply due to environmental plasticity (sensu Hindar and Jonsson 1993). Sympatric lake trout morphotypes resembling the three general forms described from the Laurentian Great Lakes have recently been documented to occur in other large North American lakes. For example, several morphologically distinct forms of shallow-water lean lake trout have been described from Great Bear Lake (Northwest Territories, Canada; Blackie et al. 2003; Alfonso 2004). In Great Slave Lake (Northwest Territories), a siscowet-like or fat deepwater morphotype has been described that differs from the sympatric lean, shallow-water form but is similar to the fat morphotype described from Lake Superior (Zimmerman et al. 2006). In Lake Mistassini, Quebec, a deepwater, light-colored, humper-like morphotype is distinct from a dark, shallow-water, lean morphotype (Zimmerman et al. 2007). The consistent occurrence of the three general lake trout forms across North America—resembling the leans, siscowets, and humpers that were originally described from the Great Lakes— suggests that common selection pressures and ecological opportunities may result in multiple morphs. The occurrence of sympatric morphotypes of lake trout therefore extends beyond the Laurentian Great Lakes. A few studies have compared life history (in terms of patterns in age, growth, and maturity) among lake trout morphotypes from the Great Lakes (e.g., Rahrer 1965; Burnham-Curtis and Bronte 1996). However, life history characteristics have not been compared between the newly described sympatric lake trout morphotypes that occur elsewhere in North America. Differences in age structure and growth rates could reflect the unique appropriation of ecological opportunities and niche space by sympatric morphotypes and suggest the need to consider separate management strategies for each morph. We compared life history characteristics (age, growth, survival, and maturity) between deepwater humper and shallowwater lean forms of lake trout in Lake Mistassini, Quebec, to determine whether the putative humper and lean morphotypes may represent resource polymorphism. The collection of otoliths, lengths, and weights from the fish used in the original description of the Lake Mistassini lake trout forms (Zimmerman et al. 2007) offered an opportunity to study life history differences between sympatric humper and lean morphotypes residing in a lake outside of the Great Lakes. Lake Mistassini humpers occupied mostly deep waters and were characterized by a narrow caudal peduncle, light body color, greater buoyancy, and deep anterior body, whereas leans occupied mostly shallow waters and were streamlined in shape, darker in color, and less buoyant (Zimmerman et al. 2007). Furthermore, the humpers ate more invertebrates, especially opossum shrimp, and appeared to mature at a smaller size than the piscivorous leans (Zimmerman et al. 2007). We compared humpers and leans for length frequency, weight–length relationships, age frequency, length–age relationships, and maturity–age relationships. Lake trout were classified as either humper or lean based on the characteristics described by Zimmerman et al. (2007). We expected to find that humpers and leans in the lightly exploited Lake Mistassini would be older in age (due to low mortality), slower growing, and later maturing than exploited populations (Johnson 1972, 1976) but would differ in life history characteristics (age, growth, survival, and maturity) because of the different habitat use (food and temperature) in Lake Mistassini than in D ow nl oa de d by [ U SG S L ib ra ri es P ro gr am ] at 0 5: 18 0 9 O ct ob er 2 01 2