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

Abstract

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.

Traditional ecological knowledge reveals the extent of sympatric lake trout diversity and habitat preferences

“…Dubois and Lagueux (1968) documented the age and size of lake trout from unknown localities. Zimmerman et al (2007) identified two forms that differed in age, growth, and maturity (Hansen et al 2012) in a small area of the lake. Most recently, Marin et al (2016) sampled the entire lake and used morphological and DNA analyses to determine that Mistassini harbors at least five weakly differentiated subpopulations of sympatrically occurring lake trout with striking phenotypic variation, including differences in body size, coloration, and to a lesser extent, body depth, caudal peduncle size, eye position, and head shape.…”

Life history differences between fat and lean morphs of lake charr (Salvelinus namaycush) in Great Slave Lake, Northwest Territories, Canada

“…From full-body photographs (Muir et al, 2012b), most fish were assigned to a morph using a combination of statistical analysis of head and body measures and visual assignment, as described by Muir et al (2014). However, for some fish, statistical assignments differed between head and body measures, and visual consensus was not reached, so these fish were classified by visual comparison to the library of statistical and visual assignments (Hansen et al, 2012).…”

“…Sagittal otoliths were used because they have been validated for age estimation of lake charr to an age of at least 50 years (Campana et al, 2008). One otolith from each fish was embedded in epoxy, and a thin transverse section (400 lm) was cut, mounted on glass slides, polished, and photographed for age and growth assessment (Hansen et al, 2012). Annuli were counted by three independent readers using criteria described by Casselman & Gunn (1992), and specimens were excluded from analysis if the coefficient of variation (CV) of age estimates was more than 5% (Campana, 2001).…”

“…In Great Slave Lake, a fat morph occupied mostly deep waters, ate mostly invertebrates, especially opossum shrimp, had a narrow caudal peduncle, light body color, greater buoyancy, and deep anterior body, whereas a sympatric lean morph occupied mostly shallow waters, ate mostly fish, were streamlined in shape, darker in color, and less buoyant (Zimmerman et al, 2006(Zimmerman et al, , 2009. In Lake Mistassini, Quebec, a deep-water light-colored humper-like morph was distinct from a dark, shallow-water lean morph (Zimmerman et al, 2007;Hansen et al, 2012). The presence of lean, fat, and humper morphs across North America suggests that common selection pressures and ecological opportunities have resulted in multiple morphs that are more widespread than just in the Laurentian Great Lakes (Eshenroder, 2008;Muir et al, 2015).…”

“…The presence of lean, fat, and humper morphs across North America suggests that common selection pressures and ecological opportunities have resulted in multiple morphs that are more widespread than just in the Laurentian Great Lakes (Eshenroder, 2008;Muir et al, 2015). Differences in life history (age, growth, and maturity) among sympatric lake charr morphs from the Laurentian Great Lakes (e.g., Rahrer, 1965;Burnham-Curtis & Bronte, 1996) and other North American lakes (Hansen et al, 2012;Chavarie et al, 2015b;Muir et al, 2015) may reflect unique use of ecological opportunities and niche space by sympatric morphs, and suggest a possible need for different management strategies for each morph.…”