Mara S. Zimmerman

An Ecosystem Perspective on Re-establishing Native Deepwater Fishes in the Laurentian Great Lakes

Herein we examine scientific questions related to successful re-establishment of native deepwater fish communities in the Laurentian Great Lakes, and we (1) propose a conceptual model for native deepwater fish communities; (2) review current research and identify research hypotheses for lake trout Salvelinus namaycush, pelagic ciscoes Coregonus spp., and sculpins (Cottus spp. and Myoxocephalus spp.); (3) pose research questions emerging at community and ecosystem levels; and (4) identify high-priority research topics related to population re-establishment. The conceptual model is based on a generalized life cycle nested within processes at the population, metapopulation, community, and ecosystem levels. The conceptual model assumes that variation in population abundance is a natural phenomenon and that biodiversity contributes to ecosystem stability. Key research topics related to lake trout re-establishment include understanding recruitment variation over space and time, identifying sources of early life history mortality, determining the level of genetic differentiation among morphotypes and populations, and comparing the life history and ecology of lean lake trout with those of other lake trout morphotypes. Key research topics related to re-establishing deepwater species of cisco include resolving uncertainties in species identifications, understanding processes maintaining distinctiveness among species, understanding processes leading to cyclic and erratic recruitment, and developing effective propagation methods. Key research topics related to re-establishment of sculpin species include comparing spawning ecology and early life history of each species, determining population structure, and evaluating the feasibility of live-transfer methods. Key research topics related to community- and ecosystem-level interactions include understanding large-scale directional influences on community function, expanding current views on ecological succession in large lakes, and determining the evolutionary role of diel vertical migration in phenotypic diversification of deepwater fishes. Finally, we propose that research in four topical areas are most pertinent to re-establishing native deepwater fishes: life history bottlenecks, population and metapopulation dynamics, matching fish phenotypes to stocking sites, and propagation and stocking methods.

Phenotypic Diversity of Lake Trout in Great Slave Lake: Differences in Morphology, Buoyancy, and Habitat Depth

Abstract Little is known about the phenotypic diversity of lake trout Salvelinus namaycush in large North American lakes outside the Laurentian Great Lakes. This study tested the hypothesis that phenotypic diversity in Great Slave Lake, Northwest Territories, Canada, is associated with water depth, as was observed during similar studies of lake trout in Lake Superior. We describe the association of body size with color, buoyancy, and morphology; compare these phenotypic traits among depth strata; and establish whether lake trout phenotypes occur as discrete groups. Phenotypic diversity increased among fish longer than 43 cm standard length. In water less than 50 m deep, large lake trout (≥43 cm) were light in color, buoyantly heavy, and streamlined and possessed short pectoral fins. In water deeper than 50 m, large lake trout were dark in color, buoyantly light, and deep bodied (less streamlined) and had long pectoral fins. Without assigning descriptions to individuals before the analysis, we identified t...

Ontogenetic niche shifts and resource partitioning of lake trout morphotypes

Resource polymorphisms are widely observed in fishes; however, ontogenetic contributions to morphological and ecological differences are poorly understood. This study examined whether ontogenetic c...

Morphological and Ecological Differences Between Shallow-and Deep-water Lake Trout in Lake Mistassini, Quebec

ABSTRACT Lake trout (Salvelinus namaycush) in Lake Mistassini, Quebec, were investigated to determine whether they resembled the lean and siscowet morphotypes of Lake Superior and Great Slave Lake. Lake trout caught in deep water were predicted to resemble the siscowet morphotype and to be better adapted for vertical migration (i.e., low percent buoyancy) than those caught in shallow water. The research objectives were to 1) identify groups based on shape, and 2) determine whether shape was associated with other morphological traits (fin length, buoyancy, color), ecology (habitat depth, diet), and life history (size at adulthood). Eighty-five lake trout were collected from three depth zones. At least two phenotypes exist in Lake Mistassini. A shallow-water form (< 50-m depth), identified by its streamlined shape, was dark in color and high in percent buoyancy. A deep-water form (> 50-m depth), identified by a deep anterior-body profile, was light in color and lower in percent buoyancy than the shallow-water form. Absolute buoyancies were relatively high in both forms; therefore, the deep-water form did not appear well-adapted for vertical migration. Opossum shrimp (Mysis relicta) were more frequent and abundant in stomachs of deep-bodied trout. All deep-bodied trout (minimum 32-cm SL) had reached adulthood, whereas immature streamlined individuals were as long as 49 cm in SL. The deep-bodied form resembled humper lake trout, a lesser-known third morphotype from Lake Superior. A humper-like morphotype in Lake Mistassini, and the apparent absence of a siscowet-like morphotype, challenges the previously-held hypothesis that humpers resulted from an introgression of leans and siscowets.

Ecomorphological Diversity of Lake Trout at Isle Royale, Lake Superior

AbstractFour Lake Trout Salvelinus namaycush morphs were identified from Isle Royale, Lake Superior; the morphs differed in shape, traits linked to feeding and locomotion, buoyancy, and physical habitat use. Lean, humper, and siscowet Lake Trout generally conformed to previous descriptions, and we report, for the first time, quantitative evidence of a fourth morph, previously described anecdotally as the “redfin.” Jackknife classification of individuals to morphs based on body shape were 94% correct. High variation within and low variation among morphs led to moderately low percent agreement among visual identifications and high uncertainty in Bayesian model groupings of morphs. Eight linear measures of phenotypic traits linked to feeding (i.e., head and eyes) and locomotion (i.e., fin lengths and caudal peduncle shape) varied among morphs, consistent with specialized adaptations for trophic and physical resource use. Habitat differed among morphs with leans being most abundant in the 0–50-m depth stratum...

Atypical swimbladders of lake charr, Salvelinus namaycush, from Great Slave Lake, Northwest Territories, Canada

Swimbladder walls of lake charr, Salvelinus namaycush, from Great Slave Lake (GSL), Northwest Territories, Canada, were unusually thick for the species. The thinnest sections of the GSL bladders (mean = 2.44mm, range = 1.1–4.4mm) were significantly thicker (P = 0.001) than lake charr swimbladders collected from two small Province of Ontario lakes (means = 0.65 and 0.92mm), whose populations were assumed to be representative of the species. Variance in wall thickness was also greater in GSL lake charr than in charr from two small lakes (P < 0.02). Within individuals, some of the GSL bladder walls were markedly irregular in thickness, but whether these anomalies exist in situ or were artifacts of preservation remains uncertain. The bulk of the tissue in the thickest sections of the GSL swimbladders was in the tunica serosa (outer layer). The extent of the modification of the GSL swimbladders is extraordinary for northern fishes in postglacial lakes.

Monitoring Climate Impacts: Survival and Migration Timing of Summer Chum Salmon in Salmon Creek, Washington

AbstractIn rivers of the Pacific Northwest, climate change is predicted to increase flow variability and water temperature, which may ultimately affect salmonid survival and the seasonal timing of key life history transitions. Summer Chum Salmon Oncorhynchus keta, native to tributaries flowing into Hood Canal and Strait of Juan de Fuca in Washington State, are particularly vulnerable to flow and temperature changes given their early spawn timing, yet relatively little is known regarding their juvenile life history. We investigated how flow and incubation temperatures influenced juvenile survival and timing of Chum Salmon in Salmon Creek between 2008 and 2016. Egg-to-migrant survival ranged from 0.9% to 46.3%, and was negatively related to the peak flow experienced during egg incubation from November 1 to January 31. Warm temperatures advanced emergence timing, as the number of days between the median spawning date and the median juvenile migration date was negatively related to average stream temperature ...