Donald J. Stewart

Forage Fishes and Their Salmonid Predators in Lake Michigan

Abstract Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax dominate the planktivorous fish fauna of Lake Michigan and are now the primary food of lake trout Salvelinus namaycush and introduced salmonids. Their fluctuations in abundance have been a concern due to their effects on native species and their present role as forage species. Each has been implicated as an important factor in the local reduction or extinction of important native species. Mechanisms for these interactions include competition for food and predation on eggs and larvae. Bioenergetic modeling simulations of alewife consumption by stocked salmonids suggest that as much as 20 to 33% of the annual alewife production may be consumed in some years. Increasing stocking rates of salmonids in Lake Michigan yield a predator-prey system in which predator numbers become relatively independent of prey dynamics. This suggests possible declines in alewife production, changes in major forage available to predators, and perhaps destabiliz...

Corroboration of a Bioenergetics Model for Sockeye Salmon

Abstract We constructed a bioenergetics model for sockeye salmon Oncorhynchus nerka and evaluated its sensitivity to parameter error. When used to predict annual growth, the model was most sensitive, in declining order of importance. to changes in the intercept of the dependence of consumption on body weight, the proportion of maximum consumption, the energy density of prey, low temperature and its associated proportion of maximum consumption in the temperaturedependence function, the intercept of the energy density relationship to predator weight, and the intercept of the relationship between body weight and respiration. Estimates of consumption from the model, when consumption was constrained by fixed growth, were quite insensitive to perturbation of all parameters except the energy density of prey. We computed consumption rates and energy budgets with the model and compared these with independently derived estimates for populations in Lake Dalnee, USSR; Lake Washington, USA; and Babine Lake, Canada. Th...

Dynamics of Consumption and Food Conversion by Lake Michigan Alewives: An Energetics-Modeling Synthesis

Abstract We developed an energetics model for the alewife Alosa pseudoharengus to evaluate consumption and conversion efficiency processes. We estimated alewife standard metabolism (R) as a function of wet body weight (W, g) and water temperature (T, °C): R (g˙g−1˙d−1) = 0.0073 W−0.215e0.0548T. We estimated maximum daily ration for 15-g, yearling alewives at 20°C to be as much as 37% of their body weight in experiments of 24 h. Modeling simulations based on observed growth in Lake Michigan indicated that yearling alewives may attain close to 70% of their estimated maximum consumption rate during September and October. Simulation results indicated wide seasonal variations in consumption rates; almost 50% of the yearly consumption by an individual adult alewife (age ≥ II) occurs in September and October, suggesting abundant food and possibly relaxed competition during that season. In contrast, adult alewives lose weight during the summer when stratification of Lake Michigan would permit orientation to water...

Modeling Steelhead Population Energetics in Lakes Michigan and Ontario

Abstract We developed a bioenergetics model for steelhead Oncorhynchus mykiss that simulates growth and consumption by separate life history forms in Lakes Michigan and Ontario. We estimated abundances of hatchery and wild smolts during 1975–1990 for both lakes based on stocking rates, survival schedules, and discrimination of the proportion of the lake population that was naturally recruited. Recruitment of wild steelhead varied as a function of presmolt winter severity in both lakes. Wild smolts accounted for 6–44% (during 1982–1989) and 18–33% (during 1978–1985) of total annual smolt production in Lakes Michigan and Ontario, respectively. We developed an age-structured population model with separate accounting for run timing (summer, fall, and spring) using the proportions of life history stages observed at weirs and estimated adult survival rates. We summarized data on growth, diet, water temperature, and energy contents of predator and prey to model lakewide prey consumption in both lakes during 1975...

Ecological Energetics of Rainbow Smelt in the Laurentian Great Lakes: An Interlake Comparison

Abstract We developed an energetics model for rainbow smelt Osmerus mordax to quantify its role as a producer and a predator in Great Lakes ecosystems. We measured the weight (W, g) and temperature (T °C) dependences of routine metabolism: R (cal·g–1·h–1) = 0.3646·W –0.216. e 0.036·T. We evaluated seasonal and ontogenetic patterns of rainbow smelt energy density. Seasonal and size-dependent diet composition was estimated based on an extensive analysis of smelt diets in Lakes Michigan and Ontario and a synthesis of published diet data for all of the Great Lakes. For each lake, we employed lake-specific temperature regimes, growth rates, mortality rates, and abundance estimates. Model simulations for the average individual rainbow smelt in the various lakes revealed gross food conversion efficiencies (GCE) of 13.8–15.8% averaged over all age-classes. For the youngest age-class, GCE was usually close to 20% and two or three times greater than for age-classes 4 and 5. An empirical estimate of daily ration in ...

Zooplanktivory by Alewives in Lake Michigan: Ontogenetic, Seasonal, and Historical Patterns

Abstract We extended a bioenergetics model of growth for the average individual alewife Alosa pseudoharengus to the Lake Michigan population of this species in the mid-1970s. We used the model to estimate patterns of total consumption of zooplankton by alewives. About 10% of total annual zooplankton consumption by Lake Michigan alewives could be attributed to larval fish feeding during the first 40 d of life. Young-of-year and larval fish together accounted for 50% of total annual consumption by alewives. Typical adult (age-3 and older) fish accounted for only 21% of total consumption. We evaluated the sensitivity of estimates of consumption by alewife larvae by using upper and lower bounds on metabolic rate and energy density of larvae. Total consumption estimates based on the low and high metabolic rates differed by a factor of 3. Consumption was relatively insensitive to energy density. Modeling a 29-d, normally distributed spawning period versus a single-day spawning period resulted in a 9% increase i...

Fish assemblages in a river with unusual gradient (Luongo, Africa-Zaire system), reflections on river zonation, and description of another new species

SynopsisFish distribution in a river within the drainage system of Zaire headwaters and with an unusual gradient was studied. The Luongo River has high gradient upper and lower sections, separated by a long middle section of no gradients, and bordered by rapids and waterfalls. On the basis of Luongos hydrographic history, topography and fish taxocenes the river is divided into saltatory zones, each zone inhabited by separate homeostatic fish communities. The problem of river natural zonation is argued. Luongo River is, furthermore, unique for its high richness (49 species), biogeographical correlations, and the number of undescribed species found (14%), of which one,Chetia mola, is described in this study. The probable reasons for the richness and for the discontinuous fish assemblages are discussed.

Seasonal and Habitat Influences on Fish Communities within the Lower Yasuni River Basin of the Ecuadorian Amazon

We sampled lagoon, river and forest stream habitats during the rising water, wet, falling water, and dry seasons in the lowland region of the Yasuni National Reserve in the Ecuadorian Amazon. We collected 195 species, increasing the current number of species for the Napo River basin to approximately 562. The steady rate of species accumulation per sample suggests that the fish fauna is still undersampled. Lagoon, river and forest stream fish communities are highly diverse and variable, composed of common species found within several habitats, of characteristic species found throughout the year, and of seasonally migrating species. Characteristic lagoon species were mainly the curimatids Curimata vittata, Psectrogaster amazonica, Potamorhina altamazonica, P. latior and Cyphocharax plumbeus. The characins Hyphessobrycon copelandi and Hemigrammus cf. lunatus and the catfishes Nemadoras humeralis, Pimelodella sp. C and Sorubim sp. A were characteristic river species. Characteristic forest stream species included Hoplias malabaricus, Hyphessobrycon copelandi, Pimelodella sp. B and Sternopyugus macrurus. During the dry season, lagoon and river habitats had the highest number of individuals and species, as fishes were concentrated in decreasing habitat area. In contrast, stream habitats had the highest species richness and abundance during the rising water and falling water seasons. Species collected included vital food fishes and seasonal migrants. The migratory catfishes Brachyplatystoma vaillantii, Hemisorubim platyrhynchos, Platynematichthys notatus, Platystomatichthys sturio and Sorubim lima were collected during the falling water season, which suggests that these species may begin migrating earlier than expected. These findings highlight the importance of seasonality for both adequately assessing aquatic biodiversity and for developing research and conservation programs encompassing whole river ecosystems.

AGE AND SIZE AT FIRST REPRODUCTION OF FISHES: PREDICTIVE MODELS BASED ONLY ON GROWTH TRAJECTORIES

Age and size at first reproduction have broad implications for studying population and community ecology. From general functions of body growth we developed models for estimating both age and size at first reproduction when growth trajectories are the only reliable information. We evaluated our models using empirical data, including 85 species of marine and freshwater fishes, with 235 populations or sex groups. From the data set, we found the following two sets of relations. (1) There is a nonlinear negative relation between the von Bertalanffy growth coefficient (K) and the age at first reproduction (Tr). (2) The ratio of body length at first reproduction (Lr) to asymptotic body length (Linf) may increase in two possible ways: one is increases in K and, consequently, decreases in Linf; the other is increases in Tr and delay of the age at first reproduction. Our model integrates the above complex relations into a simple linear function. The set of life-history invariants summarized by E. Charnov and by A....

Fish Community Patterns of Lagoons and Associated Tributaries in the Ecuadorian Amazon

We analyzed fish distribution patterns of 10 lagoons and six of their tributaries between 200 m and 295 m altitude along two parallel headwater tributaries of the upper Amazon, within the Napo River basin of eastern Feuador. Multivariate methods of Detrended Correspondence Analysis (DCA) and Two-Way Indicator Species Analysis (TWINSPAN) revealed community patterns that corresponded primarily to altitude and secondarily to habitat. We identified two altitudinal regions, the lowland between 200 m and 220 m and the piedmont between 235 m and 295 m. We recognized four community types associated with lowland lagoon, lowland tributary, piedmont lagoon, and piedmont tributary habitats. Tributary faunas were more closely associated with lagoon faunas within the same region, than with tributary faunas of differing regions. Lagoons and associated tributaries had high species diversity and evenness. Lowland sites showed higher species richness and had more characteristic species than piedmont sites, resulting in a pattern of species addition and species replacement along the altitudinal gradient. Decreased species richness in higher altitudes, coupled with unchanging diversity and evenness, is similar to published findings for riverine sandy beach assemblages of the Napo River basin. Faunal turnover between lowland and piedmont regions was greater for tributaries than for lagoons. Faunal turnover between tributary and lagoon habitats was greater in the lowland region than in the piedmont region. In contrast to observations for riverine sandy beach assemblages of the Napo basin and floodplain lagoons of the Orinoco River basin, separation of lagoon communities based on water type (clearwater vs blackwater) was not well supported. Comparisons among lagoon, tributary, and riverine sandy beach communities of the Napo River basin and Rio Negro (Brazil) communities revealed low similarity. Comparison of numerically dominant characiforms of the Napo, Negro, and Apure River (Venezuela) drainages revealed few shared species but approximately 50% shared genera.