David O. Evans

Size-Dependent Winter Mortality of Young-of-the-Year White Perch: Climate Warming and Invasion of the Laurentian Great Lakes

Abstract White perch Morone americana invaded Lake Ontario about 1946 and are now found in Lakes Erie, St. Clair, and Huron, and in Green Bay, Lake Michigan. The indigenous marine distribution of white perch along the Atlantic coast of North America and analysis of climatological data suggest that the northern Gulf of St. Lawrence in the vicinity of the Gasp& Peninsula is too cold to permit white perch to establish local populations or to invade the Great Lakes via the St. Lawrence River. High mortalities of white perch have occurred in Lake Ontario during very cold winters, further suggesting that distribution of white perch is limited by low tolerance of cold temperature. Warmer-than-average summer and winter temperatures during the late 1940s coincided with the invasion of white perch into the Great Lakes via transportation canals in the state of New York. Tolerance of young-of-the-year white perch for low temperature was tested in the laboratory in overwinter experiments at constant temperatures of 2....

Notes: Temperature Constraints on Overwinter Survival of Age-0 White Perch

Abstract We evaluated the relative importance of energy depletion and osmoregulatory stress as possible mechanisms regulating overwinter mortality of age-0 white perch Morone americana. Fish used less energy, took up more water, and had much higher mortality at 2.5°C than at 4.0°C. Mortality, energy use, and water uptake were all related to body size. Relationships of empirically derived endurance time (ET, days to 50% mortality) to body mass were allometric with weight exponents of 0.29 at 2.5°C and 0.77 at 4.0°C. Theoretically derived weight exponents were 0.82 for ET models based on starvation and 0.18 for models based on osmotic mechanisms. The theoretical and empirical models suggest that overwinter mortality of white perch is caused primarily by starvation at 4.0°C and by osmoregulatory dysfunction as well as starvation at 2.5°C.

Limitations to Lake Trout (Salvelinus namaycush) Rehabilitation in the Great Lakes Imposed by Biotic Interactions Occurring at Early Life Stages

We examine evidence that biotic factors, particularly predation, may be limiting early survival of wild lake trout (Salvelinus namaycush) juveniles in many areas of the Great Lakes. The Great Lakes contain numerous potential predators of lake trout eggs and fry, some of which are recent invaders, and most of which were probably absent when lake trout most recently re-invaded the Great Lakes after the last ice age. Simple quantitative models of predation suggest that plausible assumptions about prey densities, predator feeding rates, and duration of exposure of predator to prey can lead to very high estimates of predation mortality, in some instances approaching 100%. Indirect evidence from inter-Great Lake comparisons and inland lake examples also suggest that biotic factors may impede successful lake trout colonization. Our synthesis of the evidence leads to recommendations for research to better define field feeding rates of lake trout egg and fry predators and comparative studies of densities of potential egg and fry predators on lake trout spawning reefs. Management options should be designed to provide useful information as well as achieve short-term goals. From a management standpoint we recommend that: newly constructed lake trout reefs should be placed well away from concentrations of potential predators; offshore spawning reefs should be stocked; salmonine stocking, nutrient abatement, and commercial harvest of alewives should all be considered as options to enhance survival of young lake trout; hatchery lake trout should not be stocked at sites where wild lake trout are showing signs of recovery; and exotic species expansions or introductions must be curtailed to maintain or improve on our recent successes in lake trout rehabilitation.

Metabolic Thermal Compensation by Rainbow Trout: Effects on Standard Metabolic Rate and Potential Usable Power

Abstract Metabolic models of thermal acclimation of fishes are generally based on routine rates of oxygen consumption and, therefore, are confounded by metabolic changes due to variation in random activity. My objectives were to describe the amount, direction, and time course of change in the standard metabolic rate of rainbow trout Oncorhynchus mykiss while the fish acclimated to warm and cold temperature, and to account for the energy costs of random swimming. Rainbow trout (100–250 g) were acclimated to 10 and 20°C and tested at 10, 15, and 20°C. Random activity and oxygen consumption were monitored at acclimation temperatures immediately after acute temperature exposures and, in some cases, for several days after temperature changes. Random swimming activity and standard metabolic rate were strongly influenced by both recent thermal history and acute temperature exposure. The initial activity response depended on the extent of the temperature change, and included an orthokinetic reaction when the new ...

Introduction of Lake Trout (Salvelinus namaycush) to Inland Lakes of Ontario, Canada: Factors Contributing to Successful Colonization

We evaluated the success of 183 introductions of lake trout (Salvelinus namaycush) in small inland lakes in Ontario, Canada. Our purpose was to identify variables associated with successful versus failed natural recruitment after introductions of hatchery-reared lake trout. Origin of donor stocks, angling regulations, geophysical and water quality characteristics, and fish species presence-absence of the recipient lakes were evaluated as possible factors contributing to colonization success. Origin of donor stock had some minor influence on the success of introduced lake trout in these lakes, but other factors, including angling, were also important. Closure of lakes to fishing was strongly associated with some successful introductions. MANOVA revealed differences in the geophysical and water quality variables, and fish community structures among the native, successful, and failed lakes. The lake groups were further distinguished by canonical variate analysis using either geophysical and water quality variables and/or presence-absence of fish species. Lakes were correctly classified to one of the three lake groups with 63-83% accuracy using a cross-validated, chance corrected, discriminant analysis. Lakes in which new lake trout populations failed to become established were smaller and shallower, had higher total dissolved solids, larger littoral areas, smaller hypolimnions, and richer fish communities, including more lake trout egg and juvenile predators, than lakes that were successfully colonized. Intensity of pre-dation by indigenous species was a possible cause of failure of introduced lake trout to establish self-sustaining populations. Similar fish communities in native and failed lakes suggested that invasion and colonization by lake trout was possibly determined by the order of arrival of colonizing species. This suggested that the present day Great Lakes fish communities, which have experienced a recent net gain in species, might be resistant to reestablishment of self-sustaining lake trout stocks.

The state of Lake Simcoe (Ontario, Canada): the effects of multiple stressors on phosphorus and oxygen dynamics

Abstract Lake Simcoe, the largest lake in southern Ontario outside of the Laurentian Great Lakes, is affected by numerous stressors including eutrophication resulting from total phosphorus (TP) loading, climate change, and invasions of exotic species. We synthesized the long-term responses of Lake Simcoe to these stressors by assessing trends in water quality and biological composition over multiple trophic levels. Evidence for climate change included increasing thermal stability of the lake and changes in subfossil diatom communities over time. Although the deep water dissolved oxygen (O2) minimum has increased significantly since TP load reductions, it is still below estimated historical values and the Lake Simcoe Protection Plan end-of-summer target level of 7 mg O2 L-1. Low deep water O2 concentrations corresponded with a decline in coldwater fish abundance. Since 1980, some nutrient concentrations have decreased (spring TP) while others have increased (silica), but many show no obvious changes (ice-free TP, nitrate, ammonium). Increases in water clarity, combined with declines in chlorophyll a and phytoplankton biovolumes in Cook’s Bay, were temporally consistent with declines in TP loading and the lake-wide establishment of dreissenid mussels as a major component of the Lake Simcoe ecosystem. Using an investigative tool, we identified 2 periods when abrupt shifts potentially occurred in multiple parameters: 1986 and 1995-1997. Additional ecosystem level changes such as declines in zooplankton, declines in offshore benthic invertebrate abundance, and increased nearshore invertebrate abundance likely reflect the effects of invasive species. The interaction of these multiple stressors have significantly altered the Lake Simcoe ecosystem.

Invasive Mussels Alter the Littoral Food Web of a Large Lake: Stable Isotopes Reveal Drastic Shifts in Sources and Flow of Energy

We investigated how establishment of invasive dreissenid mussels impacted the structure and energy sources of the littoral benthic food web of a large temperate lake. We combined information about pre- and postdreissenid abundance, biomass, and secondary production of the littoral benthos with results of carbon and nitrogen stable isotope analysis of archival (predreissenid) and recent (postdreissenid) samples of all common benthic taxa. This approach enabled us to determine the importance of benthic and sestonic carbon to the littoral food web before, and more than a decade after dreissenid establishment. Long term dreissenid presence was associated with a 32-fold increase in abundance, 6-fold increase in biomass, and 14-fold increase in secondary production of the littoral benthos. Dreissenids comprised a large portion of the post-invasion benthos, making up 13, 38, and 56% of total abundance, biomass, and secondary production, respectively. The predreissenid food web was supported primarily by benthic primary production, while sestonic material was relatively more important to the postdreissenid food web. The absolute importance of both sestonic material and benthic primary production to the littoral benthos increased considerably following dreissenid establishment. Our results show drastic alterations to food web structure and suggest that dreissenid mussels redirect energy and material from the water column to the littoral benthos both through biodeposition of sestonic material as well as stimulation of benthic primary production.

Fourteen years of dreissenid presence in the rocky littoral zone of a large lake: effects on macroinvertebrate abundance and diversity

Abstract Establishment of dreissenid mussels in aquatic systems is often accompanied by major changes in the abundance, diversity, and community composition of benthic invertebrates. However, few studies have been published that address the effects of long-term dreissenid presence on the littoral benthos inhabiting hard substrata in lakes. We present the results of a depth-stratified, quantitative survey of littoral benthos conducted at 4 sites in 1993, just before dreissenid invasion, and in 2008, 14 y after the establishment of dreissenids in Lake Simcoe, Ontario. Average densities of nondreissenid invertebrates were 45× greater in 2008 than in 1993. Amphipods, isopods, chironomids and oligochaetes underwent the largest increases in absolute abundance. The taxonomic diversity of the benthic invertebrate community (&agr; diversity) increased significantly. Community structure was more similar within and between depths and sites in 2008 than in 1993, a result implying lower &bgr; diversity of the postdreissenid littoral benthos. In addition, fewer differences were found in the densities of organisms between sampling locations in 2008 than in 1993. We suggest that dreissenids increase resource availability to benthic organisms and homogenize the littoral benthos by increasing the evenness of the distribution of food and physical-habitat resources across sites and depths. The transformation of the littoral benthic community of Lake Simcoe reflects a major change in the distribution of energy in the lake and is consistent with a dreissenid-mediated redirection of production from the pelagic to the littoral zone.

Assessment and Management of Fish Community Health in the Great Lakes: Synthesis and Recommendations

Abstract Our objective was to provide a perspective on fish community health to serve as a conceptual framework for assessing the effects of toxic chemicals and other anthropogenic influences on fish communities in the Great Lakes. We discuss the hierarchical structure and homeostatic mechanisms of aquatic ecosystems, and describe a general ecosystem stress syndrome (GESS) that characterizes patterns of ecosystem response in terms of alarm, resistance, and exhaustion. Anthropogenic stressors operate at various levels within the organizational hierarchy of aquatic ecosystems, and their effects are filtered and propagated throughout the system by physical, chemical, and biological processes. Determination of causal relationships between stressors and system responses has proven difficult because of the multiplicity of factors that influence system behavior. Methods employed for fish health investigations in the Great Lakes span molecular to ecosystem approaches, reflecting the diversity of stressors operating on the fish communities, but have tended to be applied within relatively narrow disciplinary perspectives. We perceive a need for broader perspectives and interdisciplinary investigation of the effects of toxic chemicals and other stressors. Attainment and maintenance of healthy fish communities, which we define as relatively stable, self-sustaining assemblages of fishes providing sustained economic, social, aesthetic, and ecological benefits, requires an ecosystem-based fisheries management strategy. The strategy must incorporate the integral roles of humans and other terrestrial species in the Great Lakes ecosystem in recognition of feed-back mechanisms involving resource utilization and waste and material inputs, thereby incorporating health concerns for all high risk populations including humans. We recommend adoption of a field-oriented epidemiological approach for monitoring and assessment of fish community health, supported by transdisciplinary investigative teams for ecosystem problems requiring diagnostic and remedial activities. We suggest that Areas of Concern under Annex 2 of the 1978 Great Lakes Water Quality Agreement be used as sites for comparative management experiments involving deliberate manipulation of ecosystem processes, not only to rehabilitate these areas, but also to improve understanding of the structural and functional properties of these systems, and to provide feedback for adjustment of the selected management options.

Increased dependence on nearshore benthic resources in the Lake Simcoe ecosystem after dreissenid invasion

Abstract Changes in the lower trophic level of Lake Simcoe, Canada, coincident with the invasion of dreissenids have been well documented, but little is known regarding the effects of these changes on the pathways of energy flow to higher trophic levels. To evaluate pathways of energy flow, we analyzed stable isotopes of zooplankton, benthic invertebrates, and fish over a 26-year period. Using stable isotopes of carbon (C) and nitrogen (N), we found evidence for a dramatic increase in the importance of benthic-derived nearshore primary production in Lake Simcoe after the invasion of dreissenids; δ13C of benthos collected at 5 and 10 m depth increased by 4–5‰ on average, and benthic warmwater fishes demonstrated a similar increase. In contrast, δ13C of profundal (≥20 m) benthos and pelagic zooplankton were not significantly different during this time period, while offshore pelagic and profundal fishes changed more subtly in magnitude but in the same direction as nearshore benthos and warmwater fishes. The range of δ13C values observed across the fish community increased from 3 to 10‰, primarily due to a positive temporal shift among warmwater fishes. Our study highlights the increase in functional heterogeneity in Lake Simcoe after dreissenid invasion, likely due to an increase in nearshore benthic production. Accounting for increased contributions of benthic-derived C with time is critical in accurately modelling C and energy transfer in the lake, and for better understanding the degree of nearshore–offshore coupling in the lake.