Reservoir Fish Habitats: A Perspective on Coping with Climate Change

Figures & data

Figure 1. Climate warming impacts to reservoirs may include higher water temperatures and shifts in precipitation and hydrology that can result in altered physicochemical characteristics leading to changes in biological assemblages.

Table 1. Projected consequences of climate-induced changes in temperature and precipitation on physical, geochemical, and biological properties of reservoir environments.

Property	Increasing temperature	Changing precipitation
Physical
Catchment	Alter land use and vegetative assemblages in the catchment and riparian contour	Increased upstream erosion during floods; reduced organic loading from forested landscapes during droughts
Horizontal habitat connectivity	Alter plant/soil communities in floodplains, mostly in upper reaches of reservoirs	Reduced connectivity to tributaries, floodplains, and wetlands due to lower water levels and increased sedimentation
Storage	Earlier snow melt and ice break up changes timing of water arrival and storage period	Earlier rains and peak flows increase demands for storage; changes in timing of scarce water
Water residence time	Increased residence time during dry seasons can exacerbate the effects of warmer temperatures and extend stratification	Shorter wet and longer dry seasons may increase residence times and eutrophication
Water level fluctuations	Increased evapotranspiration may increase rates of drawdown especially in shallow reservoirs	Intermittent severe floods/droughts lead to intense fluctuations; lower levels enhance sediment resuspension, release of sediment-bound nutrients, and reduced DO; loss of habitat diversity as water levels recede into barren contours; encroachment of upland vegetation down to lower reservoir contours
Geochemical
Stratification	Earlier onset and longer periods of stratification. Simulations estimate warming may lower oxygen in the epilimnion by <2 mg/L, but more in hypolimnion	Onset and extent of stratification lessened by increased seasonal inflows but exacerbated by reduced inflows in summer and fall
Water quality	Increased biotic respiration reduces dissolved oxygen that controls various geochemical reactions that transform water quality. Accelerated eutrophication, increased algal growth, bacterial metabolism, and nutrient cycling rates	The effects of water pollution, including sediment, nitrogen, phosphorus, and pesticide loadings amplified by projected seasonal changes in precipitation. Increase water residence reduce nutrient flushing; where rainfall rises, runoff hasten nutrient inputs, but effect may be countered by seasonally high flushing rates
Anoxia	Rises in temperature can drop dissolved oxygen and hasten respiration rates of biota. Algal blooms associated with eutrophication may increase diel and seasonal lows and highs	Low flows in summer and fall are expected to intensify anoxia events. With increased anoxia, reservoirs may expect higher incidence of summer and fall fish kills
Biological
Aquatic macrophytes	A longer growing season may promote latitudinal and altitudinal expansion and extend number of days plants experience favorable growth. Possibly higher plant density and diversity and altered community composition	Increasingly variable water levels in some regions could prevent aquatic plants but favor moist soil growth in the exposed littoral
Cyanobacteria	Winter and spring algal phytoplankton biomass could increase. Cyanobacteria could become dominant, and harmful blooms more frequent	Increased residence times in summer and fall when temperatures are favorable for cyanobacteria could worsen blooms
Macroinvertebrates	Increased stratification and hypoxia due to warmer temperatures and eutrophication could make large sections of the benthos uninhabitable	Benthic macroinvertebrates are discouraged by water‐level fluctuations. Species able to migrate when water levels change (e.g., chironomids) may increase in abundance and representation
Fish assemblages	Cool- and coldwater fishes most impacted, especially in shallow or eutrophic reservoirs, with some populations extirpated. Warmwater taxa may expand northwards. Changes in vertical occupancy may encourage competition. Smaller and more numerous fishes, fewer piscivores, and reduced top-down control	Drought and high-water events may favor selected species and reshape local assemblages. In shallow reservoirs and in regions where drought is expected, richness and diversity may decline. Variable water levels could promote “boom and bust” fisheries
Fish invasions	Northward expansions. Winters less of a limiting factor.	Floods and droughts could accelerate rate of invasions through a basin

Table 2. Adaptation tools potentially applicable to cope with the consequences of climate-induced changes on reservoir fish habitats.

Tools	Description
Planning
Clarify goals	Goals represent the vision of reservoir habitat conditions shaped by society and agency values. Because reservoirs are artificial environments the goals for managing fish habitats may be different than those applied in natural aquatic systems. It is important to be clear about why habitat management is necessary before developing a plan
Assess vulnerabilities	Depending on catchment, area, depth, retention time, age, etc. reservoirs differ in their vulnerability to climate change. Determining which reservoirs or habitats are most vulnerable enables managers to set priorities and allocate conservation investments
Update goals	Long-term realignments can make protecting habitats and species in their original distribution increasingly difficult, and in some cases unlikely. Some goals may have to be abandoned and new goals established. Some goals may need refocusing from preserving conditions to managing for systems that differ in habitat or species composition and structure yet continue to function
Prepare for change	Strategies to achieve near-term goals need to also consider a long-term view. Thus, management needs to be vigilant for opportunities to adjust near-term goals as blurry forecasts about upcoming climatic, ecological, and societal changes come to focus
Set priorities	Reservoir ecosystems and fisheries are not equally valuable, nor are they equally vulnerable to adverse impacts from climate change. Setting management priorities can help ensure that the management investment provides the greatest possible benefit
Ensure the needs of fish are represented	Government policy can have big impacts on fish habitats. Policies about land use, water conservation, guide curves, maintenance of aging infrastructure, and transport of exotic species can have long-lasting effects over reservoir fish habitat nationwide, and need to be a focus of environmental organizations and professional societies
Prevent arrival of new invasive species	Regulate importation of species through black-listing; implement early detection and response or eradication protocols to minimize introductions and establishment of non-native species
Incorporate uncertainty	Planning for climate change includes uncertainties about the physical, geochemical, and biological properties. Uncertainty can produce lower confidence about courses of action. Models for incorporating uncertainty into management are listed
Monitoring
Document trends	Monitoring is the backbone of management. In the face of climate change, monitoring may be even more essential given the uncertainty about possible effects and actions. Regular monitoring allows detection of changes early, guide management response, and gauge the effectiveness of management activity
Early detection	Management is most effective if change is recognized early so that actions are applied at initial stages of change. Early detection allows for a more flexible and measured response based on the speed of impact development, or potential extent of impact
Hypothesis-driven monitoring	Monitoring is most effective when applied with specific hypotheses in mind and with a threshold that will trigger a policy or a management reevaluation
Increase flexibility and adaptability	Design flexibility into monitoring so that programs can be adapted to new unforeseen events and knowledge that might arise as habitat, fish, and users respond to change
Coordinate monitoring efforts	Coordination across agencies to collect and share complementary data facilitates and expands monitoring
Enlist volunteers and new technology	Fishers and citizens are often concerned about the environment and willing to participate in monitoring activities. The advent of new technologies (e.g., smartphones, text‐messages, angler apps) provide an array of opportunities for data collection
Managing
Partner with outside organizations	Climate warming can bring complex and extensive changes that management agencies cannot address. Partnerships can allow the capacity to implement activities that promote resilient reservoirs but that are beyond agency purview. Partnering with catchment, water management, water quality, or non-government organizations can provide the structure needed to plan, fund, and complete habitat work
Reduce anthropogenic stressors	Removing or reducing existing stressors and threats to habitats (e.g., sedimentation, eutrophication) and fishes (e.g., overfishing, invasive species) can enhance resilience
Protect or restore key reservoir features	Conserve or restore critical habitats such as connected backwaters, spawning areas in tributaries, spawning and nursery habitats, and nearshore zones
Preserve representative habitats	Maintain a selection of habitats that reflect diversity in the reservoir (e.g., backwaters, flooded timber, key embayments); maintain or restore connectivity among habitats
Provide refugia	Establish protected areas with diverse and superior habitat to conserve fish assemblages; increase access to and extent of thermal refugia; provide shade nearshore; maintain diversity of depths in shallow littoral zones
Manage for change	Managing for change could mean actively managing a transformation into a new state. This could involve nurturing fisheries suited to the expected future habitat conditions because existing conditions are no longer sustainable. It may also mean applying actions that provide benefits across a broad range of likely future scenarios
Adjust expectations	Maintaining resilience requires adjusting the expectations of fishers. Agencies can foster more realistic attitudes through outreach and education designed to promote participation in resource management and a broader species preference
Keep vigil on invasive species	Management of spreading invasive species that are already established remains essential. With an expected delay between the time of arrival and time of becoming invasive some species that have already been introduced may yet become invasive. Government policy (national or state) to support risk-reduction, including education, in combination with regulation and enforcement may prevent undesirable expansions