Minimizing dams impacts on biodiversity by way of translocations: the case of freshwater mussels

ABSTRACT

Dam installation causes profound changes in the structure and quality of aquatic habitats, transforming the lotic upstream stretches in lentic habitats and severely modifying hydrologic and sedimentary patterns downstream. As many impacts cannot be avoided, conservation actions are often implemented to minimize ecological losses. Translocation is a conservation tool that can be used to mitigate dams’ impacts on freshwater species, particularly those having restricted mobility. Building from the experience of translocations actions of freshwaters mussels carried out in Portugal by the authors, and the available literature, we address the challenges of planning, executing, and following up translocations of freshwater mussels. The guidelines presented increase chances of conducting successful freshwater mussels translocations and also provide guidance for others biological groups and project types.

Graphical abstract

Keywords:

• translocation
• minimization measures
• freshwater mussels
• dams

Introduction

Dams are infrastructures used by humans to address multipurpose goals such as flood control, electrical power generation, irrigation, water for human consumption, recreation, and navigation. However, they are among the most damaging human activities in river basins, deeply modifying the physiography of watersheds (Schmutz and Moog 2018). The main impacts associated with the construction of dams are related to the interruption of river continuity (longitudinal and lateral, fish migration, sediment and nutrient transport); conversion of lotic into lentic habitats; siltation of river bed and clogging of interstitial habitats; homogenization of habitats; downstream river bed incision; alteration of river/groundwater exchange; changes in downstream flow; and water quality alteration (Ligon et al. 1995; Poff et al. 1997; Fischer and Lindenmayer 2000; McLean 2003; Weeks et al. 2011; Schmutz and Moog 2018).

Habitat changes by damming usually result in homogeneous biological communities due to the spread of non-indigenous species, that are favored by the new conditions, at the expense of locally adapted native biota (Poff et al. 2007; Clavero and Hermoso 2011). Native biota that is not favored by the new conditions either move elsewhere in search of optimal conditions or suffer local population extirpation, with the latter being most obvious for restricted or less mobile species such as freshwater mussels.

Freshwater mussels are not sessile as their marine counterparts, but their mobility is highly restricted and conditioned by the existence of pathways, fine sediments and the absence of obstacles such as rocks or logs (Kappes and Haase 2012). The construction of dams could imperil mussel populations in many ways: upstream conversion into lentic habitats can lead to short-medium term extirpation of populations due to water physicochemical degradation, namely due to anoxic conditions close to the bottom; sediment retention can limit in the medium-long term favorable substrate downstream; hydrological alteration downstream can provoke short-term population shrinking due to reduced flows or continuous pressure and death due to hydropeaking; long-term viability of populations may be jeopardized by changes in fish community, with the disappearance of native fishes that serve as obligatory hosts during the larval phase of the species.

Translocation is the human-mediated movement of living organisms from one area to another (IUCN/SSc 2013). It could be used as a tool for conservation, when its objective is a conservation benefit, favoring local populations or the recipient ecosystem (IUCN/SSc 2013); or as minimization tool within the Mitigation Hierarchy framework when conducting the Environmental Impact Assessment process, after avoidance measures are excluded (BBOP & UNEP 2010; CSBI 2015). Despite some uncertainty on the efficacy of the strategy (Fischer and Lindenmayer 2000), translocation is considered a valid conservation tool (Seddon et al. 2012), that has been increasingly adopted and pointed as a solution to modern problems such as risk of extinction posed by shifts on environmental conditions associated with climate change (Chauvenet et al. 2013).

Translocation appears as an obvious tool to mitigate dams’ impacts on freshwater biota that would face the risk of disappearance in the area to be impounded or downstream of the dam. Numerous translocations experiences have been undertaken with freshwater mussels resulting in highly variable levels of success (Cope and Waller 1995; Dunn et al. 2000; Tiemann et al. 2019) associated with differences in the organisms’ biological traits, and in operation factors such as methods and timing of collection; handling, transportation; release of individuals; habitat suitability assessment and monitoring (Cope and Waller 1995; Dunn and Sietman 1997; Dunn et al. 2000; Luzier and Miller 2009; Hart et al. 2016; Tsakiris 2016; Tiemann et al. 2019).

Insufficient planning has been cited as one of the main impediments to the success of translocations (Chauvenet et al. 2013). In this communication, we aim to contribute for the planning of freshwater mussel translocations associated with the development of dams. The authors have carried out translocation of freshwater mussels related with construction of hydropower schemes in Portugal, involving altogether translocating approximately 40,000 freshwater mussels. Based on the lessons learned, together with results from other experiences available in the literature, we discuss each of which we believe being the main steps in planning and implementing translocations as a mitigation tool. In the following sections, each step (Scoping, Spatial boundaries, Collection of individuals, Processing, maintaining and transportation, Relocation of the translocation process, and Monitoring and adaptative management) is discussed using dams and freshwater mussels as a case study, although we believe that many aspects could be useful for others types of projects and aquatic biota.

Scoping

Defining the objectives of the translocation is dependent on project characteristics and the biological context. The size of the reservoir area and the level of downstream hydrological alteration combined with the local species pool and population sizes will determine the level of impact and, subsequently, the design of the translocation action. At the biological level, one must choose the target species, given that it may not make sense to include all species, and how many individuals will be translocated. Potential candidate species to be translocated include regionally or nationally threatened and protected, for which the imperiled local population is important relative to the basin or global population. The threshold to define whether the local population is important should be discussed with specialists and the regulatory bodies, with whom the translocation plan should be reviewed and agreed upon. Once the target species are decided we must set the main goal of the translocation: merely to avoid the death of the individuals; safeguard or increase the genetic adaptive potential and restore flow among populations; restore the populations by reinforcing the existing populations elsewhere; reintroduce the species in native areas where it has disappeared; or to ensure the viability of the species by introducing it in an area outside its native range. Deciding for one of these objectives will determine how many individuals will have to be collected, and the required effort and costs of the translocation. Again, this decision must be agreed with the regulatory bodies that follow the environmental assessment process and in alignment with the national and regional species conservation targets set for the target species. Setting the objective requires a broader view of the ecological context of the species, requiring information that may be outside the project’s scope and the obligations of the project developer. However, at least basin level baselines should be conducted to grow the body of existing information and aid decision-making.

In deciding the objective for the translocation, it may also be convenient to establish a potential list of destination areas that could be used to attain distinct objectives. It is highly advisable that, besides conservation authorities, other relevant stakeholders are included in the discussion of the translocation objectives. Water management authorities should be included to ensure articulation with the local river basin management plan and objectives in the destination areas under evaluation. Lastly, the project timeframe and available resources must be considered in setting the translocation objectives. Collecting and transporting organisms can be very time and resource-consuming and a realistic evaluation of the available time and resources must be done to avoid setting unrealistic objectives causing false expectations.

It is highly recommended that a Translocation Plan is devised iteratively, that can be fed along the process, ensuring, however, that when translocation is going to be implemented all the main decisions and guidelines have already been taken. The elaboration and implementation of the translocation plan must be supervised by a freshwater mussel specialist supported by other specialists (e.g. fish ecologist, hydromorphologist, hydraulic) and field technicians.

Spatial boundaries

Source site selection

Deciding where individuals are going to be collected for translocation depends on the results of the Environmental Impact Assessment and the implementation of additional mitigation measures such as environmental flows. Besides changing habitats upstream, dams also change downstream flow and sediment patterns that may threaten mussels survival. If it is considered that even with additional measures the project is likely to impact populations downstream, both upstream and downstream impacted areas will have to be considered as donor areas. Subsequently, a more detailed selection of collection areas will have to be made based on the distribution of target species, access and safety conditions of sites, and habitat characteristics, namely those that are related to dimensioning capture effort such as depth and substrate. Ideally, some of the required information to delimit the capture zones, as the species distribution, is already available from the Environmental Impact Assessment baseline. However, additional desktop effort and preliminary field surveys to evaluate local habitat and safety conditions will probably be necessary. The collected information should be used to rank sites by maximizing the combination of species abundance and easiness of collection. However, the final ranking must consider the timeframe and number of individuals to collect. For instance, it may be the case that sites harder to sample, such as deeper sites, may be sampled first to profit from low flows, or that more abundant sites are close to the destination site and could be sampled later. A unique formula does not apply to all projects and the collection planning must be devised according to the project and site specificities.

Destination site

Selection of the destination sites will have first to be considered at the macroscale during the planning phase and, at the meso and microhabitat scales when implementing the translocation plan. Selection at the macroscale is dependent upon the objective of the translocation; if the objective of the translocation is merely to rescue individuals, the destination site could be the nearest site providing suitable habitat conditions, as, for instance, immediately upstream the reservoir tail. However, if the objective is to reinforce populations, reintroduction within the native range or expand species ranges, other criteria must be considered. These include the current and historical native range; the status of existing populations of the focal species and host fish species; the distribution of species predators or competitors, particularly of alien species with whom the species has not evolved with; the current habitat ecological integrity; known water management objectives and issues; potential future threats; and if climate predictions and the expected hydrological patterns for the relocation area are compatible with the species requirements (Luzier and Miller 2009; Schwartz and Martin 2013; Hart et al. 2016).

Although the prioritization of destination sites must be project-specific, translocating species to places where healthy populations occur, especially with indications of recent recruitment, gives more confidence on the success of the translocation, as it assures the existence of suitable structural, physicochemical and biological (e.g. hosts, predators) habitat conditions. However, if the goal of the translocation is to reinforce populations, a site where healthy populations occur may not be the first choice. Choosing sites with less healthy populations or unoccupied areas is more uncertain, demands a more thorough evaluation of the local conditions, and eventually undertaking complementary actions of local habitat restoration. However, if successful, the gain of the action is greater than simply reinforcing a healthy population.

Sites within the same watershed, or at least within the native range, should be given preference as there is a greater chance that some habitat requirements will be met (e.g. water mineral characteristics, fish hosts), while minimizing the potential for detrimental effects in the recipient ecosystem. Translocated individuals may carry biological contaminants, cause erosion of local genetic pool (Hughes et al. 2003), or harm local sensitive species (Luzier and Miller 2009). Another issue to consider when choosing the destination site is if species secondary spread across the landscape from the relocation site is desired and the probability of an undesired spread (Olden et al. 2011).

Collection of individuals

Prior to start collecting animals, legal permits for capture, handling, and transportation must be obtained from the relevant regulatory body. Once the source sites are chosen, it is necessary to plan the collection of the mussels, considering the species life cycle, the project timeframe and the number of individuals to collect. The collection of individuals should be performed outside of periods of physiological stress for the mussels such as exposure to stressful environmental conditions (e.g. drought) or during or after the reproduction period, given it could decrease translocation success (Tiemann et al. 2019). Articulation with the project schedule is of major importance, ensuring that collection of individuals is undertaken prior to any intervention on the habitat. One must be aware that upstream interventions may impact downstream habitats. For instance, debris resulting from the clearing of vegetation from riparian and neighboring zones could be washed away to downstream river sections that were not intervened, sit on the riverbed and prevent or hamper the collection of individuals. Additionally, the project execution plan timeframe will dictate how the collection effort is distributed across time. To maximize the collection effort of freshwater mussels, or other aquatic biota, it should be undertaken during low flows, when water depth and the potential collection area are reduced. If the project execution period encompasses more than one low flow period, and if necessary, capture effort could be distributed among those. On the contrary, if collection period is restricted to 1 year or less, the collection may also need to be conducted during higher flows, which will require higher and specialized manpower.

The methods used to collect freshwater mussels will depend on the characteristics of the site, with requirements of technician’s specialization increasing with depth. In shallow places, capture can be undertaken by simply collecting the individuals by hand, while in deeper sites snorkeling or scuba diving are required. In these cases, search for mussels can be done visually when possible, and by touch in turbid waters and burrowed individuals. For manual and snorkeling collection, unexperienced personnel could be apt after two weeks of intensive training on capture and handling mussels, while for scuba diving experienced and certified personnel is required given the complexity of the technique and underwater riverine habitats. In deeper sites or where wildlife can pose a risk to humans, collection can also be performed by manual or mechanized dredging and suction pumps (FAO 2001). Whenever possible collection of individuals should be done using multiple-pass-depletion, quantitative, or semi-quantitative methods (Hart et al. 2016). As a secondary option, a qualitative approach could be used registering search effort, time, number of technicians, and sampling technique of each technician to obtain estimates of Capture per Unit of Effort. At least, when first collecting at each site a quantitative sampling should be performed to obtain estimates of mussel density to serve as guidance for release in the destination site.

Cumulative curves of Captures per Unit of Effort could then be used as an indication of the proportion of the population that has been collected at each site or globally. When the curve reaches a low slope, this could be interpreted as a significant part of the population having been caught.

Processing, maintaining and transportation

Collected individuals will have to be processed, maintained, and transported to the destination site. Processing concerns accurate identification, size and weight measurement, and tagging at least part of the collected individuals to later assess translocation success. Collection of biometric data will allow to characterize the population, and in the case of tagged individuals to assess survival and growth in the destination sites. Marking of translocated individuals should be done using individual numbered tags to distinguish from local individuals and assess individual growth and survival. Alphanumeric plastic or Passive Integrated Transponder (PIT) tags are reliable already tested solutions (Nobles and Zhang 2015; Tsakiris et al. 2017) that may be chosen according to the project budget, number of individuals to tag, size of focal species, sampling method, and time of processing. PIT tags require more processing time than plastic tags because most epoxies used to stick the tag to the shell need ~12 h to cure completely, which may increase individual stress, although ensures higher recovering rates relative to other tags, which may compensate the risk of prolonging processing time, and the effort to maintain suitable conditions during the process (Tiemann et al. 2019).

Emersion of individuals during processing should not exceed 60 min to avoid desiccation and death, being advisable to shorten exposure during drier and warmer weather conditions (Miller et al. 1995; Bartsch et al. 2000). While waiting for processing, individuals should be kept in water in mesh bags or perforated buckets or at least in humid conditions.

Maintenance of captured individuals until transportation is a crucial step that can vary in complexity. Main factors to consider when choosing how to maintain collected individuals are distance between collection and destination site, time that individuals must be kept, and the existence of facilities where animals could be kept. If capture and destination sites are close and individuals can be readily transported, this should be the preferred approach. However, when this is not the case, it is likely that a minimum number of collected individuals must be achieved to justify a trip to the destination site and individuals must be maintained in suitable conditions until then.

A convenient option is to maintain individuals in the site of capture within keepnets, mesh bags, or equivalent structures, at reasonable densities avoiding overcrowd (Dunn and Sietman 1997). However, one must evaluate the risk of theft, predation, drift by rises in flow level, or of other events such as localized pollution.

Another option is to maintain individuals in existing or specifically prepared ex-situ facilities, like aquaculture facilities. In any case, it is advised that these would be open systems receiving quality water from rivers, ensuring that food will be available for individuals. If the facility is not in the same river where mussels where collected, one must assure that outgoing water is properly treated if it is going back to the natural system. If a direct supply of river water is not available, appropriated water must be provided, by transporting water from the natural system using, for instance, tank trucks. Using ex-situ facilities may be the only solution if individuals must be subjected to quarantine to prevent biological contamination of destination sites (Cope et al. 2003a). Even if a potential source of contamination is not known quarantine may be advisable. If quarantine is conducted, one must ensure that the water in which the animals are maintained is clear from contamination, and that used water is disposed with no risk of contaminating other systems. While in quarantine, freshwater mussels may be scrubbed with the aid of chlorinated water or other appropriated chemical decontaminant (Waller and Fisher 2004) to eliminate biological contaminants (e.g. zebra mussel) or be submitted to other depuration according to the suspected contaminant (Lee et al. 2008).

The main concerns regarding transportation of freshwater mussels are to avoid desiccation and stress. Low temperatures and humid environment must be maintained during all the transport period (Yusufzai et al. 2010; Barrento et al. 2013; Tsakiris et al. 2017). Transportation can be made with individuals immersed, wrapped in humid paper or tissue in boxes with ice, preventing direct contact between ice and shells.

Relocation

Prior to transporting captured mussels, the exact release sites must be determined. Suitable sites must be selected within the selected basin, river, or stretch based on meso and microhabitat characteristics, distribution of fish host species and, when present, of local mussels. Meso and microhabitat selection must be done undertaking surveys of the destination area. It is highly unlikely that previous studies occur that could give information with the level of detail required, and a directed study should be undertaken for that purpose. Habitat selection should focus on identifying habitats with suitable substrate and flow for the target species, and that show signs of long-term stability as changes in habitat conditions are associated with low recovery rates (Dunn et al. 2000). Also, selection should prioritize sites with higher ecological integrity within minimal anthropogenic perturbations (Hamilton et al. 1997). Basin management plans, and other relevant sources should be consulted to identify potential factors of disturbance such as sources of pollution, weirs and dams, sediment extraction, etc. Another issue to consider is riverine connectivity. When the objective is to maximize connectivity among translocated and existing local populations, to facilitate gene flow, one must avoid choosing sites that are isolated by weirs or dams or natural barriers; on the contrary, if the objective is to avoid secondary dispersal, isolated sites should be used (Olden et al. 2011; Tsakiris 2016).

Distribution of fish and local mussel populations can be assessed at larger scale, even using previous information provided that assessed sites cover all the potential relocation area and that information is considered up to date. If this is not the case, specific surveys should be conducted prior to relocation. Additionally, while characterizing habitat, mussel quantitative surveys must be performed to obtain density estimates to compare release sites and guide the release of mussels.

Optimum sites for relocation should maximize the presence of suitable habitat conditions, and presence or abundance of host fish populations. In situations where natural populations exist, density of local mussel populations should also be considered by favoring sites with mussels but being careful not to overcrowd sites, respecting natural densities found at source or destination sites.

Sites that will be monitored to assess success of translocation should already be selected at the time of release, if monitoring all the destination area is not the option. Tagged individuals should only be released in sites selected for future monitoring, being advisable that an important fraction of released individuals is tagged to increase recapture probabilities in each site. Release of individuals should be accomplished by embedding them into the substrate with the anterior end down. In large areas of sandy substrates with reduced flows, this rule could be relaxed, and individuals released onto the substrate and let to burrow for themselves. However, in substrate mosaics or coarser substrates, individual borrowing should be strictly followed. If individuals are transported with the aid of ice, they should be left to or be acclimated gradually to local temperature previously to being released.

Monitoring and adaptative management

A Biodiversity Monitoring and Evaluation Plan should be elaborated and revised as necessary along the project. The plan should indicate: the roles and responsibilities in the execution of the plan; identify and map the monitoring stations; list the methods and indicators and the thresholds that will trigger adaptative responses; and the actions to develop when those are triggered. The items in the BMEP must be elaborated according to project specifications but general guidance on some of these aspects is provided below.

Monitoring sites should be chosen in order to cover habitat diversity of the release sites. This will allow to estimate average success of translocation across all the destination sites and to relocate mussels if unsatisfactory results are found in any place.

Monitoring of the success of the translocation in destination sites is done by monitoring relocated individuals to assess survival, growth, and recruitment. Monitoring should be performed in each of the selected sites using a quantitative approach, registering area or length of the river, and search effort (time and number of observers). The capture method should be adjusted to local habitat characteristics as mentioned for collection of individuals in source sites. In addition to release sites, neighboring areas could also be sampled through a qualitative approach to assess dispersal and colonization of other river sections (Strayer and Smith 2003; Hart et al. 2016).

Captured live individuals should be measured, weighted, and their tag number registered, while for dead animals only shell length should be measured and tag number registered.

Monitoring should begin within 2 months (Tiemann et al. 2019) after translocation to assess short-time survival. Medium-term success should be assessed by monitoring in the two consecutive years following translocation and evaluating survival and growth, while long-term success should be performed by spaced monitoring (each 2–3 years after the second annual monitoring) by evaluating survival, growth, and recruitment. The extension of the monitoring period would be defined by the output of the monitoring and species ecology, with species with extended life cycles and low growth rates demanding extended periods. However, if satisfactory results are obtained, low mortalities and signs of recruitment and no adaptive measure is needed within 10 years after translocation, monitoring could be ended.

Survival should be used as a short-medium time indicator of success (Rout et al. 2009); if low survival is observed this should be interpreted as a unsuccessful result and trigger a response that would implicate relocating individuals within the destination site or to another destination site. For this purpose, the rank of destinations (macroscale) and release (mesoscale) sites should be maintained and the information that based the ranking updated when necessary.

Thresholds for triggering an adaptive response based on survival are probably not available on the literature and will need to be obtained. Two approaches could be used, one is to conduct an in-situ experimental design using local and translocated individuals to assess short and medium-term survival and growth. This could be accomplished by maintaining individuals in holding structures (Cope et al. 2003b; Newton et al. 2006) in chosen sites that were ranked high in habitat stability. Another option is to mark resident individuals and assess their performance while monitoring the translocated individuals. This would require an additional effort to capture and mark individuals simultaneously to the translocation process. If the destination site does not have local populations the experiment or marking of local individuals can be accomplished in a neighboring population. Mortality rates of local individuals can then be used to assess mortality of translocated individuals.

Long-term success of populations can be evaluated through recruitment. In sites where local populations occur it would not be possible to evaluate the impact of translocated individuals on recruitment unless a genetic study is conducted. However, detecting recruitment and satisfactory survival rates can be interpreted as sign of success.

Conclusions

Although it is still a field of research, translocations are a valuable mitigation tool to minimize impacts of development projects. Maximizing the probability of translocation success starts with the design of an iterative translocation plan based on the best-collected information for the systems and species to be targeted. Experiences already carried out so far indicate that successful mussel translocations can be achieved provided that critical steps are planned and implemented correctly. Project developers and managers carrying translocations should create conditions ensuring translocation is planned and integrated since projects early phase and guarantee that human resources involved gather the necessary expertise. The recommendations stated herein approach issues that are common for other species and freshwater systems projects and therefore could serve as a basis to assist in translocations with other biological and technical scopes.