Research Article |
Corresponding author: Joy Wade ( joywadefundyaqua@gmail.com ) Academic editor: Baruch Rinkevich
© 2020 Joy Wade, Craig Stephen, Colin Robertson.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Wade J, Stephen C, Robertson C (2020) Mapping Rocky Mountain ridged mussel beds with preliminary identification of overlapping Eurasian watermilfoil within the Canadian range. Nature Conservation 42: 19-31. https://doi.org/10.3897/natureconservation.42.51081
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The Rocky Mountain ridged mussel (Gonidea angulata) is a bivalve species whose Canadian range is limited to the Okanagan Valley, British Columbia. In 2019, conflicts between habitat protection for the mussel and potential habitat alteration to control the invasive Eurasian watermilfoil (Myriophyllum spicatum) (milfoil), led to a decision to maintain the status of the mussels as Special Concern under Canada’s Species at Risk Act (SARA) rather than classify it as Endangered. Milfoil control can cause direct mortality and/or burial of the mussels, but there had been no systematic study of the impacts of milfoil control on mussel beds. The purpose of this study was to address knowledge gaps by delineating known mussel beds and potential overlap with milfoil to provide information for management decisions that balance the needs of native species protection and invasive species control. Rocky Mountain ridged mussels in three reference locations were enumerated using snorkel surveys. The presence and distribution of milfoil was documented in relation to five sites within these three locations. Milfoil was encroaching on one site, causing some changes to the substrate. At other sites, the differences in the depth and distribution of the mussel and the milfoil could allow milfoil control without damaging the mussel beds. It is recommended that, before milfoil removal near known mussel beds be undertaken, a detailed site evaluation be conducted to determine potential impacts. This study suggests presumed impediments to co-managing the mussels and controlling an invasive species should not preclude classifying the mussels as Endangered and affording protections under SARA.
Eurasian watermilfoil, western ridged mussel, conservation management, habitat, invasive species, Okanagan Valley
Freshwater mussels (unionids) are one of the most endangered groups of animals in North America (
The Rocky Mountain ridged mussel has specific habitat preferences. It can be found in both lotic and lentic streams, rivers and lakes (
In 2003, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assessed this species as Special Concern largely due to its limited distribution (
Eurasian watermilfoil can inhibit water flow (
A 2019 decision by the Canadian Federal Fisheries Minister maintained the species as Special Concern rather than reclassify it as Endangered under the Species at Risk Act (SARA). The potential socio-economic impacts of limiting Eurasian watermilfoil control, which could occur if classified as Endangered under the SARA, was cited as one of the reasons for this decision (http://gazette.gc.ca/rp-pr/p2/2019/2019-08-21/html/sor-dors287-eng.html). The article also stated that additional science has become available since the COSEWIC assessment in 2010, which would challenge a classification of Endangered. The Okanagan Basin Water Board felt there was not enough evidence that the mussel could be harmed by rototilling and noted that studies on the socio-economic impacts of terminating Eurasian watermilfoil control had not been completed (
Okanagan Lake provides many benefits to local tourism and the real estate market. The goal of the Okanagan Eurasian watermilfoil control programme is to minimise environmental impacts while enhancing public enjoyment of Okanagan lakes with a cost-effective programme. No scientific evidence could be found which documented the outcomes of this programme. There has been no systematic study of the impacts of Eurasian watermilfoil control on mussel beds or on the overlap of critical mussel habitat and Eurasian watermilfoil distribution.
The purpose of this study was twofold. First, to map the extent of Rocky Mountain ridged mussel beds and calculate the mussel bed density at three reference locations in Okanagan Lake, British Columbia in 2019. Second, to relate mussel bed locations to a qualitative description of the presence/absence of invasive Eurasian watermilfoil in and around these reference locations.
Rocky Mountain ridged mussel surveys have been conducted annually by Fisheries and Oceans Canada (DFO) since 2011. DFO selected three reference locations for ongoing population monitoring and included six index sites in Okanagan Lake within the locations. We used five of these index sites in this survey namely Dog Beach 1 and 2, Kinsmen Beach 1 and 2 and 3 Mile (Figure
Location of study area and reference sites (Peach Orchard Dog Park (Dog Beach) and Kinsmen Beach in Summerland and 3 Mile near Penticton) used for Rocky Mountain ridged mussel (Gonidea angulata) surveys in Okanagan Lake, British Columbia in August 2019.
Geolocation of five index sites at three reference sites used to map and enumerate Rocky Mountain ridged mussels (Gonidea angulata) in Okanagan Lake, British Columbia in August 2019.
Reference Site | Index sites | Latitude / Longitude |
---|---|---|
Peach Orchard Dog Park, Summerland | Dog Beach 1 | 49.606999; -119.64972 |
Dog Beach 2 | 49.607729; -119.65067 | |
Kinsmen Beach, Summerland | Kinsmen Beach 1 | 49.598930; -119.65078 |
Kinsmen Beach 2 | 49.59941; -119.65096 | |
3 Mile Park, Penticton | 3 Mile | 49.538110; -119.57644 |
The enumeration methods used in this study are the same as those used since 2017 in the DFO surveys. DFO’s methods have not yet been published, but one author (JW) has participated in these surveys for four years and replicated DFO’s methods for this study. A surveyor’s tape was placed as the baseline at or near the shore of each site, parallel to the shoreline. A leadline transect marker was pulled from the shore to depth, perpendicular to the baseline every 3 m beginning at the 0 m mark. The length of each transect varied depending on water depth and habitat suitability. Transects ended when the substrate became unsuitable for Rocky Mountain ridged mussels (e.g. excess mud) or exceeded 1.5 m depth. To confirm that transects were long enough to span the entire bed, each transect was snorkelled at least 3 m past the end of the transect to ensure no mussels were present. Transects were snorkelled from the maximum depth to as close to the baseline as possible remaining covered in water. Each transect was snorkelled holding a metre stick beside the transect line to delineate the counting area. Live mussels were counted along the transect to a width of 1 m on the right side of the transect line.
The Rocky Mountain ridged mussel burrows in the sediment with its siphon protruding. Only mussels which protruded from the substrate were counted; the substrate was not disturbed to look for mussels. No mussels were handled during any survey activities. Silt or vegetation was gently brushed aside to aid in counting. As a result of these non-invasive methods, very small mussels or those covered in heavy silt, mud, sand or gravel may not have been counted and, therefore, the abundance values are likely to be an underestimate.
Rocky Mountain ridged mussels
For each transect, the first and last mussels counted were marked with a numbered float tied to a weight. After each index site was enumerated, the location of each marker was georeferenced using a Trimble GeoXH GPS receiver (+/- 30 cm positional accuracy) and water depth was recorded to the nearest cm. By marking the first and last mussel on the transect, an accurate delineation of the mussel bed could be mapped.
Eurasian watermilfoil (Myriophyllum spicatum)
Visual examination for the presence or absence of Eurasian watermilfoil was undertaken by snorkel surveys at all five index sites. When beds of Eurasian watermilfoil were found, the perimeter was georeferenced using the GPS to delineate the overlap with the mussel bed. When individual plants or small clusters were found, a presence/absence was recorded without quantification. Where it was not possible to map the entire extent of the plant bed because of water depth and safety concerns due to risk of entangled in Eurasian watermilfoil stems, only the perimeter closest to the mussel bed was mapped.
To determine the total mussel bed area, positions of the start and end points of each baseline were determined from the GPS data. With the aid of high resolution satellite imagery, the baseline along the shore was digitised and transect starts were inferred from the length and interval spacing of 1 m. Individual transects were derived from the starting point on the baseline, the GPS-derived positions of the floats and the recorded transect length. Transect end points were then used to digitise the outer boundary of the mussel bed. Bed area was calculated from the area of the polygon formed from digitising the position of the first float on each transect, the outer boundary and the first and last transects (buffered on one side).
Mussel bed density was calculated using the R package “survey”, version 3.36 (
Visibility was excellent at all index sites with the exception of the first three transects (0, 3, 6 m at the baseline) of Kinsmen Beach 1. The substrate at these three transects was too muddy and had too much Eurasian watermilfoil to accurately count the mussels. It is normally possible in areas of muddy substrate to wait for the sediment to settle before snorkelling the transect. However, at Kinsmen Beach 1, the Eurasian watermilfoil was too dense to allow visualisation of the substrate in these three transects. Half of the Kinsmen Beach 1 site has a muddy substrate, but mussel visualisation was possible after disturbed mud settled. Rocky Mountain ridged mussel bed areas and densities are presented in Table
Delineated Rocky Mountain ridged mussel beds at index sites: a) Dog Beach 1 (southern bed) and 2 (northern bed) b) Kinsmen Beach 1 (southern bed) and 2 (northern bed) c) 3 Mile.
Results of snorkel surveys to map and enumerate Rocky Mountain ridged mussels (Gonidea angulata) at five index sites in Okanagan Lake, British Columbia in August 2019 (s.e.= standard error).
Index site | # transects surveyed | Range of number of mussels/ transect | Depth (cm) of mussel found | Bed area (m2) | Bed density (mussels/ m2) | Population Estimate (s.e.) | |
---|---|---|---|---|---|---|---|
Nearest to shore | Furthest from shore | ||||||
Dog Beach 1 | 21 | 1–12 | 53.3 | 175.3 | 1,766.78 | 0.22 | 383.00 (44.35) |
Dog Beach 2 | 18 | 0–8 | 55.9 | 127.0 | 971.44 | 0.13 | 128.88 (25.43) |
Kinsmen Beach 1 | 16 | 0–37 | 38.1 | 134.6 | 738.84 | 0.86 | 637.90 (134.08) |
Kinsmen Beach 2 | 12 | 0–27 | 40.6 | 124.5 | 619.92 | 0.79 | 490.80 (107.41) |
3 Mile | 25 | 2–35 | 43.2 | 175.3 | 1,355.15 | 1.23 | 1,668.28 (261.65) |
Mussels were found in water ranging in depth from 38.1 cm to 175.3 cm (Table
Eurasian watermilfoil could be seen within the boundaries of the mussel beds at all three reference sites and four of the five index sites (Table
Location of Rocky Mountain ridged mussel (Gonidea angulata) beds surveyed at Kinsmen Beach 1 (southern bed) and 2 (northern bed) index sites with encroaching Eurasian watermilfoil (hatched green/blue) in Okanagan Lake, British Columbia in August 2019.
Qualitative description of the co-occurrence of Eurasian watermilfoil (Myriophyllum spicatum) in Rocky Mountain ridged mussel (Gonidea angulata) index sites in Okanagan Lake, British Columbia in August 2019.
Index site | Description |
---|---|
Dog Beach 1 | Individual plants found sporadically in the bed. |
Dog Beach 2 | No plants present. |
Kinsmen Beach 1 | Individual plants found throughout the mussel bed. Extensive dense plant bed overlapping a portion of the mussel bed. |
Kinsmen Beach 2 | Individual plants found sporadically in the mussel bed. |
3 Mile | Extensive, dense plant bed within 2 m of the mussel bed |
The proposed management regime in Okanagan Lake would dictate a 100-metre buffer zone between the Rocky Mountain ridged mussel beds and Eurasian watermilfoil control activities (
Rototilling can remove the root of the plant in water up to 4.5 m deep, whereas cutting can remove it in water as shallow as 2 m. The surveyed mussel beds were in water as shallow as 38.1 cm and as deep as 175.3 cm. The less invasive cutting method would be suitable throughout the depth of these beds.
Our findings demonstrate that the five index sites varied in mussel bed density (0.13–1.23 mussels/m2) and corresponding population estimates (128.88 to 1,668.28), as did the extent of incursion of milfoil into the beds. These findings support recommendations for site-specific management actions, as opposed to generic, Lake-wide recommendations to balance the need for invasive species control and endangered species management.
For example, removal of the invasive plant at 3 Mile could occur using rototilling without disturbing the existing mussel bed. If plants are not removed from the Kinsmen Beach 1 index site, they will likely continue to take over the mussel bed. We subjectively noted an increase in the extent of mud and siltation at this site compared to surveys in previous years. Cutting of plants overlapping the mussel bed would be recommended for this area. The rest of the plant bed, further away from the mussel bed, could be removed by rototilling without impacting the mussel bed. This recommendation is for this site only and should not be applied to other locations without investigation.
Managing one ecosystem risk, such as an invasive species, can have unintended negative impacts on other goals, such as endemic species conservation. Ecosystem managers have three options when faced with conflicting invasive species and conservation goals; (1) manage the impacts of the invader and accept the collateral damage; (2) abandon management of the invader and accept its impacts or (3) seek a compromise strategy that allows both goals to be attained (
There is community, regulatory and scientific information to support the belief that the presence of Eurasian watermilfoil in Okanagan Lake is deleterious to the health of the Rocky Mountain ridged mussel, as well as to the economic and recreational use of the Lake by people.
We suggest that Eurasian watermilfoil control is one strategy to protect critical resources needed for Rocky Mountain ridged mussel resilience and, therefore, Eurasian watermilfoil control is a shared conservation and invasive species goal. Expanding this work to other areas can further delineate the nature of the overlaps of mussels and Eurasian watermilfoil. Such site-specific information may allow for an integrated strategy to address conservation goals without compromising invasive species management goals by setting evidence-based buffer zones and/or tailoring Eurasian watermilfoil control activities to the nature of site-specific mussel-watermilfoil overlaps.
This work was partially funded by the Species at Risk Program of Fisheries and Oceans Canada, Science, Pacific Region.