Yellow lampmussel (Lampsilis cariosa) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Since the discovery of the population of L. cariosa in Sydney River, Nova Scotia (Clarke and Meachem Rick, 1963), staff of the Nova Scotia Museum have made several visits to the area to check the occurrence and obtain specimens for their collection. Sites commonly visited are shown on the map, Figure 4. In 1977, 1978, 1987 and 1990 L. cariosa was found at Sites 1, 2 and 3; in 1999 it was also found at Sites 5 and 8. In 1999, S. Kavannagh (pers. comm.) reported L. cariosa throughout the system from the shore near the dam to Blacketts Lake. In 1999 the species appeared to be absent from the mud bottom and boggy shoreline at the southwestern end of Blacketts Lake and from the lower part of Meadows Brook (below Station 6 in Figure 4). Records of freshwater mussels made by S. Kavannagh will be published at sometime in the future with other records made in 1999 and 2000 by Nova Scotia Department of Natural Resources staff. In further surveys carried out by the Nova Scotia Department of Natural Resources staff in 1999 and 2000 studies throughout Cape Breton Island and northern Nova Scotia no additional sites for L. cariosa were found. A 2001 survey of the Fortress of Louisburg National Historic site and greater ecosystem and a 2002 survey of the Cape Breton Highlands National Park found no L. cariosa (Power and Gouthro 2002; Lambert et al. 2003).

A study aimed at providing quantitative estimates of the size of the Sydney River L. cariosa population was carried out by K. White in 2001 and 2002. This study focused on areas previously found to contain L. cariosa: the Sydney River above the dam including, Blacketts and Gillis Lake (Figure 4). All sampling for this study took place between July 1st and August 25th. Sampling methodology consisted of the collection of mussels from 310 1- quadrats randomly sampled from around the perimeter of Blacketts and Gillis Lake and 53 1-m² quadrats sampled along the shore of the Sydney River from the causeway at Blacketts Lake road to the dam. The majority of sampling (333 quadrats) was conducted through a glass-bottomed bucket, in wadeable water (< 1.5 m). Observation was visual or tactile depending on turbidity. As part of the search, fine sediment was brushed away, non-embedded material was lifted and loose sediment was raked with fingertips. A subset of thirty of these quadrats was double sampled with excavation (Smith et al. 2000). Excavation consisted of the removal of substrate to a depth of approximately 10 cm and sifting substrate through a mesh screen with openings of 6.4 mm. Additional sampling (30 quadrats) was also performed at depths ranging from 2-6 meters via SCUBA to determine if the estimates made at wadeable depths are representative of mussel density in deeper water. A total of 393 quadrats were sampled.

The majority of L. cariosa (with the exception of gravid females) observed in the Sydney River were completely buried in the sediments; however, their siphon opening was visible at the interface of sediment and water upon close inspection. Through double sampling, it was estimated that on average 87.5 % (±31.0) of L. cariosa were visible through surface counts. Based on this finding and the finding that L. cariosa density at water depths of 0.5 m and 2.0 m was representative of densities at greater depths (see habitat section for details) it was decided that density estimates used to estimate total population size for the Sydney River would be made from surface counts of 363 randomly placed quadrats along wadeable (< 1.5 m) shoreline.

The density of L. cariosa in Blacketts Lake was 0.8(±1.7 SD) individuals per . At Gillis Lake the density was 0.8 (±1.1 SD) individuals per m², and the lower Sydney River density was estimated at 0.4 individuals per m² (±1.2 SD). Translating these density estimates into total population abundance estimates for the Blacketts Lake and Gillis Lake requires an assumption about the area of the usable habitat within our sampling area. Based on the examination of density in relation to water depth we assume all area within depths of 0.5 and 5.0 meters of water represent usable habitat for L. cariosa within the Sydney River. Arc view software was used to estimate this area from GIS and bathymetric maps. Blacketts Lake, Gillis Lake and the Lower Sydney River were estimated to contain 0.564 km², 0.145 km² and, 1.81 km² of usable habitat respectively. Multiplying these numbers by the density estimates we come up with a total abundance estimate of 434,401 (±953,425 SD) for Blacketts Lake, 108,636 (±165,128 SD) for Gillis Lake and 723,600 (±2,225,070 SD) for the lower section of Sydney River to the dam. Total population size estimate for the Sydney River population is 1,266,637 (±3,343,623 SD). Obviously, the high error associated with these estimates renders them somewhat meaningless. The high degree of spatial aggregation seen in the Sydney River L. cariosa population is similar to that seen in other populations of unionids and it results in a high variance to mean ratio which in turn results in highly imprecise density estimates and difficulties in examining population dynamics (Kat 1983). In 2002 in an attempt to provide more precise estimates of minimum population size for the Sydney River, K. White carried out stratified sampling within two areas of high L. cariosa density within Blacketts Lake (marked sites 9 and 10 on Figure 4). Density at these sites was 3.5 individuals per m² (±2.4 SD) and 3.9 individuals per m² (±2.7 SD). The area of these ‘clumps’ was estimated at 12,500 m² and 23,200 m² with estimated population sizes of 39,875 (±25,375 SD) and 81,896 (±56,144 SD) respectively.

Although the Yellow Lampmussel continues to have wide distribution in the lower Saint John River system, there is some evidence that the species range in New Brunswick has been reduced over the past century (Sabine et al. In press). The Yellow Lampmussel may once have occurred in the now-dammed portions of the Saint John River above head-of-tide. The species currently exists immediately below the Mactaquac dam, and is found above head-of-tide on the Canaan River, occasionally on cobble substrates that were typical of the upper Saint John River prior to dam construction. Excellent Yellow Lampmussel habitat appears to be abundant in the shallow waters of the Mactaquac headpond (Sabine et al. In press). Although it remains possible that the Yellow Lampmussel still exists upstream from the Mactaquac headpond, the disappearance of L. cariosa from such areas above the Mactaquac dam would represent a reduction in the New Brunswick range and population. Additionally, two Lampsilis cariosa valves now in the New Brunswick Museum mollusc collection were taken at Darlings Lake on the Kennebecasis River, probably between 1895-1900. Mussel surveys in 2001 and 2002 failed to record L. cariosa from the Kennebecasis River. Likewise, species specific searches of several km of Darlings Lake shoreline in 2002 were also unsuccessful. Sabine et al. (In press) note that although the portion of the Saint John River between the lower limits of the current range of L. cariosa and the mouth of the Kennebecasis River appears to be too brackish to support the species; the disappearance of the Yellow Lampmussel from the Kennebecasis River itself would represent a considerable reduction in the range of the species in New Brunswick, and hence in the extent of occurrence and the area of occupancy.

Until Davis (1999) investigated the status of several species of freshwater mussels in New Brunswick and Nova Scotia in 1999, the Yellow Lampmussel was completely ignored in New Brunswick, save the brief comments on Canadian distribution provided by Clarke (1981) made on the basis of 2 worn valves collected by A.M. Rick near Fredericton in 1962 (Clayden et al. 1982). These specimens could not be located in the CMN collection for re-examination in October 2000. Davis (1999) was unable to locate L. cariosa in New Brunswick during his searches, and given the long interval since previous collections, as well as subsequent changes to the river with the operation of the 670 megawat Mactaquac hydroelectric dam starting in 1968, concluded that the Yellow Lampmussel was likely extirpated in New Brunswick. Considering that Rick’s 1962 collection site is only about 2 km downstream from the dam, this was perhaps not an unreasonable conclusion.

Nonetheless, surveys in New Brunswick in 2001 and 2002 indicate that a significant Yellow Lampmussel population continues to exist in the lower Saint John River and tributaries. While surveys to date have been directed largely at confirming the presence of L. cariosa in New Brunswick and delineating distribution in the Saint John system, it is clear that the lower Saint John and tributaries harbor most of the Canadian population (Sabine et al. In press). For example, Sabine et al. (In press) report that one of 30 sites on the Saint John system where L. cariosa occur consists of approximately 2.4 km² of ideal habitat. Should densities of L. cariosa at this site approach those of the neighbouring Nova Scotia (0.4 - 0.7/m²) or Maine populations (0.4 –2.8/m²; Wick and Huryn 2002) numbers at this site alone would approach 2 million individuals. Even assuming densities an order of magnitude lower than these would indicate a total population in the lower Saint John River system well into the millions. Preliminary density estimates collected for the Canaan River in 2002 based on the excavation of over 500 0.25 m² quadrats, suggest that densities in the Canaan are in the same range as those recorded in the Sydney River, Nova Scotia and in Maine (Sabine et al. In press). 

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