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Recovery Strategy for the Hickorynut and the Kidneyshell [Final Version]

Threats

The Round Hickorynut and the Kidneyshell, like most mussel species, are sensitive to a wide variety of stressors including exotic species, poor water quality resulting from point (industrial and residential discharge) and non-point (herbicide, pesticide and surface run-off) sources, loss of host fish species, impoundments, siltation/sedimentation, predation and urbanization.  The following discussion of threats focuses on those threats which are specific to the two remaining populations of the Round Hickorynut (St. Clair delta, Sydenham River) and three remaining populations of the Kidneyshell (St. Clair delta, Sydenham River, Ausable River) although it is likely that all of the stressors listed previously have contributed to the decline of these species in Canada.

 Threats to Extant Populations  St. Clair Delta Populations: The introduction and spread of the exotic zebra and quagga mussels throughout the Great Lakes basin has resulted in steep declines of native mussel species (Schloesser et al. 1996).  These invasive mussels are known to attach to the shells of unionids and can cause death by interfering with feeding, respiration, excretion and locomotion (Haag et al. 1993; Baker and Hornbach 1997). COSEWIC (2003b) reported that 64% of the Canadian sites where the Round Hickorynut was historically found are now infested with zebra mussels rendering much of the habitat unsuitable for unionids. The St. Clair delta population occurs in waters inhabited by zebra mussels and Round Hickorynuts were found in areas with relatively high zebra mussel infestation rates (D. McGoldrick, National Water Research Institute, Environment Canada, pers. comm., October 2003). It is not known why the mussels of the St. Clair delta have survived when other areas in Lake St. Clair have been devastated by the zebra mussel invasion (Nalepa et al. 1996) nor is it known if this population will persist (Zanatta et al. 2002). The St. Clair delta Round Hickorynut and Kidneyshell populations are very small with only 9 Round Hickorynuts and 1 Kidneyshell detected during sampling of nearly 15,000 m2 in 2003 (Metcalfe-Smith et al. 2004)).  These populations are dominated by relatively large, older individuals indicating poor reproductive success with the possibility of frequent year-class failure (COSEWIC 2003b).

Sydenham and Ausable River Populations

Water Quality: The Sydenham River flows through an area of prime agricultural land in southwestern Ontario and over 85% of the land in the watershed is in agricultural use with 60% of land in tile drainage (Staton et al. 2003). Large areas of the river have little to no riparian vegetation as only 12% of the original forest cover remains. Strayer and Fetterman (1999) identified high sediment and nutrient loads and toxic chemicals from non-point sources, especially agricultural activities, as the primary threat to riverine mussels.  Agricultural lands, particularly those with little riparian vegetation and large amounts of tile drain, allow large inputs of sediments to the watercourse. In the case of tile drained land, the sediment input is often of a very fine grain which can clog the gill structures of mussels resulting in decreased feeding and respiration rates and reductions in growth efficiency. The Sydenham River has historically shown high nutrient levels with total phosphorus levels consistently exceeding provincial water quality levels over the last 30 years while chloride levels have shown recent inclines due to an increased use of road salt (Staton et al. 2003).

            Agriculture is also the dominant land-use within the Ausable River basin with over 80% of the land in agricultural use and 71% of the land area in tile drainage (Nelson et al. 2003).  Suspended sediment levels are high throughout the river with levels in the lower main channel consistently exceeding those required to maintain good fisheries (Nelson et al. 2003). Nutrient levels (N, P, un-ionized ammonia) regularly exceed provincial Water Quality Objectives for the protection of wildlife and Canadian Council of Ministers of Environment guidelines. Recent evidence has shown that juvenile mussels are among the most sensitive aquatic organisms to ammonia toxicity (Mummert et al. 2003; Newton 2003; Newton et al. 2003).

            Dissolved oxygen (DO) levels in the East Sydenham River typically average about 10 mg/L however levels at all four Provincial Water Quality Monitoring Stations in this basin have dropped as low as 5 mg/L during the last 35 years (Jacques Whitford Environment Ltd. 2001). Over the same time period, DO levels in the Ausable River have on occasion fallen to comparable levels (2-3 mg/L) (Nelson et al. 2003). Johnson et al. (2001) have found mussel survival rates are closely related to DO levels while Tetzloff (2001) reported massive mussel die-offs in Big Darby Creek, Ohio, following a low oxygen event resulting from a chemical spill. Kidneyshells were one of the most sensitive species to these conditions with greater than 95% mortality, much of it coming rapidly after the onset of low oxygen conditions. Three years after the low O2 event many of the affected species have still not recovered to pre-event levels (pers. comm., J. Tetzloff, Darby Creek Association Inc., March 2004).  

            Water Quantity: Hydrologic regimes can affect mussels in a number of ways. High flow conditions can cause dislodgement and passive transport of mussels from areas of suitable habitat into areas of lesser or marginal habitat. Neither the Round Hickorynut nor the Kidneyshell show the typical shell adaptations associated with resistance to scour and shear stress associated with hydrologically flashy rivers (pustules, ridges, fluting) (Watters 1994). In contrast to the dislodgement associated with high flows, low flows can result in depressed dissolved oxygen levels, desiccation, and elevated temperatures. In a study of drought conditions in relation to mussel survival, Johnson et al. (2001) identified the need for instream flow protection as a critical issue for mussel conservation and protection in the southwestern U.S. Low flow events in the Ausable River often result in the stranding of mussels.

            Host Fish: The Round Hickorynut is an obligate parasite unable to complete its early life stages without a suitable host. The host species for the Round Hickorynut has not yet been confirmed in Canada although evidence indicates the greenside darter (see Reproductive Attributes section) likely functions as a Canadian host.  Clark (1977) also noticed an association between the Round Hickorynut and the eastern sand darter (Ammocrypta pellucida) suggesting a possible host relationship although this species has not been formally tested (M. McGregor, Kentucky Department of Fish and Wildlife Resources, pers. comm., January 2004).  The greenside darter is considered a species of special concern in Canada where it is found in both the Sydenham River and Lake St. Clair although it is believed to be relatively abundant and stable in the Sydenham River (Dextrase et al. 2003).The eastern sand darter is listed as a threatened species in Canada but can be found in the East Sydenham River in areas where the Round Hickorynut persists. Siltation resulting from agricultural activities has been cited as one of the main reasons for the decline of the eastern sand darter (Holm and Mandrak 1996).

Three species have been identified as hosts for the Kidneyshell: blackside darter; fantail darter; johnny darter (McNichols and Mackie 2004). Recent surveys have shown that johnny darters and blackside darters are abundant throughout the Ausable (Nelson et al. 2003) and Sydenham rivers (N. Mandrak, Department of Fisheries and Oceans, Burlington, pers. comm., March 2004) while fantail darters are neither abundant nor widespread in either system. If johnny darters or blackside darters are acting as a host for wild populations in the Ausable or Sydenham rivers then host limitation should not be a primary cause of the observed declines. Only a heavy reliance on the fantail darter as the host would appear to place these species in danger of being host-limited.

Any threats that affect the host species’ abundance, movements, or behaviour during the period of glochidial release must be considered as threats to these mussels as well.  For example,the invasive round goby has been implicated in the following declines of native benthic fish species in the lower Great Lakes: 1) logperch (Percina caprodes) and mottled sculpin (Cottus bairdi) populations in the St. Clair River (French and Jude 2001); 2) johnny darter (Etheostoma nigrum), logperch, and trout-perch (Percopsis omiscomaycus) in Lake St. Clair (Thomas and Haas 2004); and, 3) channel darter (P. copelandi), fantail darter (E. flabellare), greenside darter (E. blenniodes), johnny darter, and logperch in the Bass Islands, western Lake Erie (Baker 2005). Index trawling data from 1987 to 2004 (unpublished data, Lake Erie Fisheries Assessment Unit MNR) indicate that similar declines have occurred in the Inner Bay of Long Point Bay and the western basin of Lake Erie.  Potential causes include goby predation on eggs and juveniles, competition for food and habitat, and interference competition for nests (French and Jude 2001, Janssen and Jude 2001).  The round goby poses a real threat to host fish populations and could seriously impact the future survival and recovery of Round Hickorynut and Kidneyshell populations.

Table 1: Assessment of threats to extant populations of the Round Hickorynut and Kidneyshell. St. Clair delta and Sydenham River threats apply to Round Hickorynut and Kidneyshell populations. Ausable River threats apply only to the Kidneyshell population in the Ausable River.
ThreatRelative Impact (predo-minant / contri-buting)Spatial / Temporal (wide-spread / local, chronic / ephe-meral)Certainty (probable / specu-lative / unknown)
St. Clair deltaSydenham R.Ausable R. St. Clair deltaSydenham R.Ausable R. St. Clair deltaSydenham R.Ausable R.
Dreissenid musselspredo-minant--wide-spread chronic--probable--
Siltation-predo-minantpredo-minant-wide-spread chronicwide-spread chronic-probableprobable
Water quality – nutrients & contaminantscontri-butingcontri-butingcontri-butingwide-spread chronicwide-spread chronicwide-spread chronicspecu-lativeprobableprobable
Water quantity-contri-butingcontri-buting-wide-spread ephe-meralwide-spread ephe-meral-specu-lativespecu-lative
Decline of host fishcontri-butingcontri-buting-wide-spread chronicwide-spread chronic-specu-lativespecu-lative-
Urbanization-contri-butingcontri-buting-local chroniclocal chronic-specu-lativespecu-lative
Impoundments-contri-buting--local chronic--unknown-
Predation-contri-butingcontri-buting-local ephe-merallocal ephe-meral-unknownunknown

Threats in Historically Occupied Habitats

 WellandRiver: A single record exists for the Round Hickorynut in the Welland River consisting of a single shell collected in 1931 by an unidentified collector (COSEWIC 2003a). Its current status in this river is unknown. The small 880 km2 watershed of this river is dominated by rural land-uses and the river is subject to many of the same disturbances seen in the larger rural watersheds of southwestern Ontario which have contributed to the decline of freshwater mussels in these systems (A. Mack, Niagara Peninsula Conservation Authority, pers. comm. February 2004). Intensive agricultural activity coupled with extensive tile drainage and reduced riparian vegetation has resulted in high sediment inputs to the river, increased turbidity, elevated nutrient and bacterial levels and an overall reduction in the quantity and quality of aquatic habitat (http://www.conservation-niagara.on.ca/wellriver.htm).   

            Grand and Thames Rivers: The existence of the Round Hickorynut in the Grand River is indicated by three shells collected between 1966 and 1972 (COSEWIC 2003a). The Kidneyshell was probably more abundant in the Grand River than the Round Hickorynut as it was historically reported from 7 sites along a 50 km stretch between Caledonia and Port Maitland (COSEWIC 2003b). Recent surveys indicated no sign of live individuals of either species at 95 sites throughout the main channel and tributaries suggesting that the species may have been extirpated from the Grand River for an extended period of time. Like the Grand River population, the Thames River Round Hickorynut population is believed to have been lost as early as the turn of the 20th century (COSEWIC 2003a) with no live specimens collected since 1894. Several fresh shells of the Kidneyshell have been collected from the Thames River between London and Chatham as recently as 1997, however live specimens have never been collected (COSEWIC 2003b).

It is difficult to attribute a cause to the historic loss of populations such as those in the Grand and Thames Rivers although untreated wastewater inputs from major urban centres in these watersheds likely contributed to the declines.

Lake St. Clair, DetroitRiver, Lake Erie and Niagara River: The loss of the Round Hickorynut and Kidneyshell from historical habitat in these water bodies can be largely attributed to the detrimental effects of zebra mussels. Although, there is some indication that the Lake Erie population of the Round Hickorynut was in decline in the first half of the last century and may have been extirpated as early as 1950 (COSEWIC 2003a). Dreissenid mussels, however, pose the greatest limitation on recovery in these areas.