Recovery Strategy for the Atlantic Whitefish
1.6 Threats and Limiting Factors
Modification of the Tusket River and Petite Rivière watersheds through human activities has altered their physical habitat, hydrography and water chemistry. Species abundance has also been affected by past over-harvesting. Past and present significant threats, limiting factors, and habitat alterations include (in no particular order) (Bradford et al. 2004b; DFO 2004b):
- construction and operation of hydroelectric dams and water supply impoundments;
- acidification of habitat resulting from acid rain;
- land use practices, in particular agriculture and forestry;
- Historical fishing activities, and;
- introduction and spread of non-native fish species which may pose competitive or predation risks (e.g., smallmouth bass, chain pickerel).
While the threats faced by Atlantic whitefish in the two watersheds (Tusket and Petite) exhibit common traits, the significance of the threats varies between the two systems (DFO 2004b). In the Tusket, habitat alteration and inadequate fish passage due to hydroelectric dam construction and operation, acidification, chain pickerel and smallmouth bass predation, and over-harvesting are identified as the most significant threats. By contrast, the Petite Rivière is better buffered and thus less affected by acidification; however the construction and operation of water supply facilities and predation by smallmouth bass are identified as the most significant factors threatening the population.
These threats have been recently reviewed during a DFO Regional Advisory Process meeting to evaluate the level of mortality that would not jeopardize the survival or recovery of Atlantic whitefish and to identify the potential sources of human-induced harm. In support of this review, factors potentially resulting in mortality of Atlantic whitefish as a result of human activities were considered (Bradford et al. 2004b). The conclusions of this meeting are summarized in a Status Report (DFO 2004a) and in section 2.7 of this document. The scope for the harm assessment documented in the Status Report (DFO 2004a) was limited to the area of known occupancy (i.e., the three Petite Rivière lakes).
1.6.1 Hydroelectric Development and Water Supply Impoundment
The construction and operation of hydroelectric dams and water supply impoundments have transformed lake and riverine habitat to reservoir habitat; the resulting fluctuating water level regimes have altered the original habitat and the dams have either blocked or impeded fish passage. A chronology of hydroelectric generation on the Tusket and Petite rivers in relation to fish passage and habitat requirements can be found in Bradford et al. 2004b.
Atlantic whitefish migration in the Tusket River is thought to have been limited by the Tusket River dams and past seasonal closures of the fish ladders (Edge and Gilhen 2001). The fish ladders were closed annually in the fall interrupting or preventing the Atlantic whitefish fall spawning migration. The damming of the Tusket River at Tusket Falls (Figure 2) in 1929 interfered with the migratory movement of the Atlantic whitefish for many years. Although a fishway, originally constructed of timber, was built in 1930, and rebuilt in 1941 due to decay, the structure was considered unsatisfactory and a new fishway was constructed in 1949. Additional improvements and changes were made to the fishways at both the Main Diversion Dam (Lake Vaughn) and the Powerhouse Dam (Tusket Falls) in the 1960s and 70s to facilitate downstream passage for diadromous species (e.g., salmon and gaspereau), improve the overall efficiency of the fishways for fish migration, and reduce mortality associated with fish passing through the turbines. In the 1980s and 90s ongoing studies with DFO and other stakeholders on the Tusket River focused on manipulation of operation schedules and maintenance flows to coincide with migratory movements of fish. Changes were mostly associated with attempts to improve upstream and downstream passage for Atlantic salmon and gaspereau. Since 2003 the Powerhouse fish ladder operating period has been extended to the end of December to accommodate any possible remnant Atlantic whitefish spawning migrations.Monitoring devices were also installed in an attempt to confirm the presence or absence of Atlantic whitefish in the Tusket River system. No Atlantic whitefish have been observed migrating through this monitoring device (NSPI 2003), and the species is now considered extirpated from the Tusket River system. Were Atlantic whitefish re-established on the Tusket River, the existing fishway should be suitable providing operations accommodated its migration times.
In the Petite Rivière system, waterbodies have been impounded and diverted for various reasons since the late 1790s; those in the upper watershed now constitute the Town of Bridgewater water supply. The construction of a hydroelectric dam at the foot of Hebb Lake as early as 1901 effectively blocked any upstream migration of fishes beyond this point. Dams without fish ladders presently obstruct the Petite Rivière at the outlets of Minamkeak, Milipsigate and Hebb lakes (Figure 3). While it is not known if adult anadromous Atlantic whitefish migrated to these lakes to spawn prior to the existence of the dams, the Hebbville dam now eliminates any likelihood of upstream migration to the lakes, including any individuals attempting to rejoin the population after having fallen over the dam. Fish passage is also impeded at the former dam site at Conquerall Mills, around an existing dam at Crousetown; and, is impeded or blocked at the dams at Milipsigate and Minamkeak lakes (Figure 3). A brief description of each barrier to fish passage on the Petite Rivière is provided in Table 1 below.
Table 1: Descriptions of barriers to fish passage in the Petite Rivière (adapted from Conrad 2005).
|Crousetown||A 2.4 m high timber dam located at a former sawmill site. The dam includes a run-around type of fishway constructed from loose native stone that is considered to be inefficient for fish passage.|
|Conquerall||The dam at the former Conquerall Mills hydro site was partially dismantled, allowing a 9 m space between the remaining concrete abutments. The resulting short series of rapids constitutes a 1.2 m drop which may present a small in-stream barrier to Atlantic whitefish passage upstream.|
|Hebbville||The Town of Bridgewater water supply storage dam at Hebb Lake consists of a concrete flow-control structure and a long rock and earth fill berm. This berm is several hundred meters long and ends at a large pond. The pond is supplied by steady seepage through the berm and is drained by way of a meandering outlet channel and 1.5 m diameter culvert, finally emptying into the main channel of the river about 60 m downstream of the main concrete flow control structure. Other than the spillway, no fish passage is provided at this dam.|
|Milipsigate||A concrete dam structure operated by the Town of Bridgewater for flow regulation purposes. Other than the spillway, no fish passage is provided at this dam.|
|Minamkeak||The uppermost storage dam for the Town of Bridgewater and is used for flow regulation purposes. Other than the spillway, no fish passage is provided at this concrete dam structure.|
Acidification may be another limiting factor for Atlantic whitefish. The rivers most affected by acidification in Nova Scotia are in the Southern Upland Region, which include the Petite and Tusket, where a combination of hard-rock geology, inadequately buffered soils and prevailing weather patterns have resulted in severe acidification of the rivers and lakes. The Tusket is more affected by acid rain than the Petite. While research on the effect of low pH on various life stages of Atlantic whitefish is underway, the impacts may be comparable to those of other salmonids. Acid toxicity has been identified as a major factor in low wild salmon abundance in Southern Upland rivers (DFO 2000), for example. Data from Clair et al. (2004) indicate that the Petite Rivière as well as portions of the Tusket River possess sufficient buffering capacity for Atlantic whitefish survival (Bradford et al. 2004b).
With respect to forestry, Sayah (1999, cited by Llewellyn et al. 2000) noted overlaps between clear cut areas and waterbodies within the natural watershed boundary of the Petite Rivière drainage. Forestryactivities including roads, skidding trails, and clear cuts can cause accelerated soil erosion and siltation that can lead to a reduction in the productivity of the aquatic ecosystem and affect the rate and quantity of water runoff. All these factors can be harmful to fish habitat and lethal to fish (Birtwell 1999). Clear cutting has not, however, been reported directly surrounding the three lakes where Atlantic whitefish are found (Kendall & Llewellyn 2001). Under the Nova Scotia Forest Act (1989), there are Wildlife Habitat and Watercourses Protection Regulations which specify various requirements to protect watercourses that fall within forested areas (e.g., leaving buffer zones around watercourses). It is not known whether Atlantic whitefish have any particular sensitivity that would not accommodate forestry activities when undertaken according to the provincial guidelines and regulations. Generally, there is no indication of non-compliance in current forestry practices around the three lakes where Atlantic whitefish are currently found.
1.6.4 Historical Fishing Activities
Past harvesting practices, including poaching and incidental captures, may have been a factor in the decline of Atlantic whitefish populations. Atlantic whitefish were reportedly fished in the Tusket system prior to the 1960s and in the Petite Rivière until recently. Captured primarily by gill and dip nets, and occasionally by angling, the fish were used for human consumption, reportedly supporting a minor sport fishery and yielding a good food fish of fine flavour. They may also have been utilized for other purposes including bait for lobster traps and fertilizer (Scott and Scott 1988 and P. Longue, DFO 2001 personal communication).
Atlantic whitefish were once very abundant in the Tusket and Annis Rivers. Prior to 1940, it was reportedly not uncommon to catch 200 in a net when fishing for gaspereau on the Tusket River (Bradford et al. 2004a). The accumulation of Atlantic whitefish in the upper pools of the Tusket hydro facility fish ladders facilitated poaching in the 1950s. (Gilhen 1977; Scott and Scott 1988). Similarly, on the Annis River, incidental catches of 50 to 100 individuals during the gaspereau fishery was common as late as 1970.
In the Petite Rivière system, a small angling fishery around Milipsigate and Hebb lakes may have existed as early as 1870s (Edge and Gilhen 2001). Atlantic whitefish were reported as occasional bycatch in the May-June gaspereau fishery in the Petite estuary. There have been no legal directed or bycatch fisheries for the species, since at least 1978. Section 6 of the Maritime Provinces Fishery Regulations which specifically prohibits the retention or possession of Atlantic whitefish came into effect in 1993.
1.6.5 Interactions with Non-native Fish Species
Non-indigenous fish predators threaten Atlantic whitefish. Smallmouth bass (Micropterus dolomieu) and chain pickerel (Esox niger) have been identified as possible threats to Atlantic whitefish (Edge and Gilhen 2001). Smallmouth bass has been introduced into both systems, and chain pickerel are found in the Tusket system. The introduction and increasing range of smallmouth bass in both watersheds is also of concern, in particular the presence of this species in Minamkeak Lake, one of the three upper lakes of the Petite watershed which collectively support the only significant remaining population of Atlantic whitefish. The relationship of these introduced species to Atlantic whitefish is not well understood, but may pose competitive and predation risks (Bradford et al. 2004b).
1.7 Critical Habitat
Critical habitat as defined under section 2 of SARA is the “habitat necessary for the survival or recovery of a listed wildlife species and that is identified as the species’ critical habitat in the recovery strategy or in an action plan for the species”.
While the state of knowledge on habitat requirements of Atlantic whitefish is increasing as new scientific evidence becomes available, it is currently not possible to identify critical habitat for this species and thus critical habitat for Atlantic whitefish is not identified in this recovery strategy and will wait to be designated at a later stage in an action plan. As set out in SARA, if information is inadequate to identify critical habitat within the recovery strategy, a schedule of studies must be prepared. Such a schedule, when implemented, will yield new information to enable the species’ critical habitat to be described.
Appendix II includes a list of research and monitoring activities that collectively, constitute a schedule of studies. This schedule of studies describes the activities that are needed to define the critical habitat for this species. Despite not being able to identify critical habitat for Atlantic whitefish at this time, the following description of the habitat fundamental to the survival of the species may be useful in a future description of its critical habitat.
Atlantic whitefish, known historically to occur in the Tusket and Petite rivers, no longer exist outside of the Petite Rivière drainage. Life-cycle closure is achieved by fish resident within Hebb, Milipsigate and Minamkeak lakes (Figure 3). Species survival, and ultimately recovery, is therefore wholly contingent on the continued viability of this population whose global area of occupancy is now confined to approximately 16km2 of semi-natural lake habitat. While there are no indications that current human activities pose a threat to the survival of Atlantic whitefish, or that habitat quality within the lakes is inadequate, there is no scope for a further decline in either the abundance or the distribution of this species and the habitat it occupies (DFO 2004a).