Bull trout (Salvelinus confluentus) COSEWIC assessment and status report 2012: chapter 7

Habitat

Habitat Requirements

General

The Bull Trout is a cold water species generally found in water below 18°C, but most commonly in temperatures less than about 12°C (Dunham et al. 2003). Indeed, its southern range is limited by temperature (Dunham et al. 2003). The Bull Trout’s habitat requirements go far beyond temperature, however, being more specific than other salmonids (Rieman and McIntyre 1993). Characteristic requirements are habitat that is cold, clean, complex, and connected (USFWS 2008). Their habitat use is also strongly influenced by the presence, or absence, of other species (see ‘Interspecific Interactions’ section).

All life history stages need complex forms of cover, with Bull Trout tending to conceal themselves by remaining near or closely associating with the sub­strate, submerged wood, or undercut banks (Rieman and McIntyre 1993; Watson and Hillman 1997). Bull Trout also have specific requirements regarding channel and hydrologic stability that include depth, velocity, and substrate parameters (Rieman and McIntyre 1993; Watson and Hillman 1997). The association with substrate appears more important for Bull Trout than for other species (Nakano et al. 1992).

Although Bull Trout may be present throughout large river basins, their specific and changing habitat requirements mean that they will only be found in patches of a system (Rieman and McIntyre 1995). Large scale studies of spatial patterns of habitat patch occupancy show that persistence in stream networks is strongly dependent on patch size (stream or watershed size), connectivity, and quality (Rieman and McIntyre 1995; Dunham and Rieman 1999). The importance of habitat size and connectivity is further supported by models of Bull Trout population dynamics investigating the temporal processes driving these patterns, such as dispersal, demographic variation and environmental variability (Rieman and Allendorf 2001). Molecular genetic studies also show that disruption of connectivity can lead to lower effective size of local populations by simultaneously reducing dispersal and local adult population sizes (Costello et al.2003; Taylor and Costello 2006; Whiteley et al. 2006).

These specific habitat requirements are, in fact, Bull Trout’s most significant natural limiting factor (reviewed in Rieman and McIntyre 1993; Dunham et al. 2003). Such specificity makes Bull Trout particularly vulnerable to human induced habitat change and makes it less able to persist in the face of such change (Rieman and McIntyre 1993, 1995). Their habitat utilization varies according to both life-history stage and migratory form of the adult, as well as shifting on a daily and seasonal basis. Major transitions in habitat use over the Bull Trout’s life history are illustrated schematically in Figure 7. All of these variations are discussed below. Habitat requirements appear to be largely similar for Bull Trout across their range (Stewart et al. 2007a) and the description given herein refers to all Canadian Bull Trout DUs. In addition to specific references cited below, much of the information came from reviews given in Stewart et al.(2007a) and Rodtka (2009). Specific statements given without citation refer to these reviews.


Figure 7. Generic habitat use by Bull Trout throughout their life cycle

Schematic depicting generic habitat use by the Bull Trout throughout its life cycle (see long description below).

Modified from Stewart et al. 2007a.

Description of Figure 7

Schematic depicting generic habitat use by the Bull Trout throughout its life cycle. The schematic takes the form of a circle divided into four main segments. These segments represent the larval development, juvenile development, growth and maturation, and reproduction life stages. Each one contains text describing habitat use at a life stage. Three of the segments are further subdivided. For instance, “Larval Development” is divided into subsegments labelled “incubation,” “hatching,” and “emergence,” while “Reproduction” is divided into “mature,” “spawning/migration,” and “spawning.” “The “Juvenile Development” segment is divided into two unlabelled subsegments. Age ranges (years) and time of year covered by segments/subsegments are indicated around the outside of the schematic.

Natal streams and spawning

Bull Trout natal streams tend to be shallow, structurally diverse headwater or tributary streams with stable channels found at higher elevations (Burrows et al. 2001; Ripley et al. 2005; Decker and Hagen 2008). Their structural diversity not only meets habitat requirements of spawning adults but also provides for the changing habitat needs of rearing juveniles. These natal habitats occur as discrete patches of suitable habitat in a matrix of the larger stream network (Baxter 1997; Dunham and Rieman 1999; Decker and Hagen 2008). Watershed size appears to be a significant factor in providing essential connectivity between these habitats (Rieman and McIntyre 1995).

Once in their natal streams (following migration for adfluvial and fluvial forms), Bull Trout undergo a behavioural transition in habitat use towards a pattern of daytime concealment and nighttime emergence (Jakober et al.2000). Concealment cover includes woody debris and substrate crevices (Jakober et al.2000).

Bull Trout spawn in flowing water. Because eggs in­cubate over the winter, incubation sites are particularly vulnerable to anchor ice accumulations, as well as scouring and low flows. Females, therefore, often select spawning sites associated with groundwater sources that stabilise temperatures through the winter (Baxter 1997; Baxter and McPhail 1999; Baxter and Hauer 2000; Ripley et al. 2005). Within these areas of upwelling, they tend to select localized spots of strong down welling and high intergravel flows (Baxter and Hauer 2000). These occur over coarse gravel-cobble substrates that have low levels of fine sediment, for example, the tail-outs of pools at the heads of riffles (Baxter and Hauer 2000). The specific selection of these characteristics increases aeration of eggs. Successful incubation is dependent on several stream characteristics, including appropriate temperature (see Physiology and Adaptability’ section), gravel composition, permeability and surface flow.

Fry and young juvenile rearing

The preference of young Bull Trout for coarser substrate than is used by spawning adults appears to be heavily influenced by avoidance of predation and competition. In the spring, newly emerged young-of-the-year Bull Trout fry are denser than water and seek cover in shallow, slow-flowing stream margins with coarse cobble-boulder substrate (Pollard and Down 2001; Spangler and Scarnecchia 2001). As these juveniles grow, they tend to shift to deeper, faster flowing water, preferring pools over riffles (Bonneau and Scarnecchia 1998; Pollard and Down 2001; Spangler and Scarnecchia 2001). During the early months and years of life, when juvenile Bull Trout are rearing in their natal streams, microhabitat use shifts both daily and seasonally. During all seasons, juveniles are secretive during the day, remaining close to cover, and disperse more at night (Bonneau and Scarnecchia 1998; Jakober et al. 2000). This pattern of daytime concealment and nighttime emergence is particularly pronounced in winter (Bonneau and Scarnecchia 1998; Jakober et al. 2000). Juveniles tend to shift to deeper, slower-flowing water in the fall, where they stay in contact with coarse substrates and remain closer to cover (Bonneau and Scarnecchia 1998; Spangler and Scarnecchia 2001). This provides ice-free refuges for them throughout winter. Evidently, both shallow stream margins and deep water with low velocities provide important rearing areas for growing juveniles.

Cover use varies with latitude and elevation. As the diversity of cover type diminishes with increasing latitude and/or elevation (e.g., woody debris), juveniles have less opportunity to use shade, undercut banks and large woody debris (Mochnacz et al.2006). Instead they make more use of pocket pools, rootwads, cobbles, boulders and overhanging vegetation for shelter (Mochnacz et al. 2006).

Older juvenile and adult foraging and overwintering

Similar to younger fish, maturing and adult Bull Trout tend to use habitat for foraging and overwintering that has the appropriate combination of temperature, shelter, and foraging opportunities. However, while stream habitat use by Bull Trout has been studied in detail, the specifics of habitat use of rivers, lakes, and coastal waters by these fish are poorly understood. Both fluvial and resident Bull Trout prefer low-velocity water, often associating with the tail-outs of pools, and tend to remain close to cover (McPhail 2007). Resident forms find this habitat not far from their spawning grounds.

While radio-telemetry indicates Bull Trout need only move a few kilometers in the fall to find ice-free overwintering sites (Jakober et al. 1998), those in northern latitudes may move further into larger tributaries. Just as groundwater upwellings are a preferred location for spawning, these sites that have more stable temperature regimes than areas of surface-water recharge (i.e. warmer during winter, colder during summer) can also provide resident Bull Trout with suitably cold water throughout the year (Baxter and Hauer 2000). In streams at least, Bull Trout undergo a behavioural transition in habitat use during winter towards a pattern of daytime concealment and nighttime emergence. This is negatively correlated to temperature and fish size (Jakober et al. 2000).

Migratory forms (fluvial and anadromous) seek suitable habitat out in the larger streams and rivers (or even the sea) that they both migrate through and eventually settle in to forage and overwinter (Burrows et al.2001; Muhlfield and Marotz 2005). Based on fishing patterns, adfluvial adult Bull Trout appear to remain in deeper, cooler water during the day (mostly resting on the bottom) and then move to littoral areas for foraging at night (McPhail 2007).

Habitat Trends

Bull Trout habitat is less well characterized in the more remote reaches of its range although recent surveys in the British Columbia (Pollard and Down 2001), Yukon (Connor et al. 1999; Can-nic-a-nick Environmental Sciences 2004) and Northwest Territories (Mochnacz et al. 2006, 2009; Mochnacz and Reist 2007) are greatly improving our knowledge about habitat availability and Bull Trout distribution in these regions.

Bull Trout’s specific habitat requirements, particularly its requirement for cold, clean tributary or headwater streams and the importance of groundwater springs for spawning and rearing of young, result in a patchy distribution across its broad geographical range (Rieman and McIntyre 1995). This pattern of natural fragmentation has been exacerbated over past decades, especially in the USAwhere remnant populations have become more isolated (Rieman and McIntyre 1993). The distribution of Bull Trout has declined over the past century, particularly in the southern and eastern parts of its North American range in the USA (Rieman et al. 1997; USFWS 1999) and Alberta (Rodtka 2009). For example, many USA strongholds for Bull Trout are now restricted to higher elevation wilderness areas (Rieman et al. 1997).

It is difficult to quantify the impact that habitat change has had on this pattern of general decline (see ‘Threats and Limiting Factors’ section). Nevertheless, their environmental sensitivity, indicated by their specific habitat requirements, is clearly demonstrated by their consistent association with unmanaged landscapes and low human population influence (e.g., negative correlations with road density, see ‘Threats and Limiting Factors’ section). Habitat degradation and fragmentation are considered a primary threat to the persistence of Bull Trout populations (see ‘Threats and Limiting Factors’ section for full discussion of anthropogenic threats and DUspecific information). Migratory populations that use the largest diversity of habitat throughout their life cycle will be particularly vulnerable to general trends of habitat degradation and fragmentation. The presence of suitable corridors for movement between the different habitats they use for feeding, breeding and refuge is crucial to the persistence of this largely migratory fish (Rieman and McIntyre 1993).

An understanding of the environmental controls on the distribution of suitable Bull Trout habitat could facilitate predictions not only on their occurrence but also on habitat that is unoccupied but suitable for Bull Trout (Rieman and McIntyre 1995; Dunham and Rieman 1999). However, despite (or perhaps because of) the broad distribution of Bull Trout across western Canada, few studies have attempted to quantify trends in Bull Trout habitat across this landscape (BCMWLAP2004). Activities such as road construction have been used as surrogate measures for Bull Trout habitat disturbance (BC ME 2007), following studies that demonstrated correlations between them. Road density, in particular, has been repeatedly negatively correlated with Bull Trout occurrence (Rieman et al. 1997; Dunham and Rieman 1999; Baxter et al. 1999), including specifically in Canada (Alberta: Ripley et al. 2005; Scrimgeour et al. 2008). Given that road length has nearly doubled in British Columbia over the last two decades (82% increase between 1988 and 2005; BC ME 2007), a general decline in the quality of Bull Trout habitat in British Columbia is suggested over that time period. Based on the negative correlation between Bull Trout occurrence and levels of commercial forestry, Ripley et al. (2005) forecast the local extirpation of Bull Trout from 24% to 43% of stream reaches that currently support Bull Trout in the Kakwa River basin, Alberta over the next 20 years.

Climate change will also likely play a role in further restricting the availability of habitat for this cold water specialist in the future, as well as reducing connectivity among refuges of suitable coldwater habitat (see ‘Threats and Limiting Factors’ section for full discussion). An assessment of the Cariboo-Chilcotin region of British Columbia suggests that the thermal and precipitation effects of global warming will produce a long-term pattern of considerably decreased cold water stream habitat by the 2080s (Porter and Neritz 2009).

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