FISH ECOLOGY

Bull Trout

Several thorough reviews of bull trout literature were surveyed in preparation for this species description. Rather than repeat their work here, the following is a summary of the salient points from those reviews cited collectively, with information from other sources cited separately. The collective citation for the bulk of this description follows: Brown (1992), Rieman and McIntyre (1993), Sanborn et al. (1998), and U.S. Federal Register (1 November 1999).

The historical range of bull trout extended from the McCloud River in California to the Yukon River in Alaska, west of the Continental Divide within the contiguous United States except in tributaries of the Saskatchewan River, but east of the Continental Divide in the Saskatchewan and MacKenzie river systems in Canada. In Washington, bull trout occur within the Columbia River system, in rivers of Puget Sound, and in coastal rivers from Grays Harbor north. Two subpopulations of bull trout are considered within the Lake Washington basin: the Chester Morse Reservoir subpopulation and the Issaquah Creek-Sammamish River subpopulation (U.S. Federal Register, 1 November 1999; Washington Department of Fish and Wildlife [WDFW] 1998). In the mid- to late 1990s, less than 10 spawning sites were found, and fry abundance was low in the Chester Morse Reservoir; however, the population in the reservoir was estimated at approximately 3,000 adults based on hydroacoustic surveys, and did not include estimates of juveniles in the tributaries (Seattle Public Utilities, unpubl. data). In the past 10 years, only two "native char" [either bull trout or Dolly Varden (Salvelinus malma)] have been reported in Issaquah Creek and none have been reported in the Sammamish River (U.S. Federal Register, 1 November 1999; WDFW 1998). The USFWS is not certain that the latter subpopulation is "viable."

There is no known spawning subpopulation resident in Lake Washington or Lake Sammamish. However, subadult and adult native char are occasionally found in the lakes (USFWS 1999). Two subadult native char (300-400 mm fork length) were captured in horizontal gill nets (3-10 m deep) off the Cedar River delta in March 1985, and an adult (635 mm fork length) in the same location in April 1985; a subadult native char (300 mm fork length) was also captured in August 1984 at a depth of 60 meters (Beauchamp, Univ. of Washington, unpubl. data). Other native char have been caught or observed in Lake Washington, as well as in the Cedar River, Lake Sammamish, Carey Creek (Issaquah Creek tributary), and at the Ballard Locks (KCDNR 2000).

Life History

Several life history forms of bull trout occur, and all may be present within the same population. Fish exhibiting the resident life history strategy are non-migratory, spending their entire lives within their spawning stream. Migratory life history strategies include fluvial, adfluvial, and anadromous. Migratory bull trout reside as adults and subadults in larger rivers (fluvial), lakes or reservoirs (adfluvial), or marine waters (anadromous), and spawn and rear as juveniles in headwater tributaries. Due to differences in productivity between small headwater streams and larger rivers, lakes, and marine environments, resident fish are typically smaller than migratory fish. Resident fish seldom exceed 300 mm, while migratory forms can exceed 900 mm. Anadromous forms are common in Puget Sound drainages from the Snohomish River north (Kraemer in prep.).

The majority of bull trout spawning occurs between late August and early November. Spawning migrations occur during the summer, but may start as early as April in some systems (Ratliff et al. 1996). In river systems of north Puget Sound, spawners typically arrive in holding areas near spawning grounds from several weeks, to up to four months before spawning (Kraemer in prep.). Characteristics of holding areas are: depth of at least one meter; cover in the form of turbulent water, undercut banks, woody debris, or overhanging vegetation; and cool temperatures, often provided by groundwater input. Spawning typically does not commence until stream temperatures drop to 8° C. In the North Puget Sound region, "the downstream limit of successful spawning is always upstream of the winter snow line (that elevation at which snow is present on the ground for much of the winter)" (WDFW 1999). In Montana, no spawning occurred in 1st order streams, only limited spawning by non-migratory bull trout occurred in 2nd order streams, and the majority of spawning by migratory fish occurred in 4th order streams. Bull trout spawning habitat typically consists of gravel/cobble substrates, although spawning has been observed in sand, and also in cobbles too large to be dislodged by female digging attempts (Kraemer in prep.). Close proximity to cover, low gradient (even within a high-gradient reach), and depths greater than 10 cm appear to be important spawning site-selection criteria. Once sexually mature, resident, fluvial and anadromous bull trout in north Puget Sound spawn annually (Kraemer in prep.). Following spawning, adult bull trout move downstream quickly, remaining in deep pools in larger rivers, or in lakes for the winter. Spawned-out bull trout have been observed in November feeding on loose eggs in salmon spawning grounds (Kraemer in prep.). Beach spawning of native char in Lake Washington and Lake Sammamish is improbable. Confirmed observations of beach spawning bull trout are limited to extreme downwelling conditions in cold, high-elevation lakes (WDFW 1998); water temperatures in Lake Washington and Lake Sammamish are too high for successful incubation.

Temperature

Successful egg incubation requires temperatures less than 5° C (WDFW 1999), with maximum survival between 2 and 4° C. Incubation usually takes from 100 to 145 days, depending on temperature. Egg-to-fry survival declines with increasing percentages of fine particles in the substrate (particles smaller than 6.35 mm). Fine sediments decrease egg-to-fry survival by impeding the flow of water to the eggs or by physically preventing fry emergence (entombment). Maintaining water flow to the developing eggs is necessary to remove metabolic wastes and deliver dissolved oxygen.

Juvenile bull trout are rarely found in streams with summer temperatures that exceed 15° C. Cold groundwater seeps can provide temperature refuge for bull trout in streams with summer temperatures that exceed 15° C. Fry are closely associated with the substrate while foraging, and rely on interstitial spaces for cover. This strong association with the substrate decreases with body size and is substituted by an association with woody debris and large boulders. Juveniles are benthic foragers, feeding on aquatic invertebrates until they are large enough to become piscivores (> 100 mm). In allopatry, bull trout forage throughout the water column. Bull trout juveniles show a preference for low-velocity habitat; fry are often found in backwater areas, stream margins, and side channels, while larger juveniles occupy pools. There is some evidence for a diel habitat shift between concealment during the day and foraging in deep, fast water at night. Juveniles disperse widely from the spawning area, and should be expected even in tributaries that do not support spawning unless access is obstructed by a passage barrier. Juveniles that adopt a migratory life history strategy usually move downstream to a mainstem river, lake, or ocean following two or three years of rearing in headwater streams. Migration is possibly related to the need for a larger prey base that arises with the onset of piscivory. The timing of this migration varies between and within systems, and is not confined to spring.

Like juveniles, adult and subadult bull trout are typically found only in streams with summer temperatures that do not exceed 15° C. Pools with groundwater seeps may function as thermal refuges where stream temperatures exceed 15° C. Adults and subadults select low-velocity habitats, typically large pools, with abundant cover and large substrate. The diet of adults and subadults in streams consists primarily of fish, including juvenile salmon and trout, and whitefish (Prosopium spp.). A diel habitat shift similar to that of juveniles has been observed in adults and subadults in streams. Non-spawning movements are generally associated with thermal requirements, either seeking warmer water in winter (non-coastal populations) or colder water in summer.

The distribution of subadults and adults in lakes and reservoirs appears to be temperature mediated, with fish generally avoiding temperatures greater than 15° C, and preferring temperatures less than 10° C. Following stratification of lakes in the spring, bull trout are mostly found below the thermocline, and generally near the lake bottom. The diet of bull trout in lakes consists almost entirely of fish, and the species composition within the diet varies with the relative abundance of prey species in the lake. Cyprinids, catostomids, cottids, and salmonids [kokanee, cutthroat trout, smaller bull trout, whitefish] represent a substantial portion of the diet of lake-dwelling bull trout in various studies. The presence of warm-water prey species, such as yellow perch (Perca flavescens), in the diet of bull trout indicates that they either make occasional forays into warmer (17-20 ° C) nearshore waters or exploit these prey during winter and spring. Bull trout have also been observed aggregating to take advantage of localized prey abundance such as concentrations of spawning prey fish.Anadromous Form

The anadromous life history strategy in bull trout is not well understood. Historically, anadromous char were all considered Dolly Varden. The separation of Dolly Varden and bull trout into distinct species, and recent investigations of native char populations in Puget Sound, have suggested that Dolly Varden and bull trout in north Puget Sound are sympatric and equally anadromous. Anadromous bull trout spend two to three years in fresh water before migrating in the spring to the estuary or nearshore marine environment (Kraemer in prep.). While in the marine environment, they feed on smaller fish such as surf smelt (Hypomesus pretious), Pacific herring (Clupea harengus pallasi), Pacific sand lance (Ammodytes hexapterus), and pink (O. gorbuscha) and chum (O. keta) salmon smolts, closely following the distribution of the prey fish (Kraemer in prep.). Subadults usually spend two summers in the marine environment before they mature (Kraemer in prep.). Anadromous bull trout return to fresh water to overwinter, and immature and non-spawning adult fish migrate upstream with the spawners in late summer (Kraemer in prep.).

Local Lake Washington/Sammamish Sub-Populations

The only likely viable bull trout subpopulation in the Lake Washington watershed is the Chester Morse Reservoir subpopulation. Spawners have not been confirmed within the Sammamish River-Issaquah Creek subpopulation (WDFW 1998), and only two bull trout have been observed in the system within the last 10 years. The thermal requirements for spawning and successful egg incubation may prevent bull trout reproduction in the Sammamish River-Issaquah Creek system. Only a few streams within the Issaquah Creek system approach the winter snow line (see above), and they may be too small to be used by migratory spawners. Vestigial pockets of resident spawners could reside within thermal refugia in the upper reaches of Issaquah Creek (i.e., Holder Creek). Successful spawning by these remnants could produce migratory offspring that may never successfully reproduce. Adfluvial or anadromous offspring of remnant resident spawners could be present within the system. High summer water temperatures and an anoxic hypolimnion (http://dnr.metrokc.gov/wlr/waterres/lakes/thermo.htm) would likely deter bull trout from residing in Lake Sammamish.The Chester Morse Reservoir subpopulation is above an anadromous barrier and is a glacial relic population (WDFW 1998). The population exhibits an adfluvial life history strategy, although residents could exist in the upper watershed (WDFW 1998). Because all life history strategies can arise from the same population, it is possible that some fish emigrate from the Chester Morse Reservoir to exhibit anadromy or to reside in Lake Washington. Water temperatures in the lower Cedar River are probably too high to support a fluvial population (WDFW 1998). Cedar River tributaries below Chester Morse Reservoir likely do not meet the thermal requirements for spawning and successful egg incubation; thus, bull trout that emigrate from Chester Morse Reservoir would not represent a viable spawning population. However, these fish may spawn in a non-natal system. Char are known to exhibit "pioneering" behavior, spawning in areas other than their native stream (WDFW 1999). Anadromous bull trout and Dolly Varden overwinter in freshwater, and may overwinter in systems other than their natal system.

Bull trout that occupy Lake Washington or Lake Sammamish should exhibit similar distribution and behavior to that of bull trout observed in other lakes. Juveniles (length 150-300 mm) would migrate to the lake after rearing one to three years in headwater streams. Spawners would begin upstream migrations from April through July, including anadromous fish migrating through the lake. Adults would likely spawn annually as observed in other North Puget Sound populations (Kraemer in prep.). Immature fish residing in the lake would be likely to migrate upstream with the spawners. Those immature fish and any non-spawning adults remaining in the lake during the summer would avoid temperatures above 15° C. High temperature avoidance would likely confine bull trout below the thermocline (> 15 m) from mid-June through mid-October, with some annual variation due to climatic differences. The presence of a large prey base in limnetic [e.g., sockeye salmon and longfin smelt (Spirinchus thaleichthys)] and deep benthic (e.g., Neomysis mercedis and prickly sculpin [Cottus asper]) regions would reduce the need for summer forays into nearshore areas. Native char that were captured at the mouth of the Cedar River in March and April were eating spawning longfin smelt and outmigrating sockeye fry (Beauchamp, Univ. of Washington, unpubl. data).