Contributions to a recovery plan for westslope cutthroat trout (Oncorhynchus clarkii lewisi) in Alberta: distribution, population size and trends

<i><b>Executive Summary</b></i>The westslope cutthroat trout, <i>Oncorhynchus clarkii lewisi</i>, is native to southwestern Alberta. Recent status reports have shown the subspecies to be much reduced in abundance, and facing serious threats to its conservation status. The Committee on the Status of Endangered Species in Canada (COSEWIC) assessed the Alberta population of westslope cutthroat trout as Threatened in 2006, a federal Recovery Potential Assessment has been prepared, and westslope cutthroat trout is now being considered for listing under Canada’s Species At Risk Act (SARA). The Government of Alberta has designated westslope cutthroat trout as Threatened under the provincial Wildlife Act. Alberta Fish and Wildlife is preparing a recovery plan for the subspecies.This report supplies supporting information and ideas for preparing a recovery plan being prepared by Alberta’s Westslope Cutthroat Trout Recovery Team. It documents the historical distribution and abundance of westslope cutthroat trout in Alberta; describes the changes in distribution and abundance of genetically-pure native cutthroat since European settlement; and identifies trends in distribution and abundance to estimate the future prospects for persistence of westslope cutthroat trout in its Alberta native range.Westslope cutthroat trout originally occupied parts of Montana, Wyoming, Idaho, Oregon, Washington, British Columbia and Alberta in the upper Missouri, upper Columbia, upper Fraser and upper South Saskatchewan basins in the United States and Canada. In Alberta the native range includes the headwaters of the Bow and Oldman drainages. There also may have been a native population in the upper Milk River that just entered Canada at the USA-Canada border. The Alberta populations were both widespread and abundant within the Bow and Oldman systems. River-migratory stocks used the mainstems of the 2 main rivers and their principal tributaries; lake-migratory stocks used a few large lake-river systems: the Waterton, Crowsnest, Spray, and the Minnewanka-Cascade. Hundreds of distinct resident stocks occupied all of these, plus the hundreds of accessible small tributaries.The populations were originally widely-distributed and broadly interconnected over large parts of the native range within major river drainages. The finely-divided nature of drainage networks tends to produce numerous, semi-isolated island-like habitats. Salmonids are known to produce numerous distinctive sub-population stocks, perhaps to use such habitats more efficiently. The individual stocks can have widely-different life history, physiological and behavioural characteristics, at least some of which are genetically determined, and they may be reproductively isolated or at least have restricted gene flow among the individual stocks. Possibly as a result of these mechanisms, genetic variation in westslope cutthroat trout is distributed among stocks in such a way that each stock tends to be internally uniform, but each separate population has alleles that are unusual or unique. As a result, conserving the overall genetic integrity of westslope cutthroat trout means that we must conserve as many of these distinct individual sub-populations as possible.Numerous historical accounts attest to there being large numbers of trout in the streams from the Bow drainage to the Montana border. Complaints about declines in southern Alberta’s trout stocks began shortly after the arrival of the Canadian Pacific Railway in 1883. Residents took huge numbers of fish. Wholesale abuses included the use of explosives, nets, bullets, quicklime, traps, and angling of hundreds of fish at a time. Pollution was an issue, as were unscreened irrigation works. The first dams and weirs were put in place at this time.As a result of the heavy exploitation of the native stocks, cutthroat populations became severely depleted with the result that demands for hatcheries and stocking were acted upon. Trout hatcheries were established in Banff in 1913, Waterton Lakes in 1928, and Jasper in 1941, all of which contributed non-native trout for introduction into the native range of westslope cutthroats in Alberta. Readily-produced hatchery stocks of rainbow trout were used and distributed widely on top of the depleted cutthroat populations. These two species readily hybridize to produce viable offspring. The offspring in turn can hybridize among themselves or with either of the parental species, ultimately creating a hybrid swarm in which the 2 species’ genomes are completely intermixed. Rainbow-cutthroat hybrids were recognized in some populations by at least 1947, and were widespread by 1950. The net effect has been that westslope cutthroat trout of any description have been lost from most of the native range in Alberta, and pure populations are rare, occurring almost exclusively in extreme headwaters. All of the lake- and river-migratory stocks are extirpated in Alberta. Only a very few resident stocks remain.More specifically, of 199 westslope cutthroat trout populations sampled and analyzed adequately within the Alberta native range, just 14 (7.0%) are hybrid-free. Twelve of those are likely to be native to the lakes or streams from which they were sampled, or derived from local stocks. An additional 32 populations are “pure” by the mean WSCT ≥ 0.99 criterion. These are of interest because it has been proposed that this should be the criterion for assigning pure status to populations in geographic surveys. Hybrids in fact comprise on average 8.8% (range 2.1% - 24.1%) of these populations. The proportion of WSCT in these hybrids averages 0.9525 (range 0.864 - 0.985). Such relatively high levels of introgression in many individual hybrids, and the often high proportion of hybrids in the nominally “pure” populations, argues for a more rigourous standard if stocks are selected to be archived or used for restoration. The group of 32 populations with mean WSCT ≥ 0.99 perhaps represents the best indicator of still-extant pure populations with enough genetically pure fish that could form the basis of salvaged and restored stocks. There are 12 of these populations that have &lt; 5% hybrid individuals, and 3 of those were classified as hybrids only because of the statistical rounding rule. These dozen stocks would seem to hold the greatest promise for successful conservation efforts.The complete loss of migratory and resident populations of westslope cutthroat trout from the mainstem rivers, large mainstem lakes, and their largest tributaries has especially serious consequences for the continued existence of resident headwater populations. These small, isolated populations historically would have become locally extinct relatively frequently without replenishment from downstream. On the basis of extensive review data in the literature as well as theoretical considerations, we calculated that these populations, which are thought commonly to hold no more than 200 adults and often far fewer, have no more than a 35% chance of persisting without receiving new immigrants. Immigration from downstream, especially by migratory populations, must have been critical in maintaining westslope cutthroats in headwater habitats. Now that the mainstem stocks are gone, maintaining the headwater populations will require ongoing human intervention.The proposed status of Threatened for Alberta westslope cutthroats by federal and Alberta provincial agencies was based on an overly optimistic estimate of the number of remnant genetically-pure native populations remaining. While recovery plans should continue, the conservation status of this species should be re-evaluated to determine whether a designation of Endangered is warranted.

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