One of my areas of expertise is bull trout (Salvelinus confluentus). I became involved with bull trout population dynamics and habitat requirements about 38 years ago when they were still thought to be inland dolly varden (S. malma). Bull trout are a native salmonid (trouts, chars, whitefish) in the Northern Rocky Mountains that typically inhabit, indeed, require, colder water than many salmonids. They are important to land management and fish management programs because they were listed as "threatened" under the Endangered Species Act in 1998 making it illegal to harm them (their habitat gained similar status in 2010). They were also selected as a "Management Indicator Species" (MIS) on the Payette National Forest where I worked; the viability of MIS must be monitored by the Forest and considered for all planned actions.
Stream temperature is a critical habitat component for salmonid fishes, and possibly of overrriding importance in some cases; thus, temperature is very often monitored when designing and implementing land management actions. I was directly involved with one of the first studies of bull trout distribution using stream temperature (in conjunction with catchment size) as a driving parameter for estimating the probability that bull trout would occur in streams that had not been sampled. Subsequently, with the listing of bull trout under ESA in 1998 and the need to assess the effects of Forest actions on bull trout, I developed a preliminary predictive model for the likelihood of bull trout occurrence in streams in which their occurrence had not been properly documented. This model was preliminary, informal, and somewhat subjective, but it was also 100% effective; that is, bull trout were later found where I said they were likely to be found and they have not been found where I ruled them out*.
Climate change was and is thought to be a potentially significant factor in the long-term persistence of bull trout because of their dependence on very cold water. In Idaho, 10°C Maximum Weekly Maximum Temperature (MWMT)** in bull trout waters in summer was set by the Environmental Protection Agency (EPA) as suitable for bull trout***. Inquiries into the potential climatic warming-induced decrease in suitable habitat and distributions of various salmonid species began about 20 years ago and continues today. Inexpensive, submersible thermographs also only became readily available about two decades ago, making it relatively simple to sample and monitor stream temperatures. On the Payette National Forest we implemented an extensive temperature sampling and monitoring program and considered potential effects of stream temperatures, potential stream temperatures with climate change, and distribution of suitable habitat for bull trout based largely on stream temperatures to assess viability of local populations***. I want to point some of these assessments out here because they illustrate how climate change considerations were formally incorporated.
Our first Forest-wide viability assessment was produced in 2005. I was largely responsible for the discussion of why climate change could be a potential issue for bull trout viability:
"Fragmentation decreases the likelihood of long-term persistence of bull trout populations making them more susceptible to extinction from habitat alteration such as climate change, exotic species, grazing, road occurrence (sedimentation and culverts), water diversions or impoundments, consumptive activities (timber harvest, mining), and random events such as debris torrents." (page 3)
"Naturally isolated populations have persisted since they were isolated despite lack of interchange with other populations, but anthropogenic fragmentation within the watersheds inhabited by these populations is probably reducing their long-term viability, particularly because the amount of available suitable habitat is likely declining generally as a result of climate change (Keleher and Rahel 1996; Rieman et al. 1997; Nakano et al. 1996). As available habitat declines, connectivity among isolated populations becomes more important and simultaneously increases the likelihood of individual populations being extirpated while reducing the potential for their refounding." (page 4)
"Another potential determinant of bull trout viability, particularly in the Weiser River watershed, is climate change. Given their position in the food web as top predators, bull trout may be vulnerable to increasing temperatures because such climate change is expected to be harshest to top predators (Petchey et al. 1999). In addition, if temperatures continue warming, we would expect the downstream limits of bull trout to increase in elevation, shrinking the amount of habitat available to them; this has been modeled in the Japanese archipelago for Dolly Varden (S. malma) and white-spotted char (S. leucomaenis), two species closely related to bull trout, by Nakano et al. (1996). This would, of course, exacerbate the negative competitive effects of brook trout, which appear more thermally tolerant." (page 38)Although I mostly accepted the argument that global warming was happening, whether naturally or because of CO2 emissions, I would confess to being a skeptic about catastrophic global warming even then; I did, however, believe that the climate had been generally warming since the Little Ice Age and was comfortable assuming that trend would continue. I still believe management should be based on observed, not modeled, trends.
In 2010 I produced another extensive review of bull trout populations on the Forest with emphasis on streams proposed as critical habitat by the U.S. Fish and Wildlife Service. This assessment has a full section on climate change as a threat to bull trout persistence, but generally took a much more skeptical tone about the likelihood of climate change being a serious existential threat. There were two main reasons for this skepticism: first, invasive brook trout (S. fontinalis) posed a much clearer and intractable threat to viability over most of the Forest, and, secondly, we weren't seeing much in the way of increasing stream temperatures. This figure appears in that report:
The report had this to say about the above figure and Forest-wide stream temperatures:
"Nelson and Burns (2006) reported (using regression modeling that accounted for autocorrelation common to time series records) that most streams for which we had sufficiently lengthy time series data had increasing trends in MWMT. This analysis should be updated to determine whether the trends remain detectable with additional data and to get a wider sampling of the Forest (many of those with upward trends in Nelson and Burns [2006] had burned within 10 years of the modeling). For the few sites that have reasonably long records in the RU temperature discussions above and which haven’t had much recent wildfire, there is little evidence of warming streams." (page 10)Despite my skepticism, others remain convinced that the climate will continue to warm, perhaps drastically, and bull trout distributions will decline with increasing stream temperatures. To this end, I was also involved in coordinating and disbursing funds to develop a sampling/monitoring protocol and building a model of possible effects of warming-induced habitat reductions, though I question their ultimate utility and the Forest has been slow to apply them.
Bull trout have been around for a long time, probably reaching their maximum distribution shortly after the end of the Wisconsinan glaciation. There are many threats to their long-term persistence, most of them of anthropogenic origin. Climate has undoubtedly played some role in the biogeography of bull trout and in development of genetic diversity within and among various groups. However, it seems unlikely to me that their demise will be caused by a warming climate unless the earth gets hotter than previous warm periods through which they must have survived. There may be some extirpation of isolated populations at the margins of their range, such as here in south-central Idaho, but that contraction may have been going on for some time. In fact, there are populations on the Payette National Forest that survive in temperatures normally regarded as unsuitable, so some local adaptation may be underway as well. Only time will tell; but, in the meantime, treatable issues like habitat fragmentation, proper riparian management, reductions in road densities, etc. should receive most attention.
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* There is one technical exception to this. Bull trout were thought to occur in the Middle Fork Weiser River and this model, though not explicitly used for the Weiser subbasin, would have assigned a high likelihood of presence; extensive surveys have failed to find any.
** This is the 7-day moving average of daily maximum temperatures.
*** This is, unfortunately, a somewhat arbitrary target. The corresponding criterion in Oregon is 15°C and, in fact, 10°C in Idaho is achievable in relatively few Idaho streams in summer.
