Appendix A Recommended EPA Temperature Thresholds for use in Establishing Thermal Potential and Species Life Stage Numeric Criteria 1. Temperature Limits Recommended to Protect Salmonid Guilds In this appendix, EPA recommends water temperature threshold values that are needed to fully protect native salmonid fishes of the Pacific Northwest. EPA believes that these threshold values represent general upper limits for optimal thermal conditions to support various species and life stages of salmonids fishes. EPA recommends that states and tribes apply these values to defined salmonid guilds. The species making up the membership of each guild share similar lifestrategies and have similar habitat and temperature requirements and limitations. EPA recommends the use of three guilds for establishing temperature values: 1) A Char Guild which includes both bull trout and Dolly Varden; 2) A Cold Water Guild which includes the five Pacific salmon, coastal cutthroat and rainbow trout, and anadromous steelhead trout; 3) A Moderately Cold Water Guild which consists of interior redband trout and Lahontan cutthroat trout. Within each guild, EPA established temperature values to protect key life stages such as spawning and incubation; juvenile rearing; subadult rearing; smoltification; and adult migration. The EPA did this to ensure that the temperature requirements for life-stages that occur outside the summer period would be protected. By setting values for individual life-stages the temperature thresholds can also be tailored to; support the actual temporal patterns of fish use that occur in individual water bodies. The values EPA recommends are estimates of upper optimal physiological temperature preferences known to support various life stages and biological functions of the salmonid fishes of the Pacific Northwest. The numbers do not represent rigid thresholds, but rather represent temperatures above which adverse effects are more likely to occur. In the interest of simplicity, important differences between various species of Pacific salmon are not reflected in the recommendations. The values thus reflect the characteristics of the guild in general, and any one species may appear to be a little more or a little less protected when their needs are compared to the recommendations. The values were reviewed, however, in recognition of how the species in these guilds temporally and spatially express themselves across the landscape. For example, coho salmon tend to require slightly colder temperatures for optimal incubation than other Pacific salmon. Coho usually occur in watersheds along with other salmon that spawn earlier in the fall, however, and thus application of the recommendations will generally provide that waters are on a fall cooling trend that creates healthy conditions for both earlier and later spawning species. 2. Recommendations for the Char Guild Bull trout and Dolly Varden define the char guild in this guidance. These species are the region s most temperature- sensitive salmonids, and research has repeatedly suggested that summer Page 1 of 6
maximum temperatures are one of the most important environmental constraint affecting their health and distribution in our region s streams. It is recommended that temperature thresholds that fully protect char include the following: Spawning, Incubation, and Juvenile Rearing. Summer maximum temperatures should not be allowed to rise above a single daily maximum of 12 C (54 F). Migratory Life Stage. In waters used by migratory fish during the warmest time of the year, temperatures should not be allowed to rise above a single daily maximum of 12 C (54 F). The important distinction between the recommendation for spawning and juvenile rearing waters and the recommendation for migratory life stage is that the migratory life stage recommendation may be applied on a seasonal basis to specific portions of watersheds, whereas the spawning waters threshold value is to be applied year-round. The EPA recommends establishment of a single summer maximum temperature for waters used for spawning and tributary rearing in part to create some simplicity in its assignment and implementation and to reflect our knowledge of distribution of the different life stages. Maintaining water bodies at the farthest downstream extent of the early rearing use at a maximum summer temperature of 12 C, in recognition of natural fall cooling patterns, also assumes that sufficient water will be available for spawning and incubation in the early fall when such activity commonly begins. To reduce mortality of incubating eggs and developing larvae, temperatures must be on a downward trend and decline to 6 C in the weeks immediately following the initiation of incubation. The EPA assumes that with a seasonal drop in stream temperatures the average temperatures throughout incubation will remain in the range of 2-4 C. In order to ensure protection is occurring using this simplified framework, it will be important for states and tribes to cross check temperature patterns in watersheds against these incubation requirements to ensure that this expectation is being met on the ground. In site specific cases, where the char spawn in July or early August (during the warmest time of the year), the single day maximum will need to be adjusted to protect the spawning and incubation life-stage. lifestage. Char typically rear for the first one to three years in the same tributary where they first emerged from the gravels. The density and health of juvenile fish is supported by temperatures at or below 12 C, and the probability of juvenile occurrence is relatively high in portions of streams with daily maximum temperatures at or below 12 C. The occurrence and health of char populations quickly declines as temperatures increases above 12-13C. In recent laboratory studies examining optimal growth of char, an average daily temperature of 12 C has been shown to produce the best growth under variable diet constraints., This optimal laboratory estimate matches closely with the distributional patterns of these juvenile fish in the natural environment. This lends further strength to the 12 C single daily maximum recommendation. The recommendation of a 12 C single daily maximum temperature to protect the migratory life Page 2 of 6
stage of char is based on both the general patterns of health and occurrence noted above for juvenile fish and on numerous studies of salmonids that show larger, more mature fish do not have higher temperature thresholds than smaller, younger fish of the same species. It is also based on field studies showing that adult and sub-adult char tend to move out of waters once they warm to greater than 12 C. The flexibility to compare this value only to where and when the char actually occur in these waters is based on recognition that these older fish are more capable of migrating to more preferred waters and making use of thermal refuges along the way. This recommendation may need to be revisited in the future as more research is conducted on migratory char and their interaction with ambient temperatures in waters of the Pacific Northwest. 3. Recommendations for the Cold Water Guild The cold water guild is defined by the presence of one or more of the following species of fish: any of the five Pacific salmon, anadromous steelhead trout, resident and anadromous coastal cutthroat, or coastal rainbow trout. Waters used by species of the cold water guild are expected to be supported under the following temperature limits: Spawning and Incubation. The seven-day average of daily maximum temperatures should not exceed 13 C (55 F), and the weekly mean temperature should not exceed 10 C (50 F) during the incubation period. Juvenile Rearing. The seven-day average of daily maximum temperatures should not exceed 16 C (61 F), and the weekly mean temperature should not exceed 15 C (59 F). This temperature limit will also protect the smoltification (physiological preparation for salt water residence) of all of the species of the guild with the exception of steelhead, which would require cooler temperatures as specified below. Steelhead Smoltification. To support the smoltification ability of juvenile steelhead trout, temperatures in tributary systems containing steelhead trout should not exceed a sevenday average of the daily maximum temperatures of 14 C (64 F), and a weekly mean of 12 C (54 F). This threshold should be applied at the scale of a 4 th level HUC watershed (for example, the mouth of the John Day River in Oregon, and the mouth of the Wenatchee River in Washington). Water temperatures necessary to support actively migrating steelhead smolts in mainstem rivers such as the Columbia and Snake are unknown. EPA determined that there is considerable scientific uncertainty whether thresholds for other sensitive life stages (i.e. juvenile rearing, adult migration) will provide water temperatures that sustain migratory behavior and sea-water readiness in advanced steelhead smoltification. However, steelhead smoltification thresholds applied to 4 th level HUC watersheds are expected to improve water temperature conditions in Page 3 of 6
mainstem Columbia and Snake river migration corridors. Furthermore, recent historic spring water temperature conditions in the Columbia and lower Snake rivers will not exceed our recommendation for steelhead smoltification. Adult Migration. To support adult migration, the seven-day average of the daily maximum temperatures should not exceed 18 C (64 C), and a weekly mean of 16 C (61 C). This recommendation should be used only in portions of water bodies, or at times of the year, where the adult migration is the only use by the species of this guild. If migration and rearing were to occur simultaneously, the cooler rearing threshold would take precedence. Likewise, spawning and incubation would be more thermally sensitive uses that would take precedence over the rearing or adult migration thresholds. This cold water guild is composed of the greatest variety of key species, and EPA established recommendations in recognition of the general requirements of the species that comprise this guild, rather than basing them strictly on the most sensitive life-stage for the most sensitive species in the guild. Thus some compromise produced recommended thresholds that will cover this entire guild to result in a recommendation package that is far less complicated than what would have been necessary if each species received separate recommendations. The one clear exception is where EPA made a separate recommendation to protect steelhead trout. The research examined and experts consulted on this topic support the contention that smoltification in steelhead trout is uniquely sensitive to temperatures above 12-14 C. Initiation of smoltification and the early stages of smolt migration may be most sensitive to inhibition by warmer water temperatures. While the information reviewed suggested that a temperature threshold of 12 C would protect physiological adaptations necessary for successful sea-water entry of steelhead smolts, research also indicated that several weeks of higher peak temperatures could inhibit smolt development in steelhead trout. For this reason, EPA recommends a seven-day average of the daily maximum temperature of 14 C to limit the extent of daily maximum temperatures during steelhead smoltification. The current state of scientific knowledge concerning smoltification in steelhead trout supports the position that once steelhead smolts are actively migrating, it is unlikely that brief periods of slightly warmer than optimal water temperatures will result in precocious development, parr reversion, inhibition of migratory behavior, or significantly reduce sea-water adaptation. The EPA considered growth rates, maximum swimming speed, maximum metabolic scope, and disease risks in recommending temperature levels that support juvenile rearing of this guild. Considering growth rates, EPA recognized that we cannot assume that fish will be receiving an unlimited supply of food during the warmest period of the summer rearing season. This recognition resulted in bringing the recommendation down a couple of degrees from the maximum growth rates that have been observed in controlled laboratory experiments where the fish were fed to satiation. This decision is consistent with both the demonstrated effects of reduced rations and with the recommendations of many of the key papers reviewed. Even making this adjustment, the recommended values are expected to result in less than maximum growth during the peak summer period. However, in reviewing temperature profiles of rivers of Page 4 of 6
the Pacific Northwest it is clear that periods of sub-optimal growth due to higher temperatures will be kept sufficiently restricted and balanced with periods of sub-optimal growth due to colder water temperatures. Based on examining the growth rates occurring over longer-term experiments, the minor effects allowed with the rearing recommendation will not create a meaningful or detectable change in the maximum growth occurring at the end of the entire summer growing season. In establishing the incubation recommendation, EPA recognized that excellent incubation success has been repeatedly observed in tests that were begun at temperatures of 13-14 C but were made to cool sharply over the following approximately two weeks to levels that have been found to be optimal (6-10 C) under constant temperature exposures. Further these studies acknowledged that the average temperature regime rather than the daily maximum temperatures, within reasonable limits (e.g., within limits of approximately 2-12 C), is the most influential factor on the health and survival of eggs and embryos. Temperatures above 13 C have also been associated with significant losses in eggs even while still retained unfertilized in the body cavity of female fish, and evidence is strong that maintaining water temperatures in the range of 12-14 C on the spawning grounds reduces disease and pre-spawning mortality in salmon species. These factors combine to suggest that maximum temperatures of 14 C or higher may pose several direct risks of mortality to spawning fish and their offspring and should be avoided, and that average weekly temperatures of 10 C and lower over the course of incubation may be necessary to ensure that there are not direct losses or harm to fertilized eggs and developing embryos. The EPA recognizes that if this recommendation is applied to the early portion of a fall spawning period or towards the end of the incubation period for spring spawning stocks, it will result in conditions during the preponderance of the incubation period that are solidly in the range of what has repeatedly been determined optimal for these cold water species (6-10 C). In recommending temperatures that support adult migration, EPA particularly considered temperatures that allowed for sustained swimming performance, that minimized the risks of severe disease outbreaks, and that would not produce unreasonable metabolic demands on the energy reserves of migrating fish when experienced for short periods of time (days versus weeks) during migration through main-stem river reaches. The temperatures recommended provide that during the warmest portions of the day water temperatures still remain within what is optimal or near optimal for negotiating obstacles and swift currents, while remaining below levels that have been shown in field studies to reduced survival (i.e., increase pre-spawning mortality) of migrating adult salmon and their gametes. 4. Recommendations for the Moderately Cold Water Guild The moderately cold water guild defined by the presence of either interior nonanadromous redband trout or Lahontan cutthroat trout. The recommendations for this guild are to be compared to water bodies where members of the two colder water guilds are typically absent. Page 5 of 6
The EPA expects, waters used by species of the cold water guild to be supportive under the following temperature limits: Spawning and Incubation: The seven-day average of the daily maximum temperatures should not exceed 13 C (55 F), and the weekly mean temperature should not exceed 10 C (50 F) during the incubation period. Juvenile Rearing: The seven-day average of the daily maximum temperatures should not exceed 20 C (68 F) While species of this guild can thrive under summer water temperatures warmer than what is supportive of the cold water guild, their incubation requirements appear identical. Both these traits are at least partially due to these species spawning in the spring, which enables the species to select the period of time in the winter-to-spring period at which waters first become warm enough to support incubation. This still makes the species susceptible to harm from rapid spring warming that may exceed safe incubation thresholds, but does provide some natural resiliency. Further, the species of this guild are specific stocks or subspecies of the cold water guild that have evolved in warmer waters, and being resident fish that must cope with ambient stream conditions throughout their life-span, appear to have developed somewhat warmer optimal temperature thresholds. While these higher optimal thresholds are not dramatically higher than those exhibited by the species of the cold water guild, as compared to the preferences of warm water species, they are meaningful enough to warrant establishing separate temperature recommendations. Since these species are resident species, it appears that there is only a need to specifically protect conditions appropriate for the two discreet life-stages of incubation and rearing, although, it is possible that there may be a need to establish separate considerations if juveniles and adults fish are found to have different sensitivities to temperature. Due to the relative lack of research specifically testing the species of this guild in general, EPA needs to revisit these recommendations in the future when new research becomes available. Page 6 of 6