Physiological Responses

-->Climatic shifts create changes in the environment that first and foremost affect the physiology of organisms. The functional limitations and consequent responses of an organism’s anatomical systems represent the most primary and immediate impact of climate change on a species survival. For aquatic species, warming of the earth’s waters is one such significant physiological concern.

All organisms - terrestrial or aquatic, plant or animal – exist within a limited range of body temperatures. Molecular, cellular, and systemic processes proceed at optimal rates within specific thermal windows, and the coordination of these for large-scale operations (e.g. swimming) depends on this efficient functionality. The effects of shifts in temperature are evidenced through declines in performance, manifested in changes in behavior rate, growth, reproduction, etc. Extreme temperatures drive these operations to their functional limits, as low temperatures impede sufficient metabolic rates and high temperatures result in the denaturing of enzymes. In animals, performance is linked to aerobic scope (the increase in oxygen consumption rate from resting to maximal), as designated by Porter and Farrell; therefore, declines in temperature-related performance are explained by a temperature-dependant limited oxygen supply [10].

Coho salmon thermal tolerance windows, by life cycle stage [8].

In the same paper, Portner and Farrell describe how, for salmon species, the intensity of direct warming effects differs across the life cycle. Temperatures outside the thermal tolerance window of a particular life stage have the potential to cause cardiac collapse and subsequently mortality, with some stages exhibiting greater susceptibility due to narrower aerobic thermal windows. Spawning adults display a heightened sensitivity to temperature, both as a result of large body size as well as the increased oxygen demand of supporting a large mass of eggs. Larval fish also experience narrow thermal windows, limited in their oxygen supply due to developing respiratory and circulatory systems [10]. Given this differential vulnerability to temperature, even small temperature increases in estuaries and streams that comprise migratory and spawning habitat have the potential to disproportionately reduce species abundance by targeting the narrow thermal windows of life stages that directly determine future generations of salmon.

Crozier et al. addresses this sensitivity to temperature increases in respect to the timing of migration from the ocean to freshwater spawning grounds. Much of the Pacific Northwest rearing habitat for salmon experiences temperatures above optimal ranges, at times even reach lethal levels, so current migration times are limited by seasonal changes in temperature of rivers. Behavioral phenotypic plasticity allows salmon to persist through such unfavorable macro conditions, as evidenced in the deliberate occupation of cold-water refuges to avoid exposure to high temperatures. This ability to alter behavior to avoid warm water may buffer some of the potential affects of climatic warming, but comparative studies of populations inhabiting different thermal environments reinforce the existence of an absolute upper limit to thermal tolerance. Though populations of salmon may exhibit differential survival across a thermal gradient, all observed evolutionary adaptations only conferred increased fitness below 23°C. Adaptation to heat tolerance is possible, but the apparent upper limit is concerning when climatic warming may force water temperatures beyond the evolutionary scope of salmon thermal tolerance [11].

Average summertime temperature estimates for Washington state, 1980s and 2040s. Continuous colors indicate air temperatures and colored dots represent water temperature at specific sites [9].

             Further challenge to evolutionary adaptation to changing conditions is the anadromous nature of salmon. The use of both fresh and salt water for vital life stages involves precisely-timed transitions, so differential selection pressures could potentially result in mismatch between the adaptive needs for stages on either side of these transitions. Plastic behavioral responses can be selected for as adaptive traits by persisting environmental conditions, but evolving freshwater genetic responses may have deleterious effects in respect to marine timing or conditions, and vice-versa. In addition, Etterson et al. proposes that genetic traits under selective pressure may be antagonistically correlated with other unrelated traits. These genetic antagonists could ultimately reduce the fitness of the species if the correlated trait hinders the success of the organism more than the selected trait provides advantages under changing environmental conditions [12]. The net result of these complications to evolutionary response will most likely be a lag in the ability of salmon species to adapt quickly enough to keep pace with the speed of climate change, as well as a disruption in life cycle synchrony that will threaten the ability of species to effectively replace their populations.