The Potential Effects of Anthropogenic Climate Change on Freshwater Fisheries

THE POTENTIAL EFFECTS OF ANTHROPOGENIC CLIMATE CHANGE ON FRESHWATER FISHERIES

Ashley A. Ficke

Christopher A. Myrick, Ph.D.

Department of Fishery & Wildlife Biology

Colorado State University

Fort Collins CO 80523-1474

August 2004

Abstract

[H1]The purpose of this review is to explore the likely effects of climate change on the world’s freshwater fisheries. First, global warming will affect fish populations through direct temperature effects on physiology. All freshwater fish are poikilothermic and their physiological mechanisms are directly or indirectly temperature – dependent. The optimal physical and biological ranges of a fish species are determined by temperatures that are conducive to efficient metabolism, reproductive success, and disease resistance. Any changes in those temperatures, including those predicted to result from global climate change, will result in local extirpations and range shifts. Climate change is also expected to affect fish populations through its influence on physical environmental factors such as water chemistry and physical limnology. Warmer water contains less dissolved oxygen than colder water. Since fish metabolism increases with elevated water temperature, climate change will likely result in increased oxygen demand and reduced supply. Higher temperatures will tend to increase duration and strength of thermal stratification in temperate zones. Lentic (lake) environments also depend upon wind – driven mixing, so changes in weather patterns will affect their function. Mixing regimes strongly influence the community of primary producers that in turn influence lentic food webs and their associated fish communities. Hydrologic regimes may also be affected. Melting of polar ice caps will result in a sea level rise that would inundate important freshwater habitats. Also, fish have evolved with their current local hydrologic conditions—possible changes in these environmental constraints will present them with new challenges to survival and reproductive success. Finally, increased temperatures could also affect the toxicity and bioaccumulation of anthropogenic pollutants. The socioeconomic importance of the world’s fisheries are also briefly discussed in order to emphasize the stakes involved in a failure to manage greenhouse gas emissions.

Abstract......

Table of Contents......

Introduction – Predicting the magnitude of global climate change......

Fish Physiology......

Temperate fishes

Tropical Fishes

Water Chemistry......

Dissolved Oxygen (DO)

Eutrophication, Macrophyte Growth and Primary Productivity......

Eutrophication

Water Temperature Effects on Limnology

Thermal Habitat Space, Thermal Refuges, and Changes in Fish Communities

Fish Distributions and Temperature Barriers......

Disease and Parasitism......

Water Balance: the hydrologic Cycle......

Temperature and Toxicology......

Socioeconomic Effects......

Conclusion......

Literature Cited......

Species......

Introduction – Predicting the magnitude of global climate change

Following the Industrial Revolution, humans have increasingly relied on fossil fuels for power and transportation. Currently, about 80% of the world’s power is generated from fossil fuels (Bolin et al. 1986). While undoubtedly beneficial, combustion of fossil fuels produces carbon dioxide (CO2), nitrous oxide (NOX), and methane (CH4), all commonly referred to as “greenhouse gases.” In recent years, atmospheric greenhouse gas concentrations have been increasing; this has caused some concern because accumlations of these gases affect the global climate (Bolin et al. 1986). Simulations of the Earth’s climate using models that tracked natural variability in greenhouse gas concentrations could not account for recent climatic changes. When anthropogenic perturbations (i.e., increased atmospheric CO2 concentrations) are incorporated in the models, the predictions closely follow current conditions .

Predicting trends in the Earth’s climate is difficult, but it can be done using a variety of techniques. Mathematical simulations of the Earth’s climate such as global circulation models (GCMs) are one tool commonly used to predict changes in the earth’s climate. As is the case with all models, GCMs require validation, most commonly achieved by comparing their predictions with observed climatic conditions (Rodo and Comin 2003). These comparisons have revealed that while GCMs work reasonably well on a global scale, their inability to work at a finer resolution limits their ability to simulate climate on a regional scale (Bolin et al. 1986; Melack et al. 1997). The Intergovernmental Panel on Climate Change (2001) employed several GCMs and found an increased likelihood of a 1 – 7°C increase in mean global tempeature within the next hundred years. The magnitude of the temperature increase regionally is correlated with latitude—higher latiitudes are predicted to experience a larger temperature change than tropical and subtropical latitudes . Advances in modeling techniques and computer technology over the last decade have increased the accuracy of global circulation models, and they are now considered a powerful tool for tracking and predicting climate change. While these models provide insight into future climate change, they do not predict what that altered climate will mean for natural systems.

In order to predict the effects of climate change, techniques such as “forecasting by analogy” are used. This method examines the changes observed during natural anomalous warming events such as El Niño – Southern Oscillation (ENSO) events (Gunn 2002), or artificial situations where chronic temperature change occurs (e.g., thermal plumes from power plants) (Langford 1983; Schindler 1997). The advantage of this technique is that it gives us the opportunity to gather empirical data on how humans and environments react to thermal changes of the same magnitude as those expected with global climate change (Glantz 1996). Data collected using this technique spans short time intervals, as few anomalous events last for more than a decade. Though observational studies can offer an integrated view of the effects of “thermally enhanced” environments, mechanisms that increase water temperature carry their own disruptions in addition to elevated temperatures. For example, thermal plumes often contain pollutants (Langford 1983), and El Niño years are often characterized by anomalous weather patterns (Rodo and Comin 2003; Timmermann et al. 1999). Furthermore, it is also quite possible that climate change will alter existing weather patterns e.g. (Palmer and Räisänen 2002; Timmermann et al. 1999). Therefore, teasing out the effects attributable solely to anthropogenic forcing can prove to be difficult.[H2]

Paleoclimatic data, or information about prehistoric climatic conditions, can be obtained from trees, glacial ice cores, sediment cores, and corals (Melack et al. 1997; Spray and McGlothlin 2002). Data from these sources have shown that post–Industrial human actions have greatly changed atmospheric greenhouse gas concentrations. The concentrations of CO2, NO2, and CH4 remained more or less stable in the tens of thousands of years preceding the industrial revolution . However, once humans began burning fossil fuels, levels of these gases began to rise. Carbon dioxide concentrations are 31% higher than pre–industrial levels, NOx is 16% higher, and CH4 is 150% higher. The current concentrations are higher than any observed in the last 42,000 years .

The effects of global climate change can also be studied through the examination of recent trends in the earth’s climate. For example, the 1990’s was the warmest decade on record, and 1998, an ENSO year, was the warmest year ever recorded . Mann et al. (1998) conclude that the mean global temperature has risen by 0.3 – 0.4°C in the last 60 – 70 years (Mann et al. 1998). Judging from paleoclimatic data, current temperatures have reached maximum temperatures seen in other interglacial periods; this providessome further evidence that global warming is not natural (Spray and McGlothlin 2002). In North America, mean annual temperatures have risen by 1 – 2°C after having been fairly stable for the last 40 – 50 years (Schindler 1997). Though the majority of studies on the magnitude of climate change have focused on changes in air temperature, there should be substantial concern for aquatic ecosystems as well. Water temperatures in aquatic systems, particularly in relatively shallow (compared to the ocean) rivers, lakes, and ponds are highly dependent upon air temperature (Boyd and Tucker 1998; Meisner et al. 1988). For example, the predicted increase in temperature in British Columbia lakes under an average global [H3] warming scenario of +4°C ranged from 3 to 5°C depending upon lake depth. Water temperatures at the surface of a lake with an average depth of 10 m would likely increase by about 3°C with a 4° C increase in temperature (Northcote 1992).[H4]

Climate change has also affected hydrologic characteristics such as precipitation and evaporation. On a global scale, precipitation is expected to increase ; however, this increase in precipitation may come in the form of more frequent “extreme” events (Palmer and Räisänen 2002). On a regional scale, winter rainfall will most likely increase in the mid and high latitudes of the Northern Hemisphere[H5]. As rain replaces snow as the dominant form of precipitation in these regions, a change in hydrologic regimes can be expected. This will be discussed in more detail in subsequent sections. Precipitation is expected to increasein the African tropics, and it will likely decrease in Australia, South America, and southern Africa. Summer rainfall is expected to increase in southern and eastern Asia (IPCC 2001). Current trends also show a decrease in snowpack and ice cover. In the high latitudes of the Northern Hemisphere, snowpack has decreased by approximately 10% since the late 1960’s, and rivers and lakes have lost, on average, 2 weeks of ice cover (Change 2001). These changes in global hydrologic regimes and thermal regimes will impact the majority of aquatic ecosystems, including those that support freshwater fisheries.

The global effects of climate change mean that freshwater ecosystems, and the fisheries therein, will be affected to some degree. The purpose of this review is to address the question of how much a variety of freshwater fisheries will be affected by global climate change. The degree to which an individual system, a particular species, or even a single population will be affected is difficult to predict. Researchers can use laboratory studies to try to isolate the causal mechanisms behind temperature–related changes in behavior, physiology, or ecology. Field experiments can be used to observe population–level or ecosystem–level responses to changes in the thermal regime while integrating the effects of multiple variables[H6]. For this review we have used a synthesis of data from both types of studies to help answer the question: [H7]what are the potential effects of anthropogenic climate change on the world’s fisheries?

Fish Physiology

With the exception of a few marine pelagic species, fish are poikliotherms that thermoregulate behaviorally but not physiologically (Moyle and Cech 1988). Behavioral thermoregulation is constrained by the range of temperatures available in the environment, so a fish’s temperature can be assumed to be very similar to the environmental temperature. Because biochemical reaction rates are largely a function of temperature, all aspects of an individual fish’s physiology are directly affected by changes in temperature[H8]. Biochemical and physiological reactions occur can be quantified by the Q10, a dimensionless number that measures the magnitude of the rate change over a 10°C range (Franklin et al. 1995; Schmidt-Nielsen 1990; Wohlschlag et al. 1968). The ramifications of this are obvious: global warming will affect individual fish by altering physiological functions such as growth, metabolism, food consumption, reproductive success, and the ability to maintain internal homeostasis in the face of a variable external environment. This in turn means that global climate change will affect fish populations, and ultimately fishery and ecosystem productivity as each component species adjusts to the new thermal regime.

All fishes must allocate energy from consumed food to their energy budget, represented by the equation below (Warren and Davis 1967):

C = (Mr + Ma + SDA) + (F + U) + (Gs + Gr);

where C = energy consumption rate, Mr = standard metabolic rate, Ma = metabolic rate increase because of activity, SDA = energy allocated to specific dynamic action (food digestion and processing), F = waste losses due to fecal excretion rates, U = waste losses due to urinary excretion rates, Gs = somatic tissue growth rate, and Gr = reproductive tissue growth rates. The amount of energy allocated to each of these compartments is temperature–dependent, and generally increases with temperature.

Additionally, at any given temperature, fish must allocate energy to Mr. As long as the consumption rate exceeds Mr, a fish can allocate energy to other compartments, and when there is surplus energy, it can be used for activity or growth. This “surplus” energy is known as the “metabolic scope”, and this tends to be highest at the fish’s metabolic optimum[H9] temperature. Increases in temperature decrease the metabolic scope (Brett 1971; Elliot 1975a), through a variety of mechanisms including cardiac inefficiency (Taylor et al. 1997) and an increased cost of repairing heat–damaged proteins (Somero and Hofmann 1997). If fish are exposed to high enough temperatures that they become thermally stressed, they experience problems with osmoregulation (Boyd and Tucker 1998), possibly due to increased gill permeability at higher temperatures (Somero and Hofmann 1997). In general, a reduction in metabolic scope leads to decreased swimming performance (Brett 1971), reduced reproductive output (Van Der Kraak and Pankhurst 1997; Webb et al. 2001), lower growth rates (Brett 1971; Kitchell et al. 1977), and, in extreme cases, mortality (Kitchell et al. 1977).

All fish have a thermal range bounded on the upper end by their critical thermal maxima (CTMax) and on the lower end by their critical thermal minima (CTMin) (Becker and Genoway 1979; Fry 1971). These critical thermal limits represent temperatures that the fish can tolerate for a few minutes, at best, and they can be slightly increased or decreased if the fish is acclimated to a sub–lethal temperature approaching the lethal temperature (Myrick and Cech 2000; Myrick and Cech 2003). Temperatures that fish can tolerate for a few minutes to a few days are referred to as the incipient lower lethal temperatures (ILLT) and upper incipient lethal temperatures (UILT) (Myrick and Cech 2000). Although fish will eventually perish at these temperatures, they can tolerate them for longer intervals than their critical thermal limits. As temperatures move farther away from the incipient lethal temperatures, they enter the suboptimal range where physiological performance may be reduced, but the fish is not going to die. Finally, there is a narrow range where physiological performance is near the optimum; temperatures within this range are known as the optimal temperatures. Given enough time to acclimate to a changing thermal regime, most fishes can adjust the ranges of their critical, incipient lethal, suboptimal, and optimal temperatures up or down by a few degrees, but there are limits to the amount of and speed at which thermal acclimation can occur.

Thermal ranges are species–specific, as there are stenothermal (narrow thermal range) species like lake trout (Salvelinus namaycush) and peacock pavon (Cichla ocellaris), and eurythermal (wide tolerance range) species like common carp (Cyprinus carpio) and bluegill (Lepomis macrochirus). Climate change–related increases in global temperature are a concern because ambient thermal conditions may begin to approach [H10]suboptimal conditions for certain fishes, or, in some cases, bring them closer to their incipient lethal temperatures. Faced with such changes, one can expect fish populations to come to a new equilibrium dictated largely by the energetic costs of coping with a new thermal environment. Some species may increase or decrease in abundance, others may experience range expansions or contractions, and some species may face extinction. The fate of a particular species will depend on the following factors:

Whether it is stenothermal or eurythermal and what region it inhabits (arctic, subarctic, temperate, subtropical, or tropical).
The magnitude of the change in the thermal regime in that ecosystem.
The rate of the thermal regime change.
Changes in the abundance of sympatric species that may be prey, predators, or competitors for resources.

Temperate fishes

Fish growth is temperature–dependent and generally increases with temperature to an optimal level before decreasing again (Kitchell et al. 1977; Myrick and Cech 2000). This optimal growth temperature varies with species (Langford 1983). Fish in temperate ecosystems undergo about 90% of their annual growth in the summer months (Wrenn et al. 1979) because food availability tends to be highest and water temperatures approach growth optimums. In these cases, a slight increase in water temperature could be beneficial because of the extension of their growing season (Hill and Magnuson 1990; Kling et al. 2003), provided that food resources can support a higher consumption rate (Shuter and Meisner 1992) and interspecific interactions are not increased. Milder winters could also reduce overwintering stresses, such as food limitation, that can be significant for some temperate fishes (Boyd and Tucker 1998). An increase in over–winter survival combined with slightly elevated water temperatures could increase the productivity of fisheries that are currently limited by temperatures below the species’ growth optimum.

Regardless of the state of food resources, an increase in temperature causes an increase in metabolic rate, and a subsequent increase in the amount of energy needed. For example, food intake was found to be highly temperature dependent for five cyprinids from Lake Balaton (Hungary). The daily intake of bream (Abramis brama), silver bream (Blikka bjoerkna), roach (Rutilus rutilus), gibel (Auratus gibelio), and common carp increased exponentially when temperatures were increased from 5 to 25°C (Specziár 2002). In sockeye salmon (Oncorhynchus nerka), food consumption triples between 2.5 and 17.5°C, but decreases at temperatures above 17.5°C (Brett 1971). Elliot (1975a, 1975b) found similar results in his studies of brown trout (Salmo trutta). In food–limited environments, food intake cannot keep pace with metabolic demand. For example, common carp cultured at 35°C developed a vitamin C deficiency and grew more slowly than those cultured at 25°C when both experimental groups received the same rations (Hwang and Lin 2002). In 1975, a pair of studies using brown trout found that the temperature for optimum growth was 13–14°C when fish were fed on maximum rations (Elliot 1975b) and 9–10°C when fish were fed on 50% rations (Elliot 1975c). A 1999 study using rainbow trout (Oncorhynchus mykiss) found that fish fed limited rations experienced significantly lower growth rates when held in water 2°C warmer than ambient temperatures (Morgan et al. 1999). Since trout are often food–limited in the summer months (Morgan et al. 1999), climate change will probably lower the carrying capacity of trout–dominated systems. Studies on cold– and cool–water species like lake trout, whitefish (Coregonus commersoni), and perch (Perca spp.) only predict increased fish growth rates if the food supply meets increased demand. Otherwise, decreased growth rates can be expected (Gerdaux 1998; Hill and Magnuson 1990). It should be noted, however, that if the temperature increase is large enough, no increase in food availability will be sufficient to meet increased metabolic demand (Myrick and Cech 2000). In most species, feeding activity is depressed at temperatures above species–specific optimum (Brett 1971; Kitchell et al. 1977).