Assessing potential effects of global climate change on ... - Springer Link

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GeoJournal 28.1 21-27 © 1992 (Sep) by Kluwer Academic Publishers

Assessing Potential Effects of Global Climate Change on Tropical Freshwater Fishes Meisner, J. D., Dr., ESSA Environmental and Social Systems Analysts Ltd., 9555 Yonge Street, Suite 308, Richmond Hill, Ontario L4C 9M5, Canada; Shuter, B. J., Dr., Ontario Ministry of Natural Resources, Fisheries Research, Maple, Ontario L6A 1S9, Canada ABSTRACT: Climate change will affect both the quality and the quantity of water in freshwater environments. Changes in air temperature and precipitation will alter the annual water temperature cycle and the annual water level cycle. Initial assessments of the effects of climate change on freshwater fishes of North America have focused on the potential thermal effects of climate change because of the strong influence of water temperature on the life histories of temperate fishes. Paralleling the influence of water temperature on temperate fishes is the influence of water quantity on tropical fishes, especially on riverine fishes. The extreme seasonal fluctuations in discharge of tropical rivers, through the effects on fish habitat availability and habitat conditions, have produced a range in preferred habitat types among riverine fishes. We suggest that initial assessment of potential effects of climate change on tropical freshwater fishes should focus on changes to water quantity variables in riverine environments. Assessments should begin at a coarse level of resolution. A useful starting point would be comparative analyses of the sensitivity ofblackfishes and whitefishes to changes in habitat availability, oxygen levels and desiccation stress that will likely accompany changes in the seasonal flow regimes of tropical rivers.

Introduction Atmospheric scientists believe that the record increases in global temperatures since the early 1980s are the result of an increase in the "greenhouse effect" caused by the steadily increasing concentrations of CO2 and other radiatively active gases in the lower atmosphere. C o n c e r n for global climate change as early as the b e g i n n i n g of the next century (Bolin et al. 1986; H a n s e n et al. 1988; Hengeveld 1991; Jaeger 1988) has stimulated m u c h study of the potential effects of a "warmer" climate amongst all sectors in North A m e r i c a (eg, Topping 1989; Smith and Tirpak 1989; Wall and Sanderson 1990). A s s e s s m e n t of potential effects of climate change o n freshwater fishes and fisheries of N o r t h America has b e g u n (eg, special climate

This paper formed part of a symposium held at the XVII Pacific Science Congress in Honolulu, Hawaii June 1991.The theme of the symposium was "Aspects of Global Environmental Change: Freshwater Ecosystems" which focused on southern Pacific environments.

change issue of ~Dansactions of the A m e r i c a n Fisheries Society, Vol. 119, 1990). I m p o r t a n t global climate variables that are expected to change in the next decades with respect to freshwater fish habitat are air temperature and precipitation (Mitchell et al. 1990). Changes in these variables will affect water temperature, water quantity and water quality variables of freshwater e n v i r o n m e n t s which are the three primary linkages b e t w e e n climate and freshwater fish (Regier and Meisner 1990). Assessments of the potential effects of climate change on fishes of North America have b e e n d o m i n a t e d by potential thermal effects. The focus on the effects of changes in water temperature is due to the p r o f o u n d influence that the seasonal water temperature cycle has o n the life history and reproductive success of temperate freshwater fishes. Some i m p o r t a n t c o m p o n e n t s of the water temperature cycle, with respect to temperate freshwater fish, that are expected to change with climate are seasonal m a x i m u m and m i n i m u m temperatures, and the timing of critical events such as freeze-up and the spring thaw. In this investigation we use the experience

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gained in these earlier studies to identify approaches for assessing the impact of climate change on tropical freshwater fishes.

Assessment of Effects of Climate Change on Temperate Fishes

Three general approaches to assessment of the potential effects of global climate change on freshwater fish and fish habitat have been identified (Meisner et al. 1987): 1) Comparative analyses of the response of taxa to the range of climates to which they are already exposed. Effects would be predicted using equations, such as the form Y = a + bx, that describe steady state responses of a biotic variable to a range in climate conditions. 2) Analyses of effects on fishes of increases in habitat temperature not due to climate change, eg, effects of riparian deforestation and nuclear power development. 3) Simulation of stream/lake temperatures in a changed climate with hydrometeorological models. Information from these models combined with relationships from 1 and 2 permit initial prediction of effects on fish. These three approaches are directed at potential thermal effects of climate change, and all depend on extensive knowledge of the influence of habitat temperature on fishes. Qualitative and quantitative relationships between habitat temperature and various aspects of the life histories of fishes, (ie, 1 and 2, above) have provided tools with which to study potential effects of changes in water temperature on fishes at different levels of organization, (ie, organismal to species geographic distribution, Regier et al. 1990). Examples of tools derived from the thermal ecology of fishes of North America include techniques for determining the thermal bounds for hydrological and geographical distributions of different fish species, mechanistic models of the response of fish population parameters (ie, growth rate, age of maturity, mortality) to habitat temperature, and empiric relations between fisheries yield and variables of water temperature. These tools, combined with models that predict the response of water temperature to changes in air temperature (eg, Brown 1969; Shuter et al. 1983), have allowed fisheries workers to develop credible, quantitative assessments of the potential effects of climate change on wild populations of several North American fish species. Fisheries workers have used these tools in assessments of the thermal effects of climate change on fishes of North America (Shuter and Meisner this issue, GeoJournal 28, 1, 7-20 [1992]). Initial assessments of effects of climate change have addressed species' hydrologic and geographic distributions (Magnuson et al. 1990; Meisner 1990 a,b; Shuter and Post 1990), growth rates (Hill and Magnuson 1990), fisheries yield (Magnuson et al. 1990), species

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invasions (Mandrak 1989), and community structure (Regier et al. 1989). Assisting with this work has been the concept of the species' thermal niche and its spatial and temporal representation as thermal habitat (Magnuson et al. 1979), and the cold- cool- and warmwater thermal guild classification (Hokanson 1977). The thermal niche provides a simple prescription for integrating species-specific knowledge of habitat preference with abiotic measures of environmental conditions into quantitative measures of habitat size that are meaningful at the population level (Christie and Regier 1988). Hokanson's classification provides a rational basis for selecting species for study that are representative of an identifiable class. We show below that there are analogous concepts for the tropics that may be useful. The influence of the seasonal water level cycle on tropical fishes parallels the influence of the seasonal water temperature cycle on temperate fishes (Lowe-McConnell 1987; Munro et al. 1990; Welcomme 1979). At temperate latitudes, the air temperature cycle is the dominant characteristic of climate and is strongly correlated with the water temperature cycle of freshwater environments (Shuter et al. 1983; Shuter and Post 1990). In the tropics, the annual variation in air temperature is small, but there is a large and predictable annual variation in precipitation (Critchfield 1974). The seasonal precipitation cycle produces wide ranges in river flow rates and water levels, which directly alters the amount of freshwater habitat available for fish and indirectly alters many critical characteristics of that habitat (eg, 02 levels, turbidity, food availability etc.). In the following sections we explore the possibility that the major effects of climate change on tropical freshwater fishes will occur through changes to seasonal water level cycles.

Key Climate-related Habitat Factors Governing Life Histories of Tropical Freshwater Fishes

Our objective here is not to review tropical fish ecology; excellent reviews are available (eg, Lowe-McConnel11987; Welcomme 1979, 1985). Our objective is to identify major attributes of tropical fish habitat and life history strategies that could provide the basis for initial assessments of the effects of climate change. For purposes of this paper we define tropical or equatorial areas as being between about 10 °N and 10 %. The magnitude of the influence of water temperature and water quantity variables on freshwater fish habitat differs between temperate and equatorial regions (Fig 1). Temperate climates are characterized by a broad seasonal range in air temperature and photoperiod with a relatively even temporal distribution of rainfall. Tropical climates, contrastingly, are characterized by a narrow range of annual air temperature and photoperiod, and highly seasonal rainfall patterns (Lowe-McConnell 1987). Seasonality of freshwater fish habitat of tropical areas is determined by precipitation patterns not air temperature

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as in temperate latitudes (Lowe-McConnell 1979). Seasonality in tropical areas increases as one moves away from the equator. At the equator there are generally two rainy seasons as the sun passes overhead during the March and September equinoxes (Lowe-McConnell 1987). North and south of the equator there is a general shift toward one rainy season which coincides with the summer period. Tropical lake and riverine habitat differ with respect to the seasonal variability in their climates. Tropical lake environments are relatively constant and influenced little by variation in climatic variables. A rule of thumb is that fishes of tropical lakes are influenced more by biotic variables than abiotic variables, whereas in riverine environments, abiotic variables have a much greater influence on fishes than abiotic variables (LoweMcConnell 1987). Given the strong influence of the seasonal water level cycle on riverine tropical fishes, and the sensitivity of regional precipitation at tropical latitudes to a "greenhouse"-based climate change (Mitchell 1990), we will focus assessment of the effects of climate change on riverine freshwater fish.

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~~ical Tropical

Temperate

I F

I M

i A

I M

I d

I d

i A

I S

I 0

i N

D

Month

RiverDischargeandArea Tropical

Riverine

Environments

Similar to temperate river systems, tropical river systems can be zoned into upper headwater areas, where currents are swift, transitional downstream reaches, and lowland reaches and deltas, where currents are minimal. In both systems, slope determines the range in water velocity and the resultant diversity of strategies of fishes to water velocity gradients along the respective water courses. However, unlike high latitude rivers systems (Hawkes 1975), temperature does not play a significant role in habitat zonation in tropical rivers. Water temperature along the headwater-river courses in tropical areas only varies significantly when the headwaters are located at high altitudes. Groundwater discharge to tropical streams at low altitudes cannot provide significant low temperature habitat during the summer as found in temperate streams (Meisner et al. 1988) because groundwater temperature has been shown to approximate local mean annual temperature (Collins 1925), which in the tropics, differs little from local mean summer air temperature. Riparian shade and rainfall are

Tab 1 Examples of annual maximum and minimum discharges and inundated areas of some tropical rivers and floodplains, resprectively (modified from Welcomme 1979)

WaterTemperature Fig 1

Generalized seasonal trends in surface water temperature and water quantity variables in north tropical and temperate riverine environments

the major factors that influence stream and river temperatures in equatorial regions (Welcomme 1979). The extreme seasonal variability in annual discharge of tropical river systems leads to tremendous areal flooding of downstream grassland and forested areas (Tab 1), formally known as the floodplains of a tropical river (Welcomme 1979). The Zaire river of Africa, the Amazon river of South America, and the Mekong river of southeast Asia are examples of major river systems that undergo large seasonal fluctuations in flow (Lowe-McConnel11975). During the rainy seasons the tributaries and downstream floodplains of these systems provide thousands of square

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Fig 2

a

Similarityof the dominant influence of water temperature and water quantity on diversity of fish habitat at temperate (a) and tropical (b) latitudes, respectively. During summer at temperate latitudes thermal habitat of fish ranges from warmwater habitat of lowland river reaches and epilimnia of lakes to coldwater habitat of groundwaterrich headwater streams and hypolimnia of lakes. During the dry seasons at tropical latitudes riverine habitat conditions range from relatively benign main channel areas to the extreme conditions of marginal flood pools and lagoons that are isolated from the main river channel.

Wmar

R~ar

b

Low Water Marginal Waters

"~

~-~/

Marginal Waters

Main Channel of River

kilometres of habitat for fish that is not available during the dry season. Tropical riverine fish habitat can be divided into main channel and marginal habitats. During the dry season, river systems are generally dominated by true riverine conditions from headwaters down to river mouths. Marginal, and usually isolated from the main river channel during the dry season, are swamps, lagoons, and floodplain pools (Welcomme 1979). During the rainy season, rivers overflow their banks and the main channel and marginal habitats become one, during which time, the riverine flow conditions of the main channel are all but obliterated. The severity of habitat conditions generally increase with distance from the main channel of a river. This gradient is sharpest during the dry season when the marginal habitats are isolated from the main channel. Conditions in the main channel areas are relatively benign and constant year-round, while the marginal waters

provide extreme habitat conditions for fishes during the dry season. Dissolved oxygen and desiccation are the two major factors governing the severity of riverine habitats (Lowe-McConnell 1987; Welcomme 1979). Dissolved oxygen is the single most important variable governing the distribution of fishes in tropical rivers (Welcomme 1979). During the dry season fishes that do not retreat with receding waters to the main river channel must endure very low oxygen levels and varying degrees of desiccation until the river water returns the following rainy season. Large seasonal changes in pH and conductivity also can occur as a response to seasonal fluctuations in water levels.

Fishes of Tropical Riverine Habitats

Fishes have developed life history strategies to cope with the annual range in habitat availability and severity

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along habitat continua from main channels of rivers to the extreme marginal habitats of the dry season. Fishes that do not venture far from the main river channel are generally more sensitive to habitat change compared to fishes that occupy standing waters far from the main channel during the dry seasons. At a coarse resolution, fishes of tropical riverine environments can be classified as either "blackfishes" or "whitefishes" (Welcomme 1979). This terminology was born in southeast Asia, however, the distinction applies to other tropical regions. Whitefishes are relatively sensitive to low oxygen levels and migrate with advancing and receding floodwaters to avoid the extreme conditions of marginal waters during the dry seasons. Blackfishes are more tolerant of low oxygen levels and tend to occupy the marginal habitat areas during the dry season. Some members of the blackfish group have developed the ability to breathe air as a strategy to overcome anoxic conditions that can develop in marginal habitats during the dry season. The lungfishes (eg, Arapaima gigas) are probably the best known of these fishes (Welcomme 1985). The physiological and anatomical features of these fishes are described in detail elsewhere (eg, Brown 1957). Included in the blackfish group are fishes that can tolerate desiccation through aestivation. Desiccation in isolated pools and lagoons during the dry season represents the most extreme condition to which some tropical fishes have become adapted. Stranding in waters isolated from the main river channel represents a major form of annual mortality in fishes, due either to anoxia, or later to desiccation (Welcomme 1979). Aestivation enables some species to endure the dry seasons. The African lungfishes (eg, Protopterus annectens) cocoon in the mud and breathe air throughout the dry season. Fishes that aestivate are part of a group called annual fishes (LoweMcConnell 1987). The eggs of these fishes "rest" in mud throughout the dry season to continue development with the return of rainy season waters. Various degrees of development to resting fry occur in these species, with each stage aestivating through the dry season period. Reproductive strategies of riverine blackfishes and whitefishes can be broadly classified as either "total spawners" or "partial spawners" (Lowe-McConnell 1975; Munro 1990). A third major classification, the "big Bang" spawners, exists, however only a few species, such as the catadromous American eel, Anquilla rostrata belong to this group. Total spawners in the extreme sense spawn large numbers of eggs once a year, normally during flood conditions. Partial spawners drop small batches of eggs throughout the year. A continuum exists between the extremes of the total and partial spawning strategies. Within both groups are numerous variants of the two strategies (Balon 1975). While total and partial spawners are found throughout riverine environments, clear correlations between these two broad reproductive strategies and riverine habitat along the main river channel - dry season marginal habitat continuum have not been developed.

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Discussion The role of the annual discharge cycle in shaping the fish communities of tropical riverine systems is analogous to the role of the annual water temperature cycle in temperate riverine systems (Fig 2). Both variables are directly linked to climate and both variables strongly govern fish habitat diversity. During the dry season at tropical latitudes the continuum of riverine fish habitat from relatively constant main channel areas to extreme marginal habitats parallels the range in thermal habitat in summer along stream-lake continua at temperate latitudes. In view of the major influence of seasonal water availability on fish habitat diversity in tropical riverine environments, and the direct link between river discharge and climate, we judge that the annual discharge cycle should be the focus in first-assessments of potential effects of climate change on tropical fishes. Key components of the discharge cycle, with respect to riverine fish, that will likely change with climate are the magnitude and extent of the high and low flow periods. The influence of the annual range in habitat availability and habitat severity of tropical riverine environments on fishes should provide the basis for constructing scenarios of the effects of climate change. Current climate change scenarios for tropical regions, derived from general circulation models (GCM) of the atmosphere, indicate that in an atmosphere with two times the present level of atmospheric CO2, precipitation at different regions across tropical latitudes could change by over 2 m m day -1 (Mitchell et al. 1990). The scenarios project increases of between 2 and 4 °C in mean winter and summer air temperature. Compared to other regions of the world, the projected change to precipitation is the greatest for the tropics, while the projected air temperature change is the smallest. The different G C M s all project that regional precipitation will change in the tropics in a "warmer" climate, however, the direction of projected change in regional precipitation varies among of the GCMs. Despite the current disagreement among climate change scenarios with respect to the direction of projected change in regional precipitation in the tropics (eg, Rasmusson 1989), the magnitude of the projected change in precipitation would likely have a great effect on tropical fish habitat in rivers. The rainy and dry seasons of tropical regions could become more severe or more moderate. Thus, we foresee a wide range of impacts of climate change on riverine fish habitat that will likely vary widely among regions. Initial assessment of effects of changes to water quantity variables of riverine environments should begin with low resolution analyses. The first step is to link the response of tropical river discharge to changes in seasonal precipitation (eg Brysoxl 1974). Relationships that link precipitation to river discharge and flooded area are needed to provide input to empiric or analytic models of the influence of water quantity on tropical fishes. Comparative analyses of the relative sensitivities of blackfish and whitefish-type life history strategies to

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changes in annual precipitation cycle

1

changes m water level cycte eg. dischargerates, extent of flooding

amount of suitable habitat

quality of existing habitat

The same approach has been used to assess the effects of changes to suitable thermal habitat on temperate fish yields (eg, Magnuson et al. 1990; Meisner et al. 1987). The classification of temperate fish guilds has been particularly useful in this work because the general patterns of a few, well studied species can be applied with considerable confidence to all other guild members. For tropical species, we think that this type of analysis could be applied in comparing the sensitivities of whitefish and blackfish to changes in critical aspects of habitat quality. For example, it would be interesting to compare the sensitivity of population abundance to changes in the relative lengths of the dry and rainy seasons. As more species-specific information on habitat requirements and physiological processes becomes available, it may be profitable to build more processoriented models, capable of predicting the impacts of climate change on aspects of individual behaviour that are important at the population level. In this context Shuter and Post (1990) used a detailed life history model of perch to determine the relative sensitivities of wild populations to changes in the length of both the summer growing season and the winter starvation period. Many tropical species also face alternating periods of resource abundance (ie, rainy season) and resource deficit (ie, dry season). Similar process-oriented models could be used to determine if existing distributional boundaries of whitefish and blackfish in river systems are determined by such factors, and the extent to which such distributional boundaries change as a response to climate change. The different analytical approaches to the assessment of the effects of climate change outlined above are summarized in Fig 3. Major components of the study approach are empirical and analytical predictors of effects at the species and population levels. The implications of global climate change for the highly valued riverine fisheries of tropical latitudes are important. The fishery of the Mekong river, of southeast Asia for example, is a critical component of the economy and livelihoods of the peoples of the six countries that this river system traverses (Welcomme 1979). The fisheries of the Mekong, as for other major tropical river systems, are most productive during the rainy season due to habitat and food availability. Changes in annual flow regimes, due to climate change, would most likely affect the floodplain fisheries of these river systems. The potential for negative effects of climate change on these resources warrants detailed investigation.

_i .hys,°.,c.,ch.r1.cter,st,cs of individual species: (i) tife cycle parameters (ii) oxygen and thermal tolerances by life stage {iii) bioenergetic parameters

emp~rical predictors of abundance and potential harvest at both the community and species level

species-specif ic predictors of individual processes {eg. growth, survival)

1

species specif ic predictors of distributional boundaries

Fig 3

Information and techniques required for forecasting the primary effects of climate change on tropical riverine fish populations

changes in say, the length of dry and rainy seasons, would be a good starting point for assessment of climate change effects. These initial coarse-level studies would help focus subsequent finer assessments such as analyses of the potential response of different reproductive strategies of riverine fishes to changes in habitat availability. Tools (sensu Shuter and Meisner this issue, GeoJournal 28, 1, 7-20 [1992]) that describe the influence of water quantity variables on aspects of the many different habitats and life histories of tropical riverine fishes will be needed. Examples for temperate fishes (eg, Fausch et al. 1988) consist of simple regression equations that relate, for example standing biomass and abundance of river fish to variables of discharge. A few similar statistical relationships exist for tropical riverine fisheries. Catch of river fish has been positively correlated with the area of the floodplains of tropical rivers, and to interannual variation in flooding within rivers (see review by Welcomme 1985). Coupled to an estimate of the effect of climate change through the precipitation-river discharge linkage, these empiric relationships can be used to provide first-estimates of the effects of climate change on fisheries yield.

Acknowledgments Special thanks Ms J. K. Pawley for preparation the figures, and for her assistance in this investigation.

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