By Simon Mom, PhD candidate, La Trobe University and Murray-Darling Freshwater Research Centre, Wodonga

Climate change poses a threat to most, if not all, Australian ecosystems. In southern Australia, droughts are predicted to increase in severity, duration and frequency. In rivers, droughts are accompanied by changes in many potential environmental ‘stressors’, such as high temperature and low dissolved oxygen (hypoxia), which will affect the condition and survival of fishes. In order to understand the extent to which such stressors affect fish populations, it is necessary to understand how they affect individuals.

Understanding exactly how drought-induced stressors affect fish will focus management attention on what can be done to preserve fish biodiversity (Helmuth et al. 2005). For example, local catchment managers want to know if lowering water temperatures through planting trees along river banks will improve the viability of fish populations. In this situation, understanding how and why temperature affects fish is really useful.

The concept of ‘resistance’ has been used by ecologists to explore the potential for animal populations to survive (i.e., resist or adapt) environmental change (Holbrook et al. 2008). According to one point of view, the resistance of a population to a disturbance is proportional to the size of the disturbance required to drive that population locally extinct. However, the measurement of resistance is extremely difficult to carry out in the field. A more practicable approach is to assess under controlled laboratory conditions the abilities of individuals to resist and respond to environmental disturbances, and draw conclusions about how populations as a whole would respond.

Laboratory aquaria, housing fish for testing tolerances to environmental stressors (photo: Simon Mom)

A number of changes in freshwater systems occur under drought conditions. Amongst the more drastic (and easily manipulated under controlled conditions) are: increased water temperature and low dissolved oxygen (environmental hypoxia). Temperature may be the single most important factor that affects how fish perform (i.e., how they move and grow) and the make-up of fish communities (Fry 1971). Environmental hypoxia not only affects individuals, populations and communities, but can have devastating effects on the whole of freshwater ecosystems and how they function. When these two stressors occur together, the impacts are likely to be severe.

We know high temperatures can affect fish dramatically. And fish kills from hypoxia are not uncommon. Tolerances to heat and low dissolved oxygen vary from species to species. Some species are very tolerant, whilst others, such as rainbow trout, are sensitive to relatively small changes.

How temperature and oxygen tolerances change with body size within a species is mostly unknown in fishes. Many critical biological rates (e.g., metabolic rate and feeding rate) change with body size (Gillooly et al. 2001; Woodward et al. 2005). These relationships can occur within and between species (Cohen et al. 1993; Gillooly et al. 2001). This strongly suggests that body size may affect how resistant a species is to a stress, like temperature or hypoxia.

Shy river blackfish in aquarium

My PhD project aims to determine how body size affects the resistance to hypoxia of river blackfish (Gadopsis marmoratus) at set points along a temperature gradient. While the project will use existing data and bring together knowledge on the impact of these stressors, it will mainly generate new data through physiological and behavioural experiments.

The initial study species will be river blackfish, which is readily available locally and has been successfully maintained in captivity at the Murray-Darling Freshwater Research Centre, Wodonga. The study may later be extended to include one or more other native species.

References: Cohen JE, Pimm SL, Yodzis P, Saldaña J (1993) Body sizes of animal predators and animal prey in food webs. Journal of Animal Ecology, 67-78. Fry F (1971) The effect of environmental factors on the physiology of fish. In ‘Fish physiology. Vol. 6. (Eds WS Hoar and D Randall) pp. 1-98. (Academic Press: New York). Gillooly JF, Brown JH, Helmuth B, Kingsolver JG, Carrington E (2005) Biophysics, physiologicalecology,and climate change: Does mechanism matter? Annual Review of Physiology 67, 177-201. Holbrook SJ, Schmitt RJ, Brooks AJ (2008) Resistance and resilience of a coral reef fish community to changes in coral cover. Marine Ecology Progress Series 371, 263-271. Woodward G, Speirs DC, Hildrew AG (2005) Quantification and resolution of a complex, size-structured food web. Advances in ecological research 36, 85-135.