Climate change in a stable thermal environment: effects on the performance and life history of coral reef fish

The effects of climate change induced ocean warming are expected to be felt most strongly by species living at low-latitudes due to the thermally stable environments that characterise these locations. This thesis examined how near-equatorial populations of coral reef fishes, particularly A. polyacanthus, are likely to respond to increases in ocean temperatures projected to occur by the end of this century. The underlying physiological measures linked to declines in organism fitness at higher temperatures were considered, and the capacity for acclimation of these traits assessed. Whether certain populations of coral reef fishes are more or less vulnerable to projected climate change scenarios was also investigated by comparing results with those obtained in previous studies conducted at higher latitudes. Initially I sought to understand how chronic exposure to temperatures expected to occur with climate change are likely to impact on the survival and metabolic performance of a low-latitude population of marine damselfish (Acanthochromis polyacanthus), and subsequently estimate their potential for long-term reversible acclimation to higher temperatures (Chapter 2). To do this, I assigned fish to one of three treatments: (1) current average ocean temperatures for the collection locations, seasonally cycling, (2) 1.5 °C higher than current average temperatures or (3) 3 °C higher than current average temperatures. Fish were kept in these treatments for approximately 10 months. Routine and maximum oxygen consumption were then directly measured and subsequently used to estimate net aerobic scope for each fish during both the summer and winter. Critical thermal maximum was also estimated during summer and fish survival was monitored both before and after metabolic testing. For these low-latitude populations of coral reef fish performance in terms of aerobic scope was maintained up to 31.5 °C, the maximum ocean temperature routinely experienced by this population. Once temperatures exceeded 31.5 °C, aerobic scope dropped significantly, as did fish survival. Survival was further decreased at much lower temperatures after the introduction of a secondary stressor (metabolic testing). As predicted, potential for reversible acclimation was limited, even after extended periods at elevated temperatures. The first data chapter of this thesis confirmed that increased temperatures are likely to significantly affect the physiological performance of low-latitude coral reef fish. Studies which consider only a single measure of fitness such as aerobic scope when estimating the impacts of thermal stress on organismal health may however not obtain the best estimate of how an organism will cope under future conditions. In chapter 3 I aimed to expand the range of physiological estimators of fitness used to determine the effect of elevated temperatures on low-latitude reef fish. This chapter compared the impacts of increased ocean temperatures on aerobic scope, haematological parameters and tissue health for Torres Strait A. polyacanthus. Haematological parameters and tissue health were analysed during the summer months and compared with the measures of aerobic scope taken in the previous chapter. Haematological traits, similarly to aerobic scope, suggested an impact on oxygen transport in fish at +3 °C, however a negative impact of temperature on tissue health was observed in fish from both +1.5 and +3 °C treatments. Aneurismal dilations in the gill tissue were larger and more numerous in fish from the warmer temperatures. Findings from chapter 3 suggest multiple thermal optima, depending on the physiological trait concerned. Of the measures considered in this chapter, gill histopathology provided the best indicator of thermal tolerance, as it was the first measure to show of a decline in organism health and corresponded with mortality observations from chapter 2. Previous research shows that increased temperatures may also effect A. polyacanthus in the future by producing populations of offspring with a strongly male biased sex ratio. This could have a significant negative impact by reducing the functional reproductive population. For a shift in sex ratio to occur, juveniles must be exposed to increased temperatures during a specific time during development, known as the thermosensitive period (TSP). In chapter 4, I aimed to determine the TSP for A. polyacanthus and subsequently predict when projected temperature increases are likely to have the greatest effect on sex ratio for this species. For this chapter only we used fish collected from a higher latitude, and so temperature treatments were adjusted accordingly. To determine the thermosensitive period of sex determination under climate change relevant temperatures for this species, a single-shift design was used to expose juvenile fish to elevated water temperatures (+1.5 and +3 ˚C) at various stages of development. Increasing grow-out temperature to 1.5 °C had no effect on the sex ratio of offspring, however an increase to 3 °C above average produced a strong male bias (average ~90%). The thermosensitive period was up to 60 days post hatching, with the bias in sex ratio greater for fish that were exposed to higher temperatures earlier in life. Average summer temperatures will need to increase by ~3°C before an effect of temperature on sex ratio is seen for this population. Temperatures high enough to drive a bias in sex ratio are likely to be first seen during January and February and would have the greatest effect on clutches produced late in the breeding season. Based on previous chapters it was determined that the ability to acclimate to higher temperatures would be critical for the persistence of low-latitude populations in a warming world. In chapter 5 I test the capacity for three low-latitude coral reef damselfishes (Acanthochromis polyacanthus, Pomacentrus moluccensis and Pomacentrus wardi), to developmentally acclimate to ocean temperatures expected to occur by the end of this century, again using metabolic fitness as an estimate of organism health. Newly settled juveniles were collected from reef locations in Torres Strait and reared for 3 months in three different temperature treatments, this time consisting of the current-day summer average (30 °C) and 1 and 2 °C above the average (31 and 32 °C). As with chapter 2, aerobic scope was estimated, this time for each fish at both their developmental temperature and at the two remaining treatments after acute exposure. Acclimation capacity differed among species and occurred at similar absolute temperatures but lower relative temperatures than for the same species at higher latitudes. There was some scope to deal with climate change relevant temperature increases in these populations, however low-latitude populations are still more vulnerable to temperature increases than their more southern counterparts. Life history and habitat choice of each species appeared to have a strong influence on their capacity for developmental acclimation, leading to the conclusion that scientists should consider the ecological niche of their study species and take care when making generalisations about the effects of climate change, even on closely related species. Overall, results from this thesis showed that despite substantial physiological effects of climate change, some near equatorial populations of coral reef fish do have a capacity to acclimate to higher temperatures when exposed to warmer conditions during early development. In the future, multi-generational research that investigates the capacity for transgenerational plasticity may reveal a further ability to acclimate in these populations, beyond what has been shown in this thesis for developmental acclimation. The importance of understanding the life history of model species was also highlighted, as it was shown that a species' capacity for acclimation is likely to be strongly effected by their ecological niche. This thesis represents the most comprehensive investigation into the effects of projected climate change on low-latitude coral reef fish to date and is the first to examine the acclimation capacity of fish at these latitudes. Although acclimation was observed in low-latitude populations, near-equatorial fish were still more vulnerable to increased temperatures when compared with higher latitude populations. Based on the findings of this thesis, low-latitude populations should be considered a high priority for research and management into the future.