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Tropical Invaders Seek a Cool Change

 Moorish idol

Larger-bodied tropical species, such as the Moorish idol, are more likely to show vagrant behaviour into high latitude regions.

By David A. Feary & David Harasti

As oceans warm, a new study has shown that certain measurable traits may help scientists predict which species of tropical fish will successfully shift into cooler temperate waters.

Changes in global climate are a substantial threat to bio­diversity. Some species are able to cope with the associated warming by shifting their geographic range. Although there is considerable variation in these responses, average range shifts up to 6.1 km/decade in terrestrial communities and up to 28 km/decade in marine communities have been reported. These shifts are being described as one of the most dramatic results of climate change.

Of all the marine fauna, we can expect that tropical fish may be particularly sensitive to increasing ocean temperatures. These communities exist in a relatively temperature-stable environment, and elevated ocean temperatures may have substantial effects on population distribution.

Australia’s south-east coast is a global hotspot for increasing water temperatures, with rises in excess of 2°C over the past century. While attention has focused on understanding the impacts of changing water temperatures on Australia’s iconic coral reefs, temperate south-eastern Australia is changing more rapidly than almost anywhere in the world. Such changes have been associated with substantial shifts in the bottom marine community structure, from macroalgal-dominated communities to one in which urchin-barren habitats dominate.

One of the strongest signals of climate change within this region is the increasing incursion of tropical fish into cold “temperate” waters. Coral reef fish larvae are known as “tropical vagrants” as they can be transported great distances south into temperate latitudes by the East Australian Current.

There is increasing evidence to suggest that populations of tropical vagrants within this temperate region are increasing. Shifts in the distribution of tropical fish will substantially alter the functioning and success of Australian marine eco­systems.

Increased numbers of tropical fish within cold temperate regions have been reported within the United States, New Zealand, Brazil, Japan and South Africa. Although the consequences of vagrant incursions are still largely unstudied, it is already clear that incursions of non-endemic marine fish into temperate Japanese waters and the Mediterranean have resulted in devastating consequences for resident fish communities and associated benthic communities.

There is little information to allow us to predict the species that are likely to shift their distributions with increasing coastal water temperature. The question we asked was: “What are the factors that may allow tropical coral reef species to track the increasing coastal temperatures?”

Are Some Tropical Fish More Likely to Succeed?

In a recent study published in Fish and Fisheries, my colleagues and I reviewed the factors that may determine the successful incursion of coral reef fish species into temperate waters. We put together the largest data set of its kind, utilising 21 datasets that encapsulated 361 tropical reef fish species reported in a temperate region since 1935. Collecting and analysing this data took more than 2 years.

We found that vagrant behaviour appears relatively uncommon within the tropical fish fauna. While 8827 tropical fish species from 233 families are known, we found records of only 360 species from 55 families within temperate regions. There was a large range of tropical families of mostly small-bodied fish species, so these may have been missed in vagrant surveys. However, as the majority of families with vagrants are also relatively small-bodied species, we surmised that other ecological traits may also be important in vagrant success.

Latitudinal Distribution

The extent of vagrancy among tropical fish may be associated with the geographic distance between a population’s tropical source and its temperate sink. Tropical species with distributions truncated in low latitudes are unlikely to form vagrant communities, while those with distributions that are closer to temperate regions are more likely to form them.

We examined the proportion of butterflyfish (Chaetodontidae) as a percentage of the total abundance of fish surveyed in both a high latitude (southern Great Barrier Reef) and low latitude (northern Great Barrier Reef) region, and compared these with butterflyfish surveyed throughout New South Wales. We found that tropical vagrants found in NSW were more likely to be abundant in high latitude regions. Therefore, the closer a fish population is to a temperate region the more likely it will show vagrant behaviour.

Reproduction

Tropical fish vary in the degree of parental care. Broadcast spawners release gametes into the water column with very little investment, while demersal spawners invest vast resources into building and defending nest sites where eggs are laid and protected until hatching. Therefore we expected that such differences in spawning mode may influence larval dispersal.

Larvae that are spawned and are released into the water column are immediately subjected to oceanographic processes that may disperse them further than spawned larvae that are associated with the sea floor. However, demersally spawned larvae have longer incubation times, are larger-bodied, and have more developed sensory and locomotor systems. Therefore, demersally spawned larvae may have a greater ability to control their position within the water column, thereby influencing the potential for dispersal.

To examine whether the spawning mode is associated with vagrancy, we compared the reproductive strategies of both tropical vagrant and non-vagrant species and classified them into one of three distinct reproductive guilds, each exhibiting different levels of parental care:

  • non-guarders (low care, pelagic spawners);
  • guarders (moderate care, guarding and caring for eggs spawned into a demersal nest); and
  • bearers (high care through brooding).

We found that both high and medium levels of parental care were negatively associated with vagrant potential, while low levels of parental care were positively associated with vagrant potential.

Body Size

We know that larger-bodied fish produce far more eggs and sperm over their lifetime than smaller-bodied fish. Therefore there is a higher probability that the gametes of larger fish (and therefore their larvae) will be more abundant within the water column, and are thus more likely to be found within vagrant assemblages.

Therefore we compared the maximum body size of tropical fish and found that those with a larger body size are significantly more likely to be found as vagrant fish than smaller fish. In fact, we found a threshold at which species are more likely to show vagrancy: 12% of tropical fish larger than 10.95 cm are tropical vagrants while only 0.02% of fish smaller than 10.95 cm are vagrants.

Swimming Ability

Species-specific differences in larval swimming behaviour have been implicated in the dynamics of dispersal within a range of reef fish larvae. Although there is considerable variation in the swimming ability of taxa, if swimming ability is associated with potential dispersal we expected vagrants to have greater larval swimming ability than non-vagrants. Therefore, we compared the average critical speed (which is the maximum sustainable swimming speed of a species) for a range of both vagrant and non-vagrant fish, and found that tropical fish that show vagrant behaviour have a much greater swimming ability than those that are never found within temperate regions.

Dietary Preferences and Functional Groups

We would also expect that the food requirements of tropical fish may be an important factor in understanding their success within temperate regions. For example, we would expect that butterflyfish that preferentially feed on live coral would be unlikely to survive within temperate habitats that are devoid of live coral.

Accordingly, we found that the overwhelming majority of butterflyfish recorded in surveys of vagrant fish within NSW are non-coral or facultative coral feeders. In fact, when we examined the diet of vagrant fish we found that the majority were feeding on food sources that were available within both tropical and temperate regions – tropical vagrants were predominantly planktivores (which feed on small plankton in the water column) and herbivores (which feed on benthic algae).

Conclusions

Our work has shown that tropical fish species are likely to show vagrant behaviour in high latitude regions if they are larger-bodied, have a pelagic spawning behaviour, have larvae that are able to swim long distances and are able to feed on resources that are available within temperate regions.

However, all of the tropical vagrants identified in temperate habitats predominantly die-off during winter, with all population expansion associated solely with larval input. Therefore, there is still no evidence to suggest that tropical vagrants are successfully reproducing in temperate regions.

However, with increasing warming of waters we can expect that the physical variables constraining reproduction will reduce (i.e. increased growth rates in individuals, development of reproductively active individuals) and viable breeding populations of tropical vagrants will increase within temperate regions.

David Feary is Chancellor’s Postdoctoral Fellow in the School of the Environment at the University of Technology, Sydney, David Harasti is a research scientist with Fisheries NSW.