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Breeding like rabbits?


 
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Dr Becky Fox, Chancellor’s Post-Doctoral Research Fellow in the School of Life Sciences at the University of Technology Sydney, investigates breeding aggregations in an important herbivorous fish on coral reefs – rabbit-fish. Dr Becky Fox is predominantly interested in the biology and ecology of herbivorous fishes on coral reefs, in particular the role they play in maintaining a healthy balance between corals and algae on our own Great Barrier Reef. Over the last few years she has been studying one particular group of those vegetarian fishes, the family commonly known as rabbit-fish. Now, working with Professor William Gladstone, she will be looking into their reproductive ecology and breeding behaviour, about which little is currently known. Using methods of acoustic tracking to follow rabbit-fish to and from their mating aggregations, she will try to uncover more about how, where and when these important fishes reproduce. Some say they get the name rabbit-fish simply because, like their terrestrial counterparts, they are vegetarians.

 
Also, their unique jaw structure gives rise to an unusual nibbling feeding action that looks similar to rabbits eating. There are currently 29 recognised species of rabbit-fish found in waters across the tropical Indo-Pacific - most being between 20-30cm long as adults. They are an excellent source of protein and are widely harvested as a foodfish in Indo-Pacific countries, including Australia.  In the Philippines, juvenile rabbit-fish are harvested, then sun dried, salted and fried, making a popular snack called Danggit.

 
Rabbit-fish are relatively fast-growing, reaching maturity in 1-2 years, and so are also popular as an aquaculture species in Asia and the Middle East. Given that they are a fisheries target species, it is important that we know more about their reproduction and how adult stocks are replenished in the wild as well as what their ecological role is in reef habitats and what ecosystem services they provide. Once thought to be a homogenous group (the lawnmowers of the reef) that kept algae in check, we now know that different species of rabbit-fish have different roles. Some are grazers, cropping the turf algae that grows across the reef substrate and keeping it in check, others feed on reef detritus, recycling essential nutrients back into the system. Others are browsers of the large stands of brown, leathery macro algae which can pose problems for the reef when they overgrow coral. We now know that some of the grazing rabbit-fish species specifically target algae that grow in cracks and crevices in the reef, something other algal grazers like parrot fish and surgeon fish can’t do.

 
In this way these herbivores are very critical to the functioning of the reef ecosystem. But for many of these ecologically critical species of rabbit-fish, nothing is currently known about how they reproduce. Partly this is due to the observational difficulties imposed by the marine environment. For some species of rabbit-fish we know that they migrate to breeding aggregations to reproduce, but this has yet to be confirmed in many species of the family and underwater it can be difficult to observe these migration events in wild populations.

 
Images:
1. Siganus coralinus: one of the species of rabbit-fish found on reefs of the Indo-Pacific
Dr J Donelson
2. harvesting of rabbit-fish
www.creativecommons.org 
3. Danggit
www.creativecommons.org
4. rabbit-fish crevice-feeding Dr J Donelson 
5. and 6. acoustic tracking
Dr J Donelson 

 
Dr Fox will be using acoustic telemetry (tracking) to effectively follow the fishes day and night and try to observe mating habits. Small transmitters are inserted into the fish and acoustic receivers deployed across the reef pick up those signals. Each transmitter has a unique identifying code, so individual rabbit-fish can be monitored. Such tracking methods have already produced some surprise results. For one species of rabbit-fish that live in pairs on the reef, the pairings were assumed to be for mating until fishes were tracked migrating long distances (at least four kilometres) to amalgamate with others to spawn before returning a few days later as a group.

 
Separate research has shown that while most pairs observed on the reef were male/female, some 25% were same-sex couplings. Evidence is starting to point towards the fact that these fish may pair for purposes other than mating, like the pairs of male cheetahs that band together for food or mating opportunities or the male baboons who team up in pairs to distract the alpha male, giving their partner brief access to the females of the harem for mating. For rabbit-fish, sticking their heads into cracks and crevices to feed is a risky action and it may be that having a partner as a look-out is a protective strategy that has evolved in response to this unique feeding behaviour. If some rabbit-fish are already living in pairs, this makes their migration to breeding aggregations even more surprising. Preliminary evidence suggests that fish are making risky migrations each month over the breeding season (Oct-Jan), swimming more than 5 km - a considerable distance for a 20cm fish.

 
Dr Fox's work will investigate the nature of the breeding aggregation sites and whether there are particular features that make them attractive enough to risk the journey. Whether, for example, there are more opportunities for currents to sweep larvae into good habitats (and away from predators) or that there is a particular characteristic of the benthos of the site. We already know from investigations of rabbit-fish spawning aggregations carried out in The Seychelles that sites which have been impacted by high sediment loads caused by dredging are now no longer used by fishes.

 
Understanding the migrations undertaking to breeding aggregation sites and the nature of sites that are being used is therefore an important part of protecting adult stocks and ensuring rabbit-fish can remain a sustainable food source worldwide, as well as continuing in their critical functional roles on our coral reefs. Populations of rabbit-fish in some parts of the world are being harvested at levels bordering on unsustainable, and these fish don't have the luxury of breeding like rabbits.

 

Dr Becky Fox was interviewed for A Question of Balance by Ruby Vincent. Images from Dr Fox. Summary text by Victor Barry, April 2015.


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