Fish Facts

FISH FACTS: Happy moments

A black rabbitfish, aka “happy moment”. Image: Dr Ben Diggles.

Irony n. “the expression of one’s meaning by using language that normally signifies the opposite, typically for humorous or emphatic effect”.

IRONIC certainly sums up the story behind the name “happy moment”. Of course, I am referring to a group of fishes known by scientists as spinefoots or rabbitfishes of the genus Siganus within the family Siganidae. There are at least 2 species of Siganus in Australia that fishers refer to as “happy moments”, but this is not because the event of their capture is a particularly joyful or cheerful experience. Instead, they are so named to describe the several hours of excruciating pain that can follow being spiked by one of these relatively small and innocuous-looking herbivores.

The two culprits most commonly encountered by recreational fishers in Australia include the dusky or black rabbitfish (Siganus fuscescens) and the forktail rabbitfish (Siganus argenteus). Both look similar in shape and their mottled colouration, but the latter has a much deeper fork to the tail. Both species occur throughout the tropical and subtropical parts of the country, with black rabbit fish extending its range further south to around Fremantle on the West Coast and southern NSW on the East Coast.

For both species (and indeed all members of the genus Siganus) the spines of the dorsal and ventral fins are not only extremely sharp, they are venomous. While being spiked by these fish can result in severe to excruciating pain for several hours (i.e. the “happy moment” experience), fortunately the venom is not lethal or as toxic as that produced by stonefish or bullrouts (scorpionfishes), so in that respect one can certainly be happy if they are spiked by a Siganid instead of a Scorpaenid.

The antidote for the envenomation is the same, however, namely placement of the affected area in water as hot as you can tolerate for 30 to 60 minutes (or until pain resolves) in order to denature the venom. The venom from rabbitfish spines is similar to other marine venoms in being composed of certain proteins and enzymes that are heat labile and which break down when heated above 45-50°C. This is starting to get near the water temperature that can cause scalds, so the usual treatment advice is use the highest temperature that can be applied safely and that is tolerable.

Happy moments are seldom encountered by anglers as they are almost entirely herbivorous, feeding mainly on filamentous algae, encrusting algae and other types of water plants. They are most often caught in cast or bait nets, but can occasionally be tempted into taking baits and even small lures, but these latter situations are exceptional rather than the rule.

A study in Moreton Bay in Queensland examined whether happy moments actively fed on a bloom of the toxic filamentous cyanobacteria Lyngbya majuscula, in order to determine whether these fish could be used to clean up nuisance algal blooms. Scientists found that the happy moments indeed ate the Lyngbya algae, but only when the active lyngbyatoxin was not detectable, so unfortunately they don’t appear to be a magic bullet for control of these nasty algal blooms (which are caused by onshore urban development and land use changes).

Despite being hard to handle and no good for algal cleanups, happy moments nevertheless still have several endearing features. They are fast growing, reaching 20 to 25cm in their first year in tropical waters, and reach a maximum size of around 40cm. They mature in their first year of life, females produce around 300,000 eggs and may spawn more than once a year, so they are relatively resilient to fishing pressure. And finally, they are reputedly quite good to eat (though I have never tried them myself) and form the basis of important commercial fisheries and aquaculture industries throughout Asia and Japan. An interesting fish!

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