Coral trout species are the main target species for reef line fishers along the Queensland coast from the Eastern Torres Strait in the north to the Capricorn Bunkers area in the south.
Coral trout are the favourite target fish for all sectors of the fishery because they are a good eating fish and command high market prices locally and overseas. The total commercial catch of coral trout was reported at over 1500 tonnes in 1998.
The term coral trout actually describes a number of different species including:
Common coral trout or Leopard trout: Plectropomus leopardus Blue-spot trout: Plectropomus laevis Footballer trout: Plectropomus laevis (a different colour morph of blue spot trout) Bar-cheeked trout or Island trout: Plectropomus maculatus Passionfruit trout or Leopard trout: Plectropomus areolatus
Coral trout belong to a family of fish known as the Serranidae. This family includes groupers and cods which are all characterised by having three spines on the gill cover and a large mouth lined with more than one row of teeth.
Studies to date suggest that coral trout move around considerably within a single reef, though this movement is often only over distances of less than 500 metres.
Movement between neighbouring reefs does not occur to a large extents; however, there have been a few cases where fish have moved from one reef to another. A lot of this movement may be the result of fish moving towards or away from spawning aggregation sites.
Documenting the size and age structures during reproduction of coral trout stocks have been a major focus of attention for the ELF Experiment as a major indication of how fish stocks respond to various levels of fishing pressure.
Coral trout are protogynous hermaphrodites. That is, they start their lives as females and change sex to become males later in life. It is not known what triggers this sex change.
ELF research has determined that the sex ratio (males to females) differs in different areas of the Great Barrier Reef and may differ between reefs opened to fishing and protected from fishing. Sex ratios are an important consideration for management as changes in these ratios could seriously affect reproduction and subsequently the number of juveniles coming into the fishery in future years.
On average, sex change occurs when fish are between 23cm and 62cm in length. The average length at sex change is 42 cm, but this is believed to happen most frequently in the months immediately following spawning.
All length classes of fish may have both male and female individuals. However, as a rule:
small fish will be females most large fish will be males
Like many fish, coral trout spawning corresponds with an increase in water temperature (from 25 0 26.5 0 C) during late spring. In the northern Great Barrier Reef , coral trout spawn between September and December, whereas in the southern regions where the water is cooler, spawning occurs between October and February. The beginning and end of spawning can vary from year to year as the change in water temperature varies.
Common coral trout generally aggregate (crowd into a dense cluster) to spawn. These aggregations are formed around reef slopes (approximately 10-15m deep) and peak during periods of the new moon.
Spawning occurs when the tidal flow is strong, particularly during ebb tides. This is thought to allow the newly released eggs to be transported well away from the reef and its associated predators. Spawning typically takes place at dusk, when the light levels make it difficult for predators to see and feed upon the eggs.
As coral trout aggregate, males establish temporary territories. They then try to entice females into their territories to spawn by means of elaborate courtship displays.
As part of this courtship ritual, male coral trout characteristically display darkened edges to their fins, which can be switched on and off almost instantly. The male will approach a female, who is usually sitting close to the bottom, with his body tilted at 45 0 90 0 (almost lying on his side in the water!) while repeatedly quivering along his full length and shaking his head from side to side. He passes close by the female's head or body with either the top or underside of his body nearest the female. This process is frequently repeated.
Spawning rushes occur after this courtship behaviour, but only if the female agrees. During a spawning rush', a pair of coral trout swim rapidly towards the surface where they release sperm and eggs into the water as they quickly turn. The cloud of sperm and eggs released during a spawning rush' is not easily seen, but its presence can sometimes be noted by the frantic feeding of small plankton-eating fish.
The spawning of coral trout generally occurs over a 30-40 minute period during sunset. Some coral trout (especially males) spawn more than once during an evening.
Like most reef fish, coral trout have a larval stage where the eggs and hatchlings (larvae) develop within the water column.
Fertilisation takes place after spawning and the fertilised eggs float just below the water surface. The incubation period for coral trout eggs is unknown, but the eggs of related grouper species hatch after 20 to 45 hours. The newly hatched larvae are not very well developed and obtain nutrients from a yolk sac. As the grow, their spines, fins gut and other internal organs develop, as do their senses (such as sight and smell). Eventually, the yolk sac is completely absorbed and the larvae begin to actively see and catch their own prey.
Having a larval stage in the water column enables fish to disperse between reefs. For example, a larva that is spawned on one reef may be transported in the plankton to a neighbouring reef, or a reef hundreds of kilometres away, where it may settle and grow into an adult.
The fastest period of growth for a coral trout occurs in the first three years of life, after which the growth rate begins to slow. The average daily growth of newly settled juveniles has been measured at 0.81mm per day. This means they reach close to 14cm in the first 6 months!
Growth rates of coral trout are very variable. As you can see on the growth chart, every age class has a wide range of sizes.
Growth is defined as the change in body mass, or size, with age. To estimate growth therefore, the age and size of a fish must be determined. Most commonly, the growth of fish throughout life is measured by collecting individuals of varying sizes, measuring their length and determining their age by the Otolith method.
Recent research at Bramble Reef has found that common coral trout (P.leopardus) reaches a maximum age of 16 years.
Another study on the Great Barrier Reef aged a blue spot trout (P.laevis) at around 18 years old and weighing 26kg!
Coral trout are voracious fish eating predators (piscivores).
Younger juvenile trout mostly eat crustaceans, especially prawns which live on or near the reef bottom.
Adult coral trout feed upon a wide diversity of reef fish. The most common type of fish eaten is Damselfish (Family Pomacentridae), and the Spiny Chromis Damselfish (Acanthochromis polyacanthus) seems to be the favourite. Adult coral trout also eat juvenile coral trout.
Individual fish usually feed once every 1-3 days, although some fish sometimes go for many days without feeding. A total of 90% of a single prey item will be digested within 24 hours.
Coral trout only feed during daylight hours and most often at dusk and dawn. They have two different types of feeding methods; ambush and prowling. They also change the colour of their skin when feeding.
They use the ambush method to hunt fish that live among the coral on the reef bottom. The trout will hide and remain very still and alert, ready to attack passing prey. The prowling method is used to hunt schooling fish higher up in the water. Here, the trout will move (prowl) slowly towards the prey and then attack at great speed.
Individual coral trout have different feeding behaviours and success at hunting. This may explain the variability in growth and maturity of individuals.
Regional and seasonal variation in diet:
Regional Variation: Studies have shown that coral trout in the southern Great Barrier Reef feed mainly on Parrot Fish (Family Scaridae) and Hardyhead bait fish (Family Atherinidae). The most common prey items further north are the Damselfish (Pomacentridae) and Fusiliers or Banana fish' (Caesionidae).
Seasonal Variation: One study showed coral trout eating schools of Fusilers in summer, and Scarids during the winter months. This seasonal variation is quite common in the diet of coral trout due to varying abundances of prey at different times of the year. Trout also tend to eat more food in winter, possibly to increase fat stores in preparation for reproduction in spring.