Serial Spawners of Coral Reefs

A research project on the reproduction of common coral trout has discovered widespread group mating rituals at selected sites along the Great Barrier Reef. After four years spent monitoring the spawning behaviour of coral trout, Department of Primary Industries' fisheries biologist Melita Samoilys has established that they are very organised in their reproductive habits.

Coral Trout Photo: GBRMPA

Common coral trout (Plectropomus leopardus) gather in groups at particular reef sites to mate. These gatherings, called spawning repeatedly for several days over a period of months. But while their mating ritual may be designed to increase chances of reproduction, it may also leave them vulnerable to overfishing.

Such visible and predictable behaviour makes spawning coral trout a potentially easy target, particularly since they are the most sought after species by both commercial and recreational fishers. On coral reefs elsewhere in the world, such as in Palau and Puerto Rico, some fisheries have collapsed as a result of targeted fishing of spawning aggregations. Although not the case on the Great Barrier Reef, this information was the impetus behind Melita's research project, originally funded by the Fisheries Research and Development Corporation (FRDC), the Queensland Fisheries Management Authority (QFMA), and then by the CRC Reef Research Centre Ltd.

"Understanding the population dynamics of coral trout to assess the effects of fishing this species is challenging due to their rather quirky reproductive behaviour," says Melita, who is also completing a doctorate study on the topic.

"Coral trout are protogynous hermaphrodites - they change sex from females to males during the course of their life - a strategy quite common amongst species of coral reef fish. As line fishing invariably targets larger animals, it may inadvertently reduce the number of males. However, this may be offset because of the current restaurant market demand for smaller plate-sized trout," she says.

It's not known what triggers sex change in coral trout but it may be induced by social behaviour that takes place when they aggregate to spawn. Theory suggests, though it remains untested in coral trout, that if fishing reduces the number of males, females will change sex sooner to compensate. This could mean that coral trout are flexible and may be able to adapt to fishing pressure. However, smaller females are much less fecund and in the long term the number of eggs may be insufficient to sustain populations.

The research project, originally based at DPI's Northern Fisheries Centre, investigated spawning aggregations on two reefs offshore from Cairns, both of which are open to fishing within the Great Barrier Reef Marine Park. Underwater visual surveys found that coral trout aggregated and spawned at the same sites and times during the September to November spawning season. Spawning occurs around the new moon, generally three times through the season, and appears to be triggered by a rise in seawater temperature. Aggregations last for an average of five days over each new moon period.

Each year the research team returned to the same 'primary' aggregation sites at two mid-shelf reefs which attracted up to 75 fish ready to spawn. Tagging fish also demonstrated individuals returned to an aggregation site over several days within the new moon period, and even returned on subsequent new moons. Some tagged individuals have returned to the main aggregation site for a second year. Coral trout are serial spawners - they produce several batches of eggs in a season, hence their repeated spawning behaviour. In fact, females have been seen spawning up to four times in one evening.

Approximately 30 other species of reef fish have also been observed spawning or aggregating at the primary sites, indicating the importance of these locations for many reef fishes. Smaller, or secondary, spawning aggregations have also been detected but appear to be less predictable.

Melita observed a daily pattern in the spawning behaviour of coral trout. "The aggregations appear to disperse during the morning, re-establish by early afternoon and then persist until dusk - actual spawning occurred during a 30 minute period spanning sunset," she said.

Male coral trout establish temporary territories within aggregation sites which they defend aggressively against other males. Females appear to select and remain within a particular male territory during the evening, where they are typically inactive, lying close to the coral. Males perform an elaborate courtship dance to females within their territories, moving repeatedly from one female to the next, and displaying a distinct colour pattern of darkened dorsal and caudal fin edges. Males court females throughout the afternoon, but actual spawning occurs at dusk. When a female is ready to respond she rises slowly above the coral. The male joins her and they then swim rapidly towards the surface exhibiting a characteristic spawning 'rush', turning abruptly at the peak of their ascent to release eggs and sperm. The eggs are then fertilised externally in the water.

The research reveals a distinct reproductive pattern for common coral trout in which they spawn in aggregations that are highly predicable. The consistent pattern suggests aggregations play an important role in their reproductive behaviour and may be related to the mechanism of sex change. The predictability of aggregations also confirms their potential susceptibility to targeted fishing.

Public concern over this issue has led QFMA to investigate the use of seasonal spawning closures as part of its management of coral reef fish stocks.