Black bream (Acanthopagrus butcheri) is one of the most important recreational and commercial species in the estuaries of south-western Australia. Black bream complete their whole lifecycle within an estuary and can cope with salinity and temperature changes that would kill many other species. As hardy as they are, they are still ultimately reliant on healthy rivers and estuaries for their survival.
Black bream belong to the sea bream family (Sparidae). In Western Australia, related species include tarwhine (silver bream), pink snapper and western yellowfin bream.
They have a silver/olive brown upper body with brown/black fins. They grow to about 60 cm and 4 kg.
Distribution and habitat
Black bream are common in rivers and estuaries throughout southern Australia. In WA, they are found as far north as the Murchison River.
Black bream almost never leave the estuary. This has led to genetically distinct populations within each estuary.
They cope well with salinity changes from freshwater to ‘hypersaline’ (highly salty) water. Juvenile bream tend to inhabit shallower waters. The preferred adult habitat includes overhanging banks among the branches of dead trees, found in the bottom of deep pools in most rivers in WA.
In late summer and early autumn, after the spawning period (when sperm and eggs are released into the water), black bream juveniles and adults are common in the upper estuary. They are often flushed downstream with the first rains in late autumn.
In open estuaries, adults are sometimes flushed out to sea after very heavy rain, but usually return to the estuary.
Although a hardy species, black bream can be seriously affected by poor water quality. In many WA estuaries, reduced river flow and increased nutrient run-off have adverse effects. Particularly on WA’s west coast, these factors can cause harmful algal blooms and hypoxia (low dissolved oxygen in the water) in summer, resulting in fish kills.
On the south coast, where many estuaries are closed by sand bars in summer, low river flow and high rates of evaporation can cause estuaries to become extremely hypersaline, again resulting in fish kills.
Unlike many other estuarine species, black bream don’t migrate to the ocean to spawn.
They typically spawn in spring or summer, at the boundary of the salt wedge (where freshwater from the river runs over a ‘wedge’ of denser saltwater from the ocean). At this time, the salt wedge is typically located in the upper estuary.
(As rudimentary hermaphrodites, black bream have both immature ovaries and testes when young, but will turn either male or female before their first spawning.)
Females release tens of thousands of eggs in several batches during the spawning season. However, many eggs and larvae won’t survive to become mature fish. As they grow, females produce more eggs. Really big females may produce more than six million eggs per year.
The larvae hatch from the free-floating eggs after 2.5 days. After about four weeks, the larvae are about 10 mm in length. They then develop into juveniles and settle to the bottom of the estuary.
They mature at two to five years of age, when about 15-20 cm long, and the lifecycle begins again.
They are opportunistic feeders and eat everything from small crabs and fish to vegetation. They also use their peg-like teeth to prise mussels and barnacles from rocks and pylons.
Illustration © R. Swainston/www.anima.net.au