AS a young kid during family holidays I remember being fascinated by these funny looking striped bream that we’d sometimes catch around seagrass beds in estuaries, with occasional bigger versions being encountered near rocks along oceanic surf beaches in southern QLD and northern NSW. These fish looked like the usual yellowfin bream (Acanthopagrus australis), but had a rounder head profile, and a dozen or more thin golden stripes running along the length of each flank. Of course, before too long I learned these fish were not bream at all, they were a closely related sparid species called tarwhine (Rhabdosargus sarba).
Also known as goldlined sea bream, tarwhine occur on both the east and west coasts of Australia. On the east coast they can be found from around Townsville in north Queensland all the way down to Mallacoota Inlet in eastern Victoria. The population on the west coast occurs from around Bremer Bay in the south to as far north as Exmouth Gulf. Tarwhine are not restricted to Australia, however, and in fact occur throughout the Indo-West Pacific region from East Africa (Red Sea to South Africa) eastwards to Asia, the South China Sea and Japan. In South Africa tarwhine have been reported to grow to over 70 cm long and 5 kg in weight, however in Australia a tarwhine above 50 cm long and 2.5 kg is virtually unheard of. There are 6 species of Rhabdosargus recognised worldwide, with tarwhine from South Africa being the largest of the genus, however some scientists consider it’s possible the mega-tarwhine from Africa are actually a different species to the ones found in Australia – with the answers to this intriguing question awaiting further genetic studies.
Juvenile tarwhine recruit from the plankton into estuaries where they are usually found singularly or in small schools around seagrasses, where they feed on small bivalve and gastropod molluscs, amphipods (a type of small crustacean) and aquatic plants and algae. As they grow and mature they move out of the estuaries into deeper inshore waters around rocky headlands and reefs where they tend to become more herbivorous, making them harder to catch using conventional recreational fishing methods. In Australia maturity occurs in their third year at around 22-23 cm long, after which growth slows dramatically, with a 30 cm tarwhine likely to be 8-10 years old with a typical lifespan of around 13-15 years. Tarwhine, like many other sparids, are protandrous hermaphrodites with the majority of fish maturing first as males, before some change sex into females within a few years. Spawning occurs in winter or early spring, usually at river entrances or nearby inshore reefs. Studies in Perth’s lower Swan River Estuary found that tarwhine are multiple spawners, with individual fish spawning on average every 2.7 days and up to 45 times during a single spawning season. In the Swan River spawning occurred at the top of the tide at night during spring tide periods, a timing which facilitates dispersal of fertilised eggs into the nearshore reefs and beach areas during the runout tide.
While the tarwhine’s spawning methods and timing overlap with those of the closely related (and often more numerous) black and yellowfin breams, they appear to avoid direct ecological competition with bream via what is called dietary partitioning. Simply, this means tarwhine have evolved to utilise a quite selective diet, preferring to eat a range of items that breams do not. Only through this dietary specialisation is such a morphologically similar species as tarwhine able to survive in the same environment as the far more numerous and omnivorous/adaptable bream.
