Ask Dr Dazza | Yellowfin Bream Part 2

IN THE last column, I discussed the movements of yellowfin bream around Moreton Bay. In this edition, I’ll cover age and growth, reproductive biology and diet.

Much of the early work on the reproductive biology of yellowfin bream in Moreton Bay was undertaken by Dr Barry Pollock who grew up on Redcliffe peninsula. The reproductive biology of yellowfin bream is complex, involving a form of sequential hermaphroditism called protandrous (male first) sex inversion. This is not uncommon among fish species, and it is an approach which can maximise the reproductive output of a population.

Yellowfin bream possess an ovotestes in which the testis and ovary occur in separate zones within the same individual. Most juveniles become functional males, but a small proportion of juveniles develop directly into functional females (primary females). Protandrous sex inversion commences after the fish mature when male fish change into female fish; however, some fish remain as males (primary males) throughout their life. The upshot of all this is that most small adult fish are males and most large adult fish are females.

Yellowfin bream are thought to become mature as males between 17.5–20.5cm fork length. The minimum legal size of 25cm total length allows over 50 per cent of fish to spawn once. This is a standard benchmark when setting a minimum legal size.

The age of most species of fish, including yellowfin bream, can be estimated by examining otoliths (ear bones). Yellowfin bream are relatively slow growing, but not as slow growing as their cousin the black bream which is found in southern NSW and Victoria. There is substantial variation in how old a yellowfin bream is for a given size. For example, age estimates by the Queensland Department of Primary Industries identify that a 30 m yellowfin bream would likely be seven years of age, but it could quite likely be as young as four or as old as 12. Six-year-old yellowfin bream comprise the most common age group harvested by recreational fishers. Overall, fish in the four to 10-year-old age groups account for most of the harvest (86 per cent).

It is fair to say that yellowfin bream are highly opportunistic in what they eat, and this is well known to anglers who can use a myriad of baits or lures to catch them. Their natural diet though is highly dependent on their habitat at the time. For example, the diet of fish in mangrove and saltmarshes is dominated by shore crabs while terrestrial animals such as insects, spiders and lizards are also consumed.