The annual spawning aggregation of dusky flathead 'Platycephalus fuscus' at Jumpinpin, Queensland

Dusky flathead 'Platycephalus fuscus' form seasonal spawning aggregations where estuaries meet the ocean in eastern Australia. The present study at Jumpinpin in south Queensland shows that dusky flathead have a protracted spawning period with serial spawning during summer (November to April). They are rudimentary hermaphrodites with sex determined at an early juvenile stage. Sex ratios are skewed with males most common in the smaller size-classes (< 50 cm TL). Mid-sized females (45 cm - 69 cm TL) dominate the production of eggs due to their abundance in the spawning aggregation. Within the female component of the spawning aggregation, the occurrence of individuals exceeding the current maximum size limit of 75 cm TL is low (2.6%). Parasitic nematodes (philometrids) occurred in 8% of ovaries. Degenerated ovaries, in which atretic oocytes are common, are present in half of the very large females (70 cm - 75cm TL) examined. The current minimum size restriction of 40 cm TL for dusky flathead provides protection for 73% of males and 15% of females within the spawning aggregation.