Do Dusky Flathead, Platycephalus fuscus (Cuvier, 1829), Spawn in Upper Estuarine Areas?

Pollock, B. R. (2020)


This study is based on monthly samples of dusky flathead (n = 87) in the size range 40 cm TL (total length) to 75 cm TL, collected by angling methods from upper estuarine areas in southern Queensland during the spawning period (September to April). Ovaries at two stages of development were identified by macroscopic and microscopic examination: unyolked (translucent) ovaries, the most common form, in which oocytes are undergoing mass atresia up to α stage; and yolked (vitellogenic) ovaries which are also undergoing mass atresia to both α and β stages. An unusual finding is mass atresia of previtellogenic oocytes, showing multiple irregular vacuoles within the oocyte cytoplasm, which commonly occurs in both ovary types. Mature males have testes that are small and degraded. No ripe or running ripe gonads were found in the upper estuarine fish during the spawning period. It is concluded that these dusky flathead are not spawning, in contrast to the spawning aggregation fish at the Jumpinpin estuarine/oceanic interface which is 10 km to 20 km distant from the study site. Given the major differences in gonads and oocytes between spawning aggregation fish and those from upper estuarine areas, it is unlikely that mixing of the two subpopulations occurs during the spawning period. A review of size at maturity of dusky flathead estimated that L50 (length at which 50% of the size class has reached maturity) for females is 35 cm TL to 39 cm TL, and 30 cm TL to 34 cm TL for males.