Australian Bass - Reproduction

Reproduction

Australian bass spawn in estuaries in winter, generally in the months of July or August.

The salinity range in which Australian bass spawn is still not clear. Estuaries are dynamic habitats with daily fluxes in salinity due to tides, and are also affected by droughts, floods and freshes (minor, temporary rises in flow), making measurements of preferred spawning salinities for wild Australian bass difficult.

Australian bass spawn in salinities of 8–12 parts per thousand (salt water is approximately 36 ppt), based on capture of recently spawned larval and juvenile Australian bass in estuaries. Australian bass sperm have no viability at or below 6 ppt, but are most viable at 12 ppt, the latter probably being the most relevant fact. However, it has been reported that Australian bass spawned in salinities of 12–18 ppt, with this statement based on fishermens' reports of observing wild Australian bass spawnings and some unpublished data gathered by the NSW Fisheries Department.

Artificial breeding of Australian bass is carried out at much higher salinities than natural.

Australian bass are highly fecund, with a reported mean fecundity ("fertility") of 440,000 eggs from the mature wild female specimens examined, and one very large specimen yielding 1,400,000 eggs. The eggs are reported as being demersal ("sinking") in natural spawning salinities, in which case estuarine vegetation such as sea grass almost certainly play an important role in "trapping" and protecting eggs. Larvae hatch in 2–3 days. Juvenile Australian bass migrate into the freshwater reaches after spending several months in estuarine waters.

Despite spawning in estuaries, Australian bass rely on floods coming down river systems into the estuaries throughout the winter period, both to stimulate migration and spawning in adult Australian bass and for strong survival and recruitment of Australian bass larvae.

Australian bass adults and larvae may also enter the sea (the latter perhaps involuntarily) during winter spawning in times of flood. It has been reported:

The presence of field-caught larvae of both species on incoming tides in Swansea Channel indicates that the larvae have spent some time in the ocean... Macquaria novemaculeata adults move downstream into estuaries to spawn in water of suitable salinity. In low rainfall years, the spawning location is further upstream than in wet years, when spawning can occur in shallow coastal waters adjacent to estuaries (Searle, pers. comm.). Mature M. novemaculeata adults can be found outside of estuaries in wet years (Williams, 1970). This is verified by the collection of mature adults by trawl in July 1995 in 11–17 m of water off Newcastle, NSW (AMS I.37358-001).

This kind of movement leads to some genetic interchange between river systems and is important in maintaining a high degree of genetic homogeneity ("sameness") in Australian bass stocks and preventing speciation. However, this movement has not prevented distinct genetic profiles and subtle morphological ("body shape") differences developing in different river systems. These findings indicate it is important to use the appropriate regional Australian bass stocks for artificial breeding and stocking projects.