Murray Cod - Reproduction

Reproduction

Murray cod reach sexual maturity between four and six years of age. Sexual maturity in Murray cod is dependent on age. Therefore, roughly 70% of wild river Murray cod, with their slower growth rate, have reached sexual maturity by 50 centimetres (20 in) in length. Wild Murray cod in impoundments like Lake Mulwala, with their faster growth rates, do not reach sexual maturity until they are well over 60 centimetres (2 ft) in length. This data strongly indicates that the 60 centimetres (20 in) size limit for Murray cod is inadequate and should be increased substantially to allow greater chance of reproduction before capture.

Large female Murray cod that are in the 15–35 kilogram (35–80 lb) range are the most important breeders because they produce the most eggs and for other reasons; research is now showing large females in most fish species are also important because they produce larger larvae with larger yolk sacs, and are also more experienced breeders that display optimal breeding behaviours. Such large females may also have valuable, successful genes to pass on. All of these factors mean the spawnings of large female fish have far higher larval survival rates and make far greater reproductive contributions than the spawnings of small female fish.

Female Murray cod, upon first reaching sexual maturity, will have egg counts of no more than around 10,000. Very large female Murray cod can have egg counts as high as 80,000–90,000, although a recent, very large 33 kg specimen yielded an egg count of 110,000 viable eggs. Egg counts in female Murray cod of all sizes are relatively low compared to many fish species.

Murray cod spawn in spring, cued by rising water temperatures and increasing photoperiod (daylight length). Initially, fish biologists working with Murray cod considered spring floods and temperatures of 20–21° Celsius (68-70°F) to be necessary and that spring flooding is critical for successful recruitment (i.e. survival to juvenile stages) of young cod by providing an influx of pelagic zooplankton and early life-stage macroinvertebrates off the flood plain into the main river channel for first feeding, but more recent research has shown Murray cod breed annually, with or without spring floods, and at temperatures as low as 15° Celsius (59°F). Additionally, recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in non-flooded lowland rivers, traits in Murray cod larvae that should allow survival in a variety of challenging conditions, and a significant proportion of Murray cod larvae feeding successfully in non-flooded rivers.

Latest research has also shown that Murray cod in fact live their entire life cycle within the main channel of the stream. Earlier ideas that Murray cod spawn on floodplains, or that Murray cod larvae feed on floodplains, are incorrect. Murray cod breed in the main river channel or, in times of spring flood, the inundated upper portion of the main channel and tributary channels, but not on floodplains. Murray cod larvae feed within the main river channel or, in times of spring flood, on the inundated upper portion of the main channel and the channel/floodplain boundary, but not on the floodplain.

Spawning is preceded by significant upstream migrations if high spring flows or floods allow. Radio-tracked Murray cod in the Murray River have migrated up to 120 kilometres (75 mi) upstream to spawn, before returning to exactly the same snag they departed from, an unusual homing behaviour in a freshwater fish. Decades of observations by recreational and commercial fishermen suggest such spring spawning migrations are common across the Murray cod's geographical range. Spawning is initiated by pairing up and courtship rituals. During the courtship ritual a spawning site is selected and cleaned — hard surfaces such as rocks in upland rivers, and logs and occasionally clay banks in lowland rivers, at a depth of 2–3 metres (6–10 ft), are selected. The female lays the large adhesive eggs as a mat on the spawning surface, which the male fertilises. The female then leaves the spawning site. The male remains to guard the eggs during incubation, which takes six to ten days (depending on water temperature), and to guard the hatched larvae for a further week or so until they disperse. Larvae disperse from the nest site by drifting in river currents at night, and continue this behaviour for around four to seven days. During this dispersal process larvae simultaneously absorb the remainder of their yolk sac and begin to feed on pelagic zooplankton, small, early life-stage macroinvertebrates and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.

This information shows that the relationship between river flows and Murray cod recruitment are more complex than first thought, and that in less regulated rivers, Murray cod may be able to recruit under a range of conditions including stable low flows. (Significant recruitment of Murray cod in low flow conditions in less regulated lowland rivers has now been proven.) This information also suggests that non-river-regulation related causes of degradation are playing a larger role in the survival and recruitment of Murray cod larvae than first thought; competition from extremely large numbers of invasive carp larvae are negatively affecting the survival and recruitment of Murray cod larvae to a much greater degree than first thought; and that decades of overfishing is playing a far larger role in the current state of Murray cod stocks, through depletion of spawning adults, than first thought.

These findings do not mean that river regulation and water extraction have not had adverse effects on fish stocks. Rather, river regulation has been a major factor in the decline of Murray cod and other native fish. Thermal pollution is also a major problem (see below), there is evidence that strong Murray cod recruitment events (which may be important for sustaining Murray cod populations over the long term) can result from spring flooding, and copious evidence that the health of Australian lowland river ecosystems generally rely on periodic spring flooding. Also, due to the regulation of most of the rivers in the Murray-Darling river system, mainly for irrigation purposes, only exceptional spring floods manage to "break free". The long term viability of wild Murray cod, other native fish species and river ecosystems, in the face of this fact, are of great concern.