Editing Murray cod (section)

==Reproduction==
[[Image:Small Murray cod.JPG|thumb|A small Murray cod from a run in an upland river]]
Murray cod reach sexual maturity between four and six years of age, generally five years.<ref name=Rowland2005/><ref name=Rowland1998a>
{{cite journal
  | last = Rowland  | first = S. J.
  | title = Aspects of the reproductive biology of Murray cod, ''Maccullochella peelii peelii''
  | journal = Proceedings of the Linnean Society of New South Wales
  | volume =120
  | pages = 147–162|url=http://biostor.org/reference/68505
  | year = 1998 }}
</ref><ref name=Rowland1998b>
{{cite journal
  | last = Rowland  | first = S. J.
  | title = Age and growth of the Australian freshwater fish Murray cod, ''Maccullochella peelii peelii''
  | journal = Proceedings of the Linnean Society of New South Wales
  | volume = 120|url=http://biostor.org/reference/68506
  | pages = 163–180
  | year = 1998 }}
</ref>  Sexual maturity in Murray cod is dependent on age.<ref name=Rowland1998a/><ref name=Rowland1998b/> Therefore, roughly 70% of wild river Murray cod, with their slower growth rate, have reached sexual maturity by {{convert|50|cm|in|abbr=on}} 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 {{convert|60|cm|ft|abbr=on}} in length.<ref name=Rowland2005/><ref name=Rowland1998a/><ref name=Rowland1998b/> These data strongly indicate the 50-cm (20-in) size limit for Murray cod is inadequate and should be increased substantially to allow for a greater chance of reproduction before capture.

Large female Murray cod in the 15– to 35-kg (35– to 80-lb) range are the most important breeders because they produce the most [[Fish egg|eggs]] and for other reasons;<ref name=Rowland2005/> 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.<ref name=Stuart&Koehn2007/><ref name=Trippel1995>
{{cite journal
  | last = Trippel  | first = E. A.
  | title = Age at Maturity as a Stress Indicator in Fisheries
  | journal = BioScience
  | volume = 45
  | pages = 759–771
  | year = 1995
  | doi = 10.2307/1312628
  | issue = 11
  | jstor = 1312628 }}
</ref><ref name=MarteinsdottirSteinarsson1998>
{{cite journal
  | last = Marteinsdottir  | first = G.
  |author2=Steinarsson, A.
  | title = Maternal influence on the size and viability of Iceland cod ''Gadus morhua'' eggs and larvae
  | journal = Journal of Fish Biology
  | volume = 52|doi=10.1111/j.1095-8649.1998.tb00969.x
  | pages = 1241–1258
  | year = 1998
  | issue = 6 | bibcode = 1998JFBio..52.1241M
 }}
</ref><ref name=MarteinsdottirBegg2002>
{{cite journal
  | last = Marteinsdottir  | first = G.
  |author2=Begg, G.A.
  | title = Essential relationships incorporating the influence of age, size and condition on variables required for estimation of reproductive potential in Atlantic cod ''Gadus morhua''
  | journal = Marine Ecology Progress Series
  | volume = 235
  | pages = 235–256
  | year = 2002
  | doi = 10.3354/meps235235 | bibcode = 2002MEPS..235..235M
 | doi-access = free
  }}
</ref> Such large females may also have valuable, successful genes to pass on.<ref name=Stuart&Koehn2007/> 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.<ref name=Stuart&Koehn2007/><ref name=Trippel1995/><ref name=MarteinsdottirSteinarsson1998/><ref name=MarteinsdottirBegg2002/> Not surprisingly, there is no truth to claim made by some recreational fishers that "large Murray cod don't breed".

Female Murray cod, upon first reaching sexual maturity, have egg counts of no more than 10,000. Very large female Murray cod can have egg counts as high as 80,000–90,000,<ref name=Rowland1989/> although a recent, very large 33-kg specimen yielded an egg count of 110,000 viable eggs.<ref name=Stuart&Koehn2007/> Egg counts in female Murray cod of all sizes are relatively low compared to many fish species.<ref name=Stuart&Koehn2007/><ref name=KoehnOConnor1990>
{{cite book
  | last = Koehn  | first = J. D.
  |author2=O'Connor, W. D.
  | title = Biological Information for Management of Native Fish in Victoria
  | publisher = Victorian Government Printing Office
  | location = Melbourne
  | year = 1990
  | isbn = 0-7306-0590-6}}
</ref>

Murray cod [[Spawn (biology)|spawn]] in spring, cued by rising water temperatures and increasing [[Photoperiodism|photoperiod]] (daylight length). Initially, fish biologists working with Murray cod considered spring floods and temperatures of {{convert|20|–|21|C|F}} to be necessary<ref name=KoehnOConnor1990/> 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 [[invertebrate|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 {{convert|15|°C|°F|abbr=on}}.<ref name=McDowall1996/><ref name=Rowland2005/><ref name=Rowland1998a/><ref name=Rowland1998b/><ref name=Humphries2005>
{{cite journal
  | last = Humphries  | first = P.
  | title = Spawning time and early life history of Murray cod, ''Maccullochella peelii peelii'' (Mitchell) in an Australian river
  | journal = Environmental Biology of Fishes
  | volume = 72
  | pages = 393–407
  | year = 2005
  | doi = 10.1007/s10641-004-2596-z
  | issue = 4 | s2cid = 23107553
 }}
</ref> Additionally, recent research has shown abundant epibenthic/epiphytic (bottom dwelling/edge clinging) prey in unflooded lowland rivers,<ref name=Humphries2005/><ref name=King2004>
{{cite journal
  | last = King  | first = A. J.
  | title = Density and distribution of potential prey for larval fish in the main channel of a floodplain river: Pelagic versus epibenthic meiofauna
  | journal = River Research and Applications
  | volume = 20
  | pages = 883–897
  | year = 2004
  | doi = 10.1002/rra.805
  | issue = 8 | bibcode = 2004RivRA..20..883K
 | s2cid = 26620366
 }}
</ref> traits in Murray cod larvae that should allow survival in a variety of challenging conditions,<ref name=King2005>
{{cite journal
  | last = King  | first = A. J.
  | title = Ontogenetic dietary shifts of fishes in an Australian floodplain river
  | journal = Marine and Freshwater Research
  | volume = 56
  | pages = 215–225
  | year = 2005
  | doi = 10.1071/MF04117
  | issue = 2 }}
</ref> and a significant proportion of Murray cod larvae feeding successfully in unflooded rivers.<ref name=King2005/><ref name=KaminskasHumphries2009>
{{cite journal
  | last = Kaminskas | first = S.
  |author2=Humphries, P.
  | title = Diet of Murray cod, (''Maccullochella peelii peelii'') (Mitchell) larvae in an Australian lowland river in low flow and high flow years
  | journal = Hydrobiologia
  | volume = 636
  | pages = 449–461
  | year = 2009
  | doi = 10.1007/s10750-009-9973-8| s2cid = 32045501
 }}
</ref>

Latest research has also shown that Murray cod in fact live their entire lifecycle within the main channel of the stream. Earlier ideas that Murray cod spawn on floodplains, or the 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.<ref name=Koehn-in-prep>{{Citation  | last = Koehn | first =  J. D.  | title = In preparation}}</ref>{{better source needed|date=August 2023}}

Spawning is sometimes preceded by upstream or downstream movements.<ref>{{Cite journal |last=Koehn |first=J. D. |last2=McKenzie |first2=J. A. |last3=O’Mahony |first3=D. J. |last4=Nicol |first4=S. J. |last5=O’Connor |first5=J. P. |last6=O’Connor |first6=W. G. |date=2009 |title=Movements of Murray cod ( Maccullochella peelii peelii ) in a large Australian lowland river |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1600-0633.2009.00375.x |journal=Ecology of Freshwater Fish |language=en |volume=18 |issue=4 |pages=594–602 |doi=10.1111/j.1600-0633.2009.00375.x |issn=0906-6691|url-access=subscription }}</ref> Radio-tracked Murray cod in the Murray River have moved up to {{convert|120|km|mi|abbr=on}} upstream to spawn, before returning to exactly the same snag from where they departed, an unusual homing behaviour in a freshwater fish.<ref name=Koehn-in-prep/> Decades of observations by recreational and commercial fishermen suggest such spring spawning movements are common across the Murray cod's geographical range.<ref name=nfacod/> 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 impoundments, and logs and occasionally clay banks in lowland rivers, at a depth of {{convert|2|–|3|m|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 10 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 around four to seven days.<ref name=Rowland2005/><ref name=Rowland1998a/><ref name=Rowland1998b/><ref name=Humphries2005/><ref name=Lake1967>
{{cite journal
  | last = Lake  | first = J. S.
  | title = Rearing experiments with five species of Australian freshwater fishes. I. Inducement to spawning
  | journal = Australian Journal of Marine and Freshwater Research
  | volume = 18
  | pages = 137–153
  | year = 1967
  | doi = 10.1071/MF9670137
  | issue = 2 }}
</ref><ref name=Cadwallader1977>
{{Cite journal
  | editor-last = Cadwallader | editor-first = P. L.
  | title = J. O. Langtry's 1949–50 Murray River Investigations
  | journal = Fisheries and Wildlife Paper
  | publisher = Ministry for Conservation, Victoria
  | year = 1977}}
</ref><ref name=Rowland1983>
{{cite journal
  | last = Rowland  | first = S. J.
  | title = Spawning of the Australian freshwater fish Murray cod ''Maccullochella peeli'' (Mitchell), in earthen ponds
  | journal = Journal of Fish Biology
  | volume = 23
  | pages = 525–534
  | year = 1983
  | doi = 10.1111/j.1095-8649.1983.tb02932.x
  | issue = 5 | bibcode = 1983JFBio..23..525R
 }}
</ref><ref name=KoehnHarrington2005>
{{cite journal
  | last = Koehn  | first = J. D.
  |author2=Harrington, D. J.
  | title = Collection and distribution of early life stages of the Murray cod (''Maccullochella peelii peelii'') in a regulated river
  | journal = Australian Journal of Zoology
  | volume = 53
  | pages = 137–144
  | year = 2005
  | doi = 10.1071/ZO04086
  | issue = 3 }}
</ref> During this dispersal process, larvae simultaneously absorb the remainder of their yolk sac and begin to feed on small, early life-stage [[invertebrate|macroinvertebrates]] and epibenthic/epiphytic (bottom dwelling/edge clinging) microinvertebrates.<ref name=King2005/><ref name=KaminskasHumphries2009/> It may be that Murray cod are the first freshwater fish identified as having long-term pair-bonding in its repertoire of [[Animal sexual behaviour#Mating systems|mating strategies]] in the wild.<ref>{{cite journal |author1=Couch AJ |author2=Dyer FJ |author3=Lintermans M. |title=Multi-year pair-bonding in Murray cod (''Maccullochella peelii'') |journal=PeerJ |date=2020 |volume=8:e:e10460 |pages=e10460 |doi=10.7717/peerj.10460 |pmid=33354425 |pmc=7733648 |doi-access=free }}</ref>

The relationship between river flows and Murray cod recruitment are more complex than first thought, and 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 nonriver-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;<ref name=Tonkin-et-al2006>
{{cite journal
  | last = Tonkin  | first = D. Z. |author2=Humphries, P. |author3=Pridmore, A. P.
  | title = Ontogeny of feeding in two native and one alien fish species from the Murray-Darling Basin, Australia
  | journal = Environmental Biology of Fishes
  | volume = 76
  | pages = 303–315
  | year = 2006
  | doi = 10.1007/s10641-006-9034-3
  | issue = 2–4 | bibcode = 2006EnvBF..76..303T | s2cid = 998756 }}
</ref> 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.<ref name=kearney&kildea2001/>

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.<ref name=Allen-et-al2002/> Thermal pollution is also a major problem, evidence indicates strong Murray cod recruitment events (which may be important for sustaining Murray cod populations over the long term) can result from spring flooding, and the health of Australian lowland [[river ecosystem]]s generally rely on periodic spring flooding.<ref name=kearney&kildea2001/><ref name=King_Env._Flow_2008>
{{cite journal
  | last = King | first = A. J. |author2=Tonkin, Z. |author3=Mahoney, J.
  | title = Environmental flow enhances native fish spawning and recruitment in the Murray River, Australia
  | journal = River Research and Applications
  | volume = 25
  | pages = 1205–1218
  | year = 2008
  | doi = 10.1002/rra.1209
  | issue = 10| s2cid = 38777514 }}
</ref> 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.<ref name=kearney&kildea2001/>