The Mary River Cod Research and Recovery Plan

Robert Simpson and Peter Jackson
Queensland Department of Primary Industries, Fisheries Group

2. Review of available information

2.1. General biology and ecology of the Mary River Cod

2.1.1. Taxonomy and systematics

Diagnosis: Dorsal fin XI, 15; Anal fin III, 13 (III, 12-13); Pectoral fin 18 (19-20); Pelvic fin I, 5; Caudal fin 18 (16-18); Precaudal vertebrae 15; Caudal vertebrae 19 (19-20); Predorsal bones 2 (2-3). Distinguished from the nominal subspecies (M. p. peelii) by the combination of longer pelvic fins, deeper and shorter caudal peduncle, shorter extension of first anal pterygiophore towards vertebral column, larger sagittal otoliths and distinctive colouration, and from M. ikei by the combination of deeper caudal peduncle, greater postorbital head length, smaller orbit, larger interorbital width, fewer scale rows below lateral line, shorter fifth-sixth dorsal spine and shorter extension of the first anal pterygiophore (Rowland 1993).

The Mary River cod belongs to the Australian endemic freshwater fish genus Maccullochella, which is thought to have evolved from marine percichthyid ancestors and later invaded Australian freshwaters (MacDonald 1978). There are three species of Maccullochella, one of which comprises two subspecies: viz. M. peelii peelii (Murray cod), M. peelii mariensis (Mary River cod), M. macquariensis (trout cod), and, M. ikei (eastern freshwater cod) (Rowland 1993) (Table 1).

It is hypothesised that the Mary River cod is derived from ancestral stock that occurred in western drainage streams (ie. west of the Great Dividing Range) (Rowland 1993). Isolation of the cod into eastern streams could be explained by 'stream capture', where the headwaters of western drainage streams were successively 'captured' by eastern coastal streams during westward migration of the dividing range (Herbert 1980). Small founder populations of cod may have become isolated from the parent stock during this process, and subsequently evolved into the eastern drainage forms. It is estimated that eastern cod (ie. Mary River cod and eastern freshwater cod) diverged from the ancestral form between 1.7 and 0.8 million years ago.

The close similarities between the Mary River cod, eastern freshwater cod, and Murray cod previously led to the assumption that they were a single species. For example, reviews of the genus Maccullochella by Berra and Weatherly (1972) and of Australian percichthyids by MacDonald (1978) stated that the range of the widely distributed Murray cod included the Mary River in south-east Queensland and the Clarence River in north-east New South Wales. Rowland (1985, 1993) compared cod from the Murray-Darling, Clarence and Mary systems using electrophoretic protein analysis, morphometric analysis, osteology, otolith structure, and cross-breeding experiments. Although there were several features, which appeared to distinguish the two eastern drainage forms from the Murray cod (ie. phenotypes of muscle general protein and length of pelvic fins), overall, the results indicated a close relationship between the Mary River cod and the Murray cod. These results led to the description of the Mary River cod as a subspecies of the Murray cod, viz. M. peelii mariensis, and the eastern freshwater cod as a distinct species, M. ikei (Rowland 1993).

The Mary River cod is an elongate, percoid fish with concave head profile and protruding lower jaw. Back colouration varies from golden-yellow to green to dark brown, overlaid with a black to dark green mottling which sometimes extends onto the grey or whitish ventral

Table 1: Comparison of the four currently recognised taxa within the genus Maccullochella.
Name Distribution Main external distinguishing features Maximum known weight A.S.F.B. Conservation status (8) Conservation actions undertaken
Mary River cod M. peelii mariensis Mary River (s.e. Qld); possible former distribution in Brisbane-Stanley, Albert-Logan and Coomera Rivers (4,5,6,7) (b) Concave head profile
Protruding lower jaw
Relatively long pelvic fins
Relatively deep, short caudal peduncle
Coloration distinct from Murray cod and trout cod (6)
23.5 kg (4) (d) endangered Possession limit (in Mary River system): 0.
Possession limit (upstream of the walls of Maroon, Moogerah, Hinze, Bill Gunn, Lake Clarendon, Wivenhoe, North Pine, Cressbrook and Somerset Dams): 1
(minimum size limit of 50 cm applies in these waters)
Fingerlings stocked into impoundments throughout s.e. Qld since 1983 (Table 4), and in the Mary River and tributaries since 1998.
Recovery Team formed and Recovery Plan produced
Murray cod M. peelii peelii Throughout most of the Murray-Darling River system, becoming scarcer towards headwaters (1,2,3,4) (a) Concave head profile
Jaws equal, or lower jaw protruding
Coloration distinct from all other taxa (6)
113.5 kg (3) Not listed, but see Rowland (1989) for discussion of decline in distribution and numbers Minimum size limit: 50 mm.
Possession limit: 5
Closed season on the taking of any Maccullochella spp. in NSW and Victoria from 1 September to 30 November.
Fingerlings restocked into numerous waters in s.e. Australia.
Eastern Freshwater cod
M. ikei
Clarence and Richmond River systems (n.e. NSW) (4,5,6) Concave head profile
Protruding lower jaw
Relatively long pelvic fins
Coloration distinct from Murray cod and trout cod (6)
41 kg (6) endangered Five-year moratorium on the taking of any Maccullochella spp. from waters north of the Macleay River to the Queensland border.
Fingerlings restocked into Clarence and Richmond Rivers to re-establish populations.
Critical cod habitats entered on Register of the National Estate.
Trout cod M. macquariensis Murray River below Yarrawonga Weir; upper Seven Creeks (Vic) (2,3,4,9) (c) Straight head profile
Protruding upper jaw
Coloration distinct from all other taxa (6)
16 kg (1) (e) endangered Total prohibition on the taking of any trout cod from NSW, Victorian and ACT waters.
Critical trout cod habitat in the Murray River closed to all angling from 1 September to 30 November each year.
Critical trout cod habitat in Seven Creeks (Vic) closed to all angling.
Recovery Team formed and Recovery Plan produced
Fingerlings restocked in streams and impoundments.
  1. Murray cod have also been stocked in numerous waters outside their natural range.
  2. Mary River cod have been stocked in impoundments throughout southeast Queensland (see Table 4).
  3. Although Seven Creeks is within the presumed historic range of the trout cod, this population was established in the 1920's by translocation from the Goulburn River (7). Trout cod have also been stocked in several waters outside their natural range (7)
  4. Unconfirmed reports indicate Mary River cod may grow to at least 37 kg (see Table2).
  5. Unconfirmed reports indicate trout cod may grow to at least twice this weight (Cadwallader and Backhouse 1983).

Sources of information: 1. Lake 1971; 2. Berra and Weatherley 1972; 3. Llewellyn and McDonald 1980; 4. Merrick and Schmida 1984; 5. Rowland 1985; 6. Rowland 1993; 7. Wager and Jackson 1993; 8. Jackson 1993; 9. Douglas et al. 1994.

surface. Dorsal, pectoral, caudal and anal fins are clearish to dark, with dark mottling on their bases. The soft dorsal, anal and caudal fins have thin, whitish margins, and the whitish pelvic fin has white filaments.

Merrick and Schmida (1984) reported that two distinct forms of cod are recognised by anglers in the Mary system - 'sharp-nose' and 'boof-headed'. Boof-headed forms tend to be spotted rather than mottled. This difference in morphology and coloration was reportedly not related to size or sex. Rowland (1993) did not identify these two forms, and experience with cod broodstock at Lake Macdonald Hatchery indicates that the spotted markings, and possibly also the blunter head profile, are features that develop with age (personal communication, Gerry Cook).

Mary River cod do not grow as large as Murray cod. Merrick and Schmida (1984) list the largest known Mary River cod at 23.5 kg, although reports from local residents suggest that specimens up to 38 kg have been caught (Table 2). Cod up to 36 kg have also been reported from the Brisbane River (McKay and Johnson 1990). Murray cod have been recorded up to 113.5 kg (Llewellyn and MacDonald 1980).

2.1.2. Historical distribution

The historical distribution of freshwater cod in southeast Queensland included the Mary, Brisbane-Stanley, Albert-Logan, and Coomera River systems (Wager and Jackson 1993) (Figure 1). Cod are now very rare or extinct in all but the Mary system.

In the Mary system, cod were common throughout the main river and most tributaries in the early 1900s, and until much later in some areas (Simpson 1994; Willett [DPI Fisheries], unpublished data) (Table 2). The Queensland museum has few records of Mary River cod (Table 3), and there is very little other published information on the distribution and status of fish populations within the Mary system.

One of the earliest accounts of cod in the Mary River system can be found in the 'Report on the Scientific Results of the Voyage of the H.M.S. Challenger' during the years 1873-76 (Wyville Thomson 1880). One of the fish captured in the Mary River near Tiaro was recorded as Oligurus macqariensis (the name then assigned to all species of Maccullochella), however no further details of numbers or sizes were reported. Macleay (1883) collected Oligurus macquariensis from the Mary River near Maryborough, and stated that

'...I cannot detect any difference between this fish and the well known 'Cod' of our western rivers...' (p.200).

In 1890, W. Saville-Kent, then Commissioner of Fisheries in Queensland, reported that

'...The fish most highly valued for sport and for consumption is ... a form apparently allied to, if not precisely identical with, the celebrated Murray cod, Oligorus macquariensis; this species is most abundant in Tinana Creek and other tributaries of the Mary River and is not infrequently caught weighing as much as 30 or 40lb....' (p.2).

Ogilby's (1893) discussion of the Murray cod, reported that

'...the same species is also found in the Mary River, Queensland...'(p.19).

De Vis (1901) pointed out that the cod populations in the Mary and Brisbane River systems were separated from the Murray-Darling population by mountain ranges, but offered no discussion on the possible implications of this observation. Most references since 1900 simply repeat the false premise that Murray cod also occur in the Mary River (eg. Ogilby 1916; McCulloch and Whitley 1925; Whitley 1937; Roughley 1951; Grant 1965).

Table 2: Summary of anecdotal information relating to the past and present distribution of Mary River cod in the Mary River system. Information was obtained from riparian landowners (L), other residents of the Mary valley (R), recreational anglers (A), forestry workers (F), and fish hatchery operators (H).
Location Summary of anecdotal information Source
Mary River - Conondale - cod once caught in Mary River and small tributaries, up and downstream from Conondale
- decline attributed to loss of pool habitats (siltation)
Mary River - Conondale - grandfather used set lines to make regular catches of cod, up to ~10kg weight
- major siltation didn't occur in this area until the 1955 flood - cod had disappeared long before this due to blasting with gelignite
Mary River - Kenilworth - '...The Mary River has filled in since the 1920's when codfish were caught from the first bridge at Lower Kenilworth (Gheerulla) crossing...'
- '...Fine cod were once plentiful in the Mary River from Tiaro to the headwaters. Now they are rare indeed...'
Mary River - Kenilworth - aboriginals and early settlers fished for cod
- none seen since at least ~1960
Mary River - 'Blackfellows Creek' - 83 lb cod caught in ~1926
- '...Cod were plentiful in those days, and it was nothing to get two on a set line...'
Mary River - Tuchekoi - cod abundant in 1930's
- decline attributed to loss of pool habitats (siltation)
Mary River - Tuchekoi - 75 lb cod caught ~1920 L
Mary River - Gympie - occasional cod still caught in town reaches  
Mary River - Curra Ck junction - small cod (~2kg) caught about 1989 L
Mary River - Gundiah (Emery's Bridge) - cod sufficiently abundant to be targeted by anglers prior to ~1950's
- target species are now catfish and mullet
Mary River - Miva (Stantons Dairy) - small fish caught 1991 that 'may' have been a cod L/A
Mary River - Tiaro - no cod caught since at least 1985, despite regular angling A
Obi Obi Creek - gorge - several cod up to ~5kg caught since 1992 A
Obi Obi Creek - Kenilworth - cod once reasonably common in lower reaches of Obi Obi Ck
- no known cod captures since at least 1980, but heard of some upstream in the gorge area
Booloumba Creek - reasonably well known for cod, but no authenticated captures since at least ~1970
- report of a small cod caught ~1990, but sceptical as to authenticity
Yabba Creek - '...Cod were never very plentiful in Yabba Creek...' H
Yabba Creek - Yabba falls - cod catching expeditions to Yabba falls made on horseback by land-owners from Brisbane River catchment in 1930's/40's L
Yabba Creek - below Borumba Dam - cod once plentiful, but generally considered to have disappeared around 1960's L/A/T
Yabba Creek - below Borumba Dam - small cod have reappeared in creek since stocking in Borumba Dam H/A/R
Kandanga Creek - several land-owners knew that cod were once present in the creek, but had heard of no recent captures L
Amamoor Creek - cod present up until at least 1990 L
Amamoor Creek - various sites once known as good 'cod-holes', but no definite captures since at least 1980 F
Glastonbury / Widgee / Station Creeks - a few cod still present (D. Willett 1990, unpublished data) L/A/H
Wide Bay Creek - Kilkivan - large, deep pools once contained cod
- decline attributed to loss of pools (siltation)
Wide Bay Creek - Sexton - last known cod capture in ~1964 in a pool that has since silted up
- admitted to netting the creek in the past, but said that only mullet and catfish were ever caught
Munna Creek - Miva area - severe drought conditions in early 1940's led to large fish kills, including many cod
- cod have been very rare or absent since
Munna Creek - Miva area - cod once present, but none seen since ~1950's L
Munna Creek -'Miva Station' - land-owner stated cod were still present in the creek, but denied fisheries officers access to sample (1992) L
Tinana Creek - 'Kia Ora' - '...Cod were quite plentiful (~1940's)...some of the farmers would spear them at night, the eyes showing red in the torchlight...' L
Tinana Creek - cod still present upstream of Tallegalla Weir F/H/A
Coondoo Creek - cod still present, and caught from time to time F/H/A
Six Mile Creek - cod still present, and fished for by locals L/A/H
Table 3: Summary of Mary River cod specimens lodged at the Queensland Museum.
Date Lat/Long Location
18.10.33 26.42/152.52 Mary River - near Witta, via Maleny
18.10.33 26.42/152.52 Mary River - near Witta, via Maleny
22.09.75 26.44/152.42 Mary River - headwaters
20.12.88 26.20/152.50 Six Mile Creek - Cooran
11.05.89 26.05/152.47 Tinana Creek - below weir
30.04.92 25.48/152.42 Tinana Creek - Bungawatta Station
30.04.92 26.00/152.50 Coondoo Creek
30.04.92 26.00/152.50 Coondoo Creek
30.04.92 26.00/152.50 Coondoo Creek
30.04.92 26.16/152.45 Six Mile Creek - 10 km upstream junction

There are reports that cod are, or have been, present in the Dawson River in central Queensland (Roughley 1951; Grant 1965). The lack of earlier references to this in the literature suggest that cod are not endemic to this river system, although they may have been introduced there. Recent surveys of the fish fauna of the Dawson River indicate that cod do not occur there (personal communication, Peter Long).

2.1.3. Present distribution

There are three areas within the Mary River system where cod are relatively abundant. These are Tinana-Coondoo Creek upstream from Tinana Barrage, Six Mile Creek downstream from Lake Macdonald, and upper Obi Obi Creek (Figure 2) (Simpson 1994, and personal observation, R. Simpson). Cod have also been reported from Widgee, Glastonbury, Amamoor, and Yabba Creeks, and parts of the Mary River since 1990. However, surveys conducted by DPI Fisheries suggest that numbers are very low (personal observations, R. Simpson). Cod are very rare or absent in many areas where they were once common.

Mary River cod have been stocked in impoundments, both within and outside the Mary River system, since 1983 (Table 4). Most stockings outside the Mary system have been into areas that once contained cod (eg. Brisbane-Stanley and Albert-Logan catchments).

Table 4: Summary of official stockings of Mary River cod in impoundments in southeast Queensland.
Impoundment Period stocked Number stocked
Baroon Pocket Dama 1991-94 4600
Borumba Dama 1992-93 2150
Lake Macdonalda 1983-98 30000
Wivenhoe Damb 1992-98 5700
Somerset Damb 1988-98 6907
Cressbrook Damb 1989-98 4603
Moogerah Damb 1990-98 2100
Hinze Damc 1991-94 6560
Maroon Damd 1987-98 2510
Lake Samsonvalee 1993-95 1960

(a = Mary River catchment; b = Brisbane River catchment; c = Nerang River catchment; d = Albert-Logan River catchment; e = North Pine River catchment)

2.1.4. Life history

The present knowledge of reproduction and early life history of the Mary River cod is based on experience with captive fish in hatcheries. There are no recorded observations of the spawning behaviour of wild fish, and no studies into reproductive physiology have been conducted. Most information has come from the work of Gerry Cook at a fish hatchery in Cooroy. The following section summarises Gerry's experiences with cod breeding over the last twenty years.

Mary River cod form pairs and spawn annually around spring. The male selects and guards the nest site, which is a hollow pipe or purpose-built nesting box in hatchery ponds. It is therefore presumed that hollow logs are used as nests in the wild. Spawning takes place soon after the water temperature rises to 200C, and may involve considerable aggression between the spawning pair. The female is frequently injured before she can escape the guarding male. The eggs are deposited as a layer inside the nest where they adhere to the hard surface of the pipe or log, although they are sometimes scattered around the nest site following fighting. The opaque eggs which measure 3.0 - 3.5 mm in diameter are guarded by the male. Hatching commences towards the end of the fourth day at 210C, and is usually completed by the end of the seventh day. Newly hatched cod larvae are 5 - 7 mm long. The male continues to guard the brood until they disperse to search for food around seven to nine days after hatching. The fecundity of Mary River cod is not well known, but experience suggests that around 2000 eggs per kilogram of the females weight can be expected. Some females may spawn more than once in a season. Mary River cod fry may grow to 50 mm in less than 10 weeks in plankton-rich hatchery ponds. They are harvested for stocking at 30 to 50 mm, by which time they are aggressive predators.

It is not known if Mary River cod will breed in impoundments, however breeding success and recruitment among impoundment populations of other Australian percichthyid fishes is often very low or non-existent (Barlow, 1991). Natural spawning of Mary River cod in hatchery ponds at least suggests that there is a potential for impoundment populations to become self-sustaining, given that suitable conditions are provided.

Observations by anglers and in hatcheries indicate that the Mary River cod feeds mainly on fish and large crustaceans (yabbies and shrimp). There are also reports of small birds, bats and water rats in the stomach contents of angled specimens.

2.1.5. Movements and homing

Merrick and Schmida (1984) reported that Mary River cod migrate from the main river into smaller tributaries in late winter. Other accounts suggest that cod become very active and disperse to the upper reaches of tributary streams during high stream flows in summer (personal communication, Gerry Cook). Recent radiotracking work (unpublished data, R. Simpson) indicates that both of these may be true to some extent. Adult cod may move in excess of 30 km either upstream or downstream during high stream flows at any time of year. There is a tendency for cod to move upstream in spring and summer and downstream in winter. However, some fish are much less apt to move than others and may stay within a restricted area, or 'home range', for several years. Home ranges of cod measure from 100 to 1000 m in length, and usually include two to four 'core areas' where a fish spends a large proportion of its time. The core area often comprises a large log or log pile. Feeding and patrolling movements within the home range are most common from dusk to dawn.

Homing behaviour is common among Mary River cod. Individual fish may return to a previous home range after an absence of at least eight months and a return journey of at least 70 km. The same core areas are often utilised upon return. Cod are also able to find their way back to a specific tributary after an extended period in the main river or in other tributaries. The ability to home is probably achieved by a combination of olfactory and visual senses as has been demonstrated with other fishes.

2.1.6. Habitat preference

Mary River cod occur in a variety of habitat types within the Mary River system, from high gradient, rocky, upland streams, to large, slow-flowing pools in lowland areas. Anecdotal accounts by anglers and landowners often describe the ideal cod habitat as comprising deep, shaded, slow flowing pools with plenty of snags and log-piles. Similar habitat types are utilised by the closely related Murray cod and trout cod in the Murray River system (Cadwallader and Backhouse 1983; Douglas et al. 1994).

Simpson (1994) described the habitats occupied by cod in the Mary River system. Six Mile Creek and Tinana-Coondoo Creek, which are considered the best remaining areas for cod, provide abundant instream timber and are heavily shaded by overhanging vegetation compared to many other parts of the Mary system. Streambed substrates are usually fine sand or mud. Conversely, cod habitats in Obi Obi Creek are deep and rocky, with little instream timber or overhanging vegetation. Therefore, cod may inhabit a variety of broad habitat types, however they have a strong preference for areas that provide heavy cover. In a radiotracking study of cod conducted over two years and involving more than 600 observations (unpublished data, R. Simpson), the tagged fish were located within 1 m of instream timber more than 90% of the time. There was a strong preference for log piles and large individual logs over smaller logs or other types of cover. Areas of open water were usually avoided. Relatively shallow parts of the stream were sometimes occupied provided there was abundant cover. Cod were frequently found immediately downstream of a constriction of the stream (eg. a riffle) where food was presumably concentrated by the water flow.

Physicochemical parameters vary widely across the range of the cod. The waters of Six Mile and Tinana-Coondoo Creeks tend to be acidic (pH 6.0 - 6.5), relatively high conductivity (eg. 0.8 mS/cm), often low in dissolved oxygen due to low flows, and typically stained dark by organic leachates. Conversely, Obi Obi Creek tends to be alkaline (~pH 7.3), low conductivity (eg. 0.1 mS/cm), high in dissolved oxygen, and relatively clear (Simpson 1994). Although the physicochemical preferences of the Mary River cod are not known, they appear to tolerate a relatively wide range of conditions. Like many native fish, the cod's tolerance of variable water quality is probably an adaptation to the drought/flood cycles that occur in Australian waterways.

2.2 Review of existing populations

2.2.1. Tinana-Coondoo Creek

Tinana-Coondoo Creek drains 1310 km2 in the northeast of the Mary basin (Figure 2) and has a mean annual discharge of approximately 313 000 ML (Queensland Water Resources 1993). This represents 13.7% of the Mary River catchment area and 12.1% of the total discharge volume of the Mary River. Land use is dominated by state forest pine plantations, sugar cane production along the lower reaches of Tinana Creek, and mixed agriculture and livestock production in the upper Tinana catchment. Most of Coondoo Creek and its tributaries flow through exotic pine plantations.

Tinana-Coondoo Creek provides one of the best refuges for cod in the Mary system. The sub-catchment is relatively large and well forested (including extensive areas of exotic pine trees), and human population density is low. Significant riparian buffer strips of tall, native vegetation remain intact in most areas. These provide abundant shading of the streambed, a diversity and abundance of instream cover (logs, logjams, branches, overhanging vegetation) and help to maintain bank stability. Large, deep, permanent pools are present throughout Coondoo Creek, and Tinana Creek below its junction with Coondoo Creek, although these are often separated by long stretches of relatively shallow stream habitat (Simpson 1994). Streams in this area have not been affected by anthropogenic siltation to the extent seen in many parts of the Mary catchment.

Existing water storages on Tinana Creek include Tinana Barrage (AMTD 1.6 km), Teddington Weir (AMTD 15.8 km), and Tallegalla Weir (AMTD 37.5 km) (Figure 2). These supply water for irrigation within the Tinana Creek area, and urban use in Maryborough (Queensland Water Resources 1993). Fishways have been constructed on Tinana Barrage and Teddington Weir, however both are considered largely ineffective in passing fish (Hajkowicz and Kerby 1992). The Queensland Department of Natural Resources plans to upgrade the fishway on Tinana barrage (personal communication, Errol Beitz). These reservoirs, and the tidal barrage on the lower Mary River, limit the potential for movement and interbreeding of cod from Tinana Creek with those in the rest of the Mary system.

Australian bass (Macquaria novemaculeata) have recently been recorded at several sites in Tinana and Coondoo Creeks (personal observation, R. Simpson). This species was not found during electrofishing surveys carried out in 1992-3. Although the bass occurs naturally in the lower reaches of Tinana Creek, recent captures in upstream areas may be the result of stocking by recreational anglers. Introduced golden perch (Macquaria ambigua) are present in Tinana Creek, however their abundance appears to be very low (personal observation, R. Simpson).

Tinana-Coondoo Creek is a well-known source of Mary River cod for recreational anglers and hatchery operators. Regulations now prohibit the taking of cod from any waters in the Mary catchment, and also prohibit any angling in two key areas of Tinana-Coondoo Creek However the large areas of State Forest in the Tinana-Coondoo catchment make enforcement difficult. Evidence of angling activity, including forked sticks, discarded fishing line, lures, and even maggot bags suspended over the water, can still be found (personal observation, R. Simpson).

The range of Mary River cod in Tinana-Coondoo Creek extends at least thirty kilometres up Coondoo Creek, and downstream in Tinana Creek to at least Tallegalla Weir (and probably further) (Figure 2). This makes a total stream length of at least 70 km, only 25-30% of which (ie. 17-21 km) comprises large pool habitats that are likely to provide permanent habitat for cod (Simpson 1994). Based on this data and electrofishing surveys, the cod population in this creek system is estimated at around 250 individuals (personal communication, J. Koehn).

2.2.2. Six Mile Creek

Six Mile Creek drains 310 km2 in the east of the Mary basin (Figure 2) and has a mean annual discharge of 136 000 ML (Queensland Water Resources 1993). This represents 3.2% of the Mary catchment area and 5.3% of the total discharge volume of the Mary River. Much of the Six Mile Creek catchment area is used for agriculture and livestock production.

Although large areas of the Six Mile Creek catchment have been cleared for crop and animal production, riparian vegetation is generally in good condition. Stream habitats tend to be heavily shaded, and provide abundant cover for cod (Simpson 1994). Pools in Six Mile Creek are not very large or deep (compared, for example, to many in Tinana Creek), and are often separated

by long stretches of shallow riffle and run habitats. However, the cod population appears to be relatively healthy. The stocking of cod in Lake Macdonald since the early 1980s has probably helped supplement the riverine population downstream. Golden perch and Australian bass are also relatively common in Six Mile Creek as a result of stocking in Lake Macdonald.

Lake Macdonald, at AMTD 55 km on Six Mile Creek, provides water for urban use in the adjacent Sunshine Coast area (Queensland Water Resources 1990). The dam wall restricts the upstream movement of fish (including cod), although downstream movements are thought to occur (personal communication, G. Cook). A water supply treatment plant at Lake Macdonald discharges effluent into Six Mile Creek. Accidental spillage of alum sludge from the treatment plant was thought to be responsible for a fish kill in Six Mile Creek in 1990 (unpublished data, Queensland Fisheries). The reduced downstream flows resulting from diversion of water from Lake Macdonald is considered a potential threat to the Six Mile Creek cod population (Simpson 1994).

Six Mile Creek is well known as a cod stream by recreational anglers and hatchery operators. Accessibility is relatively good via the numerous road crossings or private property, although the heavily vegetated and steep banks probably help to limit fishing opportunities.

Figure 2: The Mary River system, south-east Queensland

Figure 2, The Mary River system

Landowners report that it is not uncommon to see anglers walking along the creek.

Mary River cod inhabit most of Six Mile Creek below Lake Macdonald (Figure 2). This represents a stream length of approximately 40 km, less than half of which comprises permanent pool habitats (Simpson 1994). Based on this data and electrofishing surveys, the cod population is estimated at around 250 individuals (personal communication, J. Koehn).

2.2.3. Obi Obi Creek

Obi Obi Creek drains 202 km2 in the south of the Mary catchment (Figure 2) and has a mean annual discharge of 156 000 ML (Queensland Water Resources 1993). This represents 2.1% of the Mary River catchment area and 6.1% of the total discharge volume of the Mary River. Dairy farming and some agricultural production dominate the lower Obi Obi valley. The area between these downstream floodplains and Baroon Pocket Dam (AMTD 26.4 km) is steep, rugged country with limited access, and includes Obi Obi Gorge National Park.

The Baroon Pocket Dam wall (AMTD 27 km) is an impassable barrier to upstream movement of fish. Cod occurred upstream of the dam site prior to its construction, and were stocked in Lake Baroon in 1992 and 1993. There are concerns that the changed flow regime and increased siltation in Obi Obi Creek resulting from the construction of Baroon Pocket Dam may adversely affect the cod population (personal communication, R. Wager).

Introduced golden perch relatively abundant in downstream parts of Obi Obi Creek. Recent reports by anglers indicate that golden perch and Australian bass (Macquaria novemaculeata), which have been stocked in Baroon Pocket Dam, are present in Obi Obi Gorge.

Although access to Obi Obi Gorge is difficult, the resident cod population has been subject to recent angling pressure. Due to concerns over the effects of this pressure, a closure to all angling is now in place from the Baroon Pocket Dam wall downstream to Skene's Creek, a distance of approximately 4 km.

An electrofishing survey carried out in Obi Obi Creek (Simpson 1994) and information received from local landowners suggests that cod are now very rare or absent below the gorge. The total length of Obi Obi Creek inhabited by cod may therefore total considerably less than 10 km. Assuming cod are largely restricted to the Obi Obi Gorge area, the population size may be as small as 50-70 individuals. Unconfirmed evidence from anglers indicates that cod of less than 0.5 kg have been caught since 1994, suggesting that some recruitment may have occurred in recent years.

2.2.4. Overall status of cod in the Mary River system

Occasional captures of cod are reported from a number of other sites in the Mary River system, including Amamoor, Widgee, and Yabba Creeks and parts of the main river. However, surveys by DPI Fisheries indicate that numbers are very low. The total estimated number of cod from Tinana-Coondoo, Six Mile, and Obi Obi Creeks (see above) is less than 600 individuals, however the total population in the river system is likely to be considerably larger than this. There is insufficient reliable data to make a reasonable estimate of the size of the cod population in the Mary catchment.

The distribution of cod within the Mary River system appears to have declined. Anecdotal information suggests that cod do not occur in many areas where they once did, however, these observations could be equally well explained by reduced abundance and a lack of targeted sampling. Also, individual cod may move long distances within and among tributaries (unpublished data, R. Simpson) so that absence of cod from an area may be a temporary situation that changes seasonally or from year to year.

There has been a significant reduction in the size of individual cod in the Mary River system since the turn of the century. Reports of cod larger than 10 kg are now confined to historical records and anecdotal accounts from the first half of this century (Table 2). Today, cod larger than 5-6 kg are uncommon captures in the Mary River system.

Given the history of cod decline throughout southeast Queensland, the fragmentation of remnant Mary River populations, the low numbers of cod estimated to occur in the Mary, and the decline of large specimens, the Mary River cod is a critically endangered species.

2.3. Reasons for decline of the Mary River Cod

2.3.1. Habitat changes

Extensive siltation and in filling of pools has occurred in parts of the Mary River system, particularly along the main river channel. Erosion of cleared farmland and stream banks has deposited large amounts of sand into stream channels, and the poor state of riparian vegetation in many areas contributes to continuing bank erosion during even relatively small flow events. Grazing and disturbance of banks by cattle is common, and inhibits the natural regeneration of native vegetation. The loss of deep, shaded pool habitats is probably a major reason for the rarity or absence of cod in many areas (Simpson 1994).

Other impacts commonly associated with the loss of vegetation include reduced abundance of instream timber, changes to the instream temperature regime (Theurer et al. 1985), changes to instream productivity (Murphy et al. 1981; Hawkins et al. 1983), loss of a nutrient source for the stream community (Cummins 1986), and changes to the availability of certain food items (Harris 1985; Arthington 1992). All of these may have had either a direct or an indirect adverse effect of cod populations.

River 'improvement' works, including desnagging, have been carried out in parts of the Mary River in the past, although the extent of these actions is not well documented. Cadwallader (1978) suggested that snag removal in parts of the Murray-Darling River system may have limited the potential for native fish, such as the Murray cod, to breed. The same is probably true for the Mary River cod.

The Mary River cod is, at times, a highly mobile fish, and may traverse long distances to return to a previous home range (unpublished data, R. Simpson). Migration and homing are behavioural adaptations to enhance the success of a species or population, and restricting these movements may lead to the degradation of stocks (Pellett et al. 1998). The potential for movement of cod in the Mary River system has been limited by large dams (eg. Borumba, Baroon Pocket, and Lake Macdonald), weirs (eg. Gympie, Teddington, Tallegalla), tidal barrages (ie. Mary River and Tinana Creek), and numerous culverts and road crossings. More large impoundments are planned to meet increasing water demands in the region. It is likely that barriers to movement have, and will continue to, put pressure on cod stocks.

Changes to the instream temperature and flow regime downstream of impoundments can also adversely affect fish stocks and stream communities in general (Cadwallader 1978; Johnson 1992). Given the close correlation between cod movements and flows (unpublished data, R. Simpson), and between cod spawning and water temperature (personal communication, G. Cook), the potential for impacts on cod populations is high. The main areas of concern in regard to these impacts are Yabba Creek (Below Borumba Dam), Obi Obi Creek (below Baroon Pocket Dam), and Six Mile Creek (below Lake Macdonald).

2.3.2. Overfishing

Large numbers of cod were taken from the Mary River system in the late 1800s and early 1900s for human and animal consumption (Table 2 and Rowland 1985). The use of gelignite, nets and set lines is widely reported and considered by residents in some areas to be a major reason for the decline in cod numbers.

Mary River cod remain a popular target for recreational anglers, although the number of dedicated cod anglers has probably decreased in the latter part of this century following the decline of the species. Anglers targeting cod today concentrate most of their effort towards the three or four streams known to support remnant populations (eg. Six Mile, Tinana, and Obi Obi Creeks). Anglers may still take one cod over 50 cm (total length) per day from stocked impoundments outside the Mary River system. There is a total prohibition on the taking of cod within the Mary River catchment.

2.3.3. Introduced fishes

The effects of introduced fishes on Mary River cod, or stream communities in general, in the Mary River system are unknown. Exotic fishes previously recorded in the Mary River system include the guppy (Poecilia reticulata), swordtail (Xiphophorus helleri), and mosquito-fish (Gambusia holbrooki) (Pusey et al. 1993). The mosquito-fish has been implicated in the decline of many freshwater fishes in Australia and worldwide (Arthington and Lloyd 1989; Lloyd 1990; Arthington 1991).

Australian fishes that have been translocated into the Mary River system include the golden perch (Macquaria ambigua), silver perch (Bidyanus bidyanus) and saratoga (Scleropages leichardtii). Golden perch are present, and sometimes abundant, in most areas known to support populations of cod (Simpson 1994). Like cod, they are relatively large ambush predators, and there is likely to be considerable overlap in their preferences for food and space.

The Australian bass (Macquaria novemaculeata) occurs naturally in the lower Mary River, and has become more abundant in upstream reaches following stocking. Like the golden perch, the bass may compete with cod for food and space, and prey on juveniles.

There are reports that Murray cod have been introduced into the Mary system on several occasions. An article from the Maryborough Chronicle, dated 4.6.1870, outlined a proposal to introduce Murray cod into the Maryborough Water Reservoir. Whether this occurred is not clear. Murray cod from the Condamine River were reportedly introduced into the Mary system at Obi Obi Creek in 1880 (Merrick and Schmida 1984; Rowland 1985). The cod were said to have been brought overland in milk churns, although the Kenilworth Historical Society point out that no dairying was carried out in the district until the 1890s, and the likelihood of cod surviving such a long trip is low. Also, it is unclear why such an introduction would be attempted given the reported abundance of Mary River cod. Perhaps it was because Murray cod grow larger. More recently, Murray cod are reported to have been released in the Mary River in the 1970's (personal communication, G. Cook). Rowland (1989) stated that, although Murray cod were introduced into the Mary, they can no longer be found there.