Threats to Murray cod John D. Koehn Department of Zoology University of Melbourne Parkville 3053 and Arthur Rylah Institute for Environmental Research Department of Sustainability and Environment 123 Brown St. Heidelberg 3084 [email protected] Abstract Introduction Many of the threats to Murray cod are well Most of the threats to Murray-Darling Basin fish documented, although not necessarily quantified species have long been recognised and well in a scientific manner. Most have previously documented (e.g. Cadwallader 1978), although been covered in a range of forums, (e.g. previous their impacts have not necessarily been quantified MDBC workshops) and publications (including or mitigated. Many of these threats have been the Native Fish Strategy), often with suggested discussed in detail at previous Murray-Darling solutions. Such threats include: habitat loss, Basin Commission forums with a range of sedimentation, changes to flows, barriers to management recommendations suggested in the movement, interactions with alien species, subsequent publications (Blanch 2001; Phillips fishing (commercial, recreational and illegal), 2001, 2003; Murray-Darling Basin Commission changes to water quality (temperature, salinity, 2004; Lintermans & Phillips 2004; Lintermans et suspended sediment, oxygen), loss to irrigation al. 2005). Murray cod Maccullochella peelii peelii systems and stocking of hatchery fish, including are also threatened by many of the issues that potential genetic impacts. are impacting native fish in general (Kearney & Kildea 2001), and indeed, undertaking the Other threats have not been so widely recommendations made in previous Murray- recognised, have less direct effects, and are Darling Basin Commission workshops would difficult to quantify or have not been widely greatly assist Murray cod populations. There are considered. These include: incremental changes however, less direct and less well-known threats to habitats, reductions in riparian vegetation, that ultimately also impact on Murray cod. isolation of the river from the floodplain, These include non-ecological issues and our ecosystem changes, low population numbers, approaches to management. This paper examines deviations from sustainable population structures all threats to Murray cod, provides information and diseases. Some of these threats are direct, for the basis of workshop discussions and others indirect, and some operate at a regional management outcomes, and provides scale while others are Basin-wide. Other recommendations for immediate actions to management issues such as the philosophy of restore populations of this icon species. waterway and fisheries managers, a lack of ownership and effective legal responsibility by the public agencies, and a lack of recognition and understanding of the life history traits of Murray cod, all indirectly threaten the species. This paper examines all threats to Murray cod and concludes that the greatest overall threat is that there is currently no effective management for this species. This is highlighted by our lack of knowledge of population sizes, recruitment patterns and angler take from populations, inaction in regulating non-natural mortality, a lack of ability to address known threats, and poor responses to recent major fish kills. 30 MANAGEMENT OF MURRAY COD IN THE MURRAY-DARLING BASIN – CANBERRA WORKSHOP, JUNE 2004 2800 km of stream in the Murray-Darling Basin Recognised threats to (Ryan et al. 2003). This may affect spawning, egg Murray cod and larval survival, swimming speeds, feeding and growth rates. For example, juvenile Murray Changes to flows cod held at 24.2 °C grew almost twice as long and 3.5 times as heavy as Murray cod held at Most debate regarding the importance of floods 12.6 °C over a 3-month period (Ryan et al. and a natural flow regime for native fish involves 2003). High turbidities and salinities may also the contribution flow makes to conditions that have adverse physiological or behavioural enhance recruitment. While the applicability of effects. Stratification may occur in pools due to the Flood Pulse Concept (Junk et al. 1989) to temperature or salinity gradients and result in Australian fishes has recently been questioned de-oxygenated, saline bottom layers (Anderson (Humphries et al. 1999), it has been suggested & Morison 1990). that strong Murray cod recruitment follows years with high flow levels (Ye et al. 2000; Rowland Barriers to movement 1998). Murray cod have been categorised as a main channel specialist (Humphries et al. 2002) There are more than 3600 structures that can that will utilise floodplain channels when they impede fish movements in the Murray-Darling are flowing (Koehn 1996), but they do not Basin. Recent research has increased our appear to utilise the flood plain proper (King understanding of the movements of Murray 2002). Changes to flows can alter downstream cod, both for adults and larvae (Koehn 1996; larval travel times and hence distances (Koehn Koehn & Nicol 1998; Humphries et al. 2002; et al. 2004), and can therefore affect population King 2002), both in upstream and downstream distributions. Variations in flow, often removed directions. Movements of adult fish in both with the delivery of irrigation water, can upstream and downstream directions provides stimulate fish movements (Mallen-Cooper et al. challenges to ensure that large adult, pre- 1996) and assist in recolonisation. spawning fish can negotiate fishways, then return downstream safely. The downstream Loss to irrigation systems drift of larvae then provides a recolonisation and redistribution process for these young fish. Losses of native fish, including Murray cod, into Agencies that manage Murray cod need to irrigation systems has recently been recognised recognise the mobility of various life stages for larvae, juveniles and adult fish (Koehn & and provide for adequate fish passage (Koehn Nicol 1998; Koehn et al. 2004), with presumed et al. 2005). high mortalities when the channels are drawn down at the end of the irrigation season. Habitat loss There is a need for quantification of this risk and improved management of irrigation systems The removal of structural woody habitat has to reduce its impacts. been widespread in Murray-Darling Basin rivers, particularly in lowland reaches over a large Changes to water quality number of years (Mudie 1961; Phillips 1972; Gippel et al. 1992; Treadwell et al. 1999). We now Suspended sediment, low oxygen levels, have a better understanding of the importance herbicides and altered water temperatures have of this habitat to adult and juvenile Murray cod all been suggested as possible causes of recent fish (Koehn 1996). Reinstatement of woody habitat is kills involving Murray cod (Koehn 2005; Sinclair now recommended as a priority action for river 2005). These kills however, have probably been restoration (Murray-Darling Basin Commission the result of a number of factors, exacerbated by 2004), and our understanding of its effects and a lack of flow, and have highlighted the fact that fish-habitat relationships is increasing (Nicol et al. water quality problems remain a threat to this 2002). In general, more habitat can mean more species. While such fish kills provide a graphic fish, and suitable habitats are needed for all life reminder of the critical impact of water quality stages. The infilling of undulations and holes by changes, non-critical changes are more common sedimentation may also impact on cod habitats and may have greater overall impacts. Cold-water and could blanket spawning substrates. pollution from low-level dam releases (see Phillips 2001) has been estimated to impact on at least MANAGEMENT OF MURRAY COD IN THE MURRAY-DARLING BASIN – CANBERRA WORKSHOP, JUNE 2004 31 Interactions with alien species widespread and can be problematic in fish Eleven introduced fish species are also present in culture conditions (Langdon 1988; Rowland & the Murray-Darling Basin (Murray-Darling Basin Ingram 1991), but their occurrence or impact in Commission 2004). Of these carp Cyprinus carpio, the wild is unknown. Redfin virus, epizootic redfin Perca fluviatilis and Gambusia Gambusia haematopoietic necrosis (EHN) (Langdon et al. holbrooki are the most widespread. Carp receive 1986) has been shown to be lethal to some the most public attention and are often blamed native species, and Murray cod is susceptible and for many of the ills of the river. Recent reviews could be a carrier (Langdon 1988). Outbreak of a (Koehn et al. 2000; Koehn 2004) indicate that new iridovirus infection in cultured Murray cod they are a typical invasive species, which is tough in Victoria remains unconfirmed for wild fish and well adapted to making the most of already (Prof. Richard Whittington unpubl. data). degraded riverine environments. Carp now Potential impact of Barramundi Encephalitis comprise a majority of the fish biomass in the Virus (BEV) is also unknown. Murray-Darling Basin (Harris & Gehrke 1997) and may compete in some way with Murray Fishing cod for habitat space. There is no evidence for Past commercial catches of Murray cod have any other form of competition between Murray obviously impacted on populations (Reid et al. cod and carp, although Murray cod are known 1997), although these fisheries are now closed. to prey on juvenile carp (Koehn et al. 2000). The loss of commercial fisheries however, also In contrast, redfin are likely to have caused means the loss of the most comprehensive predation pressure on young Murray cod, and so population data available