Conditioning to predators improves survival of stocked Murray cod (Maccullochella peelii) fingerlings

SummaryDegradation of instream habitats in the northern Murray–Darling Basin has occurred through numerous stressors, including siltation, clearing of bankside vegetation, intrusion of livestock and impacts of pest species. A better understanding of habitat preferences of native fish species could help guide future instream habitat restoration actions. The habitat choices of seven native fish species, juvenile Murray Cod (Maccullochella peelii), juvenile Golden Perch (Macquaria ambigua ambigua), juvenile Silver Perch (Bidyanus bidyanus), adult Murray–Darling Rainbowfish (Melanotaenia fluviatilis), adult Olive Perchlet (Ambassis agassizii), adult Un‐specked Hardyhead (Craterocephalus stercusmuscarum fulvus) and adult carp gudgeons (Hypseleotris spp.) were tested in preference troughs to help inform potential habitat restoration actions in the Condamine catchment. Each species was given a choice between pair combinations of open sandy habitat, submerged macrophytes, emergent plants and rocky rubble. Habitat preferences varied between species. Murray Cod, Golden Perch, carp gudgeons and Olive Perchlets preferred structure over open sandy habitat, whilst juvenile Silver Perch, Un‐specked Hardyhead and Murray–Darling Rainbowfish did not avoid open sandy habitats. Juvenile Murray Cod preferred rocky rubble habitat over all other habitat choices. Use of complex rock piles to provide nursery habitat for Murray Cod populations is a potential restoration option. Introduction of rock could also benefit Golden Perch and carp gudgeons. Use of emergent plants, submerged macrophytes and rocky rubble for habitat restoration all appear to have merit for one or more species of small‐bodied fishes or juvenile stages of larger sized fishes. Rocky rubble or floating attached macrophytes could be viable restoration options in areas too turbid to establish submerged macrophytes. These habitat interventions would complement existing actions such as re‐snagging and provision of fish passage to assist with sustainable management of native fish populations.

Variation in river flow is a strong behavioural determinant for the movement of many freshwater fish species and often is linked to key aspects of their life cycle. The alteration of natural flow regimes to meet human water demands can result in changes to this variability, and cause declines in water‐dependent biota. Environmental flows are used as a remediation tool in some regulated rivers with the intention of restoring aspects of the natural flow regime to benefit riverine species, although empirical data are required to inform the efficacy of these interventions. Using acoustic telemetry, we quantified the movement responses of two large‐bodied native fish species (freshwater catfish, Tandanus tandanus and Murray cod, Maccullochella peelii) to variations in river flow over 4 years in two intermittent regulated rivers in the northern Murray–Darling Basin, Australia. Both rivers received periodic environmental flow releases and also there were several large natural flow events during the study period. Both species displayed a range of intra‐specific movement behaviours. Analysis of individuals' movements revealed five distinct functional groups, which were represented in both species. We found that periods of environmental flow delivery played an important role in the movement behaviour of both species. Murray cod were more likely to move during periods of environmental flow releases, whereas freshwater catfish were less likely to move on an environmental release following higher antecedent flows. No large‐scale philopatric movements were observed, yet Murray cod were more likely to move during the breeding period, indicative of nest site selection. We also found that the likelihood of movement in both species was higher in the smaller of the two rivers for a given magnitude of flow. Our results suggest that environmental flows may benefit certain fish species by facilitating, rather than cueing breeding behaviours, allowing individuals improved access to, and provision of, higher‐quality nesting habitats. These findings will aid water managers in creating economical and targeted environmental flow releases, timing larger flow pulses for the species with flow‐cued reproductive strategies, and providing a continuity of smaller baseflows for species whose reproduction is not dependent on flow cues but may be enhanced by greater connectivity.

Context Dryland rivers are unique ecosystems, where drought and flood play an important role in shaping the ecosystem. River regulation has altered the natural flow regime in many of these systems, affecting migration cues and connectivity for many species. Aims To quantify the discharge-related movements of Murray cod and golden perch within the Condamine–Balonne River subject to differing levels of river regulation. Methods We quantified flow regime variability, river regulation and fish movement to develop generalised additive mixed models to predict movement probability for Murray cod and golden perch. Results Both species showed strong positive relationships between discharge and movement. Murray cod did not show any association with river regulation; however, medium-sized individuals were significantly more likely to move than were smaller or larger fish. Golden perch movements varied among levels of regulation, were more likely to move as body weight increased and showed seasonality of movement, moving less during winter. Conclusions This study presents the largely unobserved movement behaviours of fish across a gradient of river regulation and environmental conditions in the northern Murray–Darling Basin. Implications This information is valuable for informing policy and management decisions that may affect species’ life-history requirements in analogous river systems.

Combining bio-telemetry and underwater imagery to elucidate the reproductive behaviour of a large, long-lived Australian freshwater teleost

Tracking wild animals over long periods of time is a non-trivial challenge. This has caused a bias in the availability of individual-based long-term datasets with the majority including birds and mammals. Visual Implant Elastomer (VIE) tags are now a widely used technique that may facilitate the collection of such data for fish and amphibians. However, VIE tags might have important drawbacks. Overall, four potential issues with VIE tags have been proposed: tag loss or misidentification, limited number of individual identifiers, enhanced mortality risk, and effects on intra-specific interactions. Here, we present three experiments in which we investigated these potential problems with VIE tagging in small freshwater fish both in the laboratory and in the wild, using the cooperatively breeding Lake Tanganyika cichlid Neolamprologus pulcher. We find VIE tags to be generally suitable for work with these fish as they did not impair survival, were recognisable up to 2 years after injection, and did not generally disturb group formation. Nevertheless, we identify specific issues of VIE tagging, including colour- and position-dependent variation in tag identification rates, and indications that specific colours may influence social behaviour. Our results demonstrate the suitability of VIE tags for long-term studies on small freshwater fish, while also highlighting the need of validating this method carefully for any species and study.Significance statementInformation on the survival, dispersal, and reproductive success of wild individuals across their lifespan is among the most valuable data in Behavioural Ecology. Because tracking of free-ranging individuals over extended periods of time is challenging, there exists a bias in the taxonomic distribution of such long-term datasets. Here, we investigate the suitability of visible implant elastomers (VIE) as a tracking technique to allow for the collection of such data also in small tropical freshwater fish. We show that VIE tags neither alter social behaviour in our study species, nor do they reduce survival, but they enable the tracking of wild individuals across years. We also identify colours and tag positions that are less suitable. We conclude that VIE tags can help produce long-term datasets also for small fish, provided certain precautions are met.

Stock enhancement is a management tool used for fishery recovery worldwide, yet the success of many stocking programs remains unquantified. Murray cod (Maccullochella peelii) and golden perch (Macquaria ambigua) are important Australian recreational target species that have experienced widespread decline. Stocking of these species has been undertaken for decades, with limited assessment of effectiveness. A batch marking and recapture approach was applied to assess stocked Murray cod and golden perch survival, contributions to wild fisheries, and condition in rivers and impoundments. Stocked fish were marked with calcein. Marked fish were detected during surveys undertaken 3 years and 10 months from initial marking, and it is probable that marks will persist beyond this time. The proportion of calcein marked fish in the population sub-sample whose age was equal to, or less than, the number of years since release, varied by 7–94% for Murray cod, and 9–98% for golden perch. Higher proportions of marked fish were found in impoundments than rivers. Marked Murray cod had significantly steeper length–weight relationships (i.e. higher weight at a given length) to unmarked fish. Our results show that application of methods for discriminating stocked and wild fish provides critical information for the development of adaptive, location-specific stocking strategies.

Simple SummaryPassive integrated transponder (PIT) and visible implant elastomer (VIE) tags are commonly used to identify reptiles, amphibians, and fish. The aim of this study was to evaluate for pathologic changes associated with these tags in the marine toad (Rhinella marina). For the 56 toads in this study, all PIT tags were functional and 95% remained at the insertion site with little to no damage to the tissue. However, only 48% of VIE tags were functional, i.e., visible through the skin. Although there was little to no damage to the skin at the site of placement, the VIE was found to have migrated to the kidneys in 98% of toads as well as along the legs and sporadically in other internal organs. VIE in the kidney caused inflammation and damage to the vasculature that progressed in severity over time. Based on these findings, the authors recommend the use of PIT tags over VIE tags for identification of adult anurans, when feasible.Passive integrated transponder (PIT) and visible implant elastomer (VIE) tags are commonly used to identify reptiles, amphibians, and fish. Despite reports of good retention rates and little to no negative effect on survival time, migration remains a concern and histologic changes have not been widely evaluated. Fifty-six wild-caught marine toads (Rhinella marina) were marked with a PIT tag in the left caudal thigh and a VIE tag over the right gastrocnemius muscle prior to transport to the North Carolina Zoo. Fourteen toads were then humanely euthanized on day 9, 15, 32, and 62 for postmortem examination and histopathology which were compared to 10 control toads with no tags. All PIT tags were functional and 95% remained at the insertion site with minimal to no histologic changes. Externally, 48% of VIE tags were visible through the skin at the original site of injection under fluorescent or UV light. Upon gross examination of the tissues, VIE tags had an overall retention rate of 62% at the injection site, with similar retention rates across time points. Migrated VIE material was visible grossly and histologically in the kidneys of 98% of toads and along the right leg, proximally and distally, in 25% of toads. VIE material was also found sporadically in mesentery, colon, and free in the coelomic cavity. Histologically, VIE material in the skin was surrounded by minimal to mild granulomatous inflammation while in the kidney it was associated with dilation of the small vessels, edema, and granulomatous nephritis that progressed in severity over time. Based on these findings, the authors recommend the use of PIT tags over VIE tags for identification of adult anurans, when feasible.

Recreational fishery management aims to prevent species decline and provide sustainable fisheries. Overfishing has been frequently suggested as a cause of historic fishery declines within the Murray–Darling Basin (MDB), Australia, but there have been few quantitative surveys for providing fishery-dependent data to gauge status. The Murray Cod Maccullochella peelii and the Golden Perch Macquaria ambigua are important species targeted by recreational fishers across the MDB. The fisheries are controlled by size and bag limits and gear restrictions (both species) as well as a closed season (Murray Cod only). A complemented fisher survey design was used to assess the recreational fishery for both species in a 76-km reach of the Murrumbidgee River in 2012–2013. Progressive counts were used to quantify boat- and shore-based fishing effort. Catch and harvest rate information was obtained from shore-based fishers via roving surveys and from boat-based fishers via bus route surveys. Murray Cod catch rates (fish/angler-hour) were 0.228 ± 0.047 (mean ± SE; boat based) and 0.092 ± 0.023 (shore based), and harvest rates (fish/angler-hour) were 0.013 ± 0.006 (boat based) and 0.003 ± 0.001 (shore based). Golden Perch catch rates were 0.018 ± 0.009 (shore based) and 0.002 ± 0.001 (boat based), and harvest rates were 0.006 ± 0.002 (shore based) and 0.001 ± <0.001 (boat based). The Murray Cod fishery had maximal catch and harvest during the 5-month period after the closed season ended. The closed season aims to protect spawning Murray Cod, but this strategy's effectiveness may have been influenced by high fishing effort and deliberate bycatch during the closure period. To sustain and improve these MDB fisheries, we suggest quantification of catch-and-release impacts on spawning Murray Cod, provision of fish passage, re-stocking of Golden Perch, and education on fishing techniques that minimize Murray Cod bycatch during the closed season. Received October 22, 2014; accepted March 6, 2015

Rates of hybridization and introgression are increasing dramatically worldwide because of translocations, restocking of organisms and habitat modifications; thus, determining whether hybridization is occuring after reintroducing extirpated congeneric species is commensurately important for conservation. Restocking programs are sometimes criticized because of the genetic consequences of hatchery-bred fish breeding with wild populations. These concerns are important to conservation restocking programs, including those from the Australian freshwater fish family, Percichthyidae. Two of the better known Australian Percichthyidae are the Murray Cod, Maccullochella peelii and Trout Cod, Maccullochella macquariensis which were formerly widespread over the Murray Darling Basin. In much of the Murrumbidgee River, Trout Cod and Murray Cod were sympatric until the late 1970s when Trout Cod were extirpated. Here we use genetic single nucleotide polymorphism (SNP) data together with mitochondrial sequences to examine hybridization and introgression between Murray Cod and Trout Cod in the upper Murrumbidgee River and consider implications for restocking programs. We have confirmed restocked riverine Trout Cod reproducing, but only as inter-specific matings, in the wild. We detected hybrid Trout Cod–Murray Cod in the Upper Murrumbidgee, recording the first hybrid larvae in the wild. Although hybrid larvae, juveniles and adults have been recorded in hatcheries and impoundments, and hybrid adults have been recorded in rivers previously, this is the first time fertile F1 have been recorded in a wild riverine population. The F1 backcrosses with Murray cod have also been found to be fertile. All backcrosses noted were with pure Murray Cod. Such introgression has not been recorded previously in these two species, and the imbalance in hybridization direction may have important implications for restocking programs.

Zebrafish Injectable Plastic for Identification Tagging (ZIP IT) for larvae to adults using a fluorescent Visible Implant Elastomer

Rates of hybridization and introgression are increasing dramatically worldwide because of translocations, restocking of organisms and habitat modifications (Allendorf et al., 2001) thus determining whether hybridization is beneficial or detrimental for the species involved is commensurately important for conservation. Restocking programs are sometimes criticized because of the genetic consequences of hatchery-bred fish breeding with wild populations. These concerns are important to conservation restocking programs, including Percichthyidae. Two of the better known Australian Percichthyidae are the Murray Cod (Maccullochella peelii) and Trout Cod (Maccullochella macquariensis) which were formerly widespread over the Murray Darling Basin. In much of the Murrumbidgee River Trout Cod and Murray Cod were sympatric until the late 1970s when Trout Cod were extirpated. Here we use genetic SNP data to examine hybridization and introgression between Murray Cod and Trout Cod in the upper Murrumbidgee River and consider implications for restocking programs. For the first time we have confirmed restocked riverine Trout Cod as reproducing in the wild. We detected hybrid Trout Cod-Murray Cod in the Upper Murrumbidgee, recording the first hybrid larvae in the wild. Although hybrid larvae, juveniles and adults have been recorded in hatcheries and impoundments, and hybrid adults have been recorded in rivers previously (Douglas, Gooley & Ingram, 1994a; Douglas et al., 1995) , this is the first time fertile F1 have been recorded in the wild. The F1 backcrosses with Murray cod have also been found to be fertile. All backcrosses noted were with pure Murray Cod. Such introgression has not been recorded previously in these two species, and the imbalance in hybridization direction may have important implications for restocking programs.

The Murray cod (Maccullochella peelii peelii) is a large fish species keenly sought by anglers. However, this species has declined in distribution and abundance and is now listed nationally as vulnerable. This study was undertaken in the Ovens and Murray rivers, to collect larvae and age-0 Murray cod and determine the distribution of larval Murray cod around the mid-Murray River irrigation storage of Lake Mulwala. Murray cod larvae were collected from 17 of 18 sites: main channels and flowing anabranch channels of regulated and unregulated rivers, sites upstream and downstream of the lake, in the upper and lower reaches of the lake, and in the outflowing Yarrawonga irrigation channel. Larval Murray cod were collected only by methods that sampled drift in flowing waters. Age-0 Murray cod were collected by electrofishing in the main river, but not in off-channel waters, suggesting that cod are likely to settle into habitats in the main channel at a post-larval stage. The widespread occurrence of drifting larvae suggests that this species may be subject to previously unrecognised threats as they pass through hydro-electric power stations or become stranded in anabranch and irrigation channels. Results of this study are likely to be applicable to other species with drifting larval stages, and are relevant to other locations in the Murray–Darling Basin.

Use of coded-wire and visible implant elastomer tags for marine stock enhancement with juvenile red snapper Lutjanus campechanus

From 1997 to 2002, retention of coded wire tags (CWTs) and visible implant elastomer (VIE) tags was evaluated in a series of stock enhancement studies with common snook Centropomus undecimalis (60–230 mm fork length (FL)). These experiments were conducted in both field and laboratory settings in Sarasota, Florida. Retention rates of CWTs were stable after 30 d and remained greater than 97% even 1 year after tagging. Retention of VIE tags was compared among different body implant locations, implant techniques, and fish sizes. Body location had the strongest influence on VIE retention, and tags implanted in the fins (anal and caudal) had significantly higher retention rates (mean ± SE = 76 ± 9%) than those implanted in the head (adipose eyelid, jaw, and preorbital nose tissue; 5.6 ± 1.8%, P = 0.038) 1 year after tagging. After 1 year, however, most VIE tags—regardless of body location—were nearly indistinguishable or lost. Fish implanted with two VIE marks had consistently higher mean tag retention rates and visibility than those with single marks (86% versus 64% mean retention in the caudal fins 7–12 months after tagging). Fish size at tagging was not a significant contributor to tag loss. Tagging rates were slowest with VIEs: 250–400 fish per hour and 200–300 fish per hour when one and two VIE marks, respectively, were injected per fish. Tagging mortality was less than 1.6%. No significant differences in tag retention or mortality were found between field and laboratory trials. Overall, we recommend CWTs implanted in cheek muscle and at least two VIE marks implanted in the caudal fin as tagging methods and locations for juvenile common snook because of the ability to tag large numbers of fish, high tag retention, and low tagging mortality.

The Murray Cod Maccullochella peelii and Golden Perch Macquaria ambigua are important recreational species in Australia's Murray–Darling Basin (MDB); both species have declined substantially, but recovery is evident in some areas. Minimum length limits (MLLs)—implemented to ensure fish could spawn at least once prior to harvest eligibility—have increased three times in the past decade. We quantified variation in length at 50% maturity (LM50), age at 50% maturity (AM50), and von Bertalanffy growth parameters (k = Brody growth coefficient; L∞ = asymptotic length; t0 = theoretical age at zero length) of these species within two rivers and two reservoirs of the MDB; to investigate whether fish length is a suitable surrogate for AM50 in setting MLLs. Between 2006 and 2013, we collected 1,118 Murray Cod and 1,742 Golden Perch by electrofishing and gillnetting. Values of k and L∞ were greater for reservoir fish than for riverine fish. For both species, AM50 was generally greater in rivers than in reservoirs; for Murray Cod, LM50 was greater in reservoirs than in rivers. A yield-per-recruit model demonstrated that smaller Murray Cod MLLs would be required for rivers and that an MLL at or below 600 mm (the existing MLL) across all populations could lead to overfishing in some systems. The differences in growth rate and the onset of reproductive maturation between riverine and reservoir populations suggest that system-specific regulations would be more effective at reducing the overfishing risk and meeting fishing quality objectives. Received August 18, 2014; accepted September 10, 2015