Category Archives: Pest species

Assessing the susceptibility of previously untested basin fish species to Epizootic Haematopoietic Necrosis Virus (EHNV) and its epidemiology in the wild

Key words: Epizootic Haematopoietic Necrosis, EHN, disease, virus, native fish, Native Fish Strategy

Epizootic haematopoietic necrosis virus (EHNV) is a viral pathogen of international concern. The disease it causes is known from Redfin Perch (Perca fluviatilis) and farmed Rainbow Trout (Oncorhynchus mykiss) in some parts of the upper Murrumbidgee catchment and the lower Murray catchment in NSW, as well as from some parts of Victoria. Outbreaks have been recorded since 1985.

Despite its significance, very little was known about the natural distribution of EHNV in Australia or its real impact on native fish, as no formal surveys had ever been conducted. Although the susceptibility of native finfish species to EHNV had been suspected for some time, no work had been done to confirm this.

Objectives and methods: The objectives of this project were to identify the extent to which EHNV is a risk to native fish in the MDB and to provide scientific knowledge to aid in the development of effective management policy. The specific aims were to;

  1. validate earlier findings of susceptibility of native fish to EHNV,
  2. determine the susceptibility to infection by EHNV of a range of previously untested fish species found in the Basin,
  3. investigate the epidemiology of EHNV in wild populations of priority fish species and,
  4. develop a test to determine exposure of wild populations of priority fish species to EHNV.

Lab tests were undertaken using fish from candidate species separated into treatment and control groups, in which the former were exposed to the virus to identify native fish that are susceptible to EHNV (Fig 1). Subsequent analysis of 3622 tissue and 492 blood samples from fish collected from the field enabled the project team to look for instances of EHNV in the wild. Lastly, a new blood test was developed for Silver Perch (Bidyanus bidyanus), Murray Cod (Maccullochella peelii), Macquarie Perch (Macquaria australasica) and Redfin Perch, to detect antibodies resulting from immune responses to EHNV.

Figure 1, Experimentally infected Redfin Perch showing multiple white spots in the liver. Each spot is an area of dead tissue, where cells have become infected with and killed by EHNV.  (Photo courtesy Richard Whittington.)

Figure 1, Experimentally infected Redfin Perch showing multiple white spots in the liver. Each spot is an area of dead tissue, where cells have become infected with and killed by EHNV. (Photo courtesy Richard Whittington.)

Findings: It was concluded that EHNV is still present in the upper Murrumbidgee River catchment in the Murray-Darling Basin (MDB). During the study two separate outbreaks of disease due to EHNV occurred in juvenile Redfin Perch (at two locations) and a dead Redfin Perch infected with EHNV was detected (at one of the locations of an outbreak). EHNV appeared to be absent from Redfin Perch populations elsewhere in the MDB as there were no reports of disease outbreaks and neither virus nor antibodies against the virus were detected.

EHNV appeared to be absent from other species of fish in the MDB during the study period. Enough data were collected to be 95% confident that EHNV was present in <10% of the population of the following species: River Blackfish (Gadopsis marmoratus), Brown Trout (Salmo trutta), Mountain Galaxias (Galaxias olidus), Eastern Mosquitofish (Gambusia holbrookii), Murray Cod, Silver Perch, Southern Pygmy-perch (Nannoperca australis), Rainbow Trout and Redfin Perch.

The susceptibility of Silver Perch, Macquarie Perch and Eastern Mosquitofish to EHNV after exposure with water was confirmed. Two new susceptible species were identified: Murray-Darling Rainbowfish (Melanotaenia fluviatilis) and Freshwater Catfish (Tandanus tandanus). Species which became infected with EHNV following exposure via water, and in which some individuals survived and appeared to carry the live virus were Silver Perch, Eastern Mosquitofish, and Redfin Perch. Species with resistance to EHNV following exposure via water were Murray Cod, Golden Perch, Un-specked Hardyhead (Craterocephalus stercusmuscarum), Carp Gudgeon (Hypseleotris spp), Southern Purple-spotted Gudgeon (Mogurnda adspersa), Trout Cod and Southern Pygmy-perch.

Redfin Perch generally were highly susceptible to EHNV, but there appeared to be differences in susceptibility between populations. Fish from Blowering Dam in the known endemic region appeared to have a greater degree of resistance than others.

A new blood test was developed during this project to detect antibodies resulting from immune responses in fish against EHNV. A specific test was developed for Silver Perch, Murray Cod, Macquarie Perch and Redfin Perch with results suggesting that the blood test has application in field surveys for EHNV.

Lessons learned and future directions: The findings of this study reinforce a view that EHNV is a factor detrimental to native fish populations in the MDB and policies to reduce the risk of exposure to the disease in the MBD are justifiable and necessary to protect native fish populations.

The blood test developed in this study is versatile and opens up a new range of options to study the health of fish in the MDB. It has application in a much wider range of species, and it can be adapted to detect antibodies against many other pathogens. An advantage of the blood test is that it can reveal past exposure of a population to EHNV.

Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy and undertaken by a collaborative team from the University of Sydney and NSW DPI.

Contacts: Professor Richard Whittington, University of Sydney, Tel +61 2 9351 1619, Email: richard.whittington@sydney.edu.au.

Link: http://www.finterest.com.au/wp-content/uploads/2013/07/MD743%20EHNV.pdf

Assessing the recovery of fish communities following removal of the introduced Eastern Gambusia

Key words:  Gambusia holbrooki, pest species control, native fish recovery, Native Fish Strategy.

Threats and Impacts: Alien fish species have been recognised as one of eight major threats to native fish in the Murray–Darling Basin (MDB), and the control of these species is one of the key drivers of the Native Fish Strategy. There is growing evidence of detrimental impacts of Eastern Gambusia (Gambusia holbrooki) on native fish fauna globally, and this species has been identified as potentially one of the key alien species contributing to the decline of a number of native fish within the MDB, where it is widespread (Figs 1 and 2). The ecological impacts of the Eastern Gambusia in the MDB remain uncertain and this project addressed these research needs by integrating surveys and experimental work in natural billabong systems throughout the MDB.

Broad aim and specific objectives: The specific objectives of the project were to:

1. Review current knowledge of the impacts of Eastern Gambusia on native fishes of the MDB.

2. Provide information on the response of native fish communities following the reduction of Eastern Gambusia populations.

3. Provide a framework to evaluate the feasibility and effectiveness of such control actions and form a template for evaluating control options for other alien fishes across the MDB.

Figure 1: Mature female Eastern Gambusia (photo courtesy of Tarmo Raadik)

Figure 1: Mature female Eastern Gambusia (photo courtesy of Tarmo Raadik)

Figure 2: High density of Eastern Gambusia in a shallow backwater environment (Photo courtesy of Tarmo Raadik)

Figure 2: High density of Eastern Gambusia in a shallow backwater environment (Photo courtesy of Tarmo Raadik)

 

Methods: The project was divided into four phases. The first phase involved a review of current knowledge of the impacts of Eastern Gambusia on native fishes of the MDB. The second phase involved a broad-scale, cross-sectional study of wetland fish communities to develop hypotheses about the effect of Eastern Gambusia on native fish communities in these enclosed systems. The third phase was a field trial of Eastern Gambusia control in small isolated billabongs, to test the hypotheses through density manipulation experiments and to provide information on control options and Eastern Gambusia population dynamics. The fourth phase identified strategies to maximise the level of improvement to the native fish community through Eastern Gambusia control given a fixed budget (benefit maximisation), and to minimise the cost of achieving a defined significant improvement in the native fish community (cost minimisation). Finally, the project provided a template for evaluating control options for other alien fishes across the MDB.

Findings: The review of literature exploring impacts of Eastern Gambusia on native fishes of the MDB identified that 16 of 37 native species have major habitat or diet (or both) overlaps with Eastern Gambusia. The most significant overlaps were with small-bodied species e.g. Glassfish (Ambassidae), Pygmy-perches (Nannopercidae), Rainbowfishes (Melanotaeniidae), Hardyheads (Atherinidae), Gudgeons (Eleotridae) and Smelt (Retropinnidae). The review therefore concluded that Eastern Gambusia is likely to have contributed to the decline (in distribution and/or abundance) of the Olive Perchlet (Ambassis agassizii), Southern Pygmy-perch (Nannoperca australis), Murray-Darling Rainbowfish (Melanotaenia fluviatilis) and Purple-spotted Gudgeon (Mogurnda adspersa).

An assessment of wetland communities throughout the mid-Murray region of the MDB found that Carp Gudgeon (Hypseleotris spp.) and Eastern Gambusia were the dominant species in both abundance and distribution. The results of the survey suggest that Eastern Gambusia do not have a negative influence on abundances of the more common native species (e.g. Carp Gudgeon and Flat-headed Gudgeon (Philypnodon grandiceps) most likely due to the generalist nature of such species enabling co-existence. Gambusia were found to impact on the abundance of juveniles of several native species, and on their general health by ‘fin nipping’

Several small isolated billabongs had Eastern Gambusia removed to observe how native fish would respond. During this trial, astonishingly, a few individual Eastern Gambusia were able to re-establish populations of thousands within three or four months. Most importantly, the results of the removal trial indicate that reductions of Eastern Gambusia abundances will result in some improvements to small-bodied native fish populations, and these effects may be enhanced within billabongs without complex habitat (making Gambusia easier to catch and remove), and containing native species with quite specific diets.

In examining the cost-effectiveness and logistics of Eastern Gambusia removal, this study presents a strategy to determine the feasibility of removal for different scenarios and concluded that the highest benefits per dollar invested were for habitats with low frequency of connection to other Eastern Gambusia populations, low structural complexity and of high ecological value.

Lessons learned and future directions: This project provides fundamental ecological information necessary for management of Eastern Gambusia. This project provides managers with a decision making tool to assess the cost benefit of Eastern Gambusia removal for a range of habitat scenarios. This will result in better targeted action of controlling this pest species and maximise benefits to native fish populations. This project will raise awareness of the impacts of Eastern Gambusia on native fish and what benefits may be obtained for native fish following Eastern Gambusia removal.

Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.

Contacts: Zeb Tonkin, South Australian Research and Development Institute. Tel: + 61 3 9450 8600, Email: zeb.tonkin@depi.vic.gov.au.