Biocontrol of common carp in Australia poses risks to biosecurity

Use of spatio-temporal habitat suitability modelling to prioritise areas for common carp biocontrol in Australia using the virus CyHV-3

Common carp (Cyprinus carpio) are an invasive species of the rivers and waterways of south-eastern Australia, implicated in the serious decline of many native fish species. Over the past 50 years a variety of control options have been explored, all of which to date have proved either ineffective or cost prohibitive. Most recently the use of cyprinid herpesvirus 3 (CyHV-3) has been proposed as a biocontrol agent, but to assess the risks and benefits of this, as well as to develop a strategy for the release of the virus, a knowledge of the fundamental processes driving carp distribution and abundance is required. To this end, we developed a novel process-based modelling framework that integrates expert opinion with spatio-temporal datasets via the construction of a Bayesian Network. The resulting weekly networks thus enabled an estimate of the habitat suitability for carp across a range of hydrological habitats in south-eastern Australia, covering five diverse catchment areas encompassing in total a drainage area of 132,129 km² over a period of 17-27 years. This showed that while suitability for adult and subadult carp was medium-high across most habitats throughout the period, nevertheless the majority of habitats were poorly suited for the recruitment of larvae and young-of-year (YOY). Instead, high population abundance was confirmed to depend on a small number of recruitment hotspots which occur in years of favourable inundation. Quantification of the underlying ecological drivers of carp abundance thus makes possible detailed planning by focusing on critical weaknesses in the population biology of carp. More specifically, it permits the rational planning for population reduction using the biocontrol agent, CyHV-3, targeting areas where the total population density is above a "damage threshold" of approximately 100 kg/ha.

Scavenging by threatened turtles regulates freshwater ecosystem health during fish kills

Humans are increasing the frequency of fish kills by degrading freshwater ecosystems. Simultaneously, scavengers like freshwater turtles are declining globally, including in the Australian Murray-Darling Basin. Reduced scavenging may cause water quality problems impacting both ecosystems and humans. We used field and mesocosm experiments to test whether scavenging by turtles regulates water quality during simulated fish kills. In the field, we found that turtles were important scavengers of fish carrion. In mesocosms, turtles rapidly consumed carrion, and water quality in mesocosms with turtles returned to pre-fish kill levels faster than in turtle-free controls. Our experiments have important ecological implications, as they suggest that turtles are critical scavengers that regulate water quality in freshwater ecosystems. Recovery of turtle populations may be necessary to avoid the worsening of ecosystem health, particularly after fish kills, which would have devastating consequences for many freshwater species.

The phylogenomics of evolving virus virulence

How virulence evolves after a virus jumps to a new host species is central to disease emergence. Our current understanding of virulence evolution is based on insights drawn from two perspectives that have developed largely independently: long-standing evolutionary theory based on limited real data examples that often lack a genomic basis, and experimental studies of virulence-determining mutations using cell culture or animal models. A more comprehensive understanding of virulence mutations and their evolution can be achieved by bridging the gap between these two research pathways through the phylogenomic analysis of virus genome sequence data as a guide to experimental study.

Genomic and biologic comparisons of cyprinid herpesvirus 3 strains

Cyprinid herpesvirus 3 (CyHV-3) is the archetypal fish alloherpesvirus and the etiologic agent of a lethal disease in common and koi carp. To date, the genome sequences of only four CyHV-3 isolates have been published, but no comparisons of the biologic properties of these strains have been reported. We have sequenced the genomes of a further seven strains from various geographical sources, and have compared their growth in vitro and virulence in vivo. The major findings were: (i) the existence of the two genetic lineages previously described as European and Asian was confirmed, but inconsistencies between the geographic origin and genotype of some strains were revealed; (ii) potential inter-lineage recombination was detected in one strain, which also suggested the existence of a third, as yet unidentified lineage; (iii) analysis of genetic disruptions led to the identification of non-essential genes and their potential role in virulence; (iv) comparison of the in vitro and in vivo properties of strains belonging to the two lineages revealed that inter-lineage polymorphisms do not contribute to the differences in viral fitness observed; and (v) a negative correlation was observed among strains between viral growth in vitro and virulence in vivo. This study illustrates the importance of coupling genomic and biologic comparisons of viral strains in order to enhance understanding of viral evolution and pathogenesis.