Applied ecology and management of the invasive topmouth gudgeon Pseudorabora parva in England & Wales

Introduced species are a major threat to native biodiversity and, whilst there are socio-economic benefits often gained from introductions, there are also inherent risks to receiving ecosystems, particularly from those species that develop invasions. This is problematic, as once in the wild, their management is inherently difficult. The most effective method to avoid an invasion is preventing the initial introduction; if this is unsuccessful, then methods should aim to minimise the dispersal opportunities of the species into the wider environment. This research used the Asian fish of the Cyprinidae family, topmouth gudgeon Pseudorasbora parva, as the model species, a highly invasive fish in mainland Europe. England and Wales was the model area, where 26 P. parva populations have been recorded but the species is not yet invasive. The research identified how aspects of P. parva ecology can underpin the application of methods to prevent their wider dispersal into the environment to prevent their invasion. It covered work on: (i) the expression of their life history traits; (ii) preventing their introduction into inland waters via fish movement audits; (iii) identifying the risk of their dispersal into river catchments from aquaculture sites; and (iv) assessing how their populations can be controlled to minimise dispersal opportunities. Assessments of their somatic growth rates in relation to temperature and latitude revealed that whilst temperature was a strong regulator of growth in controlled conditions, in the wild, population density was important, with this having a stronger influence on growth rates than latitude (temperature). Individuals in recently founded populations and at low densities had very fast growth rates, with this advantageous in facilitating their establishment, with these growth rates slowing as population abundance increased. Assessments of their reproductive traits revealed that, contrary to European and Asian populations, they did not show an extended reproductive period involving fractional spawning, but instead had a discrete reproductive season that lasted no longer than three weeks in the early summer period. Approximately 8000 fish movements (i.e. removals from fisheries, stocking into fisheries) are legally consented by the Environment Agency in England each year. On each of these stockings, there is a concomitant risk of also accidentally introducing a ‘hitch-hiking’ species that has not been detected in the batch of fish. To test whether a ‘fish movement audit’ scheme could reduce this risk, an experiment was completed on batches of native fish containing known proportions of P. parva and using auditors of varying non-native fish knowledge and pre-defined searching effort levels. Outputs revealed that although the scheme could be successful at preventing 95 % of accidental introductions, this required highly experienced auditors using high searching efforts. The presence of P. parva on an aquaculture site in Southern England enabled completion of field studies on assessing their drifting rates from the farm and into an adjacent stream. Drift rates were negligible during normal summer operations. However, when aquaculture ponds on the site were drained down for husbandry purposes, the elevated flow of water displaced high numbers of mature P. parva, resulting in high dispersal rates. Whilst no P. parva were recorded in the three hours prior to ponds being drained down, over 200 were captured in drift nets in the 21 hours after draining commenced. Thus, whilst the overall risk of their dispersal was low, this was elevated considerably during specific on-site activities, and would require pro-active management to prevent accidental dispersal during these activities. A long-term field experiment, allied with a field study, revealed that in small fishing and aquaculture ponds, P. parva population densities can be maintained at very low abundances over long time periods, but required a combination of biocontrol (via manipulating of numbers of native piscivorous fish) and targeted fish removals (by trapping in spring and autumn). Fish removals alone were unable to reduce population sizes due to the compensatory responses of the remaining fish. These responses were not, however, detected when native piscivorous fish were present, probably through their predation of nest-guarding male P. parva that then inhibited large numbers of in young-of-the-year being produced. In summary, the research revealed utility of non-native fish management methods being applied to populations following their testing in more controlled conditions. It revealed that introductions into fisheries of fast growing, small-bodied, non-native fishes can be prevented by auditing, and where this is unsuccessful then their dispersal into the wider environment can be minimised, if not eliminated, by a variety of method that should reduce their invasion risk.