Simon J. Nicol

Observations on the distribution and abundance of carp and native fish, and their responses to a habitat restoration trial in the Murray River, Australia

Abstract A native fish strategy has been initiated to rehabilitate native fish populations in the Murray‐Darling Basin, Australia. The reintroduction of large woody debris (LWD) into the Basins large lowland rivers is one of the restoration activities in the strategy. The results from three separate studies undertaken on the Murray River between Yarrawonga and Tocumwal are presented on the relationship between carp (Cyprinus carpio), native species, and LWD to examine whether native species and carp compete for LWD habitat. The first study reports on the relative abundance of carp and native fish in a river reach. Since 1995 carp abundance has declined, whereas the abundance of native fish populations has remained relatively constant providing little support for the hypothesis that competition for LWD habitat is having effects at the population level effects. The second study reports on the relationship between LWD, river channel position, and its use as habitat by carp and native species. A statistically significant relationship was observed between native fish, LWD, the location within a meander, and curvature of the meander. There was no statistically significant relation between carp and any of these parameters indicating that carp utilise a variety of riverine habitats, whereas native species were strongly associated with LWD. The third study reports on an experiment that tested the response of carp to the placement of new LWD habitat. The response from carp was statistically inconclusive. The combination of these studies suggest that it is unlikely that carp and native species are directly competing for LWD habitat and it is unlikely that carp will inundate restored LWD habitats and preclude native species.

Bioeconomic modelling in the development of invasive fish policy

Abstract Invasive species policy is either explicitly or implicitly underpinned by the question “When should investment in managing the invasive species stop?” Bioeconomic modelling provides a quantitative framework for considering the benefits and costs of alternative levels of investment in invasive species management by linking the level of investment to the costs of intervention (control) and value of benefits derived. Control costs are typically the product of the number of individuals that have to be removed to either eradicate the invasive species or constrain it at some specified density, and the cost of removing each individual. Impact functions take a variety of forms, but in general are systematically related to the density of the managed population. Where impacts can be accounted in monetary terms (e.g., where an invasive species affects income), control costs and benefits can be directly compared and an optimal level of investment (usually that which maximises return on investment) can be identified. However, where impacts do not have a directly accessible monetary value (e.g., where an invasive species affects conservation values), benefits and costs cannot be directly contrasted. Under these circumstances, bioeconomic modelling can be used to identify management strategies that maximise the level of benefit that can be achieved for expenditure of a fixed budget (benefit maximisation), or minimise the cost of achieving a given level of benefit (cost minimisation).

Behavioral Responses of Murray Cod Maccullochella peelii peelii to Pulse Frequency and Pulse Width from Electric Fishing Machines

Abstract The effective electrical conductivity (Cf ) of Murray cod Maccullochella peelii peelii was evaluated as part of an investigation into effective electric fishing settings for this important species. We describe an evaluation of minimum thresholds of power applied to an electrical field required to elicit four responses (escape, forced swimming, immobilization, and narcosis) from Murray cod. Estimates of Cf varied from 46 to 80 μS/cm depending on the response threshold of interest; these results support hypotheses that a global standard for Cf could be applied to power transfer calculations. Results indicate that at 60 Hz, pulse widths (PWs) of 6.7-10 ms are most efficient; however, the electrical dose required for all responses is minimized using higher frequencies resulting in PWs of less than 2 ms. Our observations support other work suggesting that the complexity and range of responses are determined jointly by the nature of the stimulus and the orientation to the field. We suggest that a measu...