Redbait are an important component of the pelagic ecosystem, being a major consumer of zooplankton and a prey species for predators such as tunas, birds and mammals. The introduction of large-scale mid-water trawl operations (2001) to target small pelagics in Zone A of the Small Pelagic Fishery has produced catches of redbait that have no precedent locally and there is real potential for further rapid expansion. Little information is available on the biology or population dynamics of redbait, and there are no assessments of stock size on which to base TACs, currently set at 34,000t (combined species, including management triggers).
The development of a method for estimating redbait biomass is urgently required to support the setting of scientifically defensible TACs. Fishery-dependent methods of assessing fish stocks are generally unsuitable for small pelagic species due to their schooling behaviour and targeted nature of the fishing operations. Furthermore, due to the very recent development of mid-water trawling targeting redbait, no time series of data are available to detect changes in redbait stocks off Tasmania. The daily egg production method (DEPM) has been applied successfully for biomass estimation of a variety of small pelagic species, and is deemed suitable for estimating redbait biomass. There are advantages in generating biomass estimates as early as possible in the development of this fishery, if fishery impacts are to be detected and managed.
FRDC has supported the evaluation of DEPM for blue mackerel (Project 2002/061), a species that co-occurs with redbait. There are considerable efficiencies in establishing linkages with this project in terms of research focus, expertise and resources. However, as redbait have a different spawning season to blue mackerel, the timing of blue mackerel egg surveys are not appropriate and therefore additional research is required to evaluate the feasibility of using DEPM for redbait.