Otolith Ageing Project

The aim of this project is to establish the age of black jewfish using otoliths (ear bones). Between 2005-07, over 2000 fish harvested in the commercial fishery were sampled, with the size and weight of the fish measured and the otoliths, or ear bones, removed. As otoliths grow in size, they deposit growth rings (in a manner similar fashion to trees), thereby allowing the age of individual fish to be determined. The otoliths are weighed individually before being cast in resin and prepared for sectioning. The otoliths are finely sectioned on a saw and the best cut is then set on a slide and placed under a microscope enabling the reader to determine the age. Once the timing of their formation is confirmed, these rings can be used to establish an accurate picture of the age structure of the aggregating fish. This project is scheduled to be completed in mid 2007.

Morphometrics Project

The aim of this project is to identify a practical, cost effective method for monitoring the size frequency of black jewfish landed in the Northern Territory. Size monitoring is currently prohibitively costly and logistically challenging. On-board monitoring requires frequent observer trips (hindered by a lack of room on the small vessels in this fleet, and the high cost in terms of staff resources), while port-side monitoring requires fishers bring back whole fish for measuring (hindered by the large amounts of space and ice required by whole fish, and the need for two staff to be available at short notice). This project is exploring the potential of an alternative approach that may overcome these issues. The project is examining the relationship between the length of the fish versus the length of the swim bladder and various fins. If a correlation can be identified, a monitoring program can be implemented in which fishers remove and dry store the swim bladders or fins until they are able to pass on for measuring. All field work required by this project will be undertaken by RDPIFR's graduate, Dylan Campbell. This six month project is scheduled to be completed in late 2007.

Otolith Microchemistry Project

Identifying the spatial structure of local black jewfish stocks is the aim of this project. A subset of the otoliths collected in the Otilith Ageing Project were examined. One of the pair of these otoliths was dissolved in acid and the element concentration determined using an 'Inductively Coupled Plasma Mass Spectrometer’. A subset of samples was also processed with the laser ablation approach. This project is being conducted in partnership with the Australian Institute of Marine Science, Charles Darwin University and the Tasmanian Aquaculture and Fisheries Institute. All laboratory work required by this project was completed by RDPIFR's graduate, Rachel Meldrum. Analysis of the results is now underway. The Project is scheduled to be completed in late 2007(although the project may be extended to extend lasar ablation work). 

Reproductive Biology Project

This project aims to define the reproductive traits of black jewfish inhabitating the waters of the Northern Territory. The project will define the size in which black jewfish become sexually mature, as well as the time and location of spawning. Between 2005-07, almost 2000 fish were sampled (the fish same as those sampled in the Otolith Ageing Project). Fish were sourced from a large number of aggregation sites. The gonads from each fish were weighed individually and the female gonads were for sectioned to determine the stage of gonad development. The gonads of females are often examined in preference to males as they are easier to interpret. Histological work has now been completed and presentation of the results is in development. The project is scheduled for completion in mid 2007.