Developing cost-effective industry based techniques for monitoring puerulus settlement in all conditions: Phase2

A review of the Tasmanian puerulus program undertaken in 2008 involving government, industry and an external review identified that the current puerulus collectors were all on the East Coast (with the exception of King Island); despite the southern and western regions supporting the largest catches in the fishery. The review identified as a priority to "investigate options for collection on the west coast using boat-based collection and using the commercial fleet to reduce cost of collection".

In phase 1 of this project a design for a deep water collector was developed through consultation with industry and prototypes of this design were constructed and tested in aquaria with captured pueruli, on the seafloor adjacent to an existing inshore shallow collector site on the east coast of Tasmania, and in deep water on the south and southwest coasts of Tasmania. The prototype collectors were successfully deployed, retrieved and serviced by vessels in the commercial lobster fleet and vessel masters reported that the design facilitated safe and efficient handling on deck. The prototypes collected significantly more puerulus than adjacent routine collectors in deployments at the shallow site and collected puerulus for the first time on the deeper and more exposed southwest coast of Tasmania.

This phase 2 of the project saw deployment of a refined collector design onto reefs around Tasmania over 2 puerulus settlement seasons and provided evidence that; (1) puerulus settle in larger numbers in shallow inshore waters; (2) puerulus settlement in deeper water varies in space, time and depth around the Tasmanian coast (eg. Puerulus settlement was higher on the south coast than on the east coast in the 2016/2017 settlement season and puerulus settlement in waters deeper than 100m appears to be very low).

When deployed alongside traditional diver based collectors, the fisher-serviced puerulus collector captures and retains more puerulus than traditional diver-based methods and is more cost-effective per collector. However, refinements to the design would be required for its use in inshore puerulus monitoring due to siltation issues from mobile sediments in exposed inshore locations.

Despite experiencing lower catch rates than inshore settlement monitoring, the number and consistence of settlement on offshore deeper water collectors enabled similar temporal patterns of settlement to be determined. The deep water collectors also retained puerulus for similar periods to the traditional collectors. Consequently, deep water puerulus collection is a feasible alternative to costly inshore diver-serviced monitoring programs and would be expected to indicate similar trends in recruitment. Industry involvement in servicing offshore collectors during routine fishing operations greatly increases the cost- effectiveness of this approach; particularly if this support was provided without the requirement for financial compensation.