Rock lobster fisheries throughout southern Australia show substantial fluctuations in recruitment which if not carefully monitored and managed may lead to lost opportunity and substantial loss in revenue. In Australia, larval (puerulus) collectors have been established in shallow water regions to provide early warning of future changes in abundance. These collectors are serviced either by divers (SA, Tas & Vic) or from dinghies (WA) which make them expensive to service and thus limited in their regional distribution to a few sites. For southern rock lobster there has been concern over how well the observed larval settlement represents the entire fishery as sampling sites are few and limited to the East Coast whereas the majority of catch is from deeper reefs on the South and West Coasts where no collectors are deployed. To improve our understanding of the relationship between recruitment, future catches and short and long term recruitment trends, there is a need to improve spatial (region and depth) coverage.
This proposal follows on from Phase 1 which:
(1) Successfully developed a deep water collector that is easily serviceable by fishers and that captures puerulus.
(2) Developed an in-situ camera system that enables real time remote viewing of puerulus settlement
The need is to determine the sampling strategy that will provide meaningful results to industry and managers on recruitment patterns and trends to the fishery in regions important to the fishery and currently not represented in existing monitoring programs.
To meet this need, this phase aims to determine the depths, times and number/collectors of puerulus that settle in deeper water to determine the number of sites and the number of collectors per site that will provide meaningful settlement data to support management decisions.
Final report
Outcomes achieved to date
The outputs from this second phase of the project have led to the following outcomes:
1. A refined puerulus collector design that:
• Collects puerulus as effectively as traditional diver-serviced inshore collector systems
• Collects puerulus effectively from deep water (>50m)
• Can be easily and safely deployed, retrieved and serviced by vessels from the Tasmanian commercial lobster fleet during routine fishing operations
2. Deployments at various locations around the Tasmanian coast over 4 settlement seasons have shown that:
• Puerulus settlement is considerably lower in deeper offshore waters than in shallow inshore waters although sufficient to demonstrate major changes in recruitment.
• Puerulus settlement in deeper waters was higher in the 2016/2017 settlement season on the south coast of Tasmania than it was on the east coast
• Puerulus settlement rates in deep waters varied between recent seasons similarly to settlement in inshore waters
3. A cost-benefit analysis comparing traditional diver-based and deep-water fisher serviced puerulus collection strategies has shown that:
• Fisher-serviced is more cost-effective than diver-based methods for similar arrays of collectors
• The current fisher-serviced design is not suitable for deployment in inshore shallow exposed waters due to sedimentation from mobile sediments
• The fisher-serviced collection system developed in this project is a cost-effective way to monitor puerulus settlement in deep water
• Despite yielding lower catch rates than inshore settlement monitoring, the number of offshore collectors used in this project displayed similar temporal patterns of settlement with similar statistical power.
• Offshore collectors retain puerulus settlers similarly to inshore collectors
• Fisher-serviced puerulus monitoring would be even more cost effective if industry agreed to provide support without the requirement for financial compensation
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.
