Publication Date

5-2026

Series Number

FRDC Project No 2019-159

Publisher

Fisheries Research and Development Corporation

City

Perth WA

Abstract

Background

Puerulus settlement has proven to be a good forecaster of legal recruitment of Western Rock Lobster entering into the commercial and recreational fisheries. However, in 2011, this relationship broke down, with average puerulus settlement levels not being reflected in commercial catch rates four years later. Some northern areas showed unexpectedly low catch rates, while other southern areas showed unexpectedly high catch rates. Since this change followed the 2011 marine heatwave, it was assumed that this event changed some component(s) which were important to the growth and survival of juvenile lobsters. It was determined that a better understanding of this life stage, its abundance changes and interactions with habitat would be invaluable for the lobster industry, especially in the face of uncertain future climate conditions including the likelihood of increased marine heatwaves.

Aims/objectives

The project had four main aims/objectives that were designed to improve understanding of juvenile lobster survival between puerulus settlement and legal size, including the importance of their associated habitats. These were: 1. Produce accurate measures of pre-recruit abundance throughout the West Coast Rock Lobster Managed Fishery. Compare with commercial monitoring undersize lobster abundance and puerulus settlement data. 2. Design and construct a more efficient data recording tool to increase the accuracy and speed of data collection 3. Develop base-line habitat descriptions at all potting locations throughout the shallow water survey 4. Determine the relationship between sampling rate required to detect different magnitudes of marine habitat change at these survey locations

Methodology

A desktop study on fisheries independent surveys was developed and used by the project steering committee to design a best-practise survey design for monitoring juvenile lobsters in shallow waters throughout the fishery. In the first year the lobster survey design was tested, with results examined and the design modified to maximise efficiency, consistency and data quality. The improved lobster survey was then implemented in subsequent years to develop a time series of juvenile abundance, with components of this being compared to previous catches of puerulus. To increase the efficiency of data collection, a bespoke electronic data collection application was developed and implemented for use onboard the survey vessels. This form of data collection was compared to a paper-based version and shown overall to be faster and more accurate, whilst also providing the opportunity for almost real time feedback to the sampling team/skipper and to the wider industry. In the developmental phase of the surveys, benthic imagery of all potting locations was collected, categorised and mapped to create baseline habitat descriptions. Habitat imagery collections and classification has continued throughout the life of the project, with POTBot systems randomly deployed with pots during the annual ISS. These data have been used by other projects conducting wider-scale habitat mapping as well as used through a simulation analysis, to determine subsequent habitat monitoring protocol capable of identifying change.

Results/key findings

The steering committee was able to design a survey, the independent shallow-water survey (ISS), capable of collecting a better measure of the size composition and abundance of juvenile lobsters throughout the fishery that could not be produced through the monitoring of commercial catches. This was due to the use of modified pots (closed escape gaps) and the standardised nature of the ISS, making samples far more comparable between subsequent years. Using an electronic based data collection system resulted in faster and more accurate data collection. It provided the collection team with the ability to review their data in near real time, thus providing a secondary check of its accuracy and allowed post-survey analysis to be conducted within hours of a survey finishing. From the five consecutive annual samples collected during this project, strong correlations (generally R2 > 0.75) were found between puerulus settlement and ISS catch rates, with relationships varying by site, size class and lag. These strong relationships do not indicate any marked changes in survival/growth of lobster at any site over this project and that puerulus remains a good indicator of recruitment for this period. OFFICIAL Habitat imagery and classification have formed a valuable baseline database which has already been used for broader habitat mapping and the trialling of remote sensing habitat determination studies. This data has formed the basis of three published journal articles and will be used in at least two more. Additionally, it provides a record of habitat composition at a point in time across the entire Western Rock Lobster fishery, and with ongoing sampling using POTBots and future intensive sampling pulses, the start of a time series that will aid in the understanding of the links between habitat and juvenile lobster survival.

Implications for relevant stakeholders

For industry, these findings reinforce the value of puerulus indices in forecasting stock availability but highlight the need for caution where settlement-recruit relationships are weaker or more variable and provide an additional early warning system of recruitment failures. The ISS provides a more accurate measure of pre-recruit abundance than commercial monitoring data, and is this more appropriate for informing quota decisions, and managing stock sustainability. These results also demonstrate the importance of considering spatial variation in settlement–recruit dynamics across the West Coast Rock Lobster Managed Fishery.

Recommendations

Continuing the ISS time series is critical to capture broader environmental variability, understand puerulus to index relationships and how habitats influence this. Incorporating environmental drivers such as temperature and habitat quality into future analyses will further strengthen recruitment forecasts. Ongoing investment in ISS monitoring is recommended to enhance resilience of stock assessments and management decisions across the West Coast Rock Lobster Managed Fishery.

Keywords

Western Rock Lobster fishery, Panulirus cygnus, Independent monitoring, Habitat, Electronic data collection, DPIRD, FRDC

Disciplines

Aquaculture and Fisheries

Comments

Ownership of Intellectual property rights

Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this publication is owned by the Fisheries Research and Development Corporation n/s e.g. CSIRO Marine Research]

This publication (and any information sourced from it) should be attributed to: Rudd, L., De Lestang, S., Young, O. and Hebiton, B. (2025).

Creative Commons licence

All material in this publication is licensed under a Creative Commons Attribution 4.0 International Licence, save for content supplied by third parties, logos and the Commonwealth Coat of Arms. Creative Commons Attribution 4.0 International Licence is a standard form licence agreement that allows you to copy, distribute, transmit and adapt this publication provided you attribute the work. A summary of the licence terms is available from https://creativecommons.org/licenses/by/4.0/. The full licence terms are available from https://creativecommons.org/licenses/by/4.0/legalcode.en.

Inquiries regarding the licence and any use of this document should be sent to: [email protected]

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