Rock Lobster Assessment Model

Assessment Model

Biomass and egg production estimates from the 2022/23 assessment model compared to the estimated level of the stock prior to the commencement of the fishery (an unfished population) in each of the 8 assessment areas.

The southern rock lobster assessment model is used by IMAS researchers to assess the current status of the stock and project forward in time to determine the effect of proposed management strategies on the fishery, for example to formally evaluate different possible TACCs against the developed reference points. Two reference points (the limit and the target) are used as performance indicators. Limit Reference Points (LRPs) define undesirable states for the fishery.  Target reference points (TRPs) generally define desirable states for the fishery to maintain, however, in this fishery, which is rebuilding from a very low state, the TRPs are intermediate targets on a rebuilding trajectory.

Evaluation of the three limit reference points for the southern rock lobster for the 2022/23 stock assessment is provided in the table below. The required levels (limit levels) are relative to the estimated unfished stock. For example, the egg production limit requires egg production to remain above 30% of the level estimated to have been produced prior to the commencement of fishing. These limit levels must be predicted to be achieved by the Year with the required probability. The probability achieved in the 2021/22 stock assessment was above the required probability and, therefore, above the limit for a sustainable fishery.

Limit Reference Points tend to be associated with management objectives that are related to biological sustainability of the fishery. The use of LRPs to prevent recruitment overfishing in the southern rock lobster assessment is consistent with the National Status of Australian Fish Stocks (SAFS) report which uses LRPs to separate the status of “recruitment overfished” from “sustainable”. Whilst LRPs ensure biological sustainability, they doesn’t necessarily deliver good economic and social outcomes 

Total allowable commercial catches have been reduced across south-eastern Australia over the past decade to reduce fishing mortality so that biomass and catch rates in most areas are expected to increase.
SAFS 2018

The economic benefit from both recreational and commercial fisheries is related to the abundance and catch of the legal sized stock.  In this case LRPs are paired with target reference points to manage trade-offs between catch and stock abundance.  This is true for recreational fisheries where benefit is the success or enjoyment of fishing and also for commercial fisheries the benefit is the financial earnings (technically, both these benefits are forms of economic yield). For example, high levels of catch provide high revenue but reduce the legal sized biomass, while low catch rates are undesirable for both sectors because they imply high cost of fishing in the commercial sector and a greater number of unsuccessful fishing trips in the recreational sector.  Hence there is a trade-off between catch rates and catch, and the TRP attempts to optimise this balance.   

Management action is intended to be more forceful in achieving LRPs than TRPs as we are willing to accept a higher probability of not meeting economic goals than we are of risking sustainability. For the Tasmanian southern rock lobster fishery this is implemented as a requirement to achieve the LRP with a 90% probability and the TRP with a 70% probability.