Rock Lobster Fishery Harvest Strategy

(excerpt from the Victorian Rock Lobster Fishery Management Plan 2024)

Harvest strategies provide a structured framework for assessing the status of a fishery and a set of rules to determine the annual catch limits. Decisions regarding catch limits are therefore set out in advance, ensuring that fishers, fishery managers and other relevant stakeholders know what action will be taken in response to the conditions in the fishery (Sloane et al 2014).

Unlike the previous harvest strategy that focused on setting a constant exploitative rate to enable stock rebuilding, the foundation of this harvest strategy is based on establishing a clear rebuilding target to be achieved within a defined time frame. As the stock rebuilds, more conservative exploitation rates will be adopted to increase the resilience of the fishery.

Operational objectives

The harvest strategy aims to achieves three main operational objectives, all of which link to the overarching objectives for the management of the fishery. These operational objectives are:

1. Establish a clear target that guides rebuilding trajectories of the rock lobster population by setting appropriately conservative TACCs on an annual basis.

2. Ensure the agreed Target Reference Point (TRP) of 28 percent and 28.8 percent pre-fishing available biomass for the Western Zone and Eastern Zone, respectively, is reached by 2043.

3. As the stock rebuilds, gradually transition towards a more conservative exploitation rate to reduce the probability of future declines back to the current level.

Principles of the harvest strategy

• The harvest strategy is based on standardised CPUE from commercial catch and effort logbook information and is derived from the ‘fishing year data series’ (i.e. November to September). All estimates of standardised CPUE are rounded to two decimal places.
• TACCs are set by assessing the fishing zone’s performance against stock performance indicators, biological reference points and applying decision rules. These factors include the standardised CPUE from that year’s stock assessment, a pre-recruit index (PRI) and predefined TACC tables that determine whether the annual TACC is increased, maintained or decreased in the following season.
• TACCs are predetermined and have been calculated to ensure that the target reference points are achieved within the rebuilding timeframe.
• The exploitation rate is zero at or below the CPUE limit reference point.
• The TACC is calculated in each CPUE band on the basis of the exploitation rate in that band. * The TACC is capped at a specified level, consequently the exploitation rate declines once this cap is reached.
• The fishery will be closed to all fishing if the annual standardised CPUE falls below 0.30kg/pot lift for the Western Zone and 0.25kg/pot lift for the Eastern Zone (the lower limit reference point)
• To receive an increase in TACC, the annual PRI must be above the PRI threshold set for each zone. * The TACC can only be increased by one level at any time (‘one-jump rule’).
• An upper limit, or cap, on TACC levels has been included in this harvest strategy. The cap is 242 tonnes for the Western Zone and 32 tonnes for the Eastern Zone.

Figure 9.Theoretical construction of the proposed harvest strategy framework

2.1 Exploitation rates

The exploitation rates when the catch rate is at and above the upper reference point are:

• 26.3% in the Western Zone; and
• 20.5 % in the Eastern Zone.

2.2 Stock performance indicators

Performance indicators measure and track the performance of the stock against the operational objectives in this harvest strategy and are integral in determining the level at which the TACC will be set.

Three biological performance indicators are used in setting the TACC:

1. Egg production – an estimate of the spawning size of the population.  It is a crucial determinant of the health of the stock and is used in this strategy as a primary indicator in the decision rules.
2. Standardised CPUE – CPUE data comes from the catch and effort logbooks submitted by commercial fishers. It is expressed as the catch (kilograms) achieved per pot lift. In lobster fisheries, CPUE closely correlated to abundance and is widely accepted as being a proxy for abundance of legal-size rock lobsters. Standardised CPUE is calculated from logbook data that has undergone a quality control process and has been standardised for a range of factors that affect catchability, such as month, year, depth, region and fisher (i.e. the ability and practices of fishers). Throughout this document all references to CPUE refer to standardised.
3. Pre-recruit index – is the number of undersize lobsters per pot lift and is derived from the data collected through the fixed-site survey and on-board observer programs.  This provides a measure of the level of recruitment to the legal-size bimoass likely to be experienced in the next few years.

The total biomass (for all lobsters >60mm) is also available from the stock assessment model and is used as an additional performance indicator in the assessment of the fishery. This indicator does not directly impact TACC adjustments but is used as a secondary indicator to track the stock status and performance against the rebuilding trajectory.

2.3 Reference points for the performance indicators

Reference points are the benchmarks of performance that define acceptable levels of impact on a stock (Sloane et al. 2014).  Reference points are usually linked to the performance indicators and three types have been used; these are limit, threshold and target reference points.

Limit reference points

Limit reference points (LRP) act like a safety measure as they establish the point at which there is significant risk to the sustainability of the stock.  The LRPs used in this harvest strategy are:

1. Egg production LRP

   Model estimated egg production must be above the LRP of 20% of unfished levels with a 90% probability.

2. Standardised CPUE LRP

An upper and a lower LRP have been established, both of which are CPUE-based.

1. Upper LRP: 0.40kg/pot lift
2. Lower LRP: 0.25kg/pot lift

If the standardised CPUE starts to fall and then breaches the upper LRP, the harvest rates are decreased sequentially until the lower LRP is reached.  If the LRP point is breached, the fishery will be closed to all fishing.

Threshold reference point

Threshold reference points can represent a threshold value which triggers a certain management action or a pre-determined management response.

Pre-recruit index threshold

In this harvest strategy, threshold reference points have been established for the PRI. The PRI threshold is determined using data from the fixed-site surveys and on-board observer program and is averaged, weighted by region based on past commercial catch during a reference period of 2005 to 2014.

To be eligible for an increase in the TACC in an upcoming season, the PRI for that stock assessment period must be above the threshold level for that zone.

The PRI thresholds are:

• 1.81 undersize per pot lift in the Western Zone; and
• 0.32 undersize per pot lift in the Eastern Zone.

Target reference point

The target reference point defines the level or value of an indicator that is considered ideal or desirable and at which management should aim.

Maximum economic yield

A target reference point is not explicit in this harvest strategy; however, it is an objective of the management plan to develop a long-term maximum economic yield (MEY) target reference point for the stock.  MEY is the theoretical catch or effort level that maximises the profit of the commercial fishery.  MEY occurs when the total fishing revenue minus total fishing costs is maximised. Economic efficiency in a fishery suggests that the stock is protected and the net returns (i.e. the profits) for fishers are maximised (Australian Government 2007).

Decision rules to set the TACC

The annual TACC will be set using the following decision rules:

Decision Rule 1: Egg production  

Model estimated egg production must be above the limit reference point of 20% of the unfished level with a 90% probability.  This decision rule must be satisfied before the CPUE-based harvest strategy can be used to set the TACC.

If this decision rule is not met, the TACC will be determined using the rock lobster fishery model to ensure that the TACC returns the egg production to above the limit reference point within two years with a 90% probability.

Decision Rule 2: TACC Determination

When Decision Rule #1 has been met, the TACC is set using the standardised CPUE tables.  Refer to Tables 7 and 8, for the Western and Eastern Zones, respectively.

Step 1: Use the standardised CPUE from the preceding season to identify the CPUE band.

Step 2: Determine the TACC level according to the following conditions:

1. The TACC will be increased to the next level when:
2. the standardised CPUE is in a band higher than in the previous season;

   AND

3. the PRI (rounded to two decimal places) is at or above the threshold level of 1.80 undersize per pot lift for the Western Zone or 0.32 undersize per pot lift for the Eastern Zone.

One-jump rule: the TACC can only be increased one level per year.

2. The TACC will remain at the same level when:
3. the standardised CPUE remains in the current band;

   OR

4. the standardised CPUE has increased to a higher band but the PRI is below the trigger point.
5. The TACC will be decreased when:
6. the standardised CPUE has decreased into any lower band.

The TACC will be set at the level that corresponds to the standardised CPUE.

Decision Rule 3: TACC Cap

A TACC cap of 300 tonnes in the Western Zone and 70 tonnes in the Eastern Zone will be used over the life of the harvest strategy.

TACC Tables

Table 8 (below left) and Table 9 (below right). CPUE thresholds and corresponding TACC levels for the Western Zone and Eastern Zone, respectively

• Blue TACC values refer to the TACC cap
• Green TACC values refer to standardised CPUE values that are above the upper LRP and have a constant exploitation rate (26.3% in the Western Zone and 20.5% in the Eastern Zone).
• Orange TACC values refer to standardised CPUE values below the upper LRP and have incrementally decreasing exploitation