Rock Lobster Fishery Harvest Strategy

(excerpt from the Victorian Rock Lobster Fishery Management Plan 2017, amended to incorporate changes from 2019 and 2020)

Harvest strategies provide a structured framework for assessing the status of a fishery and a set of rules to determine what the annual catch limits will be.  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).

The foundation of this harvest strategy is the exploitation rate, which is the proportion of the available stock that can be caught.  Unlike the previous harvest strategy used for the fishery, it does not include a rebuilding target so conservative exploitation rates that ensure that stocks rebuild, catch rates improve, profits are maximised and the objectives of this harvest strategy and management plan are met are used.

Operational objectives

This harvest strategy aims to achieve two main operational objectives, both of which link to the overarching objectives for the management of the fishery.  These operational objectives are:

1. Continue to rebuild the rock lobster population by setting appropriately conservative TACCs on an annual basis.
2. Maintain catch rates above 0.40 kg/pot lift (standardised).

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 predetermined and have been calculated using agreed rates of exploitation.
• 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.
• The rate of exploitation is constant unless the annual standardised CPUE falls below 0.40kg/pot lift (the upper limit reference point), when it is incrementally reduced (Figure 9).
• The annual standardised CPUE will be rounded to two decimal points when it is at and above the upper limit reference point and the exploitation rate is constant (refer to the green zone in Figure 9).  It will be increased to three decimal points when the annual standardised CPUE falls between the upper and lower limit reference points (refer to the orange zone in Figure 9).
• The fishery will be closed if the catch rate falls to 0.25kg/pot lift (the lower limit reference point) or less (Figure 9).
• 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 for the Western Zone is 300 tonnes and 70 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.  In lobster fisheries, CPUE is accepted as being proxy representing the abundance of rock lobsters in the fishery above the legal minimum length.  It is expressed as the effort required to harvest a defined amount of catch. Standardised CPUE has had irregularities in the data removed and it 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)
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.  The annual PRI will be rounded to two decimal places.

A further performance indicator, available biomass, will be used in the assessment of the fishery.  This indicator however does not result in explicit TACC adjustments in the fishery in the way egg production, standardised CPUE and pre-recruit index do.  Available biomass, as well as historic exploitation rates, is used to provide a more comprehensive picture of the status of the fishery and trends over time.

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