Harvest control rules are in place for the SBPMF that are consistent with the constant escapement harvest strategy for the fishery (DoF 2014a). The control rules are directly responsive to changes in the catch rate and size-based performance measures for brown tiger prawns and western king prawns, which are critical for ensuring the sustainability of the stocks of these species.
The harvest control rules for the SBPMF are designed to meet the ecological objectives of the fishery by minimising fishing of vulnerable life stages (e.g. pre-spawning and small prawns), whilst also optimising economic efficiencies. In-season control rules govern the timing and duration of fishing in particular areas of the fishery throughout the season, with annual control rules in place to ensure that the season arrangements are effective in maintaining sufficient spawning stock so that recruitment is not impaired. Table 8.1 outlines the control rules in place for the SBPMF and Figure 8.4 illustrates how they guide the annual operations of the fishery.
Figure 8.4. Flowchart of the general annual harvest strategy operations in the SBPMF
Additionally, Figure 8.5 to Figure 8.13 (see below) provides a visual representation of the effect of control rules on harvest within in a season.
SHARK BAY ANNUAL OPERATIONS
Recruitment surveys undertaken in March and April
Mean catch rate for tiger and king prawns from the two surveys used to predict catches for the current season
The temporal and spatial extent of additional rolling area openings determined based on mean catch rate, catch predictions and size composition of tiger and king prawns
(size determined from April survey only) using in-season control rules Note: Additional recruitment surveys may be undertaken on an ‘as-needed basis’ to
provide information for rolling openings of remaining closed areas
Spawning stock surveys undertaken in North CPL and South CPL in August and September
Mean tiger prawn catch rate in the North CPL from September survey used to determine re-opening of this area using in-season control rules
Mean catch rates in North and South CPL during spawning surveys used as indices of tiger and king prawn spawning stock abundance, which are used to determine
seasonal arrangements for the following year
Spawning stock survey undertaken in North CPL following June closure North CPL key spawning area (TPSA) closed to fishing following June lunar phase Spatially-restricted season opening date determined based on spawning stock survey
results from previous season and lunar phase (usually occurs following the March lunar phase)
South CPL key spawning area (ENA) closed to fishing following August lunar phase
Denham Sound area opens to fishing following July/August moon closure
Season closes following 175 total available fishing days, usually following October lunar phase
In-season control rules 8.3.1.1
• Season opening: The timing of the March lunar phase dictates when the fishery can open. Under the control rule fishing is permitted in the area outside the CPL and north of the Denham Sound line. These decisions are based on the historical understanding of prawn biology and when migration onto the trawl grounds occurs. Fishing activities early in the season are predominantly focused on the capture of larger residual western king prawns (and in some years brown tiger prawns) and avoiding small prawns moving through areas of the fishery.
• Opening areas east of the CPL: Catch rates of western king and brown tiger prawns from both recruitment surveys determine when and what areas open to fishing during the main part of the season. Catch rates above target levels permit an area to open to fishing. Catch-rates below the threshold trigger a review of the spatial and temporal extent of areas opened. Catch rates below the limit result in an area remaining closed to fishing. The purpose of these control rules is to limit total effort through spatial and temporal closures in order to reduce exploitation of the spawning stock biomass.
• North CPL and South CPL closures: The June lunar phase triggers the closure of the North CPL. Its purpose is to protect spawning brown tiger prawns as well as western king prawns. The lunar phase around the beginning of August triggers the closure of the South CPL to fishing. Its purpose is to protect spawning brown tiger prawns and small western king prawns that move onto the fishing grounds after over- wintering in the nursery grounds.
• North CPL re-opening: This area re-opens to fishing from the third quarter moon phase of the last fishing period for the season because the prawns in this area are likely to have spawned more than once and majority of prawns (both species) have migrated out of the area.
• Denham Sound and Trawl Closure opening: The August lunar phase may trigger the opening of Denham Sound (excluding the Denham Trawl Closure) to fishing. This area opens late in the season to restrict effort and protect small sized western king prawns. High (≥ 50 kg / hr) catch rates of prawns and a low by-catch of snapper (< 200 snapper / trawl hour) may also trigger a partial opening of the Denham Trawl Closure for a period of 10 days. This control rule is similarly designed to restrict effort, and also ensures low-incidental capture of snapper, a species important to recreational anglers in Shark Bay.
• Season closure: This control rule is designed to control effort levels and protect small prawns that start to occur on fishing grounds towards the end of the season, ensuring a flow through of prawns to the next season. The season closure date is set such that fishing cannot exceed a maximum of 175 fishing nights.
Annual Control Rules 8.3.1.2
Control rules associated with spawning stock indices for brown tiger and western king prawns are in place to ensure the effectiveness of the annual operations of the fishery in
season management arrangements for the following year. Catch rates below the target level (in the threshold range) trigger a review of management arrangements for the next season, which may subsequently result in management action if sustainability is considered to be at risk. Catch rates below the limit will trigger a review of the fleet’s spatial fishing patterns and catch rates to investigate why stock abundance is low. This will either result in more severe management action to protect the stock, or a change in monitoring if it is considered to be inaccurate.
8.3.2 Accounting for Uncertainty
Harvest control rules in place for the SBPMF are highly precautionary and as such are designed to account for a wide range of uncertainties. In addition to control rules that operate on an annual basis, there are many well-defined in-season control rules. These ensure a rapid response to evidence of stock depletion and are appropriate given the biological characteristics of the target species. Also important are the existence of control rules associated with both the opening and closing of the main fishing grounds. For example, catch rates from the recruitment surveys must demonstrate that the stock is above target levels before fishing can commence within the areas east of the CPL in a given season. Control rules also govern the total number of fishing nights within the season, restricting the total level of exploitation that can occur given the fishery is a limited entry fishery and effort is partly constrained by a maximum fleet headrope allocation (as a standardised fleet).
8.3.3 Evaluation
Evidence indicates that the current harvest control rules are appropriate and effective in achieving target exploitation levels. With the exception of 2012, catch rates of brown tiger prawns from the spawning stock surveys, a key annual performance indicator, have fluctuated around the target level and above the limit level since 2002 (see Section 6.1.1 for a review of the cause of the low brown tiger prawn spawning stock index for 2012 and what the outcomes were). Catch rates of western king prawns from the spawning stock surveys have fluctuated around the target level in all years since 2002. In summary, there is strong evidence that the harvest control rules for both species are working effectively for achieving acceptable exploitation levels.