8.1 Framework
8.1.1 Design
The harvest strategy for brown tiger and western king prawns in the SBPMF is based on a constant escapement harvesting approach. The use of a this type of approach recognises that short-lived prawn species exhibit naturally variable annual recruitment and that, regardless of the level of recruitment in any year, it is necessary to ensure that sufficient spawning stock is maintained. A detailed biological understanding of the two target species in the fishery (Section 2) has underpinned the development and application of in-season temporal and spatial closures in the fishery that contribute to the constant escapement harvest strategy and also generate economic benefits for the fleet (i.e. by minimising harvest of small-sized prawns which are less valuable than larger prawns). This harvesting approach has contributed to a high level of cooperation from industry regarding adherence to regulations.
Table 8.1. Summary of the Shark Bay Prawn Managed Fishery harvest strategy brown tiger and western king prawn stocks in Shark Bay. Note the reference levels essentially prescribe the operational objective which is to maintain each resource above the threshold level.
* indicates decisions made prior to season opening and provided to fishers as part of annual season arrangements. Note that the actual starting date can be either March or April. If the latter, then opening / closing in subsequent months may be rescheduled, subject to survey results and species biology.
Component Management Objectives Species Performance Indicators Reference Levels Control Rules In-season Operations
Target
Species Ecological:
To maintain spawning stock biomass of each target species at a level where the main factor affecting recruitment is the environment.
Brown tiger
& western king prawns
Season Opening* Feb/March lunar
phase After late Feb or March full moon phase. Depending on interannual variability in moon phases, fishing may start in 1st week of March
Fishing season opens and fishing permitted in area outside CPL and north of Denham Sound.
All areas east of the CPL
Catch rates of western king and brown tiger prawns from recruitment surveys in areas inside CPL.
Target: Mean catch rate of either species is ≥ 25 kg / hr.
Threshold: Mean catch rate of either species is < 25 kg / hr and
>15kg / hr
Limit: Mean catch rate of either species is ≤ 15 kg / hr.
If the target level is met, area opens to fishing.
Review options for modifying the spatial or temporal extent of fishing operations within the area.
If the catch rate of either species is below the limit, the area remains closed to fishing and a review is triggered to investigate the reasons for the low catch rate.
North CPL (formerly TPSA) Closure* June lunar phase Start of June moon closure North CPL key spawning area (formerly TPSA) closed to fishing; may be re-opened following September spawning stock survey (see North CPL Re-opening rules below)
Component Management Objectives Species Performance Indicators Reference Levels Control Rules
South CPL (formerly ENA) Closure* August lunar phase Start of August moon closure South CPL key spawning area (formerly ENA) closed to fishing.
North CPL (TPSA) Re- opening Final fishing period. North CPL (TPSA) re-opens to fishing on the third quarter moon phase of the final fishing period each year.
Denham Sound Opening* August lunar phase July /August Denham Sound area (excluding Denham Trawl Closure) opens to fishing based on July/August survey results.
Denham Trawl Closure (Partial) Opening
Catch rates of western king and brown tiger prawns and snapper (no. of snapper per trawl hour) in fishery- independent stock surveys
Combined catch rates of both prawn species is ≥ 50 kg / hr AND pink snapper numbers at acceptable levels (< 200 snapper per trawl hour).
Partial opening of the Denham Trawl Closure by Notice for a fishing period of 10 days.
Season Closure* Number of total
available fishing nights since the season opening date
Season has been open for a maximum of 175 fishing nights depending on other performance indicators.
Fishing season closes.
Component Management Objectives Species Performance Indicators Reference Levels Control Rules Annual Operations
Target
Species Ecological:
To maintain spawning stock biomass of each target species at a level where the main factor affecting recruitment is the environment.
Brown tiger
& western king prawns
Catch rate of brown tiger and western king prawns from spawning stock surveys.
Target: Mean catch rate of each species is ≥ 25 kg / hr.
Threshold: Mean catch rate of either species is ≤ 25 and > 10 kg / hr.
Limit: Mean catch rate of either each species is ≤ 10 kg / hr.
If the target level is met, no change to season management arrangements required for the following season.
A review of season arrangements and monitoring system is triggered to investigate the reasons for the variation, which may trigger changes to the arrangements for the following season if sustainability is considered to be at risk.
If the catch rate of either species is at or below the limit, a comprehensive review of the fleet’s spatial fishing pattern and catch rates are undertaken to investigate the reasons for the low catch rate in the monitored spawning areas. This will either trigger management actions to limit fishing on that species for the following season if sustainability is considered to be at risk or a change to the monitoring system if it is considered to be inaccurate.
The SBPMF harvest strategy involves constant monitoring of stock status of brown tiger and western king prawns from just prior to the commencement of fishing in each year to the end of each fishing season, based on a combination of fishery-dependent and fishery-independent data. In-season monitoring of catch rates and size-based performance measures for the two target species is used to determine (based on defined control rules) when to open and close certain areas of the fishery, to provide protection to spawning stocks, prevent growth overfishing and help facilitate optimal economic harvesting of the stock by industry. At the end of the season, the stocks of brown tiger and western king prawns in Shark Bay are assessed based on inter-annual trends in recruitment and spawning stock indices (against specified reference points), together with additional data on annual landings, fine-scale spatial data on patterns of fishing effort and catch, and size composition data. The end-of- season assessments are linked to control rules which ensure that, if stocks are considered to be at risk of other factors adversely impacting recruitment (i.e. environment), measures are in place to provide protection for the stocks during the next fishing season.
The reference points developed for brown tiger and western king prawns are based on a detailed understanding of the biology of these species, including key aspects such as their longevity, growth, movements and reproductive biology, including SRRs relationships (Penn et al. 1995; Caputi et al. 1997). This, combined with the history of detailed monitoring of the key target species, i.e. since early 2000, has led to a sound understanding of the level of spawning stock abundance of each species required to ensure that fishing does not impact on recruitment success.
Throughout much of the history of the fishery, brown tiger prawns were the primary focus of management with respect to sustainability, as this species has been found to be the most vulnerable to overfishing (Penn & Caputi 1986). This reflects the different reproductive and behavioural characteristics of brown tiger prawns compared with western king prawns. For example, brown tiger prawns spawn over a more restricted period of the year in Shark Bay and have a greater catchability due to reduced tendency to burrow.
The suite of management measures implemented to maintain the brown tiger prawn stock in Shark Bay (e.g. the brown tiger prawn spawning closure [North CPL]), also provide a significant level of protection to western king prawn spawning stocks (as well as other retained non-target species, e.g. blue swimmer crabs). The added measure of ceasing fishing in the South CPL from August when smaller western king prawns begin to appear in the catches provides protection to these new recruits that will contribute to the spawning stock the following year as well as to the larger brown tiger prawns in this area which will contribute to the spawning stock of the same year.