Keeping a thumb on the pulse

Fig 1. Map showing the pāua quota
management areas (QMAs). The
primary areas are PAU2, 3, 4, 5A,
5B, 5D and 7).

-commercial pāua catch sampling

Our pāua fisheries are managed at the highest level by setting catch allowances based on periodic stock assessments across the eight main pāua quota management areas (QMAs) (Fig. 1).

Stock assessment outputs help to determine whether current catch levels are sustainable. They rely on a range of data inputs, such as biological data (e.g., size at maturity and growth rates) and fisheries-dependent data (e.g., catch and effort data). One of the most important fisheries-dependent data sources is the size of pāua being commercially landed across the fishery. Since 2006, the Pāua Industry Council has undertaken the Fisheries New Zealand contract to collect this data.

Since the start of the project, the primary means of sampling commercial catch measurements has been through the “Red Sack” method. When pāua are commercially harvested, divers fill hand nets of pāua, which are passed to the deckhand on the boat. The deckhand double-checks the measurements of pāua to ensure they are above the harvest size and sorts and stacks them into fish bins. Bins typically hold about 100 pāua and weigh about 25 kg. For each day’s diving, harvest crews are asked to randomly select one bin to be sampled, tagging it with details of the day’s catch (area dived and total catch). When this bin is processed at the factory, the shells from this tagged bin are kept aside and placed into a red sack with catch details attached. Shells that end up in red sacks are then measured periodically throughout the year by employed technicians (Fig. 2).

Fig. 2: The “red sack” process – random bins of pāua
are tagged by harvesters, and shells are measured after

Shells are measured using this method with an electronic shell measuring boards designed by Zebratech Ltd. Shells are placed on the board, a sliding book-end is used to measure the overall length of the shell, and a button is pressed to log the measurement. The ‘hump’ (the round bit of the shell at the back which protrudes past the underlying rim of the shell) can also be measured so that the ‘basal length’ (the length of the underlying shell rim) can be calculated by subtracting the hump from the overall length. This length is used to maintain consistency with NIWA’s data collected for stock assessment before this project, and data around hump size and frequency can also tell us about fishing patterns in some areas. Data from the measuring boards is then uploaded and processed to observe the length-frequency trends at the fishery level and is fed into stock assessment models.

In the last several years, there has been an uptick in the live export of pāua, meaning that catch length samples are no longer adequately captured by the Red Sack process alone, as live pāua obviously do not end up being processed at the factory. This has required some recent innovation to develop a system to measure pāua live as they are landed on the vessel. The solution has been with modified measuring boards designed by Scielex that enable pāua to be easily automatically recorded as they are sorted into bins. The boards have a spring-tensioned book-end that pāua can be slid through, and measurements are automatically logged as the maximum gap. An alarm can also be set to indicate if pāua measured are below the harvest size as these are being checked. The machines also record the GPS location of the samples measured, meaning data can be referenced back to the specific area of harvesting (Fig. 3).

Fig. 3: Onboard measuring of live paua.
Fig. 4: Examples of cumulative frequency plots help visualise the trends of pāua sizes
across different seasons and across QMAs. The further to the right the line is, the larger
the paua. The left hand plot shows that in PAU2 (Wairarapa) pāua have been getting
incrementally larger over recent seasons, driven by increases to the minimum harvest
size. The right hand plot compares the size of pāua across QMAs and shows the general
pattern of larger pāua in more southern QMAs.

Scieliex measuring boards are used by the primary harvest crews across each QMA. With the uptake of this new process, we are now collecting massive amounts of data that far exceed what was ever possible with the red sack method alone. In the early days of this project, 30,000 pāua were measured a season. In contrast, during the last season, over 300,000 pāua were measured.

The primary use of this data is to feed into government stock assessment processes, which help determine the status of stocks across the QMAs and inform catch settings. Further to this, assessment of the catch length data can also provide interesting insights into the performance of the fisheries around the regions. For example, suppose shell sizes are decreasing over successive seasons. In that case, it suggests the fishery is in a state of decline and that catch adjustments should be made. In most fisheries, the trend is towards increasing sizes of pāua, which is a positive sign. This has generally been driven by the move towards increasing minimum harvest size (MHS) to be more biologically appropriate than the minimum legal size of 125mm. For example, in PAU7 (Marlborough) 5 different MHS up to 145mm are implemented across the QMA commercially. Fig 4 shows how this data can be visualised and interpreted.

Overall, this project is a great example of Industry being actively involved in innovative means of data collection to ensure the overall sustainability of fisheries for all users.

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