Report - Shellfish WG

Shellfish Working Group

Meeting Report

March 2004

A report from the Shellfish Fisheries Assessment Working Group Meeting

Held in Auckland 16 March 2004

Report by John Holdsworth

22 March 2004

Overview

NIWA reported on a considerable amount of work on Scallops, the potentially high juvenile mortality, and modelling different management approaches. A breakdown in the system meant no reports for this were distributed prior to the meeting and no hard copies were available at the meeting. Full reports will be posted on the MFish working group web site.

The results of a new biomass survey of Snake Bank Cockles and the Auckland region intertidal surveys were presented.

Meeting starts 0930 hrs

Biomass survey and Preliminary stock assessment of cockles on Snake Bank, Whangarei Harbour, 2004

The 2004 NIWA survey of Snake Bank Cockles has just been completed. The results estimate that 546 tonnes of cockles are 30mm or larger. This used to be the minimum size harvested in the mid 1990s. The commercial fishers take cockles smaller that this – down to 25 mm – and the biomass of all cockles 25 mm or larger is 1500 tonnes. This result is down on the previous year and only just above the lowest level in 2001. There is no minimum size for cockles and they become sexually mature between 18 and 20 mm.

The sustainable yield estimated as Maximum Constant Yield (MCY) was 166 to 292 tonnes and the Current Annual Yeild (CAY) was 160 to 377 tonnes for the 30 mm and 25 mm or larger cockles. The current TACC is 346 tonnes for Snake Bank.

This is a fishery that has been pushed to hard for a long time prior to the introduction to the QMS. The size of cockles harvested has got steadily smaller. Now most of the fishery is based on 2 or 3 year old shellfish so the fishery is very dependant on good spat fall and recruitment each year.

Snake Bank is not a large non-commercial cockle fishery. It is a good case study of what happens to a fish stock when it is over fished. There are now more cockles on Snake Bank than in 1983 when the first survey was undertaken but they are mostly juveniles. Within a year or two of becoming large enough to harvest they are almost all taken. The average size is now so small that they are no longer of interest to non-commercial fishers.

In 2002 NIWA did a survey of other cockle beds in Whangarei Harbour. The main areas that contained appreciable numbers of cockles large enough to be of interest to non commercial fishers were Mc Donald Bank and Marsden Bay. About 35 hectares of Marsden Bay has been reclaimed recently for a new Forestry Port. In 2002 the biomass of cockles 30mm or larger on Snake Bank was 466 t and for the rest of the harbour 881 tonnes.

Pipi from Mair Bank at the entrance to the harbour are preferred small shellfish by non-commercial fishers. There is a smaller commercial fishery on this bank as well. There has not been a survey of this bank for many years.

There was discussion on the updated length based model. This particular model has worked well for paua but is not fitting the known trends in cockle biomass and length. A looking model would give more certainty about the affects of changing the harvest (TACC).

Scallops

Work from a number of MFish and FRST funded projects have be drawn together to look at collateral damage in scallop fisheries. This is the affects of fishing with a dredge on the seabed and on the mortality of juvenile scallops in the modified environment.

Areas that have been dredged heavily have less variability in physical structure, fewer large plants and animals on the bottom and overall less biodiversity.

A more structured environment provides more places to hide and a variety of habitats and places to find food. The mortality of juvenile scallops was much lower in a highly structured environment with no commercial dredging than in areas that had been dredged and had low structure. The difference in mortality between areas was used in a model that looked at what the yield from 10,000 young scallops would be at different levels of fishing effort.

The results show the incidental mortality once effort exceeds certain level leads to large wastage and if effort increases further yield is reduced. The message is that as fishing an area hard would make that area progressively less productive.

Commercial reps pointed out that the scallop fishery is closed from February to July. Martin said that in this model it was the effect of the flat bottom (less structure) that was considered not the additional effect of dredge induced mortality (smashed shells)

Commercial pointed out that Nelson/Tasman Bay was a flat dredged environment that is now very productive.   Martin pointed out that the fishery collapsed after the fishing effort got very high then scallop enhancement was used to bring it back.  

It is possible that rotational fishing would be better. Peter Todd said that Nelson /Tasman Bay is now on a less strict rotation and the fishery has performed better in recent years.

If left alone nature would restore structure to an area but it is not known how long this would take.

Scallop management strategies project

Martin Cryer also presented the results of a major modelling study of five well surveyed commercial beds, 2 in Coromandel/ Hauraki Gulf fishery and 3 in Northland.

Some of the assumptions that were included were:

• Occasional high mortality that affects all scallops in that bed
• Mortality density dependent in the first year (high density = high mortality)
• The amount of variation in the stock on a bed was estimated by the surveys
• All scallops are available to be caught

Then several different management strategies were run through the model for each bed.  A data point was generated every 3 months for 100 years and each run replicated 1000 times. Note this was a simulation project, not a stock assessment

One of the management options tested was rotational fishing by fishing just one third of the bed hard each year and then leaving that area for two years.   In the model this produced quite high yield and little risk of collapse, however it would need proving as ecologically and economically feasible.  

The current management model of having a minimum fixed TACC and annual adjustments provide to be quite a high risk option because of the harvest can be too high in bad years, so driving down further.

One of the best options that present a low risk and reasonable catch is taking a reasonably conservative proportion of available stock each year without a fixed minimum. This is similar to the situation now if the fixed minimum quota was removed and the harvest was determined by annual survey only. This assumes that the surveys are reasonable accurate and that the industry can afford them.

Just setting a low constant commercial catch still had quite a high risk attached with lower overall yield.

A very interesting project once you get your head around it. There is potential developing a combined model to provide a new insight into how to better manage the scallop fishery for all concerned.

Scallop dredge efficiency

Current scallop surveys rely on an estimate of dredge efficiency for preseason surveys. This is calculated by dive surveys in the area prior to dredging. The results can vary widely and a modelling approach was taken to improving the estimates.

There are a number of other problems estimating annual yields for this fishery which has quota measured in meat weight, reporting forms with meat weight or green weight or both and changes within and between seasons on the amount of meat in each scallop as condition changes.

Intertidal shellfish monitoring in the Auckland FMA

Kim Walshe presented the results of surveys on shellfish beds from Raglan to Pataua (Northland). This project has been going about 10 years and each year MFish with input from the working group decide on which areas to survey. The focus is on 3 species. Pipi, cockles and tutatua this year there were no open coastal beach surveyed were tuatua would be expected.

The survey design is very thorough but the Ministry had in the past specified that 300 samples had to be taken in each beach or area surveyed. It was felt that this over sampled some areas and more flexibility should be allowed in future.

Commercial reps questioned the value of the surveys. Were they being used in management? Would harvest surveys be more useful? Martin said that they had undertaken recreational harvest surveys on some beds in conjunction with biomass surveys. While significant amounts of fish were taken harvest was within the sustainable yield for those beds.   

Non commercial supported the surveys. They were collecting baseline information on resources of great importance to local communities. There are many fisheries such as kahawai where the lack of baseline information makes it very hard to quantify what we have lost. Harvest is just one factor that may cause a decline in shellfish beds. In areas such as Raglan major community initiatives are setting out to improve water quality and productivity of that harbour. MFish have imposed very restrictive catch limits (50 per person) in the Auckland /Coromandel region and closed some beaches. They should be monitoring the success or failure of this management strategy.

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