Pelagic WG Meeting Report Mar 2004

MFish Pelagic Fishery Assessment Working Group Meeting

Ministry of Fisheries chaired meeting held in

Auckland on 11th March 2004  

Report by John Holdsworth for NZ Big Game Fishing Council

Introduction

The meeting reviewed the results of recently completed research projects including two NZ Marine Research Foundation projects on kahawai and striped marlin. We also looked at two late proposals for tagging research projects.

Meeting Started 10.00am

Characterisation of the amateur fishery for kahawai in New Zealand

Paul Taylor from NIWA presented the results of this NZ Marine Research Foundation project. This report brings together existing data on the kahawai fishery and provides a more in-depth look at where and when most kahawai is caught by 40 recreational fishing zones around the country. The 1996 national survey data showed that the largest harvest is coming from the Bay of Plenty zones with the south Taranaki Bight, Taranaki itself and Hawkes Bay also important. For the number of fishers in Auckland the catch was relatively modest with most coming from the Hauraki Gulf and Manukau Harbour. See Figure 1. Most kahawai were caught in summer and autumn in The Bay of Plenty, Hauraki Gulf and Northland while on the west coast more of the catch was taken in Spring. Around the lower North Island the summer fishery took about half of the kahawai for the year. In the South Island kahawai catch is small and mostly taken over summer.

The length of kahawai measured at boat ramps was compared between 3 surveys in the 1990s (1991, 1994, 1996) and three recent surveys (2001, 2002, 2003) in KAH1. In east Northland there seems to be less of the really big fish and less small fish landed by fishers in recent surveys but the average size of kahawai has remained much the same. In the Hauraki Gulf the fish tend to be smaller with less but steady amount of larger fish. The average size landed at boat ramps in the Hauraki has declined. In the Bay of Plenty like Northland there seem to be less of the really big fish and small fish landed. The average size is about the same or a little less but the proportion of quite large fish around 50cm seems to have increased.  

Figure 1.   Kahawai harvest by recreational fishing zone and season. The size of the circle is relative to the harvest estimate for that zone from the 1996 national survey. The size of the slices are relative to the catch in each zone by season.

There were some problems with the way that the data was summarised. There was no attempt to look at the catch per angler or catch per boat across the surveys. There was a plot of the average number of kahawai per hour of interviewing on boat ramps. There is a strong possibility of bias in this method as the purpose of the recent surveys was to collect length and age data on kahawai whereas the 3 surveys in the 1990's were designed to collect a sample of average size of all species of fish. Also the data from the 2000 and 2001 recreational diary surveys was not used or was unavailable.  

The full report had not been circulated to all members of the Working Group so it was not appropriate for the working group to adopt the report. Industry reps were a little concerned how the report would be used. Neville said that most of the report draws on previous surveys that have already been reviewed and the short comings of those surveys had been noted. In summary it was recommended that this version of the report should be revised to include some cautions on the data used, some comparison of the methods used to collect data over time and some better description of trends over time. Paul asked if John H could provide written comments on the report to aid this.

There was a suggestion that to include the 2000 and 2001 survey data because it would be useful. This may require additional work as part of a linked project.

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Satellite tagging of New Zealand striped marlin

The New Zealand Marine Research Foundation funded a project to place six "smart" tags on striped marlin last season. These tags can record the water depth and temperature every 30-60 seconds and estimate the location of the fish daily. All this information is stored by the electronic tag and transmitted to Argos satellites only after the tag breaks away from the fish. The results have been worked up by Tim Sippel as part of his Master of Science degree at Massey University and in collaboration with Blue Water Marine Research.

Five of the six tags reported back after periods between 20 and 60 days attached to fish. The sixth tag due to pop off after 109 days did not report back. The tags collected information for a total of 201 days on fish. The tags transmit a summary of their data to the Argos satellite system based on specifications set by the users. The summaries are transmitted because the tags don't have enough battery life to transmit the entire data archive. However, if the tag itself is returned to researchers they can download the entire dataset from the non-volatile flash memory in the tag.  

A combined depth plot showed that these striped marlin spent about 70% of their time day and night in the surface 5 metres of water and will dive to 50 to 100 metres quite regularly and occasionally to over 200m. The preferred surface water temperature last season in New Zealand was between 21o and 23oC. One fish travelled to tropical waters and spent time in water 28oC and warmer. This project has recorded the deepest dive ever recorded by an electronically tagged striped marlin at 316 m and the coolest temperature at 14oC.  

Working out the position of these fish from day length and time of noon/midnight has been the most challenging job so far. The tag manufacturer supplies software that estimates longitude reasonably well and latitude less well. These data were run through a statistical filter to give a better estimate of longitude and then a programme that compares the surface temperature measured by the tag with the satellite sea surface temperature (SST) recorded around that time.   This gives much more reliable plots of location. A plot of all the locations plotted for all tags is shown in Figure 2.

Figure 2.   Location plots for a 120kg fish tagged at North Cape in mid February 2003. Note there may be more than one day between points.

Full results will be published in the scientific literature and compiled into a report for the NZ Marine Research Foundation. The presentation was well received with interest in how the tags performed and how the data was analysed. Some comments on how other depth recorders had performed in cold conditions were offered. A suggestion was made that for a final report it would be interesting to see how much commercial fishing effort there was where the tags were each month.

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Satellite and archival tagging project for southern bluefin tuna (SBT)

MFish has contracted Talbot Murray (ex NIWA) to plan and coordinate a project to place 50 archival tags in SBT less than 40kg off the west coast of the South Island in April/May 2004. These tags are internal and the fish must be recaptured for the data to be retrieved. Also they will be tagging 10 SBT larger than 64 kg with pop-up satellite archival tags off East Cape in June/July 2004. It is hoped these tags may track the fish back to the spawning grounds.

This is a joint project with the Australian Government which will allow some cost savings on purchase of tags and training. Observers will be trained to handle and tag the fish and placed on volunteer vessels at these times. The crews will be compensated for the fish that they release and their time helping handle the fish. There will be a reward of NZ$250 for the return of the tags.   Talbot could not remember what make of tag was being used. The total cost of the project was around $200,000. A similar project is planned next season. And the analysis of the results will be contracted out.

There was a lot of concern from the industry that they were being forced to pay for this project through cost recovery levies despite the fact that it was an international research project and that was paid for by the Government in Australia and Japan. Also the industry had some practical concerns about soak times and practicalities of conducting this project as part of "normal" fishing operations. There had obviously been plenty of discussion on these issues outside the working group and we were seeing the "tip of the iceberg".  

Pop-up satellite tagging of swordfish

Again this is a joint project with the Australians planned for this year. It will be run in the same way as the SBT project above, with tags deployed in July and August with pop-up dates up to 6 months later. The Australians will be starting sooner.

NZBGFC rep supported the project (it is officially part of a swordfish stock structure project that is currently underway) but suggested it need to be carefully thought through.  

Issues raised were;

• The timing would not tell us much about residency in New Zealand as fish were thought to start migrating at about then.
• The longer the tags are on, the more chance of not hearing back from them. Having a third of the tags at 6 month pop off was a bit risky.
• Getting the attachment right is critical for long term deployments. They might fall out.
• Geolocation of swordfish is impossible using light levels because they dive at dawn and rise at dusk keeping the light quite constant.
• Maybe they should think about using a mix of archival and straight pop off tags that only provide a location at the end, but are much cheaper.

There was no request at this meeting for assistance from recreational groups for more tags or to catch fish. Unfortunately Talbot missed the presentation on the striped marlin satellite tagging project.

Meeting ends 4.30 pm

The next Pelagic Working Group Meeting will be on the 31st of March 2004 at MFish Auckland Office .  

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