Swordfish do not breed in New Zealand waters. During the austral spring they move to warmer subtropical spawning grounds. For the first time electronic tags that store and transmit information on the temperature, depth and location have been attached to New Zealand swordfish. These are helping to reveal possible spawning locations and whether these fish subsequently return to New Zealand or disperse more widely.
Tracking Swordfish with Pop-Up Archival Satellite Tags
Funded by the Ministry of Fisheries
Researchers Blue Water Marine Research
Commercial catches of swordfish (Xiphias gladius) in New Zealand and Australian waters increased rapidly in the 1990s. In addition, catches on the high seas in the South West Pacific have also expanded. Experience in the Mediterranean, Atlantic and North Pacific has shown swordfish stocks to be susceptible to over fishing, and in recent years catch rates and the mean fish size have declined substantially in the south western Pacific.
An understanding of the amount of mixing between populations of swordfish in the South-West Pacific region is required if this highly migratory species is to be effectively managed. Knowledge of movement patterns would also contribute to more effective stock assessment of swordfish in the region
Two recaptures of conventionally tagged fish have been made from the New Zealand cooperative tagging program. The first was a 12 kg fish tagged from a Japanese longline vessel 130 nautical miles north of New Zealand in June 1991. It was recaptured in February 2002 just to the west of Wanganella Bank and was estimated to weigh 160 kg. Therefore it was caught 250 nautical miles to the west of its release location after 10 years 8 months at liberty. The second swordfish was tagged off East Cape in February 1996 and was estimated at 20 kg. It was recaptured 8 years 4 months later 113 nautical miles south of its release point. It was estimated to have grown 70 kg during this time.
More information on the seasonal movement patterns for adult swordfish, which don’t breed in New Zealand waters, can be generated by using pop-up archival transmitting (PAT) tags. Twelve of these tags were deployed from surface longline vessels north east of New Zealand between 8 July and 7 November 2006. Blue Water Marine Research personnel undertook two trips tagging 6 fish and developed a tagging procedure for Ministry of Fisheries scientific observers to use.
The pop-up tags cannot transmit information while on the fish. Location has to be estimated from the time of dawn and dusk, which gives the time at noon (longitude) and day length (latitude). The deep diving behaviour typically displayed by swordfish at dawn can significantly impair use of light level data for estimating their location and can even mean that light level changes at dawn and dusk cannot be detected at all. This has been a problem with tracking PAT tagged swordfish in other programmes.
The results from these 12 tags were quite pleasing considering the difficulties. The attachment failed on three tags, one fish died and one tag did not report. Seven tags provided between 66 and 236 days data. These tags yielded between 8 and 36 usable light level geolocations each, plus the tagging and popup coordinates giving a total of 160 locations. On average there are 5 locations per month from these tags.
The plots above are from a 130 kg fish tagged in July at North Cape (T). The track ends in mid January with the fish 200 nmiles off the New Zealand west coast. It seems to have spent quite a bit of time in water of 22 to 25 oC to the south and east of New Caledonia. Other fish moved up toward Fiji and one went as far east as the Cook Islands. Evidence from Australia shows that migrations into warmer water at this time of year (spring and early summer) is associated with spawning. Fish tagged in New Zealand waters tended to return to New Zealand after a few months in the tropics.
The PAT tags transmitted quite complete summary records of temperature and depth. Most days these swordfish spent some time at night, at or close to the surface so the maximum temperature will be similar to the average sea surface temperature detected by weather satellites. These data are useful for getting better estimates of the latitude which the fish are in. Latitude estimated from day length can have large errors especially when the fish is not at the surface.
Most fish spent the daylight hours at depths between 600 to 800 m where it was always dark and the water temperature was between 8 and 10 deg C. The maximum day time depth tended to get shallower in southern waters with cooler surface temperatures. All swordfish, except one, made occasional excursions to the surface for short periods (about 30 minutes) during the day. This behaviour is more prevalent in larger fish. This seems to be the “basking” behaviour described in PAT tagged swordfish off California. Generally they spent most of the night in the top 200 m.
Pete Saul from Blue Water Marine Research tagged one swordfish, estimated to be 130 kg, in July 2007. This tag is due to pop-up on 14 February 2008 and will transmit small packets of data to Agros satellites for about 10 days. It is hoped to get several more swordfish tagged by scientific observers in January 2008.