It is probably too early to speculate on the reasons for their reappearance, but in reality it is probably a combination of all three factors.

Management:  Since 2009 the Western and Central Pacific Fisheries Commission have banned the use of Fish Aggregation Devices (FADs) by purse seine vessels for 3 or 4 months a year in international waters to help protect the juvenile bigeye and yellowfin tuna from being taken as bycatch in the skipjack fishery.  Also, some of the distant water fishing nations have reduced the number of longline vessels fishing for bigeye.

Commercial catch in the SW Pacific, south of 10 ° S Latitude, has been stable, without the large increases seen in other areas.  The New Zealand commercial catch of yellowfin was never large and it collapsed at the same time as the recreational fishery.

Recruitment:  Yellowfin are a remarkably productive species.  Most yellowfin are mature by the time they are 2 or 3 years old and seem to spawn whenever the water temperature is above 26° C.   In tropical waters yellowfin spawn year round.  Nature has a way of making the most of favourable conditions and we may be benefiting from a year or two of good recruitment.

A new stock assessment for yellowfin was completed in 2014. It shows the spawning stock biomass of about 40% of the unfished biomass and current catches (600,000 tonnes) are about the level of the maximum sustainable yield (MSY).  Most of that catch is taken in the western equatorial Pacific, thousands of miles away from New Zealand.

Oceanographics:  There have been good patches of oceanic water and catches in January 2015.  So far this season there has also been some small marlin (60 to 80 kg) and small bigeye tuna caught by recreational fishers.  It seems that these fish are venturing further south than usual this season. Certainly they are not here every year.

Maybe this year we will see the occasional wahoo or even the first sailfish taken on rod and reel in New Zealand waters.  Now that would be exceptional.

Historical catches:  For 30 years yellowfin tuna was an important target and bycatch species for recreational fishers. Particularly in East Northland, Bay of Plenty and occasionally as far south as Hawke Bay on the North Island east coast and Kawhia on the west coast.

Yellowfin size and abundance used to fluctuate from year to year. The NZSFC produces a yearbook with New Zealand line class records and catch tallies for their 56 affiliated clubs. These national tallies show trends in catch by season.  Combined catch records of clubs affiliated to NZSFC recorded more than 1000 yellowfin for 12 out of 13 seasons between 1987–88 and 2000–01. Over 2000 yellowfin were landed in the three seasons from 1994–95 to 1996–97. Anecdotal information suggests that many more yellowfin are landed but not recorded in good seasons.  Many of these would be under 25 kg and considered small.

Recreational catch hit an historic low in 2008–09 and got progressively worse over the next five years.  This led some fishers to doubt if we would ever see a yellowfin fishery in New Zealand again.

Recommendation:  If yellowfin turn up in reasonable numbers in 2015 fishers should consider tag and release for fish in good condition, to help scientists and managers keep track of where they go and who is catching them when they are leave here.