Yellowfin tuna landings at New Zealand sportfishing clubs in the 1990s were over 1,000 fish per year, but this declined drastically from 2001/02, mainly varying between about 100 to 500 fish landed per year through 2023/24. In 2024/25, club landings shot up to about 1200 yellowfin (as shown in the graph below from Holdsworth & Gaskell 2025). What caused this recent increase? The short answer seems to be that there is no single factor causing yellowfin tuna to be more abundant in the 2024/25 summer season, according to John Holdsworth, director of Blue Water Marine Research (reported by Jones 2025).
So what are the potential causes of the changes in sportfishing landings, and what evidence is available to support or refute one factor as opposed to another? Changes in fish distribution and abundance are often a result of multiple factors acting together, so is there one factor exerting greater influence than the others?
What are the potential causes?
Commercial fishing
There is circumstantial evidence that large commercial catches in the Western Pacific (Nauru, Indonesia, & New Caledonia) as well as in the Fiji Basin may have previously reduced the abundance of yellowfin in New Zealand waters by causing contraction of their range (Blue Water Marine Research 2012). Catches of yellowfin in the Western Central Pacific Ocean (WCPO) are shown in the graph below from ISSF (2024).
Although commercial catches have increased, the latest stock assessment of yellowfin determined that the stock is not experiencing overfishing, and is not in an overfished state (Magnuson et al. 2023, Hare et al. 2023). The stock is fully exploited, but the stock is not stressed, so the increased commercial fishing is unlikely to have caused the observed decline in New Zealand sport fishing landings.
A few graphs from this document illustrate the point. Trends in catch per unit effort (CPUE) of yellowfin in the Western Central Pacific have been almost flat from the 1990s to the present, as shown in the graph below from Magnuson et al. (2023). If commercial fisheries were depleting the yellowfin stock, you would expect CPUE to fall, rather than remain steady.
If the yellowfin stock was being depleted you would expect to see declines in the size of the fish. This next graph, also from Magnuson et al (2023) shows that median fish weights have remained almost constant over the 1990s up to the present.
Recruitment is survival of the fish until the point where they are mature, or they enter the fishery, depending on how recruitment is defined. The graph below from Magnuson et al (2023) supports the conclusion that the yellowfin tuna stock is not being heavily impacted by commercial fishing, because recruitment of young tuna has remained steady from the 1990s, with indications of more positive recruitment in recent years.
Changes in migration pattern
Fish tagging has the potential to yield a wealth of information about fish movements, population structure, and even abundance, if enough fish are tagged and a significant proportion of those tagged can be recaptured. The New Zealand Gamefish Tagging Programme has provided little information about yellowfin tuna to date, but that may change soon. Although 89,472 gamefish were tagged to the end of June 2025, only 2% of those were tuna, mainly yellowfin tuna. There should be more results in the coming year since it was reported that “large numbers of yellowfin were tagged in 2025”. Numbers of yellowfin tagged since 2015/16 were less than 25 per year until 2024/25 when 368 fish were tagged. That’s a huge increase, about 15 times greater than the usual number tagged. There were no recaptures of yellowfin at all from 2007 to 2023. Total recaptures to date reported in Holdsworth & Gaskell (2025) were only 23 fish, so more results from the recent tagging are still to come in.
Based on these limited results, there is an obvious difference between the tagging results prior to 2007 compared to after 2002. The two figures below are from Holdsworth & Gaskill (2025). Prior to 2007, all of the tagged fish left NZ waters and went to warmer waters off New South Wales, between Lord Howe Island and Norfolk Island, and the Fiji Basin. In contrast, after 2002, all of the tagged fish stayed off the east coast of the North Island of New Zealand. This raises the question: why did the yellowfin stay longer in New Zealand after 2022, rather than migrating to warmer seas?
Inter-annual variability in environmental conditions
One potential reason why yellowfin might stay longer in New Zealand in recent years could be warmer temperatures. These SST maps for mid-season (December 15th) temperatures for 2005, 2006 and 2007 (top row) with the same date in 2023, 2024, and 2025 (bottom row). It’s pretty clear that SST was warmer in 2024 and 2025. In 2025, the water as warm as 21.5 degrees C was even closer to the shore than in 2024.
SST maps for mid-season (December 15th) temperatures for 2005, 2006 and 2007.
These SST maps for mid-season (December 15th) temperatures for 2023, 2024, and 2025.
The seasonal development of warm temperatures may also be important. This year, the 2025/26 season, warmed faster than last year, the 2024/25 season, as shown in the SST maps. It was cool (16 to 17 degrees C) all along the east coast on 1st November in both 2024 and 2025.
By the 1st December, SST was 19.5 to 20.5 degrees C in 2025, which was much warmer than 2024 on the same date when SST was 16.5 to 18 degrees C.
By 1st of January temperatures offshore were still warmer in 2025 compared to 2024, but were similar close to shore.
This preliminary analysis suggests that surface temperatures were warmer in 2023 to 2025 when the tagged yellowfin stayed close to the east coast of New Zealand. It also appears that the 2025/26 season warmed faster and earlier than in 2024/25 season off the east coast of New Zealand. It seems likely that warmer temperatures, and early seasonal warming are important factors driving the greater numbers of yellowfin being landed at the gamefishing clubs in the 2024/25 and 2025/26 seasons.
References
Blue water Marine Research. 2012. Trends in Yellowfin Tuna Catch in NZ Waters. Accessed (Jan 2025) at https://www.nzmrf.org.nz/trends-in-yellowfin-tuna-catch-in-nz-waters.
Hare S.R., Vidal, T., Castillo Jordán C., Day, J., Hamer P.A., Macdonald, J., Magnusson, A., Peatman, T., Scott R.D., Senina, I., Teears, T., and Pilling G.M. 2025. The western and central Pacific tuna fishery: 2023 overview and status of stocks. Tuna Fisheries Assessment Report no. 24. Noumea, New Caledonia: Pacific Community. 73 p.
https://www.spc.int/digitallibrary/get/qj59o
Holdsworth, J.C. & Gaskell, S.G. 2025. New Zealand billfish and gamefish tagging, 2022–23 to 2024–25. New Zealand Fisheries Assessment Report 2025/53
ISSF. 2024. Status of the world fisheries for tuna. Nov. 2024. ISSF Technical Report 2024-07. International Seafood Sustainability Foundation, Pittsburgh, PA, USA
Jones, N. 2025. Why are there more yellowfin tuna in NZ this season? Fishing News. Fishing.net.nz
Magnusson, A. et al. 2023. Stock assessment of yellowfin tuna in the western and central Pacific Ocean: 2023. Tech Report WCPFC-SC19-2023/SA-WP-04