New Zealand Fisheries Assessment Report 2013/3. 31 p.
Yellowtail kingfish are large semi-pelagic predators found mainly around rocky headlands, offshore islands and reef systems. 
They can grow to over 50 kg and 1.8 m long and are highly valued by recreational, customary and commercial fishers.
Commercial landings of kingfish are reported largely as bycatch of inshore set net, trawl and bottom longline fisheries.
From 1991 to late 2003 targeting of kingfish (at the time a non-QMS species) was prohibited unless the species was authorised on a fisher’s permit. A few permit holders were authorised to target kingfish, and most of their catch was taken using set nets.
Reported commercial catch in KIN 1 peaked at 378 t in 1992–93 and declined steadily to 49 t in 2003–04. Kingfish are targeted by recreational fishers as a challenging species to catch on rod and reel, especially the larger fish. Harvest surveys of recreational catch have estimated between 380 t and 800 t of kingfish are taken in KIN 1 (North Cape to Cape Runaway). There is some uncertainty about these diary surveys estimates but in all three surveys kingfish was the third largest harvest in Quota Management Area 1, behind snapper and kahawai.
Kingfish were introduced to the Quota Management System in 2003 with an allowance of 459 t for recreational fishers, 76 t for customary fishers, and a Total Allowable Commercial Catch of 91 t in KIN 1. This project estimates the age structure of the KIN 1 population by sampling recreational catch as this is the largest fishery and main target method in the area. The sampling design was based on data captured from New Zealand Sport Fishing Council club records and the Gamefish Tagging Programme. The main season was January to June and the KIN 1 area was split into two sub-regions: Bay of Plenty, and East Northland and Hauraki Gulf combined. A total of 2091 kingfish were measured in this survey. Of these, 1287 (62%) were released, 711 (34%) without being tagged. The target sample size of 1000 lengths and 250 heads from East Northland and the Hauraki Gulf was exceeded (1198 lengths and 285 otolith sets) between February and July. Anglers and skippers from Bay of Plenty collected 905 lengths and 175 otolith sets between February and November in 2010. Since this was below target, the collection period was extended from July to November to boost numbers. The distribution of sample collections across all fishery statistical areas was adequate. Many fishing clubs have a voluntary minimum size of 100 cm for kingfish. In the Bay of Plenty the survey length distribution matched the club and tagging data length distribution well.
The survey sample from East Northland had a higher proportion of fish between 100 and 106 cm and fewer large fish over 120 cm than the club and tagging records. Kingfish otolith samples were collected as a subsample of all kingfish measured and used to create age-length keys by sex and sub-region. Otoliths were aged using thin sections with emphasis on accurately identifying the first annulus. Levels of between-reader agreement for initial readings appeared moderate at 55% and 65% for East Northland/Hauraki Gulf and Bay of Plenty otolith samples respectively, with estimates of IAPE of 3.6% and 3.0%. Levels of agreement between each reader and the final agreed age estimates were higher and ranged from 80–87% for reader 1 and 73– 77% for reader 2. Most kingfish sampled from recreational catch in East Northland/Hauraki Gulf and Bay of Plenty during 2010 were less than 120 cm in length.
Young kingfish grew rapidly and started to recruit to the 2 Catch at age KIN 1 2010 Ministry for Primary Industries fishery (the minimum legal size of 75 cm) as three year olds, and were fully recruited at 4 or 5 years. The East Northland sample was dominated by young fish less than 8 years old, with few fish older than 12 years. The oldest and largest fish sampled in East Northland was a 156 cm fish (41.6 kg) caught in Bream Bay aged at 22 years. Spatial differences in age composition were evident within the Bay of Plenty samples; with fish older than 15 years poorly represented in inshore areas when compared to White Island.
The age structure of the Bay of Plenty inshore samples was nevertheless broader than those from East Northland. The oldest fish sampled in Bay of Plenty was a 170 cm fish caught at White Island aged at 24 years. Chapman and Robson estimates of total mortality (Z) differ between the two KIN 1 sub-regions; fewer older fish in East Northland implies a higher level of fishing mortality than for the Bay of Plenty. Assuming full recruitment as 5 year olds in East Northland produces a total mortality estimate of 0.77 and assuming 5 or 6 years in the Bay of Plenty produces a total mortality estimate of 0.34–0.42. Estimates of Z for offshore (i.e., White Island) and inshore Bay of Plenty samples were 0.3 and 0.38, respectively; assuming an age of full recruitment of 5 years.
Natural mortality for kingfish based on a maximum age of 23 years is about 0.20. This could mean that the Bay of Plenty stock is at about full utilisation while East Northland is overfished. However, there may be explanations, other than fishing mortality, for the lack of older fish in our sample. For example older fish may migrate offshore; as observed for White Island in the Bay of Plenty. Equilibrium fishing mortalities were derived from a per recruit analysis using growth estimates for female kingfish, natural mortality of 0.2, and age at maturity of 6 years old.
The total mortality corresponding to 40% SSB/R for females equates approximately to 0.3, a value within the likely range of estimates for the Bay of Plenty. Including historic catch in the qualitative analysis and extending the sampling area to include the Three Kings Islands and Ranfurly Bank in future programmes may help resolve uncertainty regarding the degree to which the age composition of the catch reflected that of the whole kingfish population in each region
