Fish Science - Old Man Snapper

Epi Saili of Sanford Ltd with a 71cm snapper, a fish of identical size to the 60-year-old recently aged.

Age-weight relationship for trophy-sized snapper (caught from 1990–2004) and the three 60-plus-year-olds (red) referred to in the text.

A prepared otolith (earbone) from the 60-year-old (71cm) ‘old man’ snapper caught in November 2007 from the West Coast North Island fishery (otolith preparation and photography courtesy of NIWA).

Relevant offers

Articles & Reviews

Guide to Auckland boat ramps Hooking fish on circles Give me sand, lots of sand... Horsey girls on slow-jigs Gearing up to go hard The humble kahawai Groper grappling Just a tease It's better on top! The kings of the sea

In November 2007, a snapper taken from New Zealand’s West Coast North Island stock (SNA 8) was later aged at 60 years, one of oldest ever encountered in fisheries research assessing the length and age composition of commercial catches.

The 71cm (6.6kg/14.6lb) specimen was caught by the Sanford single-trawl vessel, San Rakaia, skippered by Mark Cooksley, while fishing an area between the Manukau Harbour and Ninety Mile Beach.
This snapper was born in the spring-summer of 1948-1949, prior to the full development of the West Coast commercial fishery in the mid 1950s, and avoided capture or predation for well over half a century. Past tagging studies have shown the vast majority of snapper move only relatively small distances within a year, usually less than 75 nautical miles (~139km), with the recapture location often close to the point of release. As only a minority of fish migrate into other neighbouring stocks, it is therefore plausible that this individual stayed within the West Coast area its entire life.

Scientists and technicians from NIWA and, formerly, MAF have been routinely ageing snapper for stock-monitoring purposes for more than 30 years. The age samples, derived by counting growth rings on fish otoliths (earbones) under a microscope, are primarily used to determine the age composition of a fish species caught by a particular method. This information is fed into an age-structured mathematical model to determine the size and ‘health’ of the stock. Knowing the age of a fish and the age composition of the catch is important, because the strength of recruiting year classes (the numbers of young fish reaching adult stage in each year) in snapper stocks is highly variable.

During the past 30 years, only three snapper – from a total numbering over 80,000 individuals aged by scientists from collections taken throughout New Zealand – have been found to be 60 years or older, so this recent finding is a rarity. The oldest recorded age for a snapper is 65 years of age, a 67cm (5.6kg/12.4lb) specimen captured by a trawler fishing the Tasman Bay/Golden Bay (SNA 7) stock in November 2000. The other, aged to 62 years and of similar size (66cm, 5.4kg/11.9lb), was also taken by trawl from the West Coast stock in January 1979.

The ages determined for much larger snapper caught by recreational anglers in New Zealand are often lower, and usually range between 30 and 50 years.
Interestingly, these 60 year olds often weigh half the weight of their younger counterparts or even less.
So if this snapper was one of the oldest ever captured, why was it not the largest, too? Research suggests that snapper growth rates are affected primarily by factors such as stock size, temperature, food availability and genetic diversity, and may be highly variable both within and between stocks. Growth rates in turn directly affect the productivity of a species and are therefore important in relation to the sustainability of a fishery. As stocks are fished down over time, their productivity is generally thought to increase. As the older, slow-growing fish are removed, predominantly younger, fast-growing fish remain, which sustain harvesting at a higher rate.

This 60 year ‘old-timer’, if born on the West Coast, has undoubtedly existed through a time when the stock was virtually un-fished. Research from the 1970s has shown us that West Coast snapper then grew at a much slower rate than they do now, because the population size was comparatively large. Today, West Coast snapper of a similar size to the 71cm ‘old-timer’ can be as young as 18 years of age. Therefore, in some snapper stocks such as the West Coast, where growth rates and associated productivity are high, it appears some fish can now attain a much larger size for their given age than they may have been able to do 30–40 years ago.

So why are so few 60-year-old or older snapper ever encountered? In a perfect world, a snapper living to 60 years of age may be roughly comparable to a human living to around 110 years – which we know is a rarity.

Over the many years of research, we have on occasions aged snapper to 50-plus years, but the 60-year-old milestone is very rarely seen. Perhaps in a time when no fishing took place there may have been a reasonable chance of a fish reaching a ripe old age. It is probable this senescent 60 year old, a remnant of that era, no longer took part in the annual ‘school fish’ spawning migrations, and most likely lived out an existence just searching for food and avoiding capture in order to ¬survive.

This NIWA research was funded by the Ministry of Fisheries. The snapper sample was collected by Sanford Ltd, for NIWA.

- © Fairfax NZ News