Feature

Taking Stock Of The Sea

Estimating fish resources or the numbers of marine mammals is a difficult business, but a vital one if stocks are to be managed properly.

By Vicki Hyde, NZSM

Imagine standing in a dark room trying to catch moths with a small net, and then estimating the total number of moths by the amount you catch. This is akin to the procedures used to estimate fish stocks in New Zealand waters. But, unlike the moth example, the economic and environmental consequences of an incorrect estimate can be drastic.

MAF Fisheries is responsible for keeping an eye on 178 different stocks of marine life, from orange roughy to oysters. Prior to the 1978 establishment of New Zealand's 200-mile exclusive economic zone, fish resource management was limited. Catch controls were not in place and very little was known about marine populations. Increasing awareness of the economic and environmental value of our seas has led to more funding and more interest, but problems remain.

"It was extremely difficult to see how on earth we'd get out there and measure anything," says MAF Fisheries scientist Dr Don Robertson. A huge variety of data was needed, including the size of different fish populations, their growth rates and their mortality from both natural causes and fishing.

From the 1970s on, MAF Fisheries has run surveys of fish populations. These have involved eight to ten trawls a day over a period of four weeks. Trawl areas are carefully mapped out to provide "chunks" of ocean of a known volume. The amount, type, age and weight of fish are carefully recorded.

There have been problems with this approach. MAF was embarrassed to find one year that their trawl transects had followed a huge group of migrating fish. They'd been catching and counting the same population "who knows how many times". The ministry's scientists have also been limited by lack of a research vessel for fisheries work. They have had to hire a commercial vessel to do their surveys and pay their way in fish.

Robertson sees that as a second-best option which was very expensive in terms of fish, but adds that they had no choice. He's pleased that they have finally managed to buy a vessel of their own. The ship will give the scientists a stable base from which to work. Trawls, echo-sounders, satellite data and a host of other techniques will provide more accurate, cross-checked estimates.

On-board computer equipment will assist analysis, and Robertson says the new vessel will allow refinements of acoustic counting methods. With this, blips of sound are "pinged" off the fish, providing an estimate of abundance. This can be calibrated with direct observation from remotely operated cameras lowered from the ship.

Egg counts in spawning grounds can also provide useful data on fish numbers and location. One surprising find concerned hoki eggs taken in a DSIR plankton sample in Cook Strait. The fish had been thought confined to the West Coast area. DSIR marine scientist Rob Murdoch says that the eggs and larvae were found upwelling from one of the canyons in the strait. Seafloor conditions in Cook Strait are similar to the Hokitika Canyon area off the West Coast where hoki are more commonly found.

Quota Questions

Estimates of fish populations are used to divide fishing grounds into quotas and to set total allowable catches (TACs). Unfortunately, there is increasing evidence that the TACs have been set too high for some species.

"The alarming thing is that the [hoki] population estimate has been going down every year," says Murdoch. He doesn't believe that a good estimate of hoki numbers has been established. He has seen the size of hoki caught get smaller over the years, indicating a population under stress. The major change associated with the decline has been the rise in commercial fishing.

MAF Fisheries is well aware of the impact of fishing on fish numbers. Trawls on the Chatham Rise have confirmed suggestions of particularly slow growth rates for orange roughy. This has prompted decreases in catch limits, and commercial fishermen are required to accurately record their catches.

Robertson admits that data from commercial fleets can be suspect. Observers on each vessel would provide one means of ensuring accuracy, but is too expensive an option to contemplate.

"You're totally dependent on the provision of honest data from honest fishermen," Robertson says. Arguments between the ministry and the fishing industry over reductions in the TACs have been fierce at times. The industry has hired overseas experts to check MAF Fisheries data, suggesting that reductions were unnecessary. Robertson was pleased to find that the experts agreed with his ministry's results.

"It kind of vindicated our techniques," Robertson says.

Dolphin Debates

Controversy over population estimates has not been limited to fish. The population size and dynamics of marine mammals, such as sea lions, dolphins and whales, have also been debated. The larger size of most marine mammals makes estimating their numbers a simpler process, but one which has its own difficulties.

The establishment of the Banks Peninsula Marine Mammal Sanctuary was accompanied by intense debate over the numbers of Hector's dolphins that would benefit from the protected area. Population estimates varied widely, from 2,000 to 8,000. Gillnet fishermen opposed to the ban on set nets claimed high numbers of dolphins and low numbers of entanglements in nets.

However, a paper by Otago zoologists Steve Dawson and Elisabeth Slooten notes that significant risks can be posed even by amateur fishermen. While the chances of an individual catching a dolphin may be low, they concluded, the large number of fishermen involved could mean relatively high numbers of dolphins killed.

The dolphins, like most other cetaceans, are made vulnerable by their low rate of reproduction. Slooten and Dawson found that the most optimistic survival rate would provide a maximum growth rate for Hector's dolphins of only 2-4% per year. Surveys from 1984 to 1988 showed that these growth rates were being greatly exceeded by the number of dolphins killed in nets.

Seal Surveys

Scientists are generally less worried by fluctuations in seal populations, as these animals breed more consistently. Fur seals have done particularly well, rebounding to substantial levels since the days of open slaughter. Fur seals are now found as far north as the Three Kings Islands, and more breeding grounds are being established each year.

The Department of Conservation (DOC) in Auckland is becoming increasingly concerned over the spread of seals north into their area. Some of the more popular beaches are now being used by seals. The Auckland Conservancy is hoping that an education campaign will teach the city population how to live with seals at their doorstep.

DOC has also been concerned over fur seal entrapment in West Coast hoki fishing nets. The West Coast fur seal population is estimated at around 8,000, but the potential seal population could be as high as 20,000. It's boosted by large numbers of seals migrating up and down the coast at different times of the year.

If the fisheries are catching from the West Coast breeding population only, they could have a major effect on the local population. DOC has begun a major study and tagging campaign to assess this. Compared to their more aquatic counterparts, fur seal numbers are far easier to estimate.

"For fur seals it's basically fairly easy," says Martin Cawthorn, DOC marine mammal specialist. "The animals are amphibious and spend a lot of time on land."

The breeding season allows scientists to make accurate estimates of the population, based on the number of pups reared and the adult numbers. Each season, scientists make ground counts and compare these to numbers obtained from aerial surveys to check the accuracy. Ground counts are limited by the difficulty of getting to some of the more remote, rugged islands. The aerial surveys use cameras mounted on kites -- difficult to control but much cheaper than using aircraft, according to Cawthorn.

"Seals lie on each other and it's like trying to counts tons of sausages on a tennis court," Cawthorn notes. Double checks of counts have confirmed population estimates to within a few percent. Hooker's sea lions, living on sub-Antarctic islands, present problems of access as well as the standard difficulties associated with their more common mainland counterparts.

There have been calls for a protected area around the Auckland Islands, where the bulk of Hooker's sea lions are found. The fishing industry opposes this, as the area has extremely rich fishing grounds. They are trying to develop a number of techniques to reduce the amount of Hooker's sea lions and fur seals caught in their nets. These include speeding up net deployment and recovery, changing trawling procedures, or providing escape panels in nets. It is still too early to see whether these will reduce seal deaths significantly.

Whale Watching

"Counting whales is a totally different matter," Cawthorn says. Most data on whale numbers come from sightings by the merchant marine, the Navy and commercial fishermen. This gives a rough estimate of seasonal distribution and relative abundance, Cawthorn acknowledges, but better statistical approaches are required.

"Survey methods have been constantly refined," Cawthorn notes, citing major surveys using duplicate observers on ships running parallel courses. "The numbers coming out of these surveys now are about as good as you can get using these methods."

One common technique uses scars and markings to identify individual whales. However, marks change and observing the same area of the animal each time can be difficult. Photos of tail flukes have been used overseas as a database on whale populations, but the photos soon build up to unmanageable numbers.

One new technique which interests Cawthorn is that of population genetics. By taking a small plug of whale flesh, scientists can determine the whale's genetic relationship with other whales. Sensitive assay procedures would allow monitoring of substances in the blubber, such as persistent pesticides or other industrial contaminants.

Various approaches have been used, from grating skin samples off to collecting naturally sloughed skin. Cawthorn suggests using a crossbow to fire a corer that will take the one-centimetre deep sample, which can then be reeled back in.

"Overall the genetic approach to whale population assessment is one of the greatest advantages to whale science we've had in decades," he enthuses.

Genetic sampling may also solve the arguments that have raged about the Hector's dolphin population. Cawthorn hopes that the technique will show how much emigration and immigration there is in the local population. It will also clarify the degree to which the different populations are related.

Vicki Hyde is the editor of New Zealand Science Monthly.