Returning home
Tuna are migratory fish that roam the oceans in pursuit of their prey fish. Pacific bluefin migrate vast distances between the west coast of North America and the waters off Japan, Taiwan and the Philippines. On reaching adulthood, most bluefin swim to the coasts of Asia to spawn, arriving any time from early April to June.
Not far off the east coast of Taiwan the continental shelf falls away into an ocean trench thousands of meters deep, so bluefin arriving from across the Pacific have to come close inshore to find shallow waters to spawn. Their newly hatched fry are unable to hunt or swim for themselves, so they need waters that are rich in plankton and have strong currents. The North Equatorial Current and the Kuroshio Current pass by eastern Taiwan, creating an area of high primary productivity. Hence the "Southern Fishing Grounds" to the east of Taiwan are an important bluefin spawning ground.
The bluefin tuna that roam the vast Pacific can be divided into many subpopulations, each with their own migration paths. Migration maps compiled from the international literature by researcher Wu Chi-lun of the Taiwan Fisheries Research Institute show that each year in April, adult bluefin tuna begin to arrive off Taiwan and the Philippines from California. This marks the beginning of a tuna fishing season that lasts until early July. After a two-week spawning period, the tuna swim northward in shoals. Some also spawn in the waters off Japan.
Motherly love
Bluefin tuna lay millions of eggs at a time, and their roes can weigh over ten kilos. To protect the eggs in their inflated abdomens, the females have many layers of fat. In recent times in Taiwan, "tuna belly" has become another word for a beer belly.
For Li Yen-hung, an assistant researcher at the National Museum of Marine Biology in Pingtung, the tuna's swollen bellies are redolent of motherly love. Li says the nocturnal bluefin are generally active at depths of 50-60 meters below the sea surface, but in the breeding season, the male fish jump out of the water to court the females. In the daytime the tuna cannot dodge the gaze of predators such as dolphins and whales, so they do their courting by moonlight.
Freshly hatched tuna fry are only three to four millimeters long, and are completely at the mercy of the ocean currents. Hitching a ride on the Kuroshio Current, they drift north, surrounded by hungry predators. Those that survive stop to winter in the waters off Japan. By October to December the young bluefin are already juveniles a foot long. "In their first year they can grow to 60-70 centimeters, and become strong enough to migrate east to the northeast Pacific and California. They stay there until they are around six years old and at least 1.6 meters long. Then they return to the waters off eastern Taiwan to spawn," says Wu Chi-lun.
Bluefin tuna can live to 20 years old, and after the age of six can spawn several times a year. But compared with other fish they grow slowly and have a long life cycle. Although they lay huge numbers of eggs, their ultimate survival rate is tiny. Australian researchers studying the southern bluefin estimate that out of 10 million eggs, only two fish manage to grow safely to adulthood.
The ecology and behavior of the southern bluefin are very similar to those of the Pacific bluefin. "Hence the survival rate of the Pacific bluefin is unlikely to differ much from the southern bluefin," says Wu. He explains that bluefin tuna follow shoals of smaller fish, devouring almost everything that crosses their path. They gobble up large quantities of Pacific saury, squid and mackerel. Being at the top of their food chain, their populations cannot grow very large, and they take a long time to reach adulthood. Hence intensive fishing is likely to cause their numbers to decline rapidly.
No endless bounty
In the past, the main catches of Pacific bluefin were by fishermen from Japan and the USA. Bluefin caught by Taiwanese fishermen were mainly landed by the deep-sea fishing fleet. They did not command high prices, so few were caught. But in recent years the fashion for bluefin sashimi has caused dramatic price hikes, and many inshore fishermen have switched to tuna.
As well as the Southern Fishing Grounds east of the Philippines, as the bluefin migrate toward Japan they pass through the Northern Fishing Grounds off Su-ao, which has become second to Tungkang as a bluefin fishing harbor.
The boats that go after bluefin tuna are mostly small to medium-sized vessels of 40 to 80 tons. According to Tungkang Fishermen's Association, their number has risen from 200-plus in the past to almost three times as many today. Many boats that used to trawl for other fish have been converted for long line fishing to catch bluefin tuna.
Bluefin are mainly used for sashimi, so they are bled, gutted and refrigerated as soon as they are brought on board. The boats stay at sea for a week to ten days. Wu Chi-lun, who often goes out on the boats to study tuna ecology, says that although fishing is arduous work, the boats hunting bluefin tuna need less than ten crew, and spend only a few days at sea. "Compared with an average price of NT$1500 a kilo, their costs are low. Long-line tuna fishing has become an important high-grade inshore and offshore fishery for Taiwan," says Wu. But with the increasing numbers of bluefin being caught, if the fishery is to be sustainable, there is a need for more surveys of tuna stocks. Records need to be kept of Taiwan's annual catch and the age and size of the fish caught, and these statistics need to be analyzed in combination with those of other countries. Only then will it be possible to estimate reasonable catch numbers.
According to a survey by NTU's Institute of Oceanography, in recent years the bluefin caught in the waters from the Japanese Ryukyu Islands to eastern Taiwan have been mainly in the weight range 80-250 kilos. In earlier times it was rare for tuna below 170 kilos to be taken. "Recently there has been a trend toward smaller fish, but there is great variation in catches from year to year."
Tuna conservation
Tuna range far and wide across the world's oceans, and are among the most widely exploited fish species internationally. Except for Pacific bluefin, the populations of most tuna species are closely monitored. There are four international organizations related to tuna conservation: the Commission for the Conservation of Southern Bluefin Tuna (CCSBT), the International Commission for the Conservation of Atlantic Tunas, the Indian Ocean Tuna Commission and the Inter-American Tropical Tuna Commission. In recent years the proportion of southern bluefin of breeding age has fallen below a safe level for species survival, and the World Conservation Union (IUCN) has listed the fish as critically endangered.
Only four of the world's seven tuna species migrate through waters near Taiwan. But Taiwan's deep-sea fishing fleet roams all the world's oceans, so all seven species appear in Taiwan's annual fishing industry reports. After the CCSBT placed limits on catches of southern bluefin, although Taiwan is not a member it has voluntarily reduced its annual catch to 1450 tonnes, or little more than half the previous level.
With the gradual adoption of limits on catches of all tuna species, and the rapid rise in Pacific bluefin catches, early this year Pacific nations set up the Commission for the Conservation and Management of Highly Migratory Fish Stocks in the Western and Central Pacific Ocean. Taiwan is a member, so the bluefin it exports to Japan all have to be registered. Hence keeping proper statistics on tuna catches is essential.
"The introduction of a responsibility system is an inevitable trend in fishery resource management, so there is an even more pressing need to step up research on bluefin tuna." In Wu Chi-lun's view, the fundamental solution to enable bluefin populations to stabilize is to breed the fish artificially. "Taiwan is well placed geographically, because the females arrive here heavy with roes. We should contribute to artificial breeding." The National Museum of Marine Biology takes the same view. In May, assistant researcher Li Yen-hung went to sea twice to assess the possibility of performing artificial breeding on board ship, and then bringing the fertilized eggs back to land to hatch.
Marine jet fighters
"Breeding tuna artificially is a good idea, but it's extremely challenging." Wu Chi-lun says freshly hatched tuna fry are so small that they are barely visible to the naked eye, and finding suitable food for them is an as-yet-unsolved problem. Bluefin swim at very high speeds-up to 80 kilometers an hour. Because of their huge size, they have to move fast for their gills to filter enough water to oxygenate their blood. Forcing these "hyperactive" fish to be still would kill them.
Bringing bluefin back to land alive is immensely difficult. The earliest attempts to breed them artificially were in Japan. The fish were held in coastal cage nets. But although the enclosures were large, the tuna could not swim as freely as in the open sea, and often crashed into the nets and killed themselves. So the Japanese switched to catching one- to two-year-old juvenile fish to try to rear instead.
Although breakthroughs are needed before artificial breeding can succeed, study of bluefin ecology is proceeding apace. While fish markets are busy auctioning tuna, TFRI researchers are busy collecting their scales. "The patterns on them are just like trees' growth rings," says Wu Chi-lun. From them he can judge the age and health of the tuna, and the temperature and climatic conditions in the waters they migrated through.
In 1995 Japanese researchers began tagging bluefin tuna by implanting radio tracking devices into their bodies. Recently, the TFRI began co-operating with the Japanese, and requiring Taiwanese fisherman who catch tagged bluefin to record basic details to provide information to researchers.
Ten years ago, the gill nets used to catch tuna often trapped dolphins, which also hunt tuna. Calls from conservationists led to a ban on gill nets. In recent years, fisherman at sea have often seen their tuna catch being decimated by large numbers of dolphins. Today bluefin tuna also face a population crisis. Only by understanding and conserving them can we enable dolphins and humans to peacefully coexist on the seas.
