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Photo of Heidi Dewar Photo of Tom Williams Tom Williams & Heidi Dewar
Tuna in the Tank

The Monterey Bay Aquarium is known for pioneering new techniques in animal husbandry and its interpretation of living organisms. After the New Zoos, Aquarium veterinarian Tom Williams fielded questions about his work and Heidi Dewar responded to queries about her projects with bluefin tuna.

q I would like to know why, after you tag the tunas, you give so much money as a reward to fishermen who catch them [a thousand dollar reward].

A (Heidi Dewar): We give the fishermen $1,000 to make it worth their while to return the tags. In previous tagging projects a large reward increased the number of tags returned.

q Why did you decide to help the tuna?

A (Heidi Dewar): I decided to help the tuna for a couple reasons.

1) I have studied tuna for 10 years and find them beautiful and fascinating. So I study them because I like and am intrigued by them.

2) Tuna populations in the Atlantic and southern Pacific have declined by close to 90% in the last 20 years so they desperately need help. To make sure there are healthy populations of bluefin far into the future we need to manage them. To manage them we need to know where in the ocean they are swimming and when they move to different parts of the oceans.

q Are you optimistic about your goal of establishing new fishing quotas for the bluefin tuna? It seems like a daunting task, both because of the tuna's commercial value and because so many countries would have to agree to the new guidelines.

A (Heidi Dewar): Our primary goal is to provide the best possible scientific data for ultimate use by modelers and policy makers. If the data suggest that quotas need to be reduced, this is only the first step. All countries belonging to ICCAT (International Commission for the Conservation of Atlantic Tunas) will have to agree to new quotas, policies changes will have to be imposed and ultimately enforced. It is often during the steps beyond the science that things fall apart. I think that more political and economic pressure will have to be brought to the table and that leverage from other areas will have to be applied for any quota reduction. I am optimistic that quotas will drop, at least in the east, in the next five years.

q What kind of food do the bluefin tuna eat when they are in the wild? Is this same diet as you feed them in the tank?

A (Heidi Dewar): Bluefin in the wild will eat almost anything that will fit into their mouth. Thus, as the fish get bigger the number of things they can eat increases. In the stomachs of bluefin they have found little crustaceans like krill, lots of squid and other fish, both large and small. Right now we don't have bluefin in our tanks, only yellowfin and skipjack. We feed them fish and squid and vitamin enriched jello. Their natural diet would be more diverse and they certainly wouldn't be eating jello.

q How big are the bluefin tuna when they are born? How big do they get?

A (Heidi Dewar): Bluefin tuna are external spawners which means that the males and females shed the sperms and eggs into the water where fertilization takes place. The baby bluefin are very small when they first hatch (about the size of a sesame seed) but grow quickly. By the time they are 8 years old they weigh around 140 kg (310 lb.). Some fish reach sizes of 550 kg (1,200 lb.) or more.

q Is there anything kids can do to help the tuna and other endangered animals?

A (Heidi Dewar): There is plenty that you can do:

1) Learn about important subjects and teach others when you can.

2) Write to people in the government letting them know that you are worried.

3) Think about things that you eat and buy. For example, if you like the rain forest don't buy little horse statues made of tropical wood. If you are concerned about bluefin tuna don't support the fishery by eating it.

q Do the tuna migrate every year? And where are bluefin tuna located (what part of the ocean?)

A (Heidi Dewar): The bluefin tuna seem to migrate every year. Of the thirteen different species of tuna they appear to travel the greatest distance. What their exact routes are we don't yet know. Most of what we do know comes from the fishermen who can tell us where they caught them. The bluefin often arrive off the North Carolina coast in the winter and stay until spring. They are found off of New England in the summer. Bluefin can be found in the Gulf of Mexico, off of Norway, off of New England or Bermuda and in the Mediterranean to name a few places.

q Why are bluefin tuna so expensive?

A (Heidi Dewar): Bluefin tuna are very expensive because the Japanese are willing to pay high prices for bluefin tuna sashimi. Around 90% of the bluefin caught are sent to Japan. The more fish oil the meat has in it the more expensive it is. The tastiest part of the tuna, the meat along the belly, can sell for $75 for three slices. Most fish sell for around $10,000 but some sell for as much as $90,000. The entire market is driven by the Japanese market.

q Heidi, What is it like to do the work you do? I like animals and want to know what your career would be like.

A (Heidi Dewar): I enjoy my work for many reasons. I go out on the ocean and see many amazing animals, tuna, marlin, sharks, mola mola and flying fish. I work with tools that allow me to figure out how they live their lives, travel where they travel and do what they do. The fun stuff is about 10% of the job. I spend most of my time working on the computer, analyzing data, organizing for the next trip and writing grants to get research funds. Almost every day is different and it is seldom boring.

q At the end of the show Alan Alda said you had recovered two of the implanted tags from tuna caught off New England. Have you got back any more of the tags since this show was filmed?

A (Heidi Dewar): We have not received any more tags than the two mentioned in the show. We had hoped to recover another five this winter in the tag and release fishery off of Cape Hatteras, North Carolina. However, the Gulf Stream was pushed way off shore and few bluefin tuna were caught. We hope to get back another 5 to 7 over the next few years and will attempt to implant more tags as well.

q Do you think that it's time for consumers who care about endangered species to stop eating bluefin tuna?

A (Heidi Dewar): Yes, money talks! You are casting a "YES" vote for all practices associated with an item every time you hand over that dollar. Consumers should be more conscious about what they are supporting whether it be animal testing, excessive packaging, pollution or reckless endangerment of a species. Bluefin are probably not in danger of extinction, but they are in danger of being reduced to levels that can't recover from the current rates of exploitation. Another factor to consider is ecological extinction, whether a hole has been created in the ecosystem that bluefin tuna are now too few in number to fill.

q You mention that the tank for the bluefin tuna and mola is over 1 million gallons. When I heard this, it sounded extremely large. I was wondering how many gallons an average swimming pool is so that I could picture how large the tuna tank really is. If it is much bigger than the size of an average swimming pool, is all that room really necessary for the species in it?

A (Tom Williams): The size of an average swimming pool is 20 to 30,000 gal. Tuna need a lot of clean water as they are ram ventilators and have to constantly swim at 4 to 5 knots/hour.

q Are the molas, like the one we saw on the show, endangered?

A (Tom Williams): The Mola molas are not endangered.

q The fish in the tank seemed really ok with living in that small an area, except the ocean sunfish, or mola, that would get stressed out and try to "get out" of the tank. I was wondering why and at what would they get so upset?

A (Tom Williams): The mola mola have not tried to get out of the tank. They do gently bump their heads against the sides of tank. Now that we have only one mola in the tank, this fish does not exhibit this behavior. They appear to be very territorial and they do better with just one in the tank.

q How does the ocean sunfish, or mola, know to go to the target to feed and what does it eat in the wild?

A (Tom Williams): As soon as the mola are brought into captivity, they are shown the target whenever they are given food. They regard the target very positively. In the wild they eat mollusks (squid), crustaceans (pellagic crabs, shrimp), small fish and jellys.

q How did you change the feeding procedures for the tuna once you discovered that they were dying from being too fat?

A (Tom Williams): The Tuna Research and Conservation Center which is run by Stanford University and the Monterey Bay Aquarium does the research on husbandry, nutrition, physiology and what ever is needed to make sure that the tunas in the Outer Bay Waters tank are healthy and to do research on tuna in the wild. All the fish when they die are necropsied and a proximate analysis is done on some of the fish. This tells us mainly what is the fat, protein and carbohydrate concentration of the red and white muscle.

When the fish died and the necropsy showed pools of fat appearing in the cut surface of the muscle a few minutes later and the proximate analysis results were high fat then we changed the nutrition and husbandry to decrease their caloric intake.

a. Feed the tuna every other day instead of once a day

b. Feed fish with a lower fat content as example squid (85% of the diet) and white bait (15% of the diet) instead of herring and sardines.

c. Supplement with a gel of lower fat value.

d. Add vitamins to the gel.

q Why do the two kinds of tuna in your tank at Monterey Bay Aquarium [yellowtail and skip jack tuna] rarely do well in captivity?

A (Tom Williams): The yellow fin tuna (yellow tail is a different type of fish) and the skip jack require a large space and very clean water. The yellow fin is easier to raise in captivity than the skipjack which is very susceptible to stress. Both tunas take special techniques in capture and transportation to arrive in good health.

q At the Monterey Bay Aquarium do you operate on sick and injured fish like Dr. Krum on the Doctor Fish story on this show?

A (Tom Williams): All the major aquariums have aquatic veterinarians to do medicine and surgery on sick and injured fish. Each veterinarian usually has their specialty and then they share their findings at national and international meetings such as the International Association for Aquatic Animal Medicine (IAAAM). At the Monterey Bay Aquarium I have done a lot of work on fish anesthesia and surgery and treatment of various fish diseases. I have also worked on sea otters for about 30 years.

q How many pounds of food do the fish in the one million gallon aquarium eat every day?

A (Tom Williams): The Outer Bay Waters tank at the Monterey Bay Aquarium is about 1 million gallons. The fish are fed about 120 pounds of squid and 10 pounds of smelt on Tuesday, Thursday, Saturday and Sunday. We found by feeding every day that the fish got too fat and could not metabolize the food that fast.

q In one of your answers you said that tuna are ram ventilators. What does that mean?

A (Tom Williams): Fish that breathe by ram ventilation do not have enough muscle in their operculum to pass enough oxygen over their gills so they have to move though the water to force more water over their gills so they can extract oxygen. A gold fish, for example, can stay in one spot and by moving their operculum, force enough water over their gills and assimilate enough oxygen.

q Since tuna in the wild goes long distances between food, would it be better if you fed the tuna in the tank every week instead of every day?

A (Tom Williams): A good question about if something is true in the wild will it work in captivity. We began by feeding similar food as the tuna would eat in the wild but they became too fat. We reduced the feedings to 4 times a week and changed the diet to a lower fat fish. In the Outer Bay Waters tank, it is a multi-species tank and we have to consider all the species in the tank, not just the tuna.


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