What does science tell us about Tuna? Tuna biology.101. Erica Williams, Jacques Boubée & Wakaiti Dalton

Transcription

1 What does science tell us about Tuna? Tuna biology.101 Erica Williams, Jacques Boubée & Wakaiti Dalton

2 Types of tuna Anguillidae (Tuna, Puhi, Anguilliformes) is the family name of fish that includes all freshwater eels. Over 720 species worldwide (including marine eels). There are about 16 different types (species) of freshwater eels around the world.

3 Tuna in Aotearoa There are 3 types of eel in Aotearoa: Longfin eel Shortfin eel Australian speckled longfin While shortfins also occur in SE Australia and some Pacific Islands, longfins only occur in Aotearoa. The Australian longfin was discovered in some Aotearoa rivers around 1990, but may have arrived here as early as the 1970s. Longfin eel (Anguilla dieffenbachii) Maximum size: 2.0 m, 25 kg Shortfin eel (A. australis) Maximum size: 1.1 m, 3 kg Australian longfin eel (A. reinhardtii) Maximum size: 2.0 m, 21 kg Drawings courtesy of R. M. McDowall

4 Australian speckled longfin eel New Zealand longfin female

5 Key tuna features Pectoral fin Lateral line Dorsal fin Gill openings Shortfin Dorsal & anal fin ends are almost the same length Anal fin Dorsal fin is longer than the anal fin Longfin Copyright Peter E Smith, NSIL Copyright Peter E Smith, NSIL

6 Eels have a long and unusual life cycle which sees them travelling between the ocean, estuary and freshwater

7 Travelling upstream Tuna are amazing animals. They are the most widespread freshwater fish in Aotearoa. They start life as an egg out in the Pacific Ocean. They hatch at sea into see-through (transparent), leaf-shaped, larvae called leptocephalii and spend between 9 and 12 months drifting on ocean currents which bring them back to Aotearoa. When they reach the sea bed near Aotearoa (continental shelf) they change shape and turn into colourless eels called glass eels, which are about 60 to 70 mm long. In early spring they move into estuaries, rivers and streams where they rest to get used to their new freshwater environment. After several weeks they start to turn brown (become pigmented) and begin their journey as an elver and head upstream. Leptocephalii Elvers Glass eels

8 Elvers are amazing climbers as long as the rocks are a bit damp

9 Where do they live? Tuna are found in all sorts of freshwater habitats (places). They like choosing places where they can hide like overhanging banks, under plants, rocks, boulders and tree roots. They can squeeze into very small spaces and dig down into the mud. Adult longfin eels like fast flowing water and stony river beds, and are often found further upstream. While shortfins like slower moving water and are often found in swamps, lakes, rivers and streams a near the coast. However, both species (types) are often found in the same place. Generally, small eels like shallower water and large eels like deeper water. Longfin and shortfin eels can be found all over Aotearoa except in places where they have been unable to get past barriers - such as large dams; or where their habitat has been removed, damaged or polluted.

10 What do they eat? The longfin eel is the largest fish in Aotearoa s freshwaters, and where they are present they are the top predator. Insects that live on land Tuna will usually eat anything they can find using their awesome sense of smell. They are usually more active at night. Small eels eat a range of small insects, snails, and crustaceans (also known as invertebrates). As their mouths get bigger, they can eat larger animals such as kōura (freshwater crayfish), fish, small birds and rats. Insects that live in the water Small animals & fish

11 Tuna love it when it floods - during and after a flood tuna will eat as much as they can.

12 Travelling downstream & out to sea: After a very long life (on average between 11 & 52 years) tuna start to change and stop feeding. Their eyes get bigger, their puku turns silvery and their pectoral fins get larger and black. This is when they are known as tuna heke.? Shortfin male Shortfin female During rainy nights in autumn (and sometimes spring) they begin their long migration (or journey) to the sea and far into the Pacific Ocean where they spawn and die. Dr Jellyman releasing a tagged longfin female Tuna heke have been tracked from Aotearoa out into the Pacific Ocean and their spawning place seems to be somewhere near Fiji and New Caledonia in a region of the ocean known as the Tonga Trench.

13 Tuna numbers are falling: All over the world freshwater eel populations are in steep decline (becoming less). There are many reasons behind this loss, these include: barriers to migration, both upstream and downstream (like large dams and bad culverts); overfishing; loss of natural habitat; pollution and parasites; and climatic change. Index of juvenile stocks (% of 1960s-1970s) European eel Japanese eel American eel Year Trends in glass eel recruitment for the European eel (A. anguilla), Japanese eel (A. japonica) and American eel (A. rostrata) (from Dekker 2004). No fencing Barriers Overfishing

14 Protect the big girls: Female tuna grow a lot bigger than males. Anything you catch over about 600 mm long is probably a female. The larger the female, the more eggs she has. Female tuna can have over 20 million eggs! Tuna gonads (called ovary and testis) are found along the backbone of the tuna. The male gonad looks like a string of rosary beads; while the female gonad looks like a folded net curtain. Female gonad How fast tuna grow depends on how much kai there is, how many other eels there are (density), water temperature, season and habitat. Females are more common when there are less tuna in the area (low density) Male gonad So if you don t really need to eat it, put the big ones back so they can travel the 1,000 s of kms out to sea to breed.

15 How many tuna? There are lots of different ways (methods) to answer questions like: a) What types (species) of fish live in our awa? b) How many are there? c) Are the fish healthy? d) What types of places (habitats) do they like the best? Just like a fisherman would, scientists decide what type of fishing technique to use depending on the area they want to look at (like a lake or a shallow stream) and the type and size of fish they are interested in. The techniques we can use include: Electric-fishing, Gee-minnow traps, Seine and gill nets, Fyke nets, Hīnaki, Whitebait traps, Spotlighting at night. Hīnaki Fyke nets G-minnow traps Electric fishing

16 Picking up our fyke net in the Punakitere River to see how many tuna we caught overnight

17 What do we look for? It is important to write down all the details when we go out to sample freshwater fish. The date, name of the place we are fishing (site) and the method we use are all important to record. Once we have caught some fish we need to take some measurements. This information becomes our data which we can add to over time. Important measurements to record include: Count the number caught and identify the species. (migrant/tuna heke?) Weight (g) Length (mm) Release

18 This tuna has been put to sleep for a little while so we can measure it. It s puku is very full and it must have eaten something else that was caught in the net!

19 How old is that tuna? A close look at an otolith It is not possible to look at the size of an eel to tell it s age. A large tuna may be a young eel which has grown quickly because of where it lives (like having lots of kai around, not many other tuna around and water temperature). The only way to age a tuna is by looking at the ear bones (called otoliths). The otoliths grow as the tuna grows, and each year a new layer is added just like the rings in a tree trunk. To age the tuna you count the rings on the otoliths. Unfortunately removing the otoliths means killing the tuna. It takes practise to find the two otoliths, which are found underneath the skull behind the brain. Once you have them you should store them in a paper envelope to dry before sending to the laboratory for aging. Getting the otolith out from behind the brain

20