Oceanic migration of tropical Pacific eels from Vanuatu

Study Extent

Gaua Island (Banks Group, Vanuatu) is the emerged top of a 3000 m high and 40 km wide stratovolcano. The symmetrical cone of approx. 25 km diameter is truncated between 500 and 690 m altitude by an 8.5 × 6 km central caldera. Its active volcano Mt. Garet (979 m a.s.l., 350 m high, base 3 km) towers over the crescent-shaped Lake Letas (100 m depth, 7 × 2 km, 418 m a.s.l.), which is constantly fertilized by the inflow of nutrient-rich, warm volcanic springs. Oxygen is available all the way to the deepest point (100 m), and abundant invertebrate life provides rich food resources (Schabetsberger unpubl. obs.). Migrating eels leave the lake through the outflowing Mbe Solomul River. It falls over a 120 m high cascade (Siri Falls) and bifurcates into 1 larger (18 m3 s−1) and 2 smaller rivers (0.9 and 6.1 m3 s−1) before entering the sea. Flow rates increase significantly during heavy rain. According to the experience of local fishermen, eels predominantly leave Lake Letas during heavy rains and north-westerly winds between January and March but may emigrate throughout the year. A tropical depression hitting the island on 24 January 2012 yielded silvering eels in the lower stretches of the flooding river several days later.

Sampling

Between 17 January and 2 February 2012, Lake Letas and its outflow were surveyed for silver eels by electrofishing, hooks, hook and line, and fyke nets in collaboration with fishermen from Gaua. The research team (R. Schabetsberger, F. Økland D. Kalfatak, U. Sichrowsky, M. Tambets) then anaesthetized, photographed, and measured the fish before they were either released (17 including tagged fish) or sacrificed (49) and given to local families.

Method steps

1. Total length, distance from lower jaw to anus, to dorsal fin, and to gill opening, as well as length of mouth and length of pectoral fin were measured with a measuring tape to the nearest mm. Horizontal and vertical eye diameter was measured to the nearest 0.1 mm with ruler callipers. Weight measurements were taken on a Voltcraft HS-30 balance to the nearest 10 g. All eel species were determined through analysis of body proportions and dentition of the upper jaw (Ege 1939, p. 248–251). Colouration of the total body and the relative size of the eyes (Okamura et al. 2007) were used to classify silver eels. The stomachs of 7 Anguilla marmorata (4 yellow/3 silver) and 13 A. mega stoma (9/4) were opened and checked for contents.
2. During the last 3 days of fishing operations, 7 eels with a silvery appearance (1 Anguilla marmorata, 4 A. megastoma, 2 A. obscura) were caught for tagging by local fishermen at the banks of the main river below Siri Falls, where fish are frequently found in shallow water after plummeting over the cascade. The single A. marmorata was caught near the mouth of the river. The tags were put on 3 different species, because we were unable to catch enough silver eels of the original target species A. megastoma. Due to the strong current that prevented the use of other fishing gear, the animals were pulled out from littoral hiding places with metal hooks mounted on wooden poles and pushed into hand-nets, resulting in minor skin lesions (ca. 1 cm) behind the anus. The small wounds were not treated with antibiotics, nor were they sutured to avoid the inclusion of anaerobic germs (F. Økland pers. obs.).
3. After measuring, 2 types of satellite pop-up transmitters were deployed. Two eels were tagged with Mini-Pats (Wildlife Computers, Type 1), and the remaining 5 individuals, with X-tags (Microwave Telemetry, Type 2). Type 1 tags were provided for free by the manufacturer, and Type 2 tags were refurbished with new batteries after use in a previous project. The Type 1 tag is 11.5 cm long (excluding antenna), 40 mm wide and weighs 53 g in air. The Type 2 tag measures 12 cm in length, is 32 mm wide, and weighs 42 g.
4. The tags were pre-programmed to surface after 3 mo assuming that the eels spawn north of Fiji and migrate with a speed of approximately 15 km d−1, based on the discovery of leptocephali east of Gaua (Kuroki et al. 2008) and the migration speed of similar-sized Anguilla dieffenbachii (Jellyman & Tsukamoto 2002). In case of premature death of the host or detachment of the tag, the transmitters were programmed to initiate a failsafe release mechanism, ascend to the surface, and transmit data. A depth reading deeper than 2100 m (pressure transducer sensor range exceeded) or 4 d of equal depth readings (±10 m) would initiate this mechanism.

Eels, genus Anguilla

1. Anguilla marmorata
   rank: species
2. Anguilla megastoma
   rank: species
3. Anguilla obscura
   rank: species

Vanuatu and surrounding ocean