ANGUILLA JAPONICA Progress review on the artificial glass eel production and the nutritional research for Japanese eel, Anguilla Japonica
apanese eels, or Anguilla japonica, are members of the Anguillidae family with 19 species of freshwater eel that are all in genus Anguilla. They are mainly distributed in Taiwan, China, Japan and Korea. Japanese eel are catadromous fish, which means that they have a specific life cycle and don’t spend their whole lives in fresh water. Mature eels migrate thousands of kilometres from inland water bodies to seawaters for breeding purposes. Within this long migration period they don’t stop, normally don’t eat and sometimes crawl over land at night to get to the ocean. After breeding the parents then sacrifice their lives, the egg hatch at sea and the larvae that are named “leptocephali” are migrated by the currents to more shallow waters. Then the larvae metamorphose into translucent elvers that are also known as glass eel (Figure 1). Glass eels migrate in schools to the freshwater, estuarine habitats, lakes or rivers. For a long period of their life cycle, from juvenile to the growing stage, these fish stay in fresh water and complete their development. The Japanese eel has an elongated body that is cylindrical with small scales, plaincoloured and neither marbled nor mottled (Figure 2). Being an actual carnivore, it basically feeds on small fish, insects and also crustaceans (Arai, 2016). However, additional research is still required about the biology and life cycle of Japanese eel. Considering the high price and market demand, the Japanese eel is one of the most valuable fish species in East Asia. Unfortunately, according to the International Union for Conservation of Nature and Natural Resources (IUCN), the Japanese eel is classified as “Endangered” and is in the red list
(Jacoby & Gollock 2014). So what is the main reason for this? One of the causes of this is over exploitation as the glass eel are often caught and traded because of their high price, while predictable seasonal migrations make them easy to catch. In addition, water pollution can negatively impact migration sites, development of elvers and the abundance of food for young eel. Infectious disease has also been a reason for the current decline of Japanese eel population (van Ginneken & Maes 2005). Therefore, special attention is required to the conservation and rehabilitation of these species.
Production of Japanese eel
Aquaculture can drastically help the Japanese eel situation by conserving the genetic information, reducing the pressure of fisheries on natural resources and meeting the demand of market. This is while less success was observed in the artificial breeding of Japanese eel, thus recruiting elvers are intensively captured for use in aquaculture. During the recent decades, the aquaculture of Japanese eel has developed rapidly and has become a vigorous industry in China, Japan, Korea, Malaysia and Taiwan (Figure 3). Aquaculture is now having the highest share of eel production compared to wild catch. As for Korea, the total value of Japanese eel production rose from US$22 million in 1990s to US$192 million in 2014 (FAO, 2014). But this is still based on capture of glass eels as well as relying on wild populations. For sustainable aquaculture, development of the artificial glass eel production system seems to be vital. Studies on Artificial reproduction of eel started from 1960s; after a series of failures, Yamamoto and Yamauchi (1974) were the first ones to obtain larvae from eggs using hormone injection of the broodstock. After that, Yamauchi et al. (1976) had the chance to rear newly hatched eels for two weeks. A few years later, in 2000 the Japanese were successful in producing the first glass eel in the world. Nowadays, in Japan, thousands of glass eel are being produced annually. Although, the production cost is well above hundreds of dollars per individual glass eel (Tsukamoto 2014). In Korea, the National Research Foundation (NRF) approved the first national project on artificial breeding of A. japonica in 2002. This research was carried out by Feeds and Foods Nutrition Research Center (FFNRC) as the prior research institute at
26 | September | October 2016 - International Aquafeed