Indonesian shortfin eel

The Indonesian Shortfin Eel (Anguilla bicolor bicolor) is a tropical species of freshwater eel in the Anguillidae family and is native to Southeast Asia. They live primarily in freshwater ecosystems but can migrate to or even fully live in marine environments for spawning. Like most eels, they are carnivorous and typically reside closer to the benthic region of water ecosystems consuming small invertebrates and fish species they encounter.^[1] A. bicolor is considered one of the most economically valuable eel species in Southeast Asia and has been increasingly fished since the late 2000s.^[2] The species is potentially at risk of being overexploited due to their rapid rise in demand but with little information known about their historical or current population size, their conservation status is largely unknown.^[3]

Indonesian shortfin eel
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Anguilliformes
Family:	Anguillidae
Genus:	Anguilla
Species:	A. bicolor
Subspecies:	A. b. bicolor
Trinomial name
Anguilla bicolor bicolor McClelland, 1844

Description

Like all other members of the Anguillidae family, the Indonesian Shortfin Eel has an anguilliform body plan with an elongated, slender body that allows for high maneuverability and flexibility.^[4] Like other eels, they lack pelvic fins and have reduced pectoral fins which allow for a sleeker body shape. They have long dorsal and anal fins which are fused with their caudal fins to better propel them through the water via wave like movements. As their name implies, they are a shortfin eel which differs from longfin eels in that their dorsal fin is slightly longer along their body than their anal fin.^[5] In contrast, longfin eels have a much greater distance between the anal fin and dorsal fin along their bodies. Their body is primarily a gray and brown color, most prominent on their dorsal side, with a lighter tan color on their ventral side. Their length and body weight can vary greatly but can range from 411 to 720 mm and 126 to 1,125 grams respectively.^[6]

Indonesian Shortfin Eels primarily reside near the benthic region of water systems where they prey upon small organisms like invertebrates, fish, and crustaceans swimming by.^[1] As individuals become larger their diet too also can increase in size as larger organisms, primarily fish, become a more viable prey species. The Indonesian Shortfin Eel is primarily nocturnal and so will typically hunt at night while spending the day hiding between rocks or debris near the benthic floor.

Photograph of Indonesian Shortfin Eel, taken by Ramani Shirantha

Taxonomy

The Indonesian Shortfin Eel (Anguilla bicolor bicolor) is one of two subspecies of Anguilla bicolor with the other subspecies being the Indian Shortfin Eel (Anguilla bicolor pacifica).  The Indonesian Shortfin Eel lives exclusively in the Indian Ocean meanwhile the Indian Shortfin Eel resides in the Indo-Pacific region, most prominently Northern & Eastern Indonesia.^[7]  Even though both subspecies have been found to share some overlapping distributions in areas of Indonesia and exhibit similar morphologies, both subspecies show distinct differences in molecular genetic characters.^[8]

A. b. bicolor are part of the only genus Anguilla in the family of Anguillidae which encompass all freshwater eel species. Scientists estimate based on genetic analysis that the Anguilla genus diverged about 52 million years ago and extant anguillid species began speciating about 20 million years ago.^[9] A. b. bicolor and A.b. pacifica in particular are estimated to have diverged as sub-species about 1.6 million years ago.^[9]

Distribution

Map showcasing the approximate ranges of distribution for various Anguilla species including both subspecies of Anguilla bicolor.¹¹

A. b. bicolor are found exclusively in the Indian Ocean in a variety of habitats but are typically found in freshwater environments. As adults they primarily reside in small creeks, pools, and marsh habitats near the coast, usually avoiding larger rivers. During reproductive seasons, adults will usually migrate towards the ocean where they reside in marine waters or estuaries to reproduce. However, scientists have found that Indonesian Shortfin Eels exhibit a high degree of flexibility in their migration behavior which thus have allowed many individuals to not follow the standard strategy of living in freshwater as adults and reproducing in saltwater.^[10] Individuals can reside entirely in marine waters or estuaries, meanwhile migratory individuals can move between freshwater, brackish waters, and marine waters freely. This degree of flexibility in migration behavior fits in with the Anguilla genus as many other eel species have also been observed to exhibit a similar degree of behavioral plasticity.^[10]

Life History

A. b. bicolor, just like other members of the Anguillidae family, are facultatively catadromous which means that they primarily reside in freshwater and migrate to marine environments to spawn but have the flexibility of remaining in a variety of habitats as previously mentioned.^[11] This catadromous reproductive style likely connects to how all eel species outside of the Anguillidae family live entirely in marine environments which thus can potentially indicate how breeding in marine waters was an evolutionarily evolved behavior that has been conserved in the Indonesian Shortfin Eel.^[10] However, not enough evidence has been discovered proving if this trend in breeding behavior is due to genetics or the behavioral flexibility of Anguillidae eel species.

Catadromous life cycle of eel, drawing by Salvör Gissurardóttir

During the process of spawning for Indonesian Shortfin Eels, the female releases eggs into the marine environment which are externally fertilized by the male. The eggs eventually hatch as small, leaf shaped larvae called leptocephali that can end up measuring about 44.1 – 55mm in total length near the end of this stage.^[12] They primarily consume the fecal pellets of zooplankton and the mucus houses constructed by larvacean which are a type of filter feeding tunicate.^[13] Eventually these leptocephali undergo metamorphosis and become glass eels before they return to freshwater ecosystems.^[14] Glass eels would continue to consume detritus and small prey items such as benthic invertebrates, zooplankton, and gradually larger organisms as they grow bigger into elvers.^[15] Eventually, elvers will develop into yellow eels where they become pigmented and spend several years growing further in freshwater as young adults.^[16] A. b. bicolor mature at an approximate age range of 5.5 to 8.5 years which is notably faster compared to temperate eel species likely due to the warmer environmental conditions.^[6] At this mature adult stage, they are classified as silver eels where they can then migrate back to saltwater and produce fertilized eggs once before dying.^[16]

Importance to Humans

Indonesian Shortfin Eel is a commonly consumed species of eel that has seen increasing demand, especially in East Asia, due to the decreasing availability of other Anguilla species. Although the Japanese Eel (Anguilla japonica) is often the most preferred form of eel for consumption in many Asian countries, A. bicolor has been regarded as the second most preferred eel species to consume due to the species sharing similar taste and texture with Japanese Eel.^[17] However, the Japanese Eel has been significantly declining in population size along with other widely consumed species such as the European Eel and American Eel (Anguilla rostrata) in recent decades.^[18] As such, the significant population declines in these species have resulted in government intervention and environmental protection plans which therefore limit their ability to be fished. East Asian countries, most prominently Indonesia, have worked to fill in the gap in eel supply for Asian markets by relying more on the harvesting of tropical freshwater eels which directly includes A. bicolor. As such, the Indonesian Shortfin Eel has and will continue to grow as an important food source for millions of people in East Asia due to the high demand for eel food products.

Conservation Status

Due to increasing demand of eels for consumption, particularly in East Asia, the fishing and export of tropical freshwater eel species such as A. bicolor has grown rapidly within recent decades. In particular, the listing of European Eel (Anguilla Anguilla) on Appendix II of the 2007 Convention on International Trade in Endangered Species of Wild Fauna and Flora in the EU and complete ban of European Eel exports in 2010 has been contributed as a major influence for the rise in A. bicolor exports. The European Eel had been listed as critically endangered in 2008 by the International Union for Conservation of Nature (IUCN) with a major contributing factor being attributed to overfishing and the high consumer demand for eels in Europe and Asia.^[19] Due to these government regulations for the European Eel, the high demand for eels created a gap in supply that has been filled in by other nations, most prominently Indonesia,^[2] by relying on the export of tropical Asian eel species such as A. bicolor.^[20] Indeed, the overexploitation of local populations or entire species of eel and resulting shift towards other species has been a consistent historical trend which is likely happening again with the Indonesian Shortfin Eel.

Export of Anguilla eel species from Indonesia between 1989 – 2013. Solid black line represents 3 year averages for eel exports and dotted line represents 3 year average monetary values.²⁰

Despite the increasing exploitation of A. b. bicolor in countries such as Indonesia, this fish has not been evaluated by the IUCN since 2009. In 2009, the Indonesian Shortfin Eel was categorized then as "Least Concern" with further evaluation needed due to little information being known about their population trends. This evaluation is most likely outdated based on the rapidly increasing levels of fishing and exportation that have taken place since the late 2000s in Southeast Asia. In addition, there is no historical data tracking the wild eel populations in countries such as Indonesia which further limits the ability to approximate changes in population size.^[3] As such, further research and evaluation is needed to determine the exact extent that overfishing has had on the species.

Unfortunately, conservation efforts to either replenish Indonesian Shortfin Eel populations or restore other Anguilla species and thus reduce the effects of overfishing have largely been ineffective. The use of aquaculture or captive breeding as a means of bolstering Anguilla populations is currently not a practical option due to the fact that cultivation facilities rely entirely on capturing wild glass eels and elvers.^[3] As such, the preservation of young wild eel populations is critical for not only the conservation of the species but also to produce Indonesian Shortfin Eel for consumption. To address the risk of tropical freshwater eel populations declining in Southeast Asia, local governments need to implement stronger forms of environmental surveying, restrictions, and policies in order to ensure a safer future for species such as the Indonesian Shortfin Eel.