Seasonal changes in species composition ofglass eels of the genus Anguilla (Teleostei:

Anguillidae) recruiting to the Cagayan River,Luzon Island, the Philippines

By Jun Aoyama*, Tatsuki Yoshinaga, Akira Shinoda, Fumiaki Shirotori,Apolinario V. Yambot and Yu-San Han

AbstractTropical species of anguillid glass eels have been recently exploited to be a substitute for the commercially important A. japonica or A. anguilla, so information about their life histories is needed to facilitate conservation. To understand ecological aspects of glass eel recruitment patterns of tropical eels, Anguilla spp., monthly monitoring of species compositions at the river mouth of the Cagayan River in northern Luzon of the Philippines was carried out at different time periods from May 2008 to September 2009 and from November 2011 to December 2012. Species identifications were made for 32,178 glass eels using morphology and / or genetics and a total of five anguillid species, A. bicolor pacifica, A. celebesensis, A. japonica, A. luzonensis and A. marmorata were found. In both time periods A. marmorata was dominant followed by A. luzonensis and A. bicolor pacifica, but the proportion of A. marmorata decreased from 55.1 % in 2008-2009 to 41.2 % in 2011-2012, with A. bicolor pacifica increasing from 3.9 % to 26.3 % and A. luzonensis staying relatively stable (41.0 %, 32.5%). Two other species, A. japonica and A. celebesensis, that were reported in previous studies from the Cagayan River were extremely rare. This study indicated that selective glass eel catch of a commercially preferred tropical species in this area is not feasible, as the harvest will include a high proportion of by-catch of economically less important species, potentially having significant impacts on their populations.

*Corresponding author. E-mail: jaoyama@aori.u-tokyo.ac.jp

Pacific Science, vol. 69, no. 2October 20, 2014 (Early view)

INTRODUCTION

A total of 19 species / subspecies of the genus anguilla are presently known in the world, with most of

them being found in tropical regions (aoyama 2009, watanabe et al. 2009). The catadromous life histories

of anguillid eels have been intensively studied during the last decade, providing significant advancements

in understanding of the ecology of these fascinating species. However, most research activities have

concentrated on temperate species and their tropical counterparts have been less-studied (see Aoyama

2009).

Anguillid eels have long been a fishery resource throughout their distribution range. In particular,

Japanese eels (A. japonica) have crucial economic importance in East Asia where they have been

intensively exploited for decades (Silfvergrip 2009). Recent critical shortages of glass eels for aquaculture

have greatly impacted the eel industries in East Asia and have stimulated increasing economic interest in

tropical anguillid species that have been less exploited so far. Commercial exploitation of tropical

anguillids for international trade has therefore likely increased in recent years.

The Philippines is a principal source of glass eel supply to East Asia because of its

geographic advantage close to the final destination and the possible occurrence of A. japonica

glass eels in the northern part of Luzon Island (Crook and Nakamura 2013, Tabeta et al. 1976).

However, the recent finding of a cryptic species, A. luzonensis, from Luzon Island (Watanabe et

al. 2009) indicates that the commercial exploitations of glass eels in this area has been carried

out without clear biological or ecological information about the species being harvested. Indeed,

Yoshinaga et al. (2014) recently found that glass eels of A. luzonensis were collected by local fishermen at

the estuary of the Cagayan River for export to East Asia in 2009. These facts strongly suggest the need for

reassessment of the anguillid eels in this area in order to understand their life histories and ecology to

facilitate their management and conservation.

The East Asia Eel Resource Consortium (EASEC) consists of scientists, traders, fishermen, non-

profit organizations and governmental officials from Taiwan, Mainland China, South Korea and Japan

and was established in 1998 to share the latest information about conservation, aquaculture and trade of

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the Japanese eel. EASEC started monitoring glass eel recruitment in Japan (Aoyama et al. 2012) and

Taiwan (Han 2013) from 2009. The research initiative was named ‘Eel River Project’. Considering recent

commercial interest in tropical anguillids as an alternative to the Japanese eel or the European eel in East

Asia, EASEC concluded that an Eel River Project was needed to determine the present status of glass eels

recruiting to the Philippines. This paper reports species identities and changes in the species composition

of glass eels recruiting to the Cagayan River estuary, northern Luzon, the Philippines at both monthly and

interannual time scales.

MATERIALS AND METHODS

Samples

Glass eels were sampled monthly from a commercial fisherman who operated set nets in the river mouth

of the Cagayan River on northern Luzon Island of the Philippines during May 2008 to September 2009

(Period I) and November 2011 to December 2012 (Period II, Fig. 1, Table 1). Set nets consisted of a bag

cod-end with 5 m long and 2 m height wings on either side and were operated throughout the year for

glass eel catches.

Eels sampled for the present study varied from 103 to 2327 individuals per month depending on

the catch and price of glass eels in the field (Table 1). The specimens were preserved in 99.5 % v/v

ethanol immediately after collection and were transferred to the laboratory at ambient temperature for

subsequent analyses. It should be noted that the samples do not indicate the absolute abundance of glass

eels recruiting to the Cagayan River, and statistical records are not available for the overall catches and

trading from the local government or any fisheries agency.

Morphological species identification

Glass eels of five species, A. bicolor pacifica, A. celebesensis, A. japonica, A. marmorata and the recently

described A. luzonensis have been reported from the Cagayan River (Han et al. 2012, Tabeta et al. 1976,

Watanabe et al. 2009). The specimens collected in the present study were first classified morphologically

into two groups, the short-finned (A. bicolor pacifica) or the long-finned (the other four species), based on

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the length between verticals through the anus and origin of the dorsal fin in percent of the total length

(AD/TL), following previous studies (Ege 1939, Leander et al. 2012, Tabeta et al. 1976, Watanabe 2003,

Watanabe et al. 2004). Among the long-finned specimens (AD/TL > 5), A. japonica was identified by the

absence of cutaneous caudal pigmentation that is well developed in the other long-finned species

(Leander et al. 2012, Tabeta et al. 1976, Yoshinaga et al. 2014).

Genetic species identification

All long-finned specimens collected in Period I were genetically identified using the species-specific PCR

amplification method reported by Han et al. (2012). Specimens that showed no clear specific

amplification and some individuals (8-163) from each month that showed an AD/TL value of less than 13

% (A. luzonensis or A. celebesensis) were further identified using a partial sequence of the mitochondrial

cytochrome b gene to confirm their species identification (Table 1). In Period II, 85-96 long-finned

specimens (A. celebesensis or A. luzonensis or A. marmorata) were randomly selected from each month,

resulting in 1209 specimens being identified using a partial sequence of the mitochondrial 16S rRNA

gene. The genetic methods were described in detail elsewhere (Han et al. 2012 for Period I, Yoshinaga et

al. 2014 for Period II).

The sequences were matched with the previously described data in DDBJ/EMBL/GenBank

databases by a BLAST search. The relative abundances of the long-finned species genetically identified

from the monthly sub-sampled specimens for Period II were scaled up to estimate the proportional species

composition of each month. Genetic data obtained in the present study are available upon request to the

corresponding author.

RESULTS

A total of five species, A. bicolor pacifica, A. celebesensis, A. japonica, A. luzonensis and A. marmorata,

were morphologically and / or genetically identified among the 32,178 total glass eel specimens examined

in the present study (Table 1, Fig. 2). However, only 20 A. celebesensis and one A. japonica were found

suggesting that they are extremely rare in the Cagayan River (Fig. 2). The other three species, A. bicolor

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pacifica, A. luzonensis and A. marmorata showed consistent recruitment almost throughout the year but

with different seasonal peaks and annual fluctuation patterns.

During Period I that examined a total of 8,643 glass eels, A. marmorata was the dominant species

(55.1 % of all glass eels) followed by A. luzonensis (41.0 %) and A. bicolor pacifica (3.9 %). A.

marmorata comprised a high proportion of the glass eels sampled from October 2008 to June 2009 (52.7 -

95.2 %) while it comprised only 7.5 - 37.7 % during July to September both in 2008 and 2009 (Fig. 2A).

A. luzonensis exhibited an increase in relative abundance during July to September (60.3 - 86.6 % in 2008

and 73.3 - 88.6 % in 2009), with low abundance in the other months (2.1 - 32.7%, Fig. 2A). A. bicolor

pacifica was a low proportion of the catch (0 - 14.6 %) all year round (Fig. 2A).

In Period II that examined a total of 23,535 glass eels, A. marmorata decreased to 38.0 % and A.

bicolor pacifica increased to 29.5 %, while A. luzonensis comprised a similar proportion (32.5 %) as in

Period I. The nine month period of high A. marmorata relative abundance found in Period I was

diminished to five months from January to May in 2012 and A. bicolor pacifica significantly dominated

(more than 95 %) in November and December 2011 (Fig. 2B). A similar pattern of an increase of A.

bicolor pacifica seemed to occur in October and November 2012, although A. marmorata was found in

relatively high proportion during these months (35.0 %, 49.4%, respectively, Fig. 2B)

DISCUSSION

Glass eels in the Cagayan River

The present study was the first to make monthly observations of the species composition of glass eels

recruiting to the Cagayan River in different time periods (May 2008 -September 2009 and November

2011 - November 2012). It clearly showed that A. bicolor pacifica, A. luzonensis and A. marmorata glass

eels are consistently recruiting to the study area, but A. celebesensis and A. japonica that were reported in

the previous studies rarely occurred.

The North Pacific population of A. marmorata that is distributed from Sulawesi Island north

through the Philippines to southern Japan (Ishikawa et al. 2004, Minegishi et al. 2008) has a spawning

area in the North Equatorial Current (NEC) to the west of the Mariana Islands (Miller et al. 2002). They

5

also spawn at a variety of times throughout the year (Kuroki et al. 2009, Miller et al. 2009). Recently,

Kuroki et al. (2012) analyzed a small number of leptocephali of A. luzonensis collected in the tropical

western North Pacific region and suggested their spawning area and larval migration processes were

similar to the North Pacific population of A. marmorata . In contrast, the spawning area and subsequent

larval migration of A. bicolor pacifica in the North Pacific are almost not known, because only large

leptocephali of this species have been collected (Miller et al. 2009).

The different fluctuation patterns found for A. bicolor pacifica, A. luzonensis and A. marmorata in

the present study give new insights into their spawning ecology. Given that A. luzonensis and A.

marmorata experience the same oceanographic conditions from their similar spawning areas to northern

Luzon, these two species might be expected to show a similar fluctuation pattern in their recruitment.

However, the actual proportion of A. marmorata decreased from 55.1 % in period I to 38.0 % in period II,

whereas A. luzonensis had almost the same percentages in the two periods (41.0 % and 32.5%,

respectively). This may be due to having different spawning seasons or peaks, with the former species

spawning almost throughout the year and the latter possibly spawning in the spring season from February

to May (Kuroki et al. 2012). Considering that only five large sized leptocephali of A. luzonensis (29.2 -

51.2 mm in TL, Kuroki et al. 2012) have ever been found in the north Pacific (Shinoda et al. 2011), it is

also possible that A. luzonensis spawns somewhere different from A. marmorata, causing it to have

different larval transport patterns. Similarly the fluctuation pattern of A. bicolor pacifica found in the

present study suggested that they spawn in a different area and their larvae reach northern Luzon through

a different route that of A. marmorata and A. luzonensis.

The present study showed significant fluctuations in the species composition of glass eels

recruited to the Cagayan River. Considering the sympatric distribution of several anguillid species in the

tropics and their specific reproductive characters that vary inter-annually (See Aoyama 2009, Hagihara et

al. 2012, Miller 2009, Miller et al. 2009), it is reasonably supposed that the species composition of glass

eels in the tropics drastically changes year by year. This has also been reported for A. celebesensis in the

Poigar River, Sulawesi Island, Indonesia (Sugeha et al. 2001). Accordingly, selective catch of a

commercially preferred species, such as A. bicolor pacifica that is a plain-skinned species like the

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Japanese eel is quite difficult. The glass eel catch in the tropics is likely to include a high proportion of

by-catch of economically less important species and could potentially result in significant impact on the

by-catch species. Further observation of the actual amount of glass eels being harvested and precautionary

conservation measures for tropical anguillid eels are urgently needed.

Revisiting the species range of A. celebesensis and A. japonica

The present study clearly revealed that the glass eels of A. celebesensis and A. japonica that were

previously reported from the Cagayan River rarely occurred during the time periods of this study. Tabeta

et al. (1976) found six A. japonica among 5,404 glass eels or elvers collected at the Cagayan River during

1970 - 1974, and Yoshinaga et al. (2014) more recently identified 52 A. japonica glass eels from a total of

767 specimens sampled in 2009 from the same area. Following these reports, the present study also found

A. japonica in this area, but only one glass eel in February 2009 among the 32,178 glass eels collected

during a total of 29 months in the different years. These facts suggest that A. japonica only rarely recruits

to the northern Luzon region, probably only as a result of a specific oceanographic condition transporting

them to this area, as apparently occurred in 2009. But even after they enter the estuary, A. japonica likely

may not survive in the Cagayan River system during their growth phase, because of environmental

constraints in the tropical habitat or interspecies competition with the tropical anguillid species. Indeed,

Jamandre et al. (2007) who surveyed the anguillids in the Cagayan River system did not find A. japonica

in the river. The species range of A. japonica has long been thought to include the northern Philippines

(e.g., Silfvergrip 2009, Watanabe et al. 2004), but no adults (yellow or silver eel) have ever been found in

this region. Therefore, the northern Philippines should probably not be considered as part of the species

range of A. japonica.

The geographic distribution of A. celebesensis also needs reconsideration. This species was

thought to inhabit the western tropical North Pacific region mainly from Indonesia (Ege 1939, Silfvergrip

2009, Watanabe 2003) to the Philippines (Arai et al., 2003, Tabeta et al., 1976) to as far as Taiwan (Tzeng

1982). However, Watanabe et al. (2009) recently described A. luzonensis, which was morphologically

indistinguishable from A. celebesensis. The present study found that the northern part of Luzon rarely had

7

A. celebesensis glass eels arriving there, but A. luzonensis occurred consistently as the second-most

dominant species. Spawning areas of A. celebesensis have been found in the Indonesian waters of the

Celebes Sea and Tomini Bay (Aoyama et al. 2003) and the large leptocephali of A. luzonensis were found

in the NEC region far offshore in the western North Pacific (Kuroki et al. 2012). This suggests that A.

celebesensis is mainly distributed around Sulawesi Island and adjacent areas with their spawning sites

located nearby and being reached after a short-distance migration (Aoyama et al. 2003, Kuroki et al.

2006). In contrast, A. luzonensis that spawns in the western North Pacific, disperses westward during their

larval migration in the NEC (Kuroki et al. 2012). Arai et al. (2003) reported a significant difference in the

larval duration of A. celebesensis from Indonesia (the Poso River (98 ± 7.2 days) and the Poigar River (90

± 13.6 days) in Sulawesi Island) and the Philippines (the Cagayan River (124 ± 12.0 days) in Luzon

Island). This difference was probably because their morphologicaly-identified samples of A. celebesensis

from the Cagayan River were actually glass eels A. luzonensis, a species that was unknown at that time.

Present knowledge on A. celebesensis as well as their species range should be carefully revisited in

consideration with the newly described A. luzonensis.

ACKNOWLEDGMENTS

This study was partly supported by grants-in-aid for Scientific Research from the Japan Society for the

Promotion of Science (Nos. 21405024, 5450281), National Science Council of the Executive Yuan,

Taiwan (NSC 99-2313-B-002-021-MY3, NSC 102-2321-B-002 -073, NSC 102-2628-B-002 -023 -MY3)

and the Philippine Council for Aquatic and Marine Research and Development (PCAMRD), Department

of Science and Technology, Commission on Higher Education, Philippines for the funding.

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Figure 1. A. Map of the western North Pacific region showing the approximate location of the

offshore spawning area (shaded oval) of Anguilla marmorata and the North Equatorial Current

and adjacent major currents. B. Map showing the northern part of Luzon Island and C. the river

mouth of the Cagayan River with the glass eel collection area (square with dashed line) where the

set nets were located.

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Figure 2. Monthly changes in the relative abundance of anguillid glass eels recruiting to the

Cagayan River during Period I in May 2008 to September 2009 (A) and Period II in November

2011 to December 2012 (B). * One specimens of A. japonica was found in February 2009.

11

Literature cited

Aoyama, J. 2009. Life history and evolution of migration in catadromous eels (Genus Anguilla).

Aqua-Biosci. Monogr. 2:1-42.

Aoyama, J., A. Shinoda, T. Yoshinaga, and K. Tsukamoto. 2012. Late arrival of Anguilla japonica

glass eels at the Sagami River estuary in two recent consecutive year classes: ecology and

socio-economic impacts. Fish. Sci. 78:1195-1204.

Aoyama, J., S. Wouthuyzen, M. J. Miller, T. Inagaki, and K. Tsukamoto. 2003. Short-distance

spawning migration of tropical freshwater eels. Biol. Bull. 204:104-108.

Arai, T., M. J. Miller, and K. Tsukamoto. 2003. Larval duration of the tropical eel, Anguilla

celebesensis, from Indonesian and Philippines coasts. Mar. Ecol. Prog. Ser. 251:255-261.

Crook, V., and M. Nakamura. 2013. Glass eels: Assessing supply chain and market impacts of a

CITES listing on anguilla species. TRAFFIC Bull. 25:24-30.

Ege V. 1939. A revision of the genus Anguilla Shaw: a systematic, phylogenetic and geographical

study. Dana Rep. 16:1-256.

Hagihara, S., J. Aoyama, D. Limbong, and K. Tsukamoto. 2012. Morphological and physiological

change of female tropical eels, Anguilla celebesensis and Anguilla marmorata, in relation to

downstream migration. J. Fish Biol. 81:408-426.

Han, Y.S. 2013. Eel conservation in Taiwan. in 16th EASEC proceeding, University of Tokyo,

Tokyo.

Han, Y.S., A. V. Yambot, and H. Zhang. 2012. Sympatric spawning but allopatric distribution of

Anguilla japonica and Anguilla marmorata: sieving effects of temperature and oceanic

current. PLoS ONE 7: e37484.

Ishikawa, S., K. Tsukamoto, and M. Nishida. 2004. Genetic evidence for multiple geographic

populations of the giant mottled eel Anguilla marmorata in the Pacific and Indian oceans.

Jpn. J. Ichthyol. 51:343-353.

12

Jamandre, B.W., K. N. Shen, A. V. Yambot, and W. N. Tzeng. 2007. Molecular Phylogeny of

Philippine Freshwater eels Anguilla spp. (Actinopterygi: Anguilliformes: Anguillidae)

inferred fom Mitochodrial DNA. Raffles Bull. Zool. 14 (Supplement):43-51.

Kuroki, M., J. Aoyama, M. J. Miller, S. Wouthuyzen, T. Arai, and K. Tsukamoto. 2006.

Contrasting patterns of growth and migration of tropical anguillid leptocephali in the western

Pacific and Indonesian Seas. Mar. Ecol. Prog. Ser. 309:233-246.

Kuroki, M., J. Aoyama, M. J. Miller, T. Yoshinaga, A. Shinoda, S. Hagihara, and K. Tsukamoto.

2009. Sympatric spawning of Anguilla marmorata and Anguilla japonica in the western

North Pacific Ocean. J. Fish Biol. 74:1853–1865.

Kuroki, M., M. J. Miller, J. Aoyama, S. Watanabe, T. Yoshinaga, and K. Tsukamoto. 2012.

Offshore Spawning for the Newly Discovered Anguillid Species Anguilla luzonensis

(Teleostei: Anguillidae) in the Western North Pacific. Pac. Sci. 66:497-507.

Leander, N.J., K. N. Shen, R. T. Chen, and W. N. Tzeng. 2012. Species composition and seasonal

occurrence of recruiting glass eels (Anguilla spp.) in the Hsiukuluan River, Eastern Taiwan.

Zool. Stud. 51:59-71.

Miller, M.J. 2009. Ecology of Anguilliform Leptocephali: Remarkable Transparent Fish Larvae of

the Ocean Surface Layer. Aqua-BioSci. Monogr. 2:1–94.

Miller, M.J., J. Aoyama, and K. Tsukamoto. 2009. New perspectives on the early life history of

tropical anguillid eels: Implications for management. Pages 71-84 in J. Casselman and D.

Cairns, eds. Eels at the edge. Am. Fish. Soc. Symp.58, Bethesda, Maryland.

Miller M.J., N. Mochioka, T. Otake, and K. Tsukamoto. 2002. Evidence of a spawning area of

Anguilla marmorata in the western North Pacific. Mar. Biol. 140:809–814.

Minegishi, Y., J. Aoyama, and K. Tsukamoto. 2008. Multiple population structure of the giant

mottled eel, Anguilla marmorata. Mol. Ecol. 17:3109-3122.

13

Shinoda, A., J. Aoyama, M. J. Miller, T. Otake, N. Mochioka, S. Watanabe, Y. Minegishi, M.

Kuroki, T. Yoshinaga, K. Yokouchi, N. Fukuda, R. Sudo, S. Hagihara, K. Zenimoto, Y.

Suzuki, M. Oya, T. Inagaki, S. Kimura, A. Fukui, T. W. Lee, and K. Tsukamoto. 2011.

Evaluation of the larval distribution and migration of the Japanese eel in the western North

Pacific. Rev Fish Biol Fish 21:591–611.

Silfvergrip, A.M.C. 2009. CITES Identification Guide to the Freshwater eels (Anguillidae).

SWEDISH ENVIRONMENTAL PROTECTION AGENCY Report 5943.

Sugeha, H.Y., T. Arai, M. J. Miller, D. Limbong, and K. Tsukamoto. 2001. Inshore migration of

the tropical eels Anguilla spp. recruiting to the Poigar River estuary on north Sulawesi

Island. Mar. Ecol. Prog. Ser. 221:233–243.

Tabeta, O., T. Tanimoto, T. Takai, I. Matsui, and T. Imamura. 1976. Seasonal occurrence of

anguillid elvers in Cagayan River, Luzon Island, the Philippines. Bull. Jap. Soc. Sci. Fish.

42:421-426.

Tzeng, W.N. 1982. A new species of glass eel Anguilla celebesensis in Taiwan. Biol. Sci. 19:57-

66. (in Chinese)

Watanabe, S. 2003. Taxonomy of the freshwater eels, genus Anguilla Schrank, 1798. Pages 3-18

in K. Aida, K. Tsukamoto, K. Yamauchi, eds. Eel biology. Springer, Tokyo.

Watanabe, S., J. Aoyama, and K. Tsukamoto. 2004. Reexamination of Ege’s (1939) use of

taxonomic characters of the genus Anguilla. Bull. Mar. Sci. 74:337–351.

---. 2009. A new species of freshwater eel Anguilla luzonensis (Teleostei: Anguillidae) from

Luzon Island of the Philippines. Fish. Sci. 75:387-392.

Yoshinaga, T., J. Aoyama, A. Shinoda, S. Watanabe, V. A. Rhodra, and K. Tsukamoto. 2014.

Occurrence and biological characteristics of glass eels of the Japanese eel Anguilla japonica

at the Cagayan River of Luzon Island, the Philippines in 2009. Zool. Stud. 53:13.

14