Breed eels in captivity

Supporting evidence from individual studies

1. A replicated, controlled study in 1993 at a research facility in Japan (Ohta et al. 1996) found that captive male Japanese eels Anguilla japonica treated with a hormone (human chorionic gonadotropin) matured and produced sperm whereas untreated males did not, and sperm weight and gonad size were similar for two hormone concentrations. Male eels injected with human chorionic gonadotropin at two concentrations produced sperm (average total weight: 250 IU = 9 g/eel, 750 IU = 6 g/eel) and had developed gonads (gonad weight as a percentage of body weight: 250 IU = 10%, 750 IU = 8%), whereas untreated eels did not produce sperm and had undeveloped gonads (0.2%). Total sperm weights and gonad development were similar for the two hormone concentrations, although the results were not statistically tested. In May 1993, male eels purchased from a commercial supplier were tagged and held in 1,000-l tanks. Eels were injected 14 times weekly with a hormone (human chorionic gonadotropin at concentrations of 250 or 750 IU/eel/week; 7–10 eels/treatment) or a saline solution (eight eels). Two days after each injection, sperm was collected and weighed. Two days after the final injection, all eels were euthanized and gonads weighed. 

   Study and other actions tested

   Referenced paper

   Ohta H., Kagawa H., Tanaka H., Okuzawa K. & Hirose K. (1996) Milt Production in the Japanese Eel Anguilla japonica Induced by Repeated Injections of Human Chorionic Gonadotropin. Fisheries Science, 62, 44-49.

2. A replicated study (year not stated) at a research facility in Japan (Ohta et al. 1996) found that artificially fertilized Japanese eel Anguilla japonica eggs had variable fertilization and hatching rates, and fertilizing eggs immediately after ovulation led to higher rates compared to fertilizing after 6–9 h. Statistical significance was not assessed. Overall, 1–90% of eggs/female were successfully fertilized and 0–48% hatched. When eggs were collected from females at the first detection of ovulation, higher rates of fertilization (1–90%) and hatching (0–48%) were observed compared to when eggs were collected 6–9 h after ovulation (fertilization: 1–15%, hatching: 0–2%). Thirteen captive-reared female eels were given 9–12 weekly injections of salmon pituitary extract followed by a hormone injection (17,20β-dihydroxy-4-pregnen-3-one) to induce ovulation. Rearing temperatures were then increased from 20 to 22.5°C. Eggs were collected from 11 ovulated females at 3 h intervals for 6–9 h and mixed with diluted semen from a captive male. Six samples (each containing an average of 89 eggs) from each fertilization were placed in petri dishes with seawater, and incubated at 23°C. Average fertilization rates were estimated after 3–4 h, and hatching rates after two days (both for three samples/female/collection time). 

   Study and other actions tested

   Referenced paper

   Ohta H., Kagawa H., Tanaka H., Okuzawa K. & Hirose K. (1996) Changes in fertilization and hatching rates with time after ovulation induced by 17, 20β-dihydroxy-4-pregnen-3-one in the Japanese eel, Anguilla japonica. Aquaculture, 139, 291-301.

3. A replicated study in 1994–1996 at a research facility in Japan (Kagawa et al. 1997) found that artificially fertilized Japanese eel Anguilla japonica eggs had variable fertilization and hatching rates in each of two experiments, and rates did not differ significantly when donor females were injected with hormones in the morning or evening. In the first experiment, an average of 2–54% of eggs/female were successfully fertilized, and 0.3–40% of eggs/female hatched. Eggs from females injected with hormones in the morning had similar fertilization (13%) and hatching rates (10%) to those from females injected in the evening (fertilization: 9%, hatching: 7%). In a second experiment, average fertilization rates were 3–63%, and hatching rates were 1–55%. In each of two experiments, captive-reared female eels were injected with salmon pituitary extract followed by a hormone (17,20β-dihydroxy-4-pregnen-3-one) to induce ovulation. Hormone injections were given at 9:00 h (15 eels) or 18:00 h (18 eels) in the first experiment, and at 18:00 h only (34 eels) in the second. In both experiments, eels were checked for ovulation every 3 h. Eggs were collected and mixed with diluted semen from a captive male. Samples (containing about 100 eggs) from each fertilization were placed in petri dishes with seawater, and incubated at 23°C. Average fertilization and hatching rates were estimated for three samples/female/treatment. 

   Study and other actions tested

   Referenced paper

   Kagawa H., Tanaka H., Ohta H., Okuzawa K. & Iinuma N. (1997) Induced ovulation by injection of 17, 20β-Dihydroxy-4-pregnen-3-one in the artificially matured Japanese eel, with special reference to ovulation time. Fisheries Science, 63, 365-367.

4. A replicated study in 2006 at a research facility in Japan (Horie et al. 2008) found that Japanese eel Anguilla japonica eggs and larvae produced from spontaneous spawning and fertilization had higher fertilization, hatching and survival rates, and lower deformity rates, compared to eggs and larvae produced from manual egg extraction and fertilization. On average, eggs and larvae produced from spontaneous spawning and fertilization had higher fertilization (80%), hatching (62%) and survival rates (54%), and lower deformity rates (60%), than those produced from manual egg extraction and fertilization (fertilization = 41%, hatching = 31%, survival = 27%, deformity = 79%). In March–June 2006, eel eggs were produced from spontaneous spawning and fertilization in one experimental group (each of 12 ovulating females placed in a tank with three males releasing sperm), and from manual egg extraction and fertilization in another (eggs manually extracted from each of 15 ovulated females and inseminated with stored sperm). Female eels were wild-caught and captive-reared. Male eels were sourced from farms. Hormones were used to induce maturation, ovulation and sperm production. Samples of fertilized eggs from both groups were reared in microplates (three samples/female/treatment). Fertilization rates were estimated after 4 h, hatching rates after 28 h, and larvae survival and deformity rates after five days. 

   Study and other actions tested

   Referenced paper

   Horie N., Utoh T., Mikawa N., Yamada Y., Okamura A., Tanaka S. & Tsukamoto K. (2008) Influence of artificial fertilization methods of the hormone-treated Japanese eel Anguilla japonica upon the quality of eggs and larvae (comparison between stripping-insemination and spontaneous spawning methods). Nippon Suisan Gakkaishi (Japanese edition), 74, 26-35.

5. A replicated study in 2007 at a research facility in Massachusetts, USA (Oliveria et al. 2010) found that artificially fertilized American eel Anguilla rostrata eggs had variable fertilization rates, and larvae hatched for a quarter of females whose eggs were artificially fertilized. Eggs from six of 24 females that were artificially fertilized (fertilization rate 22–81%) successfully hatched larvae (number not reported). Hatched larvae from one female grew in length (from 2.7 mm to 3.8 mm) within four days of hatching, and survived for six days. In autumn 2007, twenty-seven female silver eels were purchased from a commercial fisher and held in 1,000-l tanks. Females were given 13 weekly injections of salmon pituitary extract, followed by a hormone (17α,20β-dihydroxy-4-pregnen-3-one) to induce ovulation. Eggs were collected from 24 ovulated females, artificially fertilized with sperm from captive males, and reared in petri dishes at 20°C. After 3 h, fertilization rates were calculated for five females (75–100 eggs/female). Hatched larvae were transferred to petri dishes containing sterile seawater (20°C) and antibiotics. Larvae from one female were measured at hatching (10 larvae) and four days after hatching (12 larvae). 

   Study and other actions tested

   Referenced paper

   Oliveira K. & Hable W.E. (2010) Artificial maturation, fertilization, and early development of the American eel (Anguilla rostrata). Canadian Journal of Zoology, 88, 1121-1128.

6. A study in 2009–2010 at a research facility in Denmark (Tomkiewicz et al. 2012) reported that wild-caught, captive female European eels Anguilla anguilla produced more larvae, which survived for longer, than farmed female eels. Results are not based on tests of statistical significance. Larvae produced in captivity by wild-caught female eels (<100–200,000 larvae/female) survived for 3–20 days after hatching, whereas larvae produced by farmed female eels (6–<100 larvae/female) survived for 1–3 days after hatching. In each of three trials in 2009–2010, eggs were stripped from female eels kept in captivity and treated with hormones. Female eels were wild-caught (11 eels, one trial) or sourced from eel farms (19–62 eels, two trials). Eggs were artificially inseminated with sperm from farmed male eels. Number and survival of hatched larvae were recorded for all female eels that produced larvae (five farmed and six wild-caught).  

   Study and other actions tested

   Referenced paper

   Tomkiewicz J., Tybjerg L., Støttrup J.G., Mcevoy F., Ravn P., Sørensen S., Lauesen P., Graver C., Munk P., Holst L., Vestbö B., Svalastoga E., Jacobsen C., Holst B., Steenfeldt S., Buelund L., Hornum T. & Kofoed T. (2012) Reproduction of European Eel in Aquaculture (REEL): Consolidation and New Production Methods. National Institute of Aquatic Resources report, DTU Aqua-report No. 249-2012.

7. A replicated study (year not stated) at a research facility in Denmark (Butts et al. 2014) found that artificially fertilized European eel Anguilla anguilla eggs had higher fertilization rates when sperm to egg ratios were increased and the time between egg collection and fertilization was reduced. Average fertilization rates were higher for sperm to egg ratios ranging from 25,000 to 1,000,000 sperm/egg (57–68%) compared to a ratio of 1,300 sperm/egg (38%). Eggs fertilized immediately after collection had higher average fertilization rates (78%) than those fertilized 15–60 min after collection (35–48%). In one experiment, eggs from six female eels were artificially fertilized at each of 12 sperm to egg ratios (ranging from 1,300 to 1,000,000 sperm/egg). In a second experiment, eggs from three female eels were artificially fertilized (25,000 sperm/egg) at 0, 5, 10, 15, 20, 30, 40 and 60 min after collection. Female eels were caught in a lake. Males were from a commercial eel farm. Hormone injections were used to induce maturation, ovulation and sperm production. In both experiments, 500-g samples of fertilized eggs were reared in containers with seawater at 20°C. After 2–5 h, fertilization rates were calculated for 70 eggs from each of three samples/female/treatment. 

   Study and other actions tested

   Referenced paper

   Butts I.A.E., Sørensen S.R., Politis S.N., Pitcher T.E. & Tomkiewicz J. (2014) Standardization of fertilization protocols for the European eel, Anguilla anguilla. Aquaculture, 426-427, 9-13.

8. A study (year not stated) at a research facility in Japan (Kazeto & Tanaka 2020) reported that treating captive Japanese eels Anguilla japonica with luteinizing hormone resulted in more semen with fewer but more mobile sperm in males and faster egg (oocyte) maturation in females compared to two other hormone types. Unless stated, statistical significance was not assessed. Males treated with luteinizing hormone produced 5–10 times more semen, with significantly fewer but more mobile sperm, compared to males treated with follicle stimulating hormone or human chorionic gonadotropin hormone (data not reported). Egg from females treated with luteinizing hormone reached the nucleus migration phase (when eggs can be collected) in 4–5 weeks, compared to 8–10 weeks for eggs from females treated with follicle stimulating hormone or salmon pituitary extract. Ovaries were 1.2–1.5 times larger in females treated with follicle stimulating hormone compared to luteinizing hormone or salmon pituitary extract. Only luteinizing hormone induced the final stage of egg maturation and ovulation. The study does not report specific methods. 

   Study and other actions tested

   Referenced paper

   Kazeto Y. & Tanaka T. (2020) Study on artificial induction of maturation and ovulation in Japanese eel using the recombinant gonadotropins: Application to the seed production. Nippon Suisan Gakkaishi (Japanese edition), 86, 364-366.

9. A replicated study (year not stated) at a research facility in Japan (Tanaka et al. 2021) found that artificially fertilized Japanese eel Anguilla japonica eggs had variable hatching rates in each of two experiments, and rearing females at different temperatures had no effect on hatching rates of their eggs. Average hatching rates were 1–36% in the first experiment, and 29–80% in the second. In both experiments, average hatching rates of eggs from females reared at different temperatures did not differ significantly (experiment 1: 20°C = 36%, 20/15°C = 22%, 15/20°C = 11%, 15°C = 1%; experiment 2: 20°C = 43%, 20/15°C = 80%, 15/20°C = 29%, 15°C = 67%). In each of two experiments, female eels were reared at four water temperature treatments for three days prior to receiving a hormone injection (17α-hydroxyprogesterone) to induce ovulation (20°C for three days, 20°C for two days then 15°C for one day, 15°C for two days then 20°C for one day, 15°C for three days). All eels received weekly salmon pituitary extract injections for 10–17 weeks prior to treatments. Treatments were assigned randomly (experiment 1: 14 eels/treatment) or according to maturation stage (experiment 2: 2–24 eels/treatment; see paper for details). Eggs were collected from ovulated females, artificially fertilized with sperm from captive males and incubated in petri dishes or microplates at 25°C. Hatched larvae were counted three days after fertilization. 

   Study and other actions tested

   Referenced paper

   Tanaka T., Adachi S., Nomura K., Tanaka H. & Unuma T. (2021) Effects of rearing temperature manipulation on oocyte maturation progress in Japanese eel. Fisheries Science, 87, 681-691.