Breeding Garden Eels

[ladydave]

Hello

My Garden Eels have been spawning kind of semi-regularly for a few years now. I have caught and hatched the eggs a few times but never been able to get the larvae past 11-14dph. I'm assuming it's down to what I'm feeding them.
I have one batch parvo calanus pods but they didn't survive past 11dph. I've given another batch rotifers and tiger pod naups and a few got to 13-14dph. I tried another batch on a paste made from egg yolk, crushed lobster eggs and masstick, only got a few days out of that batch.
What I think I'd like to try this time (I have around 100 larvae now at 1dph) is enriching the rotifer and pod naups with something. Mainly because I cannot get hold of parvo calanus for love nor money in the U.K. anymore. Any ideas on what to enrich them with (needs to be available in the U.K.) ?

Unless anyone has any better ideas? I've researched this to death but can literally find zero literature on raising the larvae. Except one webpage that made it sound bizarrely incredibly simple.

Can anyone offer any ideas at all?

 

[Jay Hemdal]

Hello

My Garden Eels have been spawning kind of semi-regularly for a few years now. I have caught and hatched the eggs a few times but never been able to get the larvae past 11-14dph. I'm assuming it's down to what I'm feeding them.
I have one batch parvo calanus pods but they didn't survive past 11dph. I've given another batch rotifers and tiger pod naups and a few got to 13-14dph. I tried another batch on a paste made from egg yolk, crushed lobster eggs and masstick, only got a few days out of that batch.
What I think I'd like to try this time (I have around 100 larvae now at 1dph) is enriching the rotifer and pod naups with something. Mainly because I cannot get hold of parvo calanus for love nor money in the U.K. anymore. Any ideas on what to enrich them with (needs to be available in the U.K.) ?

Unless anyone has any better ideas? I've researched this to death but can literally find zero literature on raising the larvae. Except one webpage that made it sound bizarrely incredibly simple.

Can anyone offer any ideas at all?

That's very cool! I've kept garden eels for 20+ years and never saw them spawn for me.

As you're aware, it is the Leptocephalus larvae that create the difficulty with raising eels. You might skip looking for info about raising garden eels and search for raising Anguilla, food eels, and see if you can extrapolate information from that. However, most/all of the eel farms that I'm aware of though, start with wild caught elvers and raise them from that point.

Jay

 

[ladydave]

That's very cool! I've kept garden eels for 20+ years and never saw them spawn for me.

As you're aware, it is the Leptocephalus larvae that create the difficulty with raising eels. You might skip looking for info about raising garden eels and search for raising Anguilla, food eels, and see if you can extrapolate information from that. However, most/all of the eel farms that I'm aware of though, start with wild caught elvers and raise them from that point.

Jay

I have found information on raising glass eels which I think are the food eels you mention? They said they use a paste made from shark eggs. Which I have no chance of replicating. It also gave no information on how dense the paste should be, how often to feed etc.

I feel like I've really hit a brick wall!

 

[ladydave]

I have found information on raising glass eels which I think are the food eels you mention? They said they use a paste made from shark eggs. Which I have no chance of replicating. It also gave no information on how dense the paste should be, how often to feed etc.

I feel like I've really hit a brick wall!

A quick search has found this disappointing piece of info...


"Because of the complex lifecycle of the eel, breeding stock in captivity is something that is not possible as yet. Eels spend months travelling to the spawning grounds to breed in the sea.

So far, artificial ways of breeding eels for commercial farming purposes has not been successful. Until methods are discovered as to how to breed hatchings from the eel spawn, the industry will not be truly sustainable and will always depend on the supply of wild glass eels for its success."

 

[ISpeakForTheSeas]

I have found information on raising glass eels which I think are the food eels you mention? They said they use a paste made from shark eggs. Which I have no chance of replicating. It also gave no information on how dense the paste should be, how often to feed etc.

I feel like I've really hit a brick wall!

The info in my quote below might be helpful, but this is one of the most pertinent quotes from the link:

"Glass eels there were formerly fed on expensive shark eggs, but these are now being substituted with a feed based on chicken eggs and fishmeal to reduce costs."

Closed-cycle eel breeding advancing in Japan

Closed-cycle breeding of glass eels is advancing in Japan, where preparations of eel such as kabayaki are culinary specialties.

www.seafoodsource.com

Not to derail the thread here too badly, but Japanese Eel (Anguilla japonica) have been aquacultured successfully (though the settlement rates are only about 10% from what I've read*) - they haven't been able to figure out how to scale it up enough for commercial production yet, though, and it's pretty expensive. I know at least the Japanese Eel is confirmed to be a polycyclic spawner (meaning they spawn multiple times, so they don't just spawn once then die) - a recent finding also suggests that this may be the case for the European Eel (Anguilla anguilla) as well.** Some morays are protandrous hermaphrodites (they start as males and end as females), but others have been shown to be simultaneous hermaphrodites (being both male and female at the same time).***

*Japanese Eel aquaculture info sources:

Closed-cycle eel breeding advancing in Japan

Closed-cycle breeding of glass eels is advancing in Japan, where preparations of eel such as kabayaki are culinary specialties.

www.seafoodsource.com

Reinventing the eel: Can the Anguilla loop be closed? - Responsible Seafood Advocate

Japanese scientists have been trying to replicate spawning conditions for eels in captivity for decades. They’re getting closer, but major hurdles remain.

www.globalseafood.org

Reproductive farming technology in Japanese eel and chub mackerel

Japanese eel (Anguilla japonica) and chub mackerel (Scomber japonicus) are commercially valuable species in Asian aquaculture. The reduction in eel an…

www.sciencedirect.com

**Polycyclic spawning info source:

Histological Evidence of Multiple Spawning in Wild Female Japanese Eel Anguilla japonica - PMC

The post ovulatory follicle (POF) is an important and reliable tissue structure used to investigate the spawning history in teleost fish. Fresh POFs shortly after spawning are comprised of cellular (follicular cells) and acellular (basement membrane ...

www.ncbi.nlm.nih.gov

***Muraenidae Hermaphroditism info source:

Hermaphroditism in fishes: an annotated list of species, phylogeny, and mating system - Ichthyological Research

Fewer than 1% of vertebrate species are hermaphroditic, and essentially all of these are fishes. Four types of hermaphroditism are known in fishes: simultaneous (or synchronous) hermaphroditism (SH), protandry (male-to-female sex change; PA), protogyny (female-to-male sex change; PG), and...

link.springer.com

 

[ISpeakForTheSeas]

A quick search has found this disappointing piece of info...


"Because of the complex lifecycle of the eel, breeding stock in captivity is something that is not possible as yet. Eels spend months travelling to the spawning grounds to breed in the sea.

So far, artificial ways of breeding eels for commercial farming purposes has not been successful. Until methods are discovered as to how to breed hatchings from the eel spawn, the industry will not be truly sustainable and will always depend on the supply of wild glass eels for its success."

Just to add (I believe this info is hidden in one of the links above, but I don't remember for sure) the Japanese Eel (Anguilla japonica) has been reared through multiple generations in captivity before (i.e. they've had wild brood spawn, they've reared the young and had those spawn, and they've reared the second generation of spawn to have them spawn too).

Edit: also what are you doing to care for the broodstock? Willing to share any info on your setup for them? For feeds to enrich the pods and rots, I'd try Nannochloropsis and/or Tetraselmis first and see how that does. If it doesn't help at all, I'd try switching to Isochrysis (T-iso), Chaetoceros, and/or Thalassiosira. You could also try Skeletonema and/or Pseudonitzschia, but those are likely going to be much harder to come by and culture.

 

[ladydave]

Just to add (I believe this info is hidden in one of the links above, but I don't remember for sure) the Japanese Eel (Anguilla japonica) has been reared through multiple generations in captivity before (i.e. they've had wild brood spawn, they've reared the young and had those spawn, and they've reared the second generation of spawn to have them spawn too).

Edit: also what are you doing to care for the broodstock? Willing to share any info on your setup for them? For feeds to enrich the pods and rots, I'd try Nannochloropsis and/or Tetraselmis first and see how that does. If it doesn't help at all, I'd try switching to Isochrysis (T-iso), Chaetoceros, and/or Thalassiosira. You could also try Skeletonema and/or Pseudonitzschia, but those are likely going to be much harder to come by and culture.

Thank you! I can't get hold of any pure T-iso, same as I can't find Parvo Calanus pods but I have got a mix of Nannochloropsis, tetraselmis, isochrysis, thalassioria and pavlova being delivered.

As far as my broodstock goes. (If my phone autocorrects that one more time to broomstick, I will scream!!!) I have 5 adults, 3 male and two females in my seahorse tank. They eat literally everything I put in there, including flake food. But they eat mainly a diet of PE mysis and whatever bits of the Gamma Marine Cuisine they fancy. SG 1.023, temp 22-23degrees Celsius. Sandbed of minimum 4 inches, rising to about 6 at the back. But they're happy in the 4" at the front.

 

[ladydave]

The info in my quote below might be helpful, but this is one of the most pertinent quotes from the link:

"Glass eels there were formerly fed on expensive shark eggs, but these are now being substituted with a feed based on chicken eggs and fishmeal to reduce costs."

Closed-cycle eel breeding advancing in Japan

Closed-cycle breeding of glass eels is advancing in Japan, where preparations of eel such as kabayaki are culinary specialties.

www.seafoodsource.com

Thank you so much for sending that.

I think I've found something similar before but I just wish they were more specific. Is it boiled chicken eggs? Yolk straight from the egg, no cooking at all? What fish meal? What percentage is egg and what percent is fish meal? How much do they feed? Everything seems so secretive or like they expect everyone to know what they're talking about!

 

[Xenia]

Perhaps from the freshwater department liquifry never hurt to try i think ?
1 or 2

 

[ladydave]

Perhaps from the freshwater liquifry never hurt to try i think ?
1 or 2

I tried that before as well. But it seems that's more for boosting infusoria than feeding directly to the larvae.

 

[ISpeakForTheSeas]

Thank you so much for sending that.

I think I've found something similar before but I just wish they were more specific. Is it boiled chicken eggs? Yolk straight from the egg, no cooking at all? What fish meal? What percentage is egg and what percent is fish meal? How much do they feed? Everything seems so secretive or like they expect everyone to know what they're talking about!

Sorry, been following a bunch of links to try and find as much info as I could (sorry it's kind of messy, there's going to be a bunch of links I drew info from at the bottom - all of this info pertains primarily to the Japanese Eel, Anguilla japonica). The links at the bottom have some good insights not listed here, particularly on how to deal with feeding slurry type feeds which have a tendency to lead to jaw deformities (which can largely be avoided using a kreisel tank) and bacterial issues.

For the original diet, they used a slurry-type feed with low-temperature dried powdered shark eggs from the Spiny Dogfish, Squalus acanthias, as the base. It also contained oligopeptides (I know very little but about chemistry, but I'd presume soy peptides, as they later developed soy peptides with "a less phytic acid effect" for use in the feed to allow for the larvae to use the vitamins and minerals better), vitamins and minerals, and (Antarctic) krill extract fluid. At some point, they started freezing the shark eggs (I'm assuming there was something of a translation issue here as it just says: "Meanwhile, frozen shark eggs before the drying process were used in place of the manufacture of low-temperature dried powdered shark eggs (the principal ingredient of conventional feed)." - I would guess this means that they used frozen rather than fresh shark eggs to be dried and powdered, but I don't know for sure). This slurry was initially fed 4 times daily, but they moved to feeding it 5 times a day. They also found that only giving the larvae access to the feed for limited amounts of time at each feeding improved the feeding response.

Chicken eggs have been used to rear the larvae successfully, but with notably lower survival rates than the shark eggs. Also, it was found that diets with lower lipid content (essentially diets with less fat) had higher success rates (they tested this using - in order from most successful to least successful - "defatted shark eggs," regular shark eggs, defatted hen egg yolks, and regular hen egg yolks).

As an interesting note here, the larvae have been kept until 26 days post hatch by giving them cow milk baths for feeding times - as in the young were literally moved into a tub of milk for a time, then moved back into the rearing tank. This seems to have gotten the right people interested in using a liquid diet, as this method of feeding prevents the jaw deformities caused by the slurry feedings, and new liquid feeding methods have better success rates than the original slurry feeding methods do. There is a chance that the two links separated from the rest at the bottom mention specifically what is used in this diets, but the full articles are behind paywalls, so I'm not sure.

Knowing what all goes into the diet may be particularly helpful, but otherwise, some of the experts on the forum here may be able to help you piece together some additives/substitutes you could use to improve the nutrition of an egg yolk based diet for the larvae using the info about the original diet here.

In the wild, it seems anguillid eel larvae feed on marine snow (essentially particulate organic matter), notably zooplankton fecal pellets and larvacean housings (basically the tunics of free-swimming tunicates). I have no idea how these measure up nutritionally, but it may be an interesting thing for someone to experiment with.

Anyway, some potentially useful related info here: after the eels hit 50mm in length, their settlement/metamorphosis would be triggered by starving them for a week (if not starved, some larvae went 200 days before metamorphosis). There may be an "optimal initial feeding time" that is determined by the reserves of the larval egg sac. The larvae of the Japanese Eel would swim away from light - so they like darker conditions - and they also had better growth and survival at 17.5ppt salinity (brackish water) than 35ppt (marine) salinity, but I doubt this would be true for garden eels too.

Progression in artificial seedling production of Japanese eel Anguilla japonica - Fisheries Science

Eel aquaculture, though thriving nowadays, is totally dependent on the successful capture of wild eel fry and glass eels for its seedlings. The declination of eel resources in recent years has resulted in an urgent need for technology development in artificial seedlings production on an...

link.springer.com

https://onlinelibrary.wiley.com/doi/full/10.1111/***.12086

Decreasing dietary lipids improves larval survival and growth of Japanese eel Anguilla japonica - Fisheries Science

Shark eggs-based diet is the only diet by which eel larvae can grow to glass eels in captivity. However, the high level of lipids in the diet is suggested to negatively affect eel larvae. This paper examines the effect of defatted shark eggs (DSE) and hen egg yolk (HY) on growth and survival of...

link.springer.com

*The liquid diet links:

A new liquid-type diet for leptocephali in mass production of artificial glass eels - Fisheries Science

Development of artificial production of glass eels still presents many problems to be solved. What and how to feed larvae is one of the most crucial problems for commercial mass production of glass eels. We evaluated a new liquid-type diet and feeding method against the conventional slurry-type...

link.springer.com

Long-term rearing of Japanese eel larvae using a liquid-type diet: food intake, survival and growth - Fisheries Science

The efficiency of a new liquid-type diet for long-term rearing of Japanese eel larvae until metamorphosis was examined, as was the effect of diet viscosity on diet intake and on the survival and growth of early larvae. The highest intake of the experimental diet by 6- and 9-day post-hatch (dph)...

link.springer.com

 

[ISpeakForTheSeas]

Should have added this, but I would personally say a liquid diet would be ideal, but a slurry diet may be easier. However, I would only try the slurry diet with a kreisel tank to try and avoid any issues such as the jaw deformities above, and I would look at how to clean up after each feeding (the links above discuss this) to avoid bacterial issues.

Also, light and light periods are known to have some effect on the spawning of eels, so what lights and light period do you have for your tank? Does your tank get exposed to any other light (such as sunlight) throughout the day?

Edit: and here is a link with super specific, detailed info on the slurry diet (Table 1):

http://nevepaling.nl/files/Image/nederlands/informatiecentrum/tanaka_glass_eels_japans_onderzoek/tanaka_glass_eels.pdf

 

[JPM San Diego]

Have you reached out to Biota or ORA? I would bet they consider their techniques proprietary. However, maybe they would be willing to share ideas with you in exchange for you giving them your protocol if you are successful. You could ask to be an offsite part of their R & D department.

 

[ladydave]

Sorry, been following a bunch of links to try and find as much info as I could (sorry it's kind of messy, there's going to be a bunch of links I drew info from at the bottom - all of this info pertains primarily to the Japanese Eel, Anguilla japonica). The links at the bottom have some good insights not listed here, particularly on how to deal with feeding slurry type feeds which have a tendency to lead to jaw deformities (which can largely be avoided using a kreisel tank) and bacterial issues.

For the original diet, they used a slurry-type feed with low-temperature dried powdered shark eggs from the Spiny Dogfish, Squalus acanthias, as the base. It also contained oligopeptides (I know very little but about chemistry, but I'd presume soy peptides, as they later developed soy peptides with "a less phytic acid effect" for use in the feed to allow for the larvae to use the vitamins and minerals better), vitamins and minerals, and (Antarctic) krill extract fluid. At some point, they started freezing the shark eggs (I'm assuming there was something of a translation issue here as it just says: "Meanwhile, frozen shark eggs before the drying process were used in place of the manufacture of low-temperature dried powdered shark eggs (the principal ingredient of conventional feed)." - I would guess this means that they used frozen rather than fresh shark eggs to be dried and powdered, but I don't know for sure). This slurry was initially fed 4 times daily, but they moved to feeding it 5 times a day. They also found that only giving the larvae access to the feed for limited amounts of time at each feeding improved the feeding response.

Chicken eggs have been used to rear the larvae successfully, but with notably lower survival rates than the shark eggs. Also, it was found that diets with lower lipid content (essentially diets with less fat) had higher success rates (they tested this using - in order from most successful to least successful - "defatted shark eggs," regular shark eggs, defatted hen egg yolks, and regular hen egg yolks).

As an interesting note here, the larvae have been kept until 26 days post hatch by giving them cow milk baths for feeding times - as in the young were literally moved into a tub of milk for a time, then moved back into the rearing tank. This seems to have gotten the right people interested in using a liquid diet, as this method of feeding prevents the jaw deformities caused by the slurry feedings, and new liquid feeding methods have better success rates than the original slurry feeding methods do. There is a chance that the two links separated from the rest at the bottom mention specifically what is used in this diets, but the full articles are behind paywalls, so I'm not sure.

Knowing what all goes into the diet may be particularly helpful, but otherwise, some of the experts on the forum here may be able to help you piece together some additives/substitutes you could use to improve the nutrition of an egg yolk based diet for the larvae using the info about the original diet here.

In the wild, it seems anguillid eel larvae feed on marine snow (essentially particulate organic matter), notably zooplankton fecal pellets and larvacean housings (basically the tunics of free-swimming tunicates). I have no idea how these measure up nutritionally, but it may be an interesting thing for someone to experiment with.

Anyway, some potentially useful related info here: after the eels hit 50mm in length, their settlement/metamorphosis would be triggered by starving them for a week (if not starved, some larvae went 200 days before metamorphosis). There may be an "optimal initial feeding time" that is determined by the reserves of the larval egg sac. The larvae of the Japanese Eel would swim away from light - so they like darker conditions - and they also had better growth and survival at 17.5ppt salinity (brackish water) than 35ppt (marine) salinity, but I doubt this would be true for garden eels too.

Progression in artificial seedling production of Japanese eel Anguilla japonica - Fisheries Science

Eel aquaculture, though thriving nowadays, is totally dependent on the successful capture of wild eel fry and glass eels for its seedlings. The declination of eel resources in recent years has resulted in an urgent need for technology development in artificial seedlings production on an...

link.springer.com

https://onlinelibrary.wiley.com/doi/full/10.1111/***.12086

Decreasing dietary lipids improves larval survival and growth of Japanese eel Anguilla japonica - Fisheries Science

Shark eggs-based diet is the only diet by which eel larvae can grow to glass eels in captivity. However, the high level of lipids in the diet is suggested to negatively affect eel larvae. This paper examines the effect of defatted shark eggs (DSE) and hen egg yolk (HY) on growth and survival of...

link.springer.com

*The liquid diet links:

A new liquid-type diet for leptocephali in mass production of artificial glass eels - Fisheries Science

Development of artificial production of glass eels still presents many problems to be solved. What and how to feed larvae is one of the most crucial problems for commercial mass production of glass eels. We evaluated a new liquid-type diet and feeding method against the conventional slurry-type...

link.springer.com

Long-term rearing of Japanese eel larvae using a liquid-type diet: food intake, survival and growth - Fisheries Science

The efficiency of a new liquid-type diet for long-term rearing of Japanese eel larvae until metamorphosis was examined, as was the effect of diet viscosity on diet intake and on the survival and growth of early larvae. The highest intake of the experimental diet by 6- and 9-day post-hatch (dph)...

link.springer.com

If I could like this post more than once, I'd like it 100 times!! Thank you SO SO much for all this incredible information!! I cannot believe how much you e managed to find out and I am so incredibly grateful!!

I'm going to have a good read and see what I can come up with to feed them. The milk bath thing is really fascinating!!!

 

[ladydave]

Should have added this, but I would personally say a liquid diet would be ideal, but a slurry diet may be easier. However, I would only try the slurry diet with a kreisel tank to try and avoid any issues such as the jaw deformities above, and I would look at how to clean up after each feeding (the links above discuss this) to avoid bacterial issues.

Also, light and light periods are known to have some effect on the spawning of eels, so what lights and light period do you have for your tank? Does your tank get exposed to any other light (such as sunlight) throughout the day?

Edit: and here is a link with super specific, detailed info on the slurry diet (Table 1):

http://nevepaling.nl/files/Image/nederlands/informatiecentrum/tanaka_glass_eels_japans_onderzoek/tanaka_glass_eels.pdf

The larvae are currently in a 20litre kreisel tank with black sides. I've put thick black paper on the front and back leaving about an inch of the glass exposed at the top and I put white paper over the opening at the top and just have a normal ikea lamp shining on it.
The room it's in is not in direct sunlight but does get daylight throughout the day but there's not a lot that gets in to their tank from the exposed inch at the top.

 

[ISpeakForTheSeas]

If I could like this post more than once, I'd like it 100 times!! Thank you SO SO much for all this incredible information!! I cannot believe how much you e managed to find out and I am so incredibly grateful!!

I'm going to have a good read and see what I can come up with to feed them. The milk bath thing is really fascinating!!!

I hope it helps - and one last thing I just found, while the growth rate wasn't great, the Japanese Eel may be able to be reared on pico-plankton such as Synechococcus spp. cyanobacteria.

Observation of the feeding behaviour of reared Japanese eel Anguilla japonica leptocephali fed picocyanobacteria Synechococcus spp - PubMed

The authors observed the feeding behaviour of artificially reared Japanese eel Anguilla japonica leptocephali, 7.5-19 mm total length (10-61 days post-hatch), fed Synechococcus sp., which is considered a potential food source of anguilliform larvae. Three strains of Synechococcus sp. (NIES-972...

pubmed.ncbi.nlm.nih.gov

 

[ladydave]

Have you reached out to Biota or ORA? I would bet they consider their techniques proprietary. However, maybe they would be willing to share ideas with you in exchange for you giving them your protocol if you are successful. You could ask to be an offsite part of their R & D department.

A friend who ran one of my lfs actually contacted a few people for me a couple of years ago.

He contacted Kevin Erikson, President of MACNA, Til Deus Biota and BOCAS Aquaculture, Tal Sweet President and co founder Marine breeding Initiative MBI, Matthew Pederson 2009 Aquarist of the year. Breeder of over 20 species of marines.

Either they did not know or didn't get back to him. You're right though - it might be worth trying again.

 

[ladydave]

I hope it helps - and one last thing I just found, while the growth rate wasn't great, the Japanese Eel may be able to be reared on pico-plankton such as Synechococcus spp. cyanobacteria.

Observation of the feeding behaviour of reared Japanese eel Anguilla japonica leptocephali fed picocyanobacteria Synechococcus spp - PubMed

The authors observed the feeding behaviour of artificially reared Japanese eel Anguilla japonica leptocephali, 7.5-19 mm total length (10-61 days post-hatch), fed Synechococcus sp., which is considered a potential food source of anguilliform larvae. Three strains of Synechococcus sp. (NIES-972...

pubmed.ncbi.nlm.nih.gov

Wow!! And there was me about to chuck a load of chemiclean in my tank!! Although I bet the cyano I have problems with isn't the right strain!

 

[ladydave]

"We recently identified a food item that is palatable to eel larvae, freeze-dried shark egg yolk (Aquaran, BASF Japan).. This is a commercially available product used to improve the nutritional value of food organisms for the culture of marine fish larvae. "


Anyone know where I can buy some Aquaran??

 

[ISpeakForTheSeas]

"We recently identified a food item that is palatable to eel larvae, freeze-dried shark egg yolk (Aquaran, BASF Japan).. This is a commercially available product used to improve the nutritional value of food organisms for the culture of marine fish larvae. "


Anyone know where I can buy some Aquaran??

Unfortunately, given how shark reproduction works, I think you'd be better off using regular chicken eggs despite their apparently inferior nutrition. That said, I found a study with more info (they compiled pretty much all of the diet info I had above and a couple more points):

"It was found that it was useful to add carbohydrates in the form of N-acetylglucosamine, glucose or maltose (see Okamura et al. 2014)."

"The optimum dietary level of the highly unsaturated fatty acid, arachidonic acid, for Japanese eel larvae was also recently determined (Shahkar et al. 2016)." (It's between 0.71% and 0.92%, but, honestly, I doubt this will be helpful info for a hobbyist.)

"Research showed that starting to feed the larvae at 5 days after hatching was advantageous compared to initial feeding at 7 or 8 days (Jinbo et al. 2013)."

Also, rather than using krill extract, one may be able to just use skinned krill, as it seems the excess fluoride in the shell is what caused enough issues for them to use krill extract instead.

Source:

The mysterious feeding ecology of leptocephali: a unique strategy of consuming marine snow materials - Fisheries Science

Leptocephalus larvae have transparent bodies with tubular intestines that usually lack identifiable food items when they are collected, so mystery has surrounded efforts to determine what they feed on. Artificially spawned and reared first-feeding larvae were found to be highly selective in what...

link.springer.com