Larval fish nutrition and live food culture

In my last article ‘Breeding tropical marine fish’, I described a larval rearing system that I have used to raise several species of ornamental marine fish to adulthood and some of the factors that should be addressed when designing and building your own larval rearing system. In this article I will cover one of the most important areas of rearing your baby fish – nutrition.

One of the greatest difficulties to overcome when trying to rear larval fish is providing the right food to enable them to grow and develop properly. The larval stages of different species of marine fish each come with there own nutritional requirements, they often form part of the oceans drifting plankton where they are able to pick and choose their food from the vast array of zooplankton around them. Unfortunately the scientific knowledge of what each species of fish naturally eats at each stage of it’s larval development is greatly lacking. Most of the information on the nutritional requirements of larval marine fish comes from research into fish used for food such as Sea Bass and Flounder. The little knowledge that there is has been applied to rearing some species of ornamental marine fish with some success but the science is still very much in it’s infancy.

Even if we did know what food each and every species of ornamental marine fish larva required during development we would still not be able to provide it for them unless we had access to an unending supply of live plankton at our back door. Instead we have to rely on live food that we can culture ourselves and this limits us largely to rotifers, Artemia (Brine shrimp) and copepods.

Not so many years ago Artemia were the only live food that were readily available to hobbyists but over the last few years rotifers and more recently copepods became available on the hobbyist market, these can all be grown quite easily in small culture vessels to use in larval fish rearing. I will cover the culture of each of these animals individually and along the way share some of the nutritional and feeding considerations for rearing some of the more common species of marine fish that will breed in our aquariums.

Rotifers
For some reason there seems to be this myth about rotifers being difficult to culture, it is simply not true. Rotifers are one of the easiest zooplankton to culture and one of the most important in larval fish rearing. Rotifers can be found in freshwater, brackish and marine environments, the species that are used in marine culture are Brachionus plicatilis (L-type) and Brachionus rotundiformis (S-type). They are small measuring 100-300 microns and naturally form part of the oceans plankton where they are readily preyed upon by a huge range of fish, corals and other filter feeders.

Rotifers are used in aquaculture because they are so easy to raise, they can be grown in almost any container that will hold water, a couple of 5 gallon glass tanks with simple aeration would be ample to culture enough to raise small broods of fish at home. Rotifers should be cultured with fresh seawater mix, try to avoid using water taken from your aquarium as you may contaminate your culture with other organisms. In a healthy rotifer culture all of the individuals will be female and they reproduce asexually with each one having a life span of 7-12 days. If environmental conditions deteriorate the females will produce eggs, if these eggs are not fertilised then they hatch into males which can go on to fertilise other female eggs. These fertilised eggs are covered in several protective layers and are known as resting eggs. It is these eggs that can be purchased dry and then hatched when added to water, however it is no longer necessary to buy rotifers in the egg stage as adult females can now be purchased over the internet in the UK.
Rotifers can be cultured at normal seawater salinity, in fact they can survive at a great range of salinities but you may want to use a salinity around 25ppt as reproduction rates will be slightly higher. Whatever salinity you culture them at should remain stable and if they are being transferred to different salinities then they should be slowly acclimated.

Rotifers have very high digestion rates and will eat almost continually consuming vast amounts of phytoplankton, they can be fed on home cultured phytoplankton but due to the low cell concentration that is usually achieved with home phytoplankton cultures you would need gallons of it just to feed a small culture of rotifers. Fortunately there are a number of ready made highly concentrated rotifer diets available on the market in the UK. Rotifers are usually fed on Nannochloropsis or a mix of Nannochloropsis and Tetraselmis species of phytoplankton, these phytoplankton produce higher reproduction rates and culture concentrations of rotifers. Reed Mariculture in the USA are one of the leading producers of high quality phytoplankton and their rotifers diets are available in the UK.

Due to the high metabolic rates of rotifers and the high concentrations that they can be cultured to, the culture water itself will become polluted quite quickly, although this is not fatal to the rotifers themselves dirty culture water should not be added to larval fish tanks for obvious reasons. Once your rotifer culture has reached harvestable concentrations (above 200 rotifers per 1ml of water) they can be collected with a fine mesh sieve, you will need a 53 micron sieve to collect rotifers from the culture vessel. When collecting rotifers in a sieve they should not be exposed to the air for more than a couple of seconds otherwise they get air trapped around their bodies and cannot submerge again.

Rotifers are the first food that is usually offered to larval marine fish, although there are some species of larval fish that are big enough after hatching to take larger organisms such as artemia or adult copepods as a first food most are unable to ingest or digest such tough large food to begin with.

The nutritional profile of most live foods that we culture is, without proper enrichment, very poor. The body of the live food item has very little nutritional value in itself, it is the stomach contents and the food that is used to enrich the live food that is important. We can think of any item of live food that we offer larval fish as a parcel for delivering the food that we use for enrichment, namely phytoplankton. If you don’t feed and enrich the live food then you are offering the larval fish an empty parcel. As the larval fish develop and get larger then we offer different and larger live food parcels. By manipulating the species of phytoplankton that we feed and enrich the live food with we are able to change the nutritional profiles of the food that our larval fish eat. This however is a complex subject and there has been extensive research on the nutritional requirements of larval fish and enriching live foods particularly for raising species of food fish. Much of this research is beyond the scope of this article and the general hobbyist but we can glean some important information for our home larval fish rearing efforts.

It follows that the nutritional requirements for a particular species of larval fish is the nutritional profile that is found in the egg yolk and it has been found that the nutritional profile of a newly hatched fish larva is very similar to the nutritional profile of the egg yolk.

Chains of fatty acids are a common component of lipids and are essential to fish for a number of reasons, they are used as an effective source of energy as well as being essential in the development of the immune system in larval fish. Fish cannot synthesise some fatty acids themselves so it is important that they are able to obtain them from their food, these are termed essential fatty acids (EFA). The eggs of marine fish have been found to have high levels of DHA (docosahexaenoic acid) which ranges from 10-40% of the total fatty acid content. They also generally have a 2:1 ratio of DHA:EPA (eicosapentaenoic acid) or higher. This suggests that the early development of the larvae requires high levels of DHA for optimal growth and survival. Many larval fish rearing techniques incorporate offering live food with a 2:1 DHA:EPA content.
It is known that the species of phytoplankton Isochrysis has high levels of DHA and Nannochloropsis has high levels of EPA, by manipulating the concentrations of these two phytoplankton that are fed to live food, it is possible to obtain nutritional profiles with varying ratios of fatty acids. In recent years the relevance of the fatty acid arachidonic acid (ARA or AA) in the dietary requirements of larval fish has been considered.

Reed Maricultures website has some excellent information on the nutritional profiles of it’s phytoplankton products that are available to hobbyists in the UK. By mixing the different species of phytoplankton that are used to enrich the live food hobbyists are quite simply able to obtain the required ratios of fatty acids. I have previously used 12 parts Isochrysis to 1 part Nannochloropsis to obtain a 2:1 ratio of DHA:EPA. Different species of fish will have different fatty acid requirements through development, although this information is rather sparse with a little time searching the internet you can find details of the different feeding regimes people have used when raising fish larva and the success they had. Personally I tried various combinations of phytoplankton and live food nutritional profiles when raising larval fish, I found it most effective to offer high levels of DHA in the first 5-9 days of development and then increasing the levels of EPA and proteins after that. I also incorporated the use of the phytoplankton Tetraselmis which is known to contain appetite stimulants.

When using rotifers as a first food for larval fish they need to be kept at a high concentration in the larval tanks, provide at least 10 rotifers per 1ml of larval tank water to ensure that there are enough for the brood to prey on. Larval fish have very small fields of vision and so a high concentration of live food is essential to ensure that each fry can constantly see items to feed on. As your baby fish develop they are going to need larger live food items, this should be done gradually by overlapping the different types of live food and gradually weaning them off of smaller live food and onto larger types. The second type of live food that is usually offered are either Artemia or copepods.

Artemia
Artemia have been used by aquarists for decades, they can be stored as resting eggs for years which can then be hatched by adding them to water. This makes them a very easy and cheap food source for larval fish rearing and they can be hatched out in huge numbers. Artemia can be purchased within their eggs or ‘decapsulated’. There are a number of different brands available with some of the Artemia strains having smaller newly hatched nauplii than others.
When Artemia hatch they leave behind the hard shell of their egg, these egg shells should never be offered to your larval fish as they can choke on them or become lodged in their digestive tracts. It is necessary then to either separate Artemia from the empty shells after they hatch or to remove the shell from them before they hatch. This process of Artemia decapsulation is actually quite simple and consists of dissolving the outer layers of the egg. There are plenty of references on the internet about how to decapsulate your Artemia eggs, or if you prefer you can purchase them already decapsulated. Decapsulating Artemia has several advantages, it helps to clean the eggs as the shells are known to be a source of bacterial contaminants and it also makes it easier for the nauplius to hatch which means they conserve energy and are a better source of nutrition.

Artemia will hatch in newly mixed synthetic seawater at room temperature after 15-20 hours. When hatching it is important that the eggs remain in suspension by aerating the water, eggs that are resting on the bottom and not in suspension will not hatch. There are a number of Artemia hatching vessels available on the market although it is quite simple to make your own out of a plastic drinks bottle. When Artemia hatch they are not able to feed as they have not yet developed a working mouth, this comes after the first moult which occurs 12 hours after hatching. The first larval stage of Artemia is referred to as the INSTAR I nauplius, they will undergo 15 moults before they reach adulthood with each stage representing a different INSTAR stage (INSTAR II, INSTAR III etc). As the Artemia go through each moult they get successively larger and so can quickly become too large for larval fish. Also because Artemia do not feed until they have developed a mouth after the first moult they become less and less nutritious to your larval fish as time goes on during the INSTAR I stage.

Directly after hatching the nutritional profile and fatty acid content of Artemia is actually quite good but within a few hours it has declined drastically. This means that you will need to feed your Artemia to your larval fish as soon after hatching as you can. During this period the Artemia have still not developed a hard carapace and are fairly easy to digest. This presents a problem when using Artemia as a second live food. Your baby fish may be several days old and have been happily feeding on rotifers but they now need a second larger food type. INSTAR II Artemia that can be enriched with phytoplankton are too large so they need to be offered Artemia that are still in the smaller INSTAR I stage, however it is only the very newly hatched Artemia that have a good enough nutritional profile. This means that you need to have several batches of Artemia eggs that are set to hatch at staggered times over the course of a day to enable you to offer your larval fish nutritious Artemia nauplii all the time.

Artemia are phototropic and will constantly swim towards light, it is possible to help newly hatched Artemia conserve energy and lengthen their early nutritional profile by keeping them in the dark. Fortunately within a couple of days your larval fish will have grown and are able to eat the larger INSTAR II nauplii that have developed mouths and have been fed and enriched on phytoplankton. The Artemia can be fed on a range of phytoplankton species to manipulate their nutritional profiles in the same way that rotifers can. The concentration of Artemia that should be used in the culture vessels is going to depend on the species of fish that you are trying to raise and how much they will eat, concentrations that have been successfully used vary from 0.5/ml to 6/ml.
It is well known that Artemia carry pathogenic bacteria on them, using decapsulated Artemia will help to reduce this but even so water used for hatching Artemia and the Artemia themselves carry huge numbers of dangerous bacteria that your larval fish will have little defence against as their immune systems are not developed in early life. Artemia should therefore always be washed before using them as food, there has also been a lot of research looking into ways of disinfecting live food before it is offered to larval fish. Ozone has long been used as a disinfectant for live food but in recent years hydrogen peroxide has been tested for it’s effectiveness as a disinfectant. Tests have shown that hydrogen peroxide based products can remove over 90% of harmful bacteria with no differences in the fatty acid composition of the live food after use. Hydrogen peroxide readily dissociates into oxygen and water and does not release any toxic products that could be harmful for your baby fish. Recent studies on a new hydrogen peroxide-based product Ox-Aquaculture on rotifers and Artemia nauplii survival and on their associated microbial population have shown good results.

It should be remembered that whatever live food you are culturing to feed your larval fish can be a dangerous source of pathogens and cultures should be maintained as cleanly as possible and you may want to disinfect all types of live food before offering them to your larval fish. Although the ease of hatching and using Artemia makes them attractive they are not a natural marine zooplankton and their tough carapace makes them difficult for young fry to digest. Accompanied by a poor nutritional profile and high pathogen levels we could do with a better second food for larval fish rearing. Fortunately in recent years copepods have become much more available.

Copepods
Copepods form a large part of the oceans zooplankton and are a major source of live food for larval fish in the wild. In recent years harpacticoid copepods have become available on the market in the UK, the most common being Tigriopus californicus. These copepods are easy to culture in any small glass tank or container using newly mixed synthetic seawater and gentle aeration, they can be kept at a range of salinities and at room temperature. These copepods can again be cultured on phytoplankton although larger species such as Isochrysis sp. are preferable over smaller phytoplankton such as Nannochloropsis sp. Unlike rotifers copepods reproduce sexually, they have a lengthy life cycle and can live for several weeks.
The nauplii of copepods are very small and can often be used as a first food for larval fish where rotifers are too large. There has been quite a lot of research lately looking into the usage of copepod nauplii as a first food for some species of ornamental marine fish such as Angelfish that are unable to take rotifers as a first food. The adult stages of copepods such as Tigriopus sp are of a similar size to artemia nauplii and make a far better food source than artemia do. Research has shown that larval fish fed on copepods as a second food develop better and with less mortality than larval fish fed on artemia as a second food. Many Seahorse breeders now rely on copepods to raise juveniles as the health and survival is far better than when using Artemia.

The only down side to using copepods or copepod nauplii instead of Artemia or rotifers is that they are slightly more difficult to culture and require more space to raise large numbers of them. However the benefits of using copepods particularly on small scale larval rearing efforts is undeniable. If you cannot produce enough copepods to adequately feed all of your baby fish then using a combination of rotifers, copepods and Artemia should produce far better results than using rotifers and Artemia alone.
The numbers of species of ornamental tropical marine fish that have been successfully reared to adulthood in captivity has greatly increased in recent years, much of this has been due to advances in live food culture and an understanding of larval fish nutrition. It is now possible for hobbyists to raise many of the species of fish that are kept in aquaria at home themselves and your experiences will only add to the pool of knowledge we have. A good understanding of your fishes nutritional requirements is essential, for those of you that would like some further reading on live food culture and larval fish nutrition I would recommend a book called ‘The Plankton Culture Manual’ by Frank Hoff & T. Snell. Published by Florida Aqua Farms (5th edition).

Being able to stock our aquariums with captive bred fish is a step in the right direction especially when we bear in mind how much damage is being done to the worlds reefs. I would encourage everyone to buy captive raised fish if they are available and if you do decide to try and raise some fish yourselves then good luck, it is not the easiest area of aquaculture but I have personally found it the most rewarding by far.