Breeding tropical marine fish

Over the years I have had countless phone calls from hobbyists that have had some of their fish spawn in their aquarium asking what they should do and how they can raise their new baby fish to adulthood. Firstly let me say congratulations to anyone that has had this happen, fish will usually only breed when they are provided with excellent conditions in which to live and regular spawning is a testament to a hobbyists care. Unfortunately the first spawnings that are seen often catch the hobbyist unawares and unless they happen to have a larval rearing system and gallons of live food tucked away in the garage ready for use there is little chance that any of the brood will survive. However all is not lost, if conditions are stable parental fish will usually spawn again and with some preparation you can be ready to raise your new baby fish to adulthood.

In this article I will cover some of the basics needed to raise larval fish to adulthood and a system that I have used to raise several different species of tropical marine fish to adulthood. In future articles I will cover other topics on fish breeding such as nutrition, live food culture, varieties of larval rearing techniques and fish breeding success stories.

There are numerous considerations when trying to raise fish larvae but most can be split into two categories, nutrition and the environment. I will come back to nutrition a little later but for now, the environment.

Larval rearing systems
Larval fish are usually very different from their parents in their behaviour and requirements, they often spend the first few weeks of their lives as part of the oceans zooplankton drifting in the currents before undergoing metamorphosis into juvenile fish and settling on the reefs. The larval fish gain their nutrition by feeding on great varieties of other zooplankton as they drift around in the currents. The water is clean but stuffed full of nutritious live food which is needed to satisfy the larval fish’s voracious appetites. So how can we replicate this type of environment in an aquarium? This is one of the more difficult aspects of raising larval fish, being able to give a constant source of the right type of nutritious food whilst keeping the water clean.

Larval fish need HUGE amounts of food, they eat constantly by day and the food that they eat must have the correct nutritional profile to meet the daily requirements needed for growth and development. In the ocean larval fish have the pick of a vast array of zooplankton to feed on and they instinctively know what to eat. Unless you are lucky enough to live by tropical seas and collect live plankton on a regular basis you are going to need to culture your own live food and a lot of it! The appetite of larval fish is astounding, when rearing Psuedochromis fridmani larvae I watched 10 day old babies feeding on a rotifer every 12 seconds! It is not easy counting the bite rate of larval fish but if this was a fairly accurate figure then a 10 day old larva would consume over 3,500 rotifers in a single day. Even if you were only trying to raise 300 larvae from a brood you would need over a million rotifers each day to feed them! This brings us to our first rule of larval fish rearing: Never try to raise more fish than you can feed! You are far better off trying to raise fewer fish and be able to feed them properly than raising a larger brood and running out of food. Young larval fish will start to die very quickly if they run out of food and even a space of a few hours without proper nutrition can lead to development problems, stress and death.

Not only this but the concentration of the live food in the larval tank must be of a certain level. Baby fish are very small, often only a couple of millimetres in length and as such they cannot see very far, usually only a few millimetres around them. This means that even if it appears there is quite a lot of live food in the water with the larva, too long can pass before an item of live food passes close enough to the fish to be seen and hence the feeding rate decreases and the larva cannot develop properly.

Rotifers are the usual first live foods used in the rearing of tropical marine fish and as a rule a concentration of 10-15 rotifers per 1ml of aquarium water can be used as the minimum concentration needed. But don’t forget this is the minimum that might be needed for the larva to spot food items regularly enough, on top of this you are going to need the rotifers that are going to be eaten that day. To give an example of how many rotifers you are going to need in one day, if you are using a 45 litre aquarium that has three hundred ten day old P. fridmani larva in it you are going to need 45 X 1,000 x 10 rotifers to get a 10/ml concentration, that equals 450,000 rotifers. On top of this you are going to need another 1 million rotifers for them to eat that day!

So how do you constantly provide nutritious live food at that concentration throughout the day whilst keeping the water clean and filtered? The system diagrams show how I managed to provide this and I will cover some of the techniques I used.  Each of the larval tanks measured 24x12x12 inches and the juvenile tanks used for growing on the young fish after metamorphosis measured 24x15x15 inches and one 30x18x18 inch tank. The tanks on the stand were placed end-on above the sump and larger juvenile tank to conserve space. Each of the larval and juvenile tanks were drilled in the side at the water line and drained into the sump where the water was filtered and returned to the tanks via UV filters.

It is not possible to use any filtration equipment in any of the larval tanks as any pumps would not only suck up and kill the live food but the baby fish as well. Neither could undergravel filters be used for the same reasons. So the only way to filter the water was to flush it through a sump, but this in itself presented problems. Firstly how do you stop the baby fish getting flushed into the sump? I solved this by making large sieves that fitted over the larval tank water outlets. These sieves had a large enough surface area so as not to create too much pressure against them as the water flowed through, this prevented the fry from becoming trapped against the sieves as the water passed out. By controlling the water flow rate into the tank I was able to set up a slow flow through the tanks that flushed out the water but not the fish.

So far so good, I had a circulation system that filtered the water but not the fish. My next problem was with the live food, I had to keep a constant high concentration of live food in the larval tanks at all times, and not only that but the live food had to be highly nutritious.

I will cover live food cultures and nutritional profiles in another article but for now just think of any live food that you offer your baby fish as a parcel and that parcel is only as good as that which it contains. All live food that we use for aquaculture has a nutritional profile that is only as good as the food that it eats. That means that if you do not feed your live food before you feed it to your baby fish it has a very poor nutritional profile, i.e. it is an empty parcel and pretty useless. The main types of live food that are used in tropical marine fish rearing are rotifers, copepods and Artemia. All of these zooplankton feed on phytoplankton and it is by manipulating the phytoplankton that the zooplankton eat that enables us to change their nutritional profiles and provide the right type of diet for the larval fish. This brings us to another rule of fish rearing: Without constant proper feeding live food will quickly become nutritionally poor and useless as a food source for larval fish. This is very important, if you remove a rotifer for example from it’s food source then within an hour it has already lost much of it’s nutritional benefit as a food, within a few hours it is almost useless.

So how do you keep the live food that is offered of a good enough nutritional profile for the baby fish? You can either provide a constant supply of newly enriched highly nutritious food and remove uneaten food as it’s nutritional profile drops or feed the live food on phytoplankton continually whilst it is in with the larval fish. After doing a quick calculation on how many rotifers I would need if I was going to constantly offer new food and flush out old I decided to go with the second option. By adding phytoplankton to the larval tanks with the baby fish and live food I was able to make sure that the live food had the best nutritional profile at all times. However this only worked if the larval tank was isolated from the system otherwise the phytoplankton would be flushed out and filtered from the water. The same problem existed with the live food, although the sieves stopped the larval fish from being flushed out, if I used a sieve mesh fine enough to stop the live food from being flushed out (i.e. 53 microns) the mesh quickly clogged up due to the high particulate matter in the water. The only option was to have the larval tanks isolated and then flushed through periodically to provide fresh clean water.

By isolating the larval tanks I could have larval fish, zooplankton and phytoplankton in together and maintain high concentrations of nutritious food. I would flush the tanks out once every 12 hours until a total water change had occurred, all of the phytoplankton and zooplankton were flushed out but to conserve live food I set up another sieve system in the sump that collected the live food as it was flushed out of the tank. By half submersing a rectangular 53micon sieve underneath the larval tank outlet in the sump I was able to collect the live food as it was flushed out and either return it to the larval tank after the flush-through had finished or into culture vessels.

It is important to make sure that the tanks are flushed through properly every 12 hours to prevent water pollution. Apart from the waste products from the fish the live food itself produces a lot of waste. Rotifers at that concentration very quickly cause ammonia levels in the water to climb, however by flushing through once every 12 hours I was able to keep the ammonia concentration below detectable levels. Bacteria levels can also quickly climb in the larval tanks if they are isolated for too long and this can cause serious problems for young fish. Larval fish take several days before their immune systems develop which leaves them very susceptible to bacterial infections, hence effective water management incorporating UV sterilizers and good protein skimming are essential in this type of larval rearing system.  The filtration that I used was a mix of fluidised sand filters, trickle filters and protein skimming. It is important to have ample biological filtration in a breeding system as there is an awful lot of waste coming not only from the larval and juvenile fish but the live food and extensive wasted dried and frozen foods as the juvenile fish are weaned off of live food and onto readily available prepared fishfoods.

Protein skimming is particularly useful in fish rearing systems due to the high amounts of phytoplankton that were used and the heavy feeding regime, protein skimming also helps to remove some of the bacteria from the water column. After the water had been cycled through the various filtration equipment in the sump it was returned to the larval tanks via UV sterilisers. As far as UV’s go the more the better, bacterial infections are a constant problem to larval fish and the less bacteria in the larval tanks the better. I returned the water to a single 45 litre larval tank via two 55w UV steralisers at a rate of around 2 litres per minute.

When I was rearing baby fish I encountered lots of problems with the larval tanks, it would seem that if you gave any opportunity whatsoever for the tiny babies to kill themselves they would take it! Bare tanks were essential, any type of substrate and the larval fish would easily become trapped and die, and substrates also lead to higher bacterial levels in the tank. Water movement and high dissolved oxygen levels are also essential to maintain. By placing an airline in each corner of the tank I was able to create water movement that helped concentrate the larval fish in the centre of the tank and dissuade them from entering the corners where they could end up swimming endlessly against the glass, becoming ‘trapped’ and die. At first I just used airlines without any airstones that produced a slow trickle of bubbles and this seemed to work quite well. However when I started to get larger numbers of Psuedochromis fry through the first week of their life I seemed to have problems when I got to days 8 and 9 of development. The fry would be developing very well up until this time and I would be getting over 95% of a 300 strong batch through, but when the laval fish reached 8 or 9 days old there would suddenly be an almost complete wipe-out of the brood. It took me several failed broods like this before I realised that it was due to depleted oxygen levels. By the time the larval fish were 8-9 days old they had become much larger and their demands for oxygen and live food were greater. Feeding levels were very high and the combined oxygen demand from both the live food and developing larval fish depleted the dissolved oxygen levels in the water past a critical point and virtually the whole brood would die within a couple of hours.

At first I tried using wooden airstones to provide greater surface area of bubbles and more water movement but I found that the bubbles produced were too small and would often become stuck to the larval fish which caused stress and death. In the end I used small white air diffusers which produced small bubbles but not small enough to become trapped on the fish. Larval fish are very fragile and too high an airflow can cause them physical damage, by using these particular air diffusers and gradually increasing the airflow as the fry developed I was able to meet their oxygen demands without causing any physical damage.

I mentioned earlier about larval fish having problems when they continuously swim against the side of the aquarium, this behaviour is known as head bumping and is a common problem. It can be caused by a number of things but results in the fish swimming continuously against the side or bottom of the aquarium, eventually the fish becomes exhausted and dies. Head bumping is often caused by stress, I would sometimes notice that if a brood had gone for a period of as little as a couple of hours without food a number of the larva would start head bumping and even if new food was introduced into the aquarium they would often continue this behaviour until they died. Another cause of head bumping is due to larval fish being phototropic, that is they are naturally attracted to light and will always move towards it. It is therefore important to black out all the sides of the larval tanks, this prevents light from entering the aquarium through the sides which would otherwise cause the larva to swim towards it and damage or kill themselves against the glass of the aquarium.

With the sides blacked out the tank should be lit from above with a low wattage bulb, I used a 55w power compact tube that lit the central surface area of water. I also found it useful to place a two inch wide frame around the top edges of the aquarium which concentrated the light in the middle and stopped any reflection of light down the insides of the glass. The final and best way to stop head bumping is to make the water green by the addition of phytoplankton which helps stop the reflection of light against the insides of the glass of the aquarium. By doing all of these things I was able to make the larva of various different species of fish congregate in the middle of the aquarium and prevent significant losses through head bumping.

Once the fish had gone through metamorphosis they could be transferred to the juvenile grow-out tanks and weaned onto dried foods. By now the difficult stages of development have passed and the juvenile fish are much hardier. This type of system for raising fish fry is by no means perfect but I found it very effective for raising several species of Clownfish, as well as Cardinals, Damsels, Dottybacks and Seahorses. At it’s best I was able to raise up to 100 Orchid dottybacks a month using the small system described. The system can be adapted to consist of tanks of various sizes to suit your requirements and the fish you are breeding and with a lot of patience and a little bit of luck you will be able to rear your baby fish to adulthood.