Chapter 6: Larval Management in the Hatchery
After hatching, larvae should be held in aquaria, tanks or hapas, until the larvae have developed enough to be able to swim horizontally and have functional mouthparts. A continuous water flow should be provided when using aquaria or tanks to help maintain water quality and wash wastes away. Aquaria have the advantage in that the larvae are easier to observe. Things to observe include: how the larvae are being carried in the water currents, how fast the larvae are developing, and the swimming pattern of the larvae.
Larvae with large yolk sacs typically will sink to the bottom of the tank and initially cannot swim up into the water column but are able to wiggle along the bottom. This may result in larvae piling up in the corners of the tank and suffocation of some larvae. Semi-buoyant larvae need enough water current to keep the larvae up in the water column, otherwise they may settle on the bottom, get covered with sediment, or pile up in corners of the tank and suffocate. Placing air stones in the tank corners and providing enough aeration to create a water current to wash the larvae out of the corners can control this. Water currents created as part of the water exchange or through aeration can keep semi-buoyant larvae suspended in the water column. Semi-buoyant larvae will initially try to swim up in the water column then drift back toward the bottom. The deeper the tank the easier it is to keep semi-buoyant larvae suspended in the water column. The current pattern should wash any larvae off the bottom of the tank and up into the water column, keeping them from piling up in corners. The current pattern should not be too strong, causing the larvae to expend too much energy swimming against it. The more energy spent swimming, the less energy is available for larval development.
The larval holding tank should have a screen with enough screen area to minimize water pressure on the screen and reduce impingement. The mesh should be large enough to allow a good water flow and not clog easily but not allowing larvae to pass through. The best mesh size will vary with the fish being cultured. Cylindrical screens placed over a standpipe are effective especially when an air diffuser ring is attached to the base of the cylinder so air bubbles will rise up along the screen surface helping to reduce impingement of fish larvae and debris on the screen. In addition, screens should be cleaned at least twice daily to remove any debris accumulation. One of the most common problems during the larval holding phase is screens getting clogged resulting in tanks overflowing and larvae being lost.