By Cecilia C. Vargas*
Among abiotic factors, visual environment is of particular importance for prey detection and foraging, hence affecting larval growth and survival. The visual environment is the result of interactions especially between light intensity, tank color and particle load. In this edition, we will focus on the topic of tank colors used for fish larval rearing.
When working at commercial fish hatcheries and/or research stations I have always wondered why fish are reared either in green, black or white tanks. While it is possible to get a tank in any color, why not to choose yellow for instance, or any color of our preference? Is there any concrete recommendation regarding color when choosing rearing tanks? Literature about possible effects of tank color in fish performance is scarce and limited to larval stages probably because of the critical phase related to the onset of exogenous feeding and prey vulnerability. Some of these experiments test the effect of tank color only while others test combined effects of tank color and light intensity, with variable results.
Fish are generally considered to be visual feeder-hunters and most species present poorly developed vision at hatching. Their sensitivity to light may be species-specific and varies during ontogeny. As tank color is one of the physical parameters that affect fish larvae in detecting and catching their prey, it is therefore important to optimize their rearing conditions. Studies in some marine fish species have related tank color to neural and hormonal processes, behavior and feeding acceptance.
For example, tank color can cause stress in species such as tiger puffer, common carp and tilapia with negative impacts like behavioral alterations, modifications to normal feeding and changes in swimming activity. Walling behavior (crowding against and swimming into tank walls) has been observed in Atlantic cod larvae when they were reared in tanks with white walls. Furthermore these “walling” larvae developed malformed jaws. On the other hand, fish larvae of several species reared in black tanks have shown enhanced larval growth and survival. The positive effects of using black tanks are probably related to the increased contrast between live prey and the tank’s background color.
Contrarily, in haddock larvae, survival was better in larval groups reared in white tanks compared to those reared in black tanks. In addition, growth was impaired in larvae reared in black tanks at low light intensity. The low reflection and transmission of the light at low intensity resulted in poor prey-to-background contrast and probably affected the prey catchability for the larvae in this treatment.
Green tanks are commonly used for several marine species like Atlantic cod, wolf fish and lump sucker although light intensities may vary between these species and there is no report on higher survival and growth compared to tanks of different color. Probably, one of the advantages of using green tanks compared to black tanks may be the ease in seeing dead larvae and uneaten feed resting on the bottom of the tanks which may facilitate cleaning during the daily tending activities.
Some practical recommendations
When using tanks of any color for fish larval rearing, one should register and monitor the light intensity and reflection and adjust these parameters based on larval behavior. Check larval fish’s gut filling as soon as exogenous feeding starts, observe swimming behavior, and try to avoid fish walling behavior.
The addition of micro algae to rearing tanks has been recommended to increase the contrast between the live prey and the surrounding water environment in addition to other benefits. This is helpful in the case of using light colored tanks (white or green). However, avoid increasing the amount of microalgae to diminish the strong reflectance caused by the interaction of white colored tanks and high light intensity. This will only increase the bacterial load in the tanks. Instead, covering the light units with bakery paper of a light brown color may help to reduce the light intensity and even the light reflection.
Cecilia Campos Vargas is currently taking the 3rd and last PhD year at the University of Nordland in BodØ, Norway. She has many years of experience in production of aquatic species like rainbow trout, Atlantic salmon, Japanese species, cod and live feed production.