Since 2001 the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) has been developing a range of novel feeding attractants to support the development of sustainable aquaculture. Feeding and growth trials at commercial aquaculture facilities show that attractants based on synthetically produced fish pheromones induce increased feeding activity in species such as the European cod, penaeid shrimp and tilapia resulting in higher rate of growth and feed uptake.

What are pheromones?

Pheromones are the molecules used for communication between animals. They are known to be involved in the behaviour of a wide range of animals from ants and moths through to mammals.

The word pheromone comes from the Greek pherein, to carry or transfer and hormōn, to excite or stimulate. In fish, “pheromone” is defined as substances, or specific mixtures of substances, which are released by individual organisms into the environment where they evoke specific adaptive and largely-innate biological responses in exposed con-specifics.

Pheromones, odorants and chemical cues are considered to play a role in shoaling, fish aggregations, social interactions (dominance), kin recognition, prey detection migration and signalling the presence of predators. More recent research, which is directly applicable to aquaculture, has demonstrated that a number of pheromones and chemical cues can induce increased feeding activity in a range of freshwater and marine fish species. In the majority of fish species the pheromones are released to the environment within the urine or across the gills. In all cases the fish receiving the pheromones detect them via the sense of smell (olfaction).

Olfaction is crucial to the detection of the pheromones and so the feeding attractants must be in a soluble liquid form. In most fish species the olfactory organs are well developed and in environments where light may be limited and vision restricted by the clarity of the water the sense of smell plays the major role in communication. The pheromones and other cues are extremely potent odorants and are detected at very low concentrations. For instance, a number of the known reproductive pheromones are detected by the sense of smell at concentrations as low as 10-12 M, and so minute amounts of these compounds are required to produce the necessary behaviourn the fish.

Developing Attractans

The development of the pheromone-based feeding attractants for aquaculture involves a number of stages involving a wide range of analytical and behavioural assays and techniques. The main stages involved are:

• Isolation of the pheromone / feeding cue
• Identification of the pheromone / feeding cue
• Production of a synthetic version of the pheromone / feeding cue
• Olfactory bioassay of the pheromone / feeding cue
• Laboratory-based behavioural bioassa to confirm feeding response
• Laboratory-based feeding and growth studies on key species
• Large-scale growth trials under commercial aquaculture conditions.

Once a compound has been confirmed as a feeding attractant then a synthetic version is produced commercially for large scale testing within the aquaculture industry. As each of the compounds is produced synthetically, the feeding attractants and commercial formulations do not contain any material directly derived from biological matter. A full environmental assessment of the feeding attractants for aquaculture has been completed and they pass a number of U.K. Government and European Commission Directives relating to the aquatic environment. The principal components of the feeding attractants have a low biological activity and degrade rapidly within the natural environment (half-life < 12 hours). The environmental assessment indicated that the feeding attractants have no harmful effects on the aquatic environment.

Application

The most effective way of applying the feeding attractants within an aquaculture environment is to spray the formulations as a liquid preparation onto the surface of the water two to five minutes prior to the feeding event. This allows the detection of the feeding attractants by the olfactory system and the release of the appropriate feeding behaviour. As a result there is an increase in the general “readiness” of the fish to feed so that on the addition of the commercial pellets the food is readily consumed. This approach has a number of advantages, the most important being that the feeding attractants are not surface coated on the feeds and are therefore not consumed. This reduces the potential for bioaccumulation of the compounds and a significant reduction in tissue residues. In addition, it can be argued that because the fish does not consume the feeding attractants, then they can be considered not to be food additives and as such are not covered by existing legislation. However, this needs to be confirmed by all the main aquaculture producing countries.

Feeding trials

Feeding attractants and formulations have been developed for a number of key aquaculture species and preliminary commercial feeding trials have now been completed with these products. These fish and shellfish species studies are the following, which have been tested in hatcheries and production facilities in Norway, China and Thailand:

1. European cod (Gadus morhua)
2. Tilapia (Oreochromis niloticus)
3. Crucian carp (Carassius carassius)
4. Whiteleg shrimp (Litopenaeus vannamei)

The principal findings from the commercial trials are as follows:

1. European Cod (Gadus morhua). The trial was conducted at the Stolt Sea Farm A/S pre-ongrowing facility at Tustna in Norway over a three-month period. The feeding attractant formulation was applied as a liquid formulation to the tank five minutes prior to feeding. At the end of the three-month study there was a significant difference in the final weights of the different groups of juvenile cod. The feeding attractant group had the highest mean weight (25.8 g) compared to the two control groups (23.2 g) and (23.4 g). The feeding attractant group also had a better growth rate (SGR- 1.95 % day-1) than the two controls (SGR -1.85 % day-1 and 1.83 % day-1). In addition this group required the application of 10% less feed than the control groups suggesting better feed utilization in these fish.
2. Tilapia (Oreochromis niloticus). The trial was conducted between April and November 2006 at a commercial tilapia farm in Zhouhai, China. 14,500 juvenile tilapia, (mean size 2.5 cm) were stocked into each of four growing on ponds (6.3 mu) at a density of 1711 fish/ mu. Subsequently, every 14 days the mean weight of 30 fish was measured. The application of the feeding attractant produced a 17% increase in the average weight of the tilapia compared to the control pond. It also increased the growth rate of the tilapia allowing the farmer to start harvesting three weeks earlier than the control pond. In addition, it was noted that in the pond treated with the feeding attractant the fish appeared healthier, the water quality was better and the secondary crop of Litopenaeus vannamei was significantly higher with less incidence of disease. Overall, the farmer received a 50% higher income from the treated pond compared to the control pond.
3. Crucian Carp (Carassius carassius). The trial was conducted between May and October 2006 at a commercial crucian carp farm in Dafeng, China. A total of 2000 juvenile crucian carp (mean weight 29.5g) were stocked into each of eight growing-on ponds (each pond 2mu). Subsequently, every 14 days the mean weight of 30 fish was measured. At the end of the study the feeding attractant group had the highest mean weight (153.8 ± 2.56g) compared to the control group (130.1 ± 1.75g).
4. Whiteleg Shrimp (Litopenaeus vannamei). The trial was conducted in Tradt, south east Thailand over a 3 month period. Ponds, 4 rai in size, were used for the trial and each stocked with 640,00 post larvae shrimp. The feeding attractant was applied coated to a commercial shrimp feed. The mean weight of 300 shrimp from each of the ponds was subsequently measured on five occasions during the 90 days. The application of the feeding attractant produced shrimp, which were 30% larger on average than the control shrimp and had a significantly faster rate of growth. Mean weight of the treated shrimp was 9.97 ± 1.94g compared to 7.17 ± 1.45 for the control group. In addition, less feed was required in the treated pond probably due to increased feeding by the shrimp. This was reflected in a lower food conversion rate (FCR) at harvest than the control pond. Although the results of the preliminary growth trials with the feeding attractants appears to be encouraging it should be recognised that there can be significant variations in the growth rates of fish even in similar pond systems. However, further trials are being carried out to replicate the original tests and obtain further information on the efficacy of the pheromone-based feeding attractants for sustainable aquaculture.

This article is based on a presentation by Dr. Moore at ‘Aquafeed Horizons’, 2007, Utrecht, the Netherlands an Aquafeed.com Conference. The on-going research and development of the pheromonebased feeding attractants is carried out exclusively at the CEFAS Fisheries Laboratory at Lowestoft, England. The laboratory has an international reputation for its research into the identification and role of pheromones in fish biology.