Seed supply is an important component of sea trout culture because an important part of the production output is sold as eggs, fry or fingerlings. Broodstock may be domesticated (originating from cultured fish) or captured from the wild. Wild-caught fish usually produce fewer eggs/kg than domesticated broodstock but they may be of interest for restocking purposes in order to stay as close as possible to the wild population. However, collection of wild broodstock is generally regulated and their regular use is generally restricted to official services or angling associations; these organizations are also less concerned by the profitability issues caused by the use of poorly productive broodstock.

As fish will mostly be used for restocking it is important when using domesticated broodstock to keep enough genetic variability by using highly effective numbers of broodstock to produce the successive generations; this avoids depleting the genetic variability of natural stocks. Another important factor is the high genetic diversity of the natural populations of sea trout, and the fact that restocking with exogenous material has already caused a lot of mixing of genetic pools. To avoid this, it has been suggested that, at a minimum, fish from the same lineage should be used, or sterile triploid fish (see later) should be used for restocking to avoid interbreeding with wild fish; this is already done in the United Kingdom.

Male and female broodstock can either be maintained separately or mixed, as they will not spontaneously spawn in holding tanks (at least for domesticated broodstock). During the reproduction season (usually November-December), females are checked weekly for ovulation by gently pressing their abdomen. Ovulated females can retain eggs in their body cavity for 8-10 days without significant loss in fertilization rate. Eggs are manually stripped in dry plastic containers. Sperm is also stripped from males and kept in dry tubes (it is important to avoid any contact with water of sperm and eggs prior to fertilization). For fertilization, sperm and eggs are mixed, and then water or a fertilization medium is added. Fertilization is completed within one minute. If several males are used, it is better to separate the spawn in batches, each fertilized by a single male, because mixing several sperms will inevitably result in heavily unbalanced male contributions due to spermatic competition; this has detrimental effects on the conservation of genetic variability in the offspring.

During the first 30 minutes following fertilization, eggs absorb water and harden. They can be manipulated between 30 minutes and 24 h following fertilization, after which any shock should be avoided until the eyed stage. Eggs are incubated in trays or in Californian incubators, with a maximum of 2 layers of eggs. Fertilized eggs should be kept in the dark, water exchange should be moderate (one renewal per hour, eggs should not move) and the dissolved oxygen level kept at 100 percent saturation. The water temperature should be stable and kept between 4 and 12 °C. Dead (white) eggs should be manually removed every day. At the eyed stage (typically 300 degree -days), eggs can be manipulated again and sold if requested.

Eyed eggs are hatched in the trays (typically 380 degree-days) and egg shells are removed at that stage. The normal hatching rate is >90 percent. Yolk resorption is completed in the same structures, still in the dark. When the fry have almost completed their yolk resorption and start swimming, they can be fed for the first time with dry feeds (0.4 mm diameter, 55 percent protein and 12 percent lipid content). When all fish are swimming, they are transferred to nursery tanks or sold as swim-up fry.

If triploid sterile fish are to be produced, they should be ‘shocked’ for 5 min post-fertilization. The shock can either be a temperature shock (28 °C for 10 minutes) or a pressure shock (500-600 atm for 3 minutes) in order to retain an additional copy of the maternal genome, which will ensure the sterility of the fish.

Nursery tanks are typically circular or oblong with a depth of 30-40 cm and have a relatively fast water current (1-2 times body length/second at the periphery). Stocking density should be kept below 15 kg/m^³ with a water renewal rate of 200 percent/hour. Fish are fed dry extruded pellets of 0.5 mm diameter (or 0.4 mm which are recommended for very small fry). In the first stage feed with a high protein content (>53 percent) and low lipid content (10-15 percent) is used. Up to a fry size of 0.5 g a feeding rate of 5 percent/BW/per day at 12 °C is followed. The protein level can be lowered to 50 percent when the fry reach 10 g, while the lipid level may be increased as high as 18 percent. The food must be distributed by automatic feeders or belt-feeders throughout the entire day.

Ongrowing in freshwater

In the first stage, after the fry have achieved 4-5 g Body Weight (BW), they can be transferred to outdoor grow-out facilities. Circular tanks with a water depth of 0.8-1 m are best suited for farming sea trout. A circular tank provides a better water flow than a raceway. A water current of one body length/second has been shown to increase weight gain (22 percent higher than the control group at zero current) and to stimulate muscle development, leading to a higher condition factor. It is preferable to cover about half of the tank surface in order to decrease the stress of the fish. When the fish exceed an average weight of 50 g, it becomes feasible to raise them in raceways.

The water exchange rate must be adequate to maintain 7 ppm dissolved oxygen and an ammonia level of <0.3 mg/litre at the outlet. Experiments have defined an optimum density of about 40 kg/m^³; above this there is a decrease in fish performance (growth, survival and FCR). Grading is unnecessary if the feeding management is optimal (sufficient quantity and distribution modalities to provide access to food for all animals). The coefficient of variation of the weight remains stable (20-25 percent) throughout the growing season. Grading often leads to decreased growth in this species, as the stress involved is very significant and may increase its susceptibility to diseases (furunculosis). The necessity to cease feeding two or three days before grading also causes a decrease in growth. Despite this, fish that are to be used for restocking must have a flawless appearance. They are therefore graded by hand before sale to eliminate unwanted fish, which can be sold for consumption or processing. Finally, for animals intended for farming at sea, an effective grading performed during vaccination against vibriosis, three weeks before transfer to sea water, is sufficient to obtain sufficient homogeneity of the stock.

Sampling fish quantity and size twice monthly allows estimations of growth rates, feed conversion, and closeness to carrying capacity to be calculated; these are essential considerations for proper farm management. The growth rate depends on ambient temperature. At 10 °C it is usually possible to rear fish to a table size (25-30 cm) within 14 months.

Ongrowing in sea water

Sea trout rearing can be an alternative to the culture of Atlantic salmon and rainbow trout when salinity and summer temperatures are too high. The transfer process at sea is always a delicate operation. Unlike the Atlantic salmon, sea trout does not really smoltify in culture (no changes in colour, swimming behaviour).

Juvenile fish (50-350 g) are directly transferred to full-strength sea water. Their adaptation is very fast (24-48 hours) but it causes profound but temporary osmotic and physiological imbalances. Sea trout withstand these changes if their physiological condition is already good and if the conditions of transfer are optimal, that is:

• Fifty grams minimum size (below this, survival is uncertain).
• Minimal distance between the freshwater and sea water sites.
• Transport tanks supersaturation (>150 percent) avoided, to prevent the gills burn.
• Minimal temperature differential between freshwater and sea water, and sea water temperature below 12 °C.

Subject to compliance with these conditions, the transfer of sea trout to sea water occurs without excessive mortality (2-3 percent). After an adaptation period, which can be very rapid (one to three days) if the transfer went well, the behaviour of sea trout at sea changes radically. It almost resembles that of rainbow trout or salmon. The growth rate that can be obtained shows that it is possible to rear fish to >2 kg after one year at sea, when using fish selected for growth and optimal farming practices are adopted. A key point is the possibility of transferring larger juveniles (average size 250-300 g) at the end of the autumn, taking advantage of the mild temperatures at that time for good growth after the adjustment phase. High summer temperatures can sometimes slow down or even completely stop growth.

Control of sex and sexual maturation

Sexual maturation results in a sharp decrease in growth, some sea mortality and a deteriorating quality of the flesh and colour, thus exhibiting an obstacle to obtaining large sized, commercially interesting animals. Raising sterile females is therefore a very interesting option. Their continued growth, not affected by sexual maturation, results in individuals weighing up to 4 kg. Sterility can be obtained by thermally induced triploidy induced by dipping fertilized eggs in a warm water bath (28 °C) for a few minutes.

Feed and feeding

Trout producers usually try to grow their fish as quickly and efficiently as possible while maintaining uniformity of growth and degrading water quality as little as possible. To accomplish these goals it is important to feed the correct amount. Fish feed requirements are generally determined by two parameters: fish size and water temperature. The general principles of the effect of water temperature are well known. Feed requirements increase with temperature up to a threshold temperature that is considered to be the optimum (16 °C for sea trout) and then decreases. Other factors may be involved, such as health status, physiological state, environment and farming system. However, even for a given temperature, size is insufficient to precisely determine feed requirements. Indeed, the requirement for maintenance is mainly a function of size, while the requirement for growth is, among other things, dependent on the genetic potential of the strain used. Precise assessment will require many observations and subsequent adjustments by the fish farmer.

Despite their imperfections the feeding tables provided by commercial feed manufacturers take into account the weight and water temperature. They only apply to the particular feed used, with the sole objective of fast growth. These tables are provided for information only and have to be adapted to the farming conditions and the characteristics of the fish. However, it is rare that the actual weight and temperature correspond exactly to those indicated on the table and linear interpolation may be necessary. The relevance of the tables to the needs of fish, is assessed by regularly monitoring performance, especially the food conversion ratio (FCR), which must remain within the limits usually accepted for the species and density energy of the food used.

The feed ration can be distributed according to several modalities. Hand feeding is labour intensive and may not be practical on a large commercial farm and, in the case of the sea trout, it is very difficult to practice due to the low domestication of this species. An automatic feeder is more practical, allows the distribution of the ration throughout the whole day, and has been shown to improve FCR. An interesting alternative for sea trout is to use a demand feeder system. This allows the fish to obtain only the amount of food that is necessary. Trout weighing more than 5 g can easily be trained to feed themselves. With careful adjustment, rapid weight gain and efficient feed utilization can be attained by the use of demand feeders. The use of demand feeders can eliminate the sharp oxygen decline that occurs when fish are fed by hand or machine a few times each day. Demand feeders also reduce the labour costs associated with daily hand feeding. The disadvantages of this system include the tendency to overfeed because of improper feeder adjustment. In this case, an electronic device can control the response time to requests from fish on a pendulum and modulate the amount distributed at each solicitation. Several days’ feed can be loaded in the hopper, but for best feeding efficiency it should not be replaced until the daily feeding period has elapsed, to avoid stressing the fish. The advantage of this method of distribution is to adapt to the voluntary feed intake of fish, but the cost of investment, especially if electronic control systems are used, may be prohibitive for small farms. An important point is that whatever feeding methods are adopted, feed should be dispersed throughout the whole of the rearing tank, pond or raceway.

Sea trout are fed the same commercially available feeds as rainbow trout, both in sea water and in freshwater. The current trend is to increase the levels of lipid, which have increased from 10 to 30 percent over the last 20 years.

Methods of harvesting vary but the fish are generally starved for up to three days beforehand. The fish are crowded in net-pens using sweep nets (or by the use of grids in raceways) and are either pumped from out alive and transported to the slaughter plant, or slaughtered at the side of the pens.

Generally fish are initially stunned using an automated stunner or a blow to the head. Bleeding is then carried out by rapidly cutting the gill arches and the fish are immersed in iced water. However, this method is widely believed to expose the fish to unnecessary pain and suffering; the industry is consequently seeking an alternative method that offers improved welfare of the fish at slaughter. Electric stunning of fish in water has been identified as a suitable method. Carcass yield greatly increases between 1 and 3 days and then more slowly up to 3 weeks. The whole process is carried out with the aim of keeping stress to a minimum, thus maximizing flesh quality. The impact of nutrition or genetics on meat quality have been shown to be less important than the technological factors (slaughter and post-mortem conditions) for the quality of the final product.