Growth and survival of European grayling reared at different stocking densities

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In Sweden, many European grayling, Thymallus thymallus, populations have decreased owing to freshwater acidification, water-flow regulation and overfishing. Thus there is an increasing demand for production of grayling for stocking purposes. Knowledge concerning intensive rearing technology for grayling is generally limited to the rearing of one-summer-old fish (Carlstein, 1993), although older grayling have been reared in natural ponds with poor results (L. -0. Eriksson, pers. comm.). There is a paucity of information about the effect of stocking density on the intensive rearing of grayling. Stocking density, i.e. the amount of fish reared per unit volume, is a key factor in fish farming economics because growth, survival and production costs are often strongly influenced by it, Optimal stocking density in intensive rearing operations varies, depending on the rearing methods as well as the species, size and age of the fish. Several studies have been performed to examine the effect of stocking density on the growth of different fish species. There is often a negative relation between density and growth, e.g. in the rearing of juvenile Atlantic salmon,SQlmo sQlQr(Refstie and Kittelsen, 1976), brook charr, Salvelinus fontinalis (Vijayan and Leatherland, 1988, 1989) and rainbow trout, Oncorhynchus mykiss (Refstie, 1977; Trzebiatowski et al., 1981; Papoutsoglou et al., 1987). By contrast, increasing the stocking density can enhance the growth of Arctic charr, Salvelinus alpinus (Brown et al., 1992; Jprrgensen et al., 1993). Baker and Ayles (1990) showed that the growth rate of Arctic charr reared in tanks increased with density, reaching an optimum at 40-60 kg m a. The positive effect of crowding in Arctic char-r probably results from a change in behaviour from agonistic interactions to shoaling as stocking density is increased (Wallace et al., 1988; Baker and Ayles, 1990). The present study was carried out to examine the effect of stocking density on the growth and survival of one-summer-old European grayling under standard hatchery rearing conditions. The study was carried out at the SPna Fish Farm between 8 January and 25 February 1992. The fish used in this experiment were the offspring of adult spawners caught in the River &terdaltilven (see Carlstein, 1993, for further details of rearing). In January 1992, one-summer-old grayling were transferred into twelve 1 m3 (1 m2 X 1 m) standard rearing tanks (EWOS type 2009) to give two replicate of six different initial stocking densities: 5,10,15,20,30 and 50 kg fish m3 (Table 1). These grayling were obtained from