Riverine spawning areas are important but restricted source habitats for large brown trout Salmo trutta (!40 cm total length). In hydrophysically different reaches within three contrasting boreal rivers, redds larger than 100 cm in length were located by direct underwater observation in 1998 and 2001 (n ¼ 157 redds; range ¼ 10-60 redds per reach and year) and were characterized based on 29 habitat variables. The microhabitat of utilized redd sites (total redd length: mean 6 SD ¼ 206 6 83 cm, range ¼ 100-615 cm) was typified by a mean total water depth of 103 6 39 cm with a wide optimum (range ¼ 23-215 cm) and a mean water velocity of 47 6 32 cm/s (range ¼ 2-124 cm/s). Mean velocity measured at 5 cm above the substrate was 29 6 20 cm/s (range ¼ 0-82 cm/s). Mean substrate particle size was 7 6 6 cm and demonstrated a narrower optimum (range ¼ 2-37 cm) than the other variables. Substantial differences among the three rivers, particularly in water depth and velocity measures, largely reflected size and gradient differences and indicated some flexibility in spawning microhabitat utilization. The use of substrate particle size was less variable. Multivariable analysis indicated that water velocity (except velocity measured just above the substrate) and water depth explained most of the variation. Depths and substrates utilized were larger than previously reported, probably reflecting the in situ habitat availability in the relatively large study rivers.

Arctic charr (Salvelinus alpinus (Linnaeus)) is a stenothermic cold‐water fish, which has been cultured in Northern Europe and North America since the 1980s. The industry has remained relatively small with an annual production between 6000 and 10 000 tonnes, and is still challenged by an unreliable offspring production. This review focuses on offspring production in Arctic charr aquaculture including holding conditions for brood‐stock, fertilisation and egg rearing until hatch. Brood‐stock requires low temperatures during summer (<12°C) with the optimum still unknown. The temperature maximum for egg incubation lies between 6 and 8°C. The composition of an optimal brood‐stock diet is debated regarding fatty acids. A demand for a freshwater‐based diet rich in omega‐6 fatty acids is indicated, but results remain inconclusive. Extensive knowledge has been gained on the timing of spawning and its manipulation through photoperiod, temperature and hormone treatments; spawning can be induced by short‐day photoperiod; and temperature drops to 5°C. Eggs are fertilised dry in ovarian fluid. Egg quality is highly variable and positively related to egg size and energy density. Contrary, little information is available on sperm quality and its impact on egg survival. There may also be profound differences between Arctic charr of stationary or anadromous origin regarding requirements for holding conditions of brood‐stock and their diet. However, these differences have received little attention, and direct comparative studies are in demand.

1. Introgression into natural salmonid populations from stocked conspecifics has been widely studied. Outcomes vary from no effect even after decades of stocking, to population replacement after only a couple of generations. Potential introgression caused by semi-supportive breeding (i.e. using a mixture of local strains as brood stock) is, however, less well studied. 2. We investigated population structure of brown trout (Salmo trutta) in a regulated alpine lake with three natural, environmentally contrasting tributaries used as spawning and rearing habitat. Massive semi-supportive breeding of admixed local strains has been implemented for decades. Stocked trout represented c. 17% of the total lake population, and a substantial post-release survival reflects a considerable potential for introgression. However, the mark-recapture studies indicate no spawning runs of stocked fish. 3. Using 13 polymorphic microsatellite loci, we found natural straying and non-native reproduction, especially among wild populations inhabiting environmentally unstable habitat. Retained genetic structure across tributaries indicated low reproductive success of wild-born non-natives. Moreover, the genetic structure among tributaries has probably not been influenced by semi-supportive breeding, because of recruitment failure of stocked trout.

Ecotype variation in species exhibiting different life history strategies may reflect heritable adaptations to optimize reproductive success, and potential for speciation. Traditionally, ecotypes have, however, been defined by morphometrics and life history characteristics, which may be confounded with individual plasticity. Here, we use the widely distributed and polytypic freshwater fish species brown trout (Salmo trutta) as a model to study piscivorous life history and its genetic characteristics in environmentally contrasting habitats; a large lake ecosystem with one major large and stable tributary, and several small tributaries. Data from 550 fish and 13 polymorphic microsatellites (H e = 0.67) indicated ecotype‐specific genetic differentiation (θ = 0.0170, p < .0001) among Bayesian assigned small riverine resident and large, lake migrating brown trout (>35 cm), but only in the large tributary. In contrast, large trout did not constitute a distinct genetic group in small tributaries, or across riverine sites. Whereas life history data suggest a small, river resident and a large migratory piscivorous ecotype in all studied tributaries, genetic data indicated that a genetically distinct piscivorous ecotype is more likely to evolve in the large and relatively more stable river habitat. In the smaller tributaries, ecotypes apparently resulted from individual plasticity. Whether different life histories and ecotypes result from individual plasticity or define different genetic types, have important consequence for conservation strategies.