Interpopulation variation in reproductive traits of anadromous female brown trout, Salmo trutta L.

We studied reproductive traits in nine anadromous brown trout, Salmo trutta L., populations in seven Norwegian rivers. Within populations we found a positive significant correlation between fish length and fecundity in all populations, and between fish length and egg diameter in five populations. There were significant differences in these relationships between populations from different rivers, and between populations from different locations within rivers. When adjusted for variation in fish length, mean fecundity and mean egg diameter showed a negative significant correlation among populations. The ratio of gonadal weight to somatic weight (gonadosomatic index) varied significantly among populations but was not associated with variation in fish length. Comparatively few large eggs were found in brown trout populations co-existing with several other fish species.     

Phylogeography of the Turkish brown trout Salmo trutta L.: mitochondrial DNA PCR‐RFLP variation

In the present study, mitochondrial DNA polymerase chain reaction‐restriction fragment length polymorphism (PCR‐RFLP) assay was used to assess the phylogenetic and phylogeographic relationships among 27 brown trout Salmo trutta populations from Turkey. The complete NADH 5/6 region and a second segment comprising the cytochrome b gene and D‐loop of mtDNA amplified by PCR were digested with six and five restriction enzymes, respectively. A total of 27 haplotypes were observed and divided into three major phylogenetic assemblages, namely Danubian (DA), Adriatic (AD) and a newly proposed Tigris (TI) lineage. The timing of the net nucleotide divergence between the major lineages along with the geological history of Turkey suggested pre‐Pleistocene isolation of the Turkish brown trout and provided evidence that Turkey could be considered as a centre of diversification for these lineages. The average haplotype diversity (0·1397) and the nucleotide diversity (0·000416) within populations were low in comparison to the observed interpopulation nucleotide diversity (0·021266). PCR‐RFLP analysis showed that most of the mtDNA sequence variation found in the Turkish brown trout populations was imputable to differences among lineages. On the other hand, there was also an obvious relationship between geographical distribution of the populations and their clustering. The present study showed that brown trout populations from Turkey are highly divergent and mainly have a unique genetic profile that could be used for conservation and management purposes.     

Comparative performance of juvenile sea trout families in high and low feeding environments

Groups of eight full-sibling wild sea trout Salmo trutta L. families, communally reared, exhibited highly significant differences in family representation in the largest 25 percentile within each tank at the end of the observation period (referred to as family performance). Despite these large differences in family performance within tanks, there were no significant differences in performances of families relative to each other between high and low feeding levels. These results are discussed in the context of domestication selection.     

Heart rate responses to predation risk in Salmo trutta are affected by the rearing environment

Both wild‐ and hatchery‐reared brown trout Salmo trutta , 18 months of age and of the same genetic origin, responded with increased heart rates (tachycardia) to a simulated predator attack on 2 consecutive days. Brown trout reared in the hatchery showed a more rapidly induced tachycardia compared with wild‐reared fish at day 1, but not day 2. During an undisturbed period several hours after attacks, hatchery‐reared brown trout maintained higher heart rates compared to wild‐reared fish on both days. Behavioural responses to the attack were very low for all fish, although hatchery‐reared fish tended to be more active than wild fish after the attack day 2. The observed differences may have had a genetic background caused by different selection regimes in the hatchery‐ and wild‐rearing environments, or could have been due to different phenotypic responses in the two environments.     

Does light intensity affect self‐feeding and food wastage in group‐held rainbow trout and white‐spotted charr?

The self‐feeding rhythms of rainbow trout Oncorhynchus mykiss and white‐spotted charr Salvelinus leucomaenis were studied when group‐held fishes ( n  = 10 per group) were fed using self‐feeders under two different light intensities (50 lx, 16 μW cm⁻² and 700 lx, 215 μW cm⁻²) during the light phase of the light‐dark cycle. Food wastage was also measured. At 50 lx, all groups of rainbow trout learned to operate the self‐feeder within 4 days, whereas it took up to 25 days for all groups at 700 lx. In contrast, all groups of white‐spotted charr learned self‐feeding within 17 days, irrespective of light intensity. These results, although non‐significant, suggest that lower light intensities can stimulate instrumental learning in rainbow trout, but not white‐spotted charr. In rainbow trout, the total number of trigger actuations for the entire experimental period was significantly higher at 50 rather than 700 lx, although this may have been related to delayed learning at 700 lx. There was no significant effect in white‐spotted charr. Growth rate (assessed using the thermal growth coefficient) was also higher in rainbow trout but not white‐spotted charr at 50 rather than 700 lx, although this difference was non‐significant. Light intensity had no significant effect on food wastage in either rainbow trout or white‐spotted charr, and it did not appear to affect the proportion of trigger actuations during the light phase. Clear diurnal feeding rhythms were observed in both species and these were classified into four categories: uniform, dawn, dusk and crepuscular. At 50 lx, fish from both species generally fed in temporally localized periods at either dawn and dusk, whilst feeding was predominantly uniform during the light phase at 700 lx.     

Density‐dependent growth in hatchery‐reared brown trout released into a natural stream

Hatchery‐reared brown trout Salmo trutta stocked in a natural stream in addition to resident wild brown trout grew more slowly than those stocked with an experimentally reduced density of brown wild trout. In both cases, hatchery‐reared brown trout grew more slowly than resident wild fish in control sections. Mortality and movements did not differ among the three categories of fish. The results showed that growth of stocked hatchery‐reared brown trout parr was density‐dependent, most likely as a consequence of increased competition. Thus, supplementary release of hatchery‐reared fish did not necessarily increase biomass.     

Trade-off between egg mass and egg number in brown trout

Individual egg mass and fecundity increased with somatic mass in first time and repeat spawning wild anadromous and freshwater resident brown trout Salmo trutta . The egg mass was larger for similar-sized trout in south (58° N) than mid Norway (63° N), whereas fecundity was higher in mid- than in south Norway, making total gonadal investment similar in the two areas. Repeat spawners had heavier eggs than similar-sized first time spawners. The egg mass of residents was c. 10% larger than that of similar-sized first time spawning anadromous trout. Common garden experiments with offspring of wild anadromous trout showed no significant correlation between egg and somatic mass in first time spawners in two of the three populations studied. In the third population, a slight positive correlation was found. Similar results were found for repeat spawners. In the three populations, fecundity increased significantly with somatic mass in both first time and repeat spawners. Wild and hatchery-reared trout showed negative correlation between egg mass and fecundity when the effect of body size was excluded, indicating a trade-off between the two parameters. In wild trout, this was caused by variation among populations, whereas in hatchery fish, within population variation was observed in egg mass over fecundity. Furthermore, the egg mass of first time and repeat spawners were positively correlated, when adjusted for fish size. Size-specific gonadal investment was significantly higher in wild anadromous than resident trout. There was no significant difference in gonadal investment between first time and repeat spawners in wild anadromous trout. However, in the hatchery-reared trout, gonadal investment was significantly higher at repeat than first time maturation. The hatchery trout did not spawn naturally, but were artificially stripped. Among populations, a part of the variation in egg mass and fecundity is phenotypically plastic, a part appears genetically determined.     

Genetic and morphological characterization of a Lake Ohrid endemic, Salmo (Acantholingua) ohridanus with a comparison to sympatric Salmo trutta

Analysis of both uni‐(two mtDNA gene sequences) and bi‐parentally (seven microsatellite loci) inherited genetic markers, together with analysis of 40 morphological characters, described Salmo ohridanus as a highly divergent member of the genus Salmo . Based on comparative substitution rate differences at the cytochrome b gene, and a rough estimated age of the Salmo trutta complex ( i.e. at least 2 million years), the S . ohridanus and Salmo obtusirostris clade probably split from a common ancestor of brown trout Salmo trutta >4 million years ago, overlapping with minimum age estimates of the formation of Europe's oldest freshwater habitat, Lake Ohrid. Comparative analysis with Lake Ohrid brown trout (known regionally as Salmo letnica ), supported the notion that these fish have more recently colonized the lake and phylogenetically belong to the Adriatic lineage of brown trout. It is further suggested that species‐specific saturation in the mtDNA control region underestimated the divergence between S . ohridanus and S . trutta . Evidence of rare hybridization between S . ohridanus and Lake Ohrid brown trout was seen at both mtDNA and microsatellite markers, but there was no support for extensive introgression.     

Body pigmentation pattern to assess introgression by hatchery stocks in native Salmo trutta from Mediterranean streams

Five qualitative and seven quantitative colouration and spotting pattern features were measured in 23 brown trout Salmo trutta populations and two hatchery stocks. Simultaneously, the LDH‐C1 *, a diagnostic locus fixed for *90 and *100 alleles in stocking and native populations from southern Europe, respectively, was analysed to classify the brown trout studied according to their origin: native, hatchery stock and hybrids. The three genotypes showed significant differences in the colouration and spotting features and a discriminant function analysis could correctly identify 79% of the individuals. The most discriminating variables were dorsal fin margin colour, number of opercular spots, presence of the preopercular mark and diameter of black spots. Given the low cost, ease and possibility of field identification of native fish, the results indicate great opportunities for the application of morphological‐based classification models on the conservation and management of native brown trout stocks.     

Within-stream variation in early life-history traits in brown trout

Significant additive genetic variance for most early life-history traits was found in brown trout Salmo trutta living in both allopatry above an impassable waterfall and sympatry (below the waterfall in the same stream) with alpine bullhead Cottus poecilopus. These traits included length, mass and yolk sac volume at hatching, and size at'button-up' (the time when yolk is enclosed within the body cavity). There were small differences in size at hatching and size at button-up among populations (adjusted for egg size). However, sympatric fry grew more rapidly and experienced lower mortality rates during the period of first feeding than allopatric fry. This might indicate behavioural differences between brown trout from the two populations. It is suggested that these phenotypic differences may be a result of adaptation to living in sympatry with alpine bullhead.     

Environmentally induced spatio-temporal variations in the fecundity of brown trout Salmo trutta L.: trade-offs between egg size and number

1. Resident brown trout Salmo trutta in the Esva River basin (north Spain) live in a patchy environment with tracts of riparian forest or meadow along stream banks. This study assessed whether the reproductive traits of brown trout from four contrasting sites reflected site-specific factors.
2. Length at maturity (10.5–11 cm of 1 + individuals) was the same in the four sites examined but slowest growers in slow-growing sub-populations delayed maturity for 1 year relative to fast-growing fish. The analysis of monthly variations in egg size and number suggest that two 'decisions' in two consecutive years are required to complete spawning. The first concerns the number of eggs, determined when trout are still 0 +, and the second concerns egg size.
3. At three sites, egg size and number did not differ significantly between years but highly significant interannual variations were apparent at another site. Fish length was the major determinant of egg size and number at all sites but for any given length, brown trout at sites where the fish exhibited higher growth rates spawned more, but smaller, eggs than those at slow-growing sites. This spatial pattern was identical to the temporal pattern exhibited by trout at another site. The combination of temporal (year-to-year) and spatial (between rivers) variations in egg size and number showed a significant negative correlation, supporting the operation of a trade-off between these two traits.
4. The trade-off between egg size and number seems to be determined by site-specific factors, with slow-growing trout at sites which are fully covered by canopy spawning fewer, but larger, eggs than fast-growers in unshaded sites.     

Among-sibling differences in the phenotypes of juvenile fish depend on their location within the egg mass and maternal dominance rank

We investigated whether among-sibling differences in the phenotypes of juvenile fish were systematically related to the position in the egg mass where each individual developed during oogenesis. We sampled eggs from the front, middle and rear thirds of the egg mass in female brown trout of known dominance rank. In the resulting juveniles, we then measured traits that are related to individual fitness: body size, social status and standard metabolic rate (SMR). When controlling for differences among females in mean egg size, siblings from dominant mothers were initially larger (and had a lower mass-corrected SMR) if they developed from eggs at the rear of the egg mass. However, heterogeneity in the size of siblings from different positions in the egg mass diminished in lower-ranking females. Location of the egg within the egg mass also affected the social dominance of the resulting juvenile fish, although the direction of this effect varied with developmental age. This study provides the first evidence of a systematic basis for among-sibling differences in the phenotypes of offspring in a highly fecund organism.     

Partial migration in a landlocked brown trout population

Population densities of landlocked lake‐migratory brown trout Salmo trutta were estimated in two distinct lotic sections, separated by a lentic segment, in the Greåna River, Sweden, and individual growth and habitat use were monitored for 835 tagged brown trout from September 1998 to June 2000. Residency dominated in the upstream section where density of 0+ and 1+ year brown trout was low and growth rate high. In contrast, >90% of the brown trout that migrated to the lake originated from the downstream section, where density was high and growth rate low. For ≥2+ year individuals, growth rate was similar between the two stream sections, but densities were higher in the upstream than in the downstream section. Lake‐migrants had higher growth rates than non‐migrants (residents) during the autumn of both years. From September to May, migrants increased their body mass by >35%, whereas non‐migrants increased by <5%. Approximately 70% of the brown trout moved <10 m and <2% moved between the two stream sections, indicating that the lentic habitat might function as a barrier for juveniles. Differences in migratory behaviour, density and growth between the upstream and the downstream section might indicate that environmental factors influence the decision to migrate. It cannot be excluded, however, that the observed differences are genetically programmed, selected by migration costs that favour migratory behaviour downstream and residency upstream.     

Regional variation in the microhardness and mineralization of vertebrae from brown and rainbow trout

Regional variation in properties of vertebral bone from brown Salmo trutta and rainbow trout Oncorhychus mykiss were explored by using microhardness tests. Statistically‐significant positive correlations were identified between the microhardness of bone and its mineral content. In both brown and rainbow trout, the vertebrae from the caudal region were harder than those of the trunk region. There was a significant difference between the species; microhardness of bone from vertebrae of rainbow trout was greater than those from brown trout.     

Performance of juvenile brown trout exposed to fluctuating water level and temperature

Individual daily food intake, mass‐specific growth rate and growth efficiency in groups of juvenile brown trout Salmo trutta were compared in tank experiments with three water level regimes (fluctuating, stable high and low water levels) and two temperature regimes (fluctuating between 10 and 14° C and constant 14° C) to simulate events during hydropeaking in regulated rivers. Fish exposed to high stable water level showed higher food intake and growth rate, and higher or similar growth efficiency than fish exposed to fluctuating or stable low water level. Both groups of slow‐growing and fast‐growing individuals fed less and grew slower at stable low and fluctuating water level than at stable high water level. Furthermore, growth and growth efficiency were lower in brown trout exposed to stable low water level and fluctuating temperature, particularly for groups of fish with slow growth. Temperature did not have any effect at high water level. For groups of fast‐growing fish, there was no difference in growth efficiency between treatments. It is concluded that fluctuating water level and temperature have a potentially detrimental effect on growth in juvenile brown trout and effects are more severe in slow‐ than fast‐growing fish.     

Winter ecology of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) and brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) in a subarctic lake,Norway

We studied habitat choice, diet, food consumption and somatic growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) during the ice-covered winter period of a subarctic lake in northern Norway. Both Arctic charr and brown trout predominantly used the littoral zone during winter time. Despite very cold winter conditions (water temperature <1°C) and poor light conditions, both fish species fed continuously during the ice-covered period, although at a much lower rate than during the summer season. No somatic growth could be detected during the ice-covered winter period and the condition factor of both species significantly declined, suggesting that the winter feeding rates were similar to or below the maintenance requirements. Also, the species richness and diversity of ingested prey largely decreased from summer to winter for both fish species. The winter diet of Arctic charr <20 cm was dominated by benthic insect larvae, chironomids in particular, and Gammarus lacustris, but zooplankton was also important in December. G. lacustris was the dominant prey of charr >20 cm. The winter diet of brown trout <20 cm was dominated by insect larvae, whereas large-sized trout mainly was piscivorous, feeding on juvenile Arctic charr. Piscivorous feeding behaviour of trout was in contrast rarely seen during the summer months when their encounter with potential fish prey was rare as the small-sized charr mainly inhabited the profundal. The study demonstrated large differences in the ecology and interactions of Arctic charr and brown trout between the winter and summer seasons.     

Evaluation of biochemical methods for the non‐destructive identification of sex in upstream migrating salmon and sea trout

Female‐specific markers of reproductive activity [plasma 17β‐oestradiol (E2), vitellogenin (VTG) and alkali‐labile phosphoprotein phosphorous (ALP)] were measured over 12 months in a captive population of brown trout Salmo trutta . During the early months of the reproductive season (February to May) and using the concentration of plasma E2 or plasma ALP as a marker for females the proportion of fish in which sex was misidentified was high (15–50%). The misidentification rate was considerably lower (1–8%) using plasma VTG. Preliminary evaluation of a commercial immunochromatographic VTG test system as a screen for the presence or absence of VTG in plasma from brown trout provided results that were consistent with those obtained from direct measurement of plasma VTG levels by enzyme‐linked immunosorbent assay (ELISA). These preliminary conclusions were verified by sampling upstream‐migrating anadromous brown trout, sea trout, and Atlantic salmon Salmo salar trapped over a 6 month period. Plasma E2 levels did not satisfactorily discriminate between male and female sea trout and Atlantic salmon. Plasma VTG levels in both species, however, were bimodally distributed and it was assumed that this divergence corresponded to male (plasma VTG levels <10 μg ml⁻¹) and female (plasma VTG levels >800 μg ml⁻¹) fishes. Plasma ALP provided a more accurate indication of sex in the wild Atlantic salmon and sea trout than was suggested by the pilot study on captive brown trout. The commercial immunochromatographic VTG test system provided results that were wholly consistent with the data obtained from the trapped fishes by direct measurement of plasma VTG.     

Food partitioning between coexisting Atlantic salmon and brook trout in the Sainte‐Marguerite River ecosystem, Quebec

Food resource partitioning between similar‐sized, sympatric Atlantic salmon Salmo salar and brook trout Salvelinus fontinalis was examined as a possible mechanism enabling their coexistence in a stream (Allaire) of the Sainte‐Marguerite River ecosystem, Quebec, Canada. Fish stomach contents and invertebrate drift were collected concurrently during three diel cycles in August to September 1996. The food and feeding habits of an allopatric brook trout population in a nearby stream (Epinette) were studied for comparison. The diel feeding rhythms of the two coexisting fish species were similar. The composition of their diet, however, showed significant differences. Atlantic salmon predominantly (60–90%) fed on aquatic insects, mainly Ephemeroptera (35–60% of the diet). The brook trout mostly (50–80%) fed upon the allochthonous terrestrial insects (mainly adults of Coleoptera, Hymenoptera and Diptera) which comprised 5–40% of the stream drift. The allopatric brook trout fed opportunistically on the more abundant aquatic insects and terrestrial insects rarely formed 25% of its diet. The allopatric trout fed nearly twice as much as the sympatric brook trout during a day. The results suggest that the differences in feeding by brook trout in the two streams (with and without Atlantic salmon) are the result of inter‐specific interaction with Atlantic salmon and are not related to the differences in food availability between the two streams. Food resource partitioning between Atlantic salmon and brook trout may be viewed as an adaptive response resulting in a greater exploitation of available resources and coexistence.     

A 27-year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown trout Salmo trutta , population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish >3 years old for period II. The gill-net catchability was correlated negatively with spawner biomass and number of trout >3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density-dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density-dependent recruitment of 1-year-old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.