Stock Transfers in Spanish Brown Trout Populations: A Long-term Assessment

Synopsis Stocking of fish from other populations has been commonly employed for enhancement of wild brown trout, Salmo trutta, populations in north Spain. Young hatchery reared brown trout of central European origin were introduced into some Asturian rivers every year since 1984. Based on variation at the isozyme locus LDH-C1* and at the microsatellite locus BFRO 002, two genetic markers race-specific in Salmo trutta, we detected introgression of foreign genomes into native gene pools in some Spanish trout populations where only pure native individuals were present 10 years ago. We strongly suggest development of alternative management policies for conservation of Spanish natural brown trout populations without endangering the traditional recreational fisheries. Jorge I. Izquierdo, Ana G. F. Castillo: These two authors contributed equally to the article.     

Phylogeography and taxonomic status of trout and salmon from the Ponto‐Caspian drainages,with inferences on European Brown Trout evolution and taxonomy

Current taxonomy of western Eurasian trout leaves a number of questions open; it is not clear to what extent some species are distinct genetically and morphologically. The purpose of this paper was to explore phylogeography and species boundaries in freshwater and anadromous trout from the drainages of the Black and the Caspian Seas (Ponto‐Caspian). We studied morphology and mitochondrial phylogeny, combining samples from the western Caucasus within the potential range of five nominal species of trout that are thought to inhabit this region, and using the sequences available from GenBank. Our results suggest that the genetic diversity of trout in the Ponto‐Caspian region is best explained with the fragmentation of catchments. (1) All trout species from Ponto‐Caspian belong to the same mitochondrial clade, separated from the other trout since the Pleistocene; (2) the southeastern Black Sea area is the most likely place of diversification of this clade, which is closely related to the clades from Anatolia; (3) The species from the Black Sea and the Caspian Sea drainages are monophyletic; (4) except for the basal lineage of the Ponto‐Caspian clade, Salmo rizeensis, all the lineages produce anadromous forms; (5) genetic diversification within the Ponto‐Caspian clade is related to Pleistocene glacial waves; (6) the described morphological differences between the species are not fully diagnostic, and some earlier described differences depend on body size; the differences between freshwater and marine forms exceed those between the different lineages. We suggest a conservative taxonomic approach, using the names S. rizeensis and Salmo labrax for trout from the Black Sea basin and Salmo caspius and Salmo ciscaucasicus for the fish from the Caspian basin.     

Maintenance of a small anadromous subpopulation of brown trout (Salmo trutta L.) by straying

1. Microsatellite and isozyme loci variation were used to study structure and dynamics of a brown trout (Salmo trutta) population heavily affected by damming. The downstream area accessible for spawning was drastically reduced to a stream 1 km long influenced by regulated discharge. 2. Stocking of hatchery‐reared juveniles failed and the population is entirely supported by anadromous adults from neighbouring populations. 3. Temporal genetic stability is reported here. Some punctual between‐river genetic differences are likely because of different contribution from each neighbouring river through years. 4. High anadromy‐mediated gene flow produces a lack of genetic substructure in the region. The role of anadromous brown trout on maintenance of endangered small populations is emphasised.     

Quantifying an overlooked aspect of partial migration using otolith microchemistry

For the first time, an overlooked aspect of partial migration was quantified using otolith microchemistry and brown trout, Salmo trutta, as a model species. Relative contributions of freshwater resident and anadromous female brown trout to mixed-stock sea trout populations in the Baltic Sea were estimated. Out of 236 confirmed wild sea trout sampled around the coast of Estonia 88% were of anadromous maternal origin and 12% were of resident maternal origin. This novel finding underscores the importance of the resident contingent in maintaining the persistence and resilience of the migratory contingent.     

Phylogenetic status of brown trout Salmo trutta populations in five rivers from the southern Caspian Sea and two inland lake basins,Iran: a morphogenetic approach

Interrelationships, origin and phylogenetic affinities of brown trout Salmo trutta populations from the southern Caspian Sea basin, Orumieh and Namak Lake basins in Iran were analysed from complete mtDNA control region sequences, 12 microsatellite loci and morphological characters. Among 129 specimens from six populations, seven haplotypes were observed. Based on mtDNA haplotype data, the Orumieh and southern Caspian populations did not differ significantly, but the Namak basin–Karaj population presented a unique haplotype closely related to the haplotypes of the other populations (0·1% Kimura two‐parameter, K2P divergence). All Iranian haplotypes clustered as a distinct group within the Danube phylogenetic grouping, with an average K2P distance of 0·41% relative to other Danubian haplotypes. The Karaj haplotype in the Namak basin was related to a haplotype (Da26) formerly identified in the Tigris basin in Turkey, to a Salmo trutta oxianus haplotype from the Aral Sea basin, and to haplotype Da1a with two mutational steps, as well as to other Iranian haplotypes with one to two mutational steps, which may indicate a centre of origin in the Caspian basin. In contrast to results of the mtDNA analysis, more pronounced differentiation was observed among the populations studied in the morphological and microsatellite DNA data, except for the two populations from the Orumieh basin, which were similar, possibly due to anthropogenic causes.     

Recapture rate and growth of hatchery‐reared brown trout (Salmo trutta v. fario,L.) in Blanice River and the effect of stocking on wild brown trout and grayling (Thymallus thymallus,L.)

Hatchery‐reared adult brown trout, Salmo trutta v. fario L., [215–335 mm standard length (L_S), n = 82] were individually tagged and released into three sections of the Blanice River in May 2007. Wild populations of brown trout and grayling, Thymallus thymallus, L., in these sections and three non‐stocked control sections were also tagged. The recapture rate of hatchery‐reared adult brown trout after 6 months (18%, n = 15) was comparable to that of wild adult brown trout in stocked (15%, n = 14) and control (14%, n = 11) sections. The recapture rates of wild brown trout and grayling after 6 months were higher in control sections than in stocked sections, but the differences were not significant. The movement of recaptured large juvenile wild brown trout from stocked sections was significantly higher (36%) than from control sections (9%). Wild brown trout growth and grayling growth were unaffected by stocking with adult hatchery‐reared brown trout.     

Genetic population structure of Norwegian brown trout

Biochemical genetic variation in populations of anadromous and resident brown trout, Salmo trutta L., was studied. Altogether 50 Norwegian populations were screened for 32 enzyme loci. Genetic polymorphism was found at the following 11 loci: AAT-4 * (E.C. 2.6.1.1), CK-1 * (E.C. 2.7.3.2), G3PDH-2 * (E.C. 1.1.1.8), IDHP-2 * (E.C. 1.1.1.42), LDH-5 * (E.C. 1.1.1.27), MDH-2 * (E.C. 1.1.1.37), MDH-3/4 * (E.C. 1.1.1.37), MEP-2 * (E.C. 1.1.1.40), GPI-2 * (E.C. 5.3.1.9). GPI-5 * (E.C. 5.3.1.9) and PGM-1 * (E.C. 5.4.2.2), giving an overall polymorphism of 34%, ranging from 3.7 to 29.6% among individual populations. The average calculated heterozygosity ranged from 1.4 to 10.2% among populations. Genetic heterogeneity was observed among anadromous populations, and significant differences in allelic frequencies were found between anadromous populations in neighbouring watercourses, among resident populations and between anadromous and resident populations inhabiting the same watercourses. Significant heterogeneity was also found among 12 populations from Lake Mjøsa, with a major division between the western and eastern populations of the lake. Differences in allelic frequencies were found between wild stocks and their hatchery derivatives, and between different hatchery derivatives originating from the same wild population. In some cases release of hatchery populations into wild stocks may have influenced the genetic characteristics of wild stocks. The data support the hypothesis of eastern as well as western postglacial colonization lines for Norwegian brown trout.     

Life history and genetic characterisation of sea trout Salmo trutta in the Adriatic Sea

1. The brown trout Salmo trutta is characterised by both anadromous (sea trout) and resident populations, naturally occurring in Atlantic and Ponto-Caspian rivers. Sea trout are currently considered absent from rivers of the Mediterranean area, probably because of the non-optimal chemical–physical characteristics of the Mediterranean Sea. However, the occasional bycatch of smoltified S. trutta in the Adriatic Sea is well known among fishermen and the biological explanation of this phenomenon is still controversial. The aim of this study was to compare the genetic diversity of freshwater and marine brown trout to try to understand the factors underlying the presence of putative anadromous brown trout in the Adriatic Sea.
2. In this study, we analysed the genetic diversity of: (1) wild brown trout collected from the Esino River (central Italy); (2) a domestic strain of brown trout used for stocking the study area; and (3) a sample of Adriatic sea trout collected near the outlet of the Esino River. Together with genetic analysis, we carried out scale analysis in order to track the freshwater/marine stages of the life cycle in the sea trout samples. The genetic characterisation was carried out by polymerase chain reaction–restriction fragment length polymorphism analysis of the mtDNA fragment ND-5/6 and the nuclear locus LDH-C1* and by genotyping 15 microsatellite loci. The genetic polymorphism obtained was used to investigate intra- and inter-population genetic diversity, rates of genetic introgression between wild and domestic samples and the origin of sea trout specimens by using assignment tests.
3. Our genetic analyses demonstrated that the sea trout analysed in this study are from the domestic strain of Atlantic origin used in central Italy for stocking activities. The level of genetic introgression between native and domestic samples is high in the Esino River. The populations more resilient to introgressive hybridisation appeared to be those living in the portion of the river network dominated by carbonate rocks. Assignment tests (GeneClass) suggest the existence of a link between stocking efforts and the freshwater origin of the sea trout. In addition, data obtained from the analysis of scales, size measurement, and sex determination showed a pattern of smolt age, size, and sex ratio very similar to those observed in other anadromous populations.
4. In conclusion, the present study highlighted that sea trout from the central Adriatic Sea originated from brown trout of Atlantic origin inhabiting the Esino River. Their seaward migratory behaviour could represent a consequence of an active migration instead of a passive displacement by water flow. Our results also showed that traditional stocking practices represent a negative activity for the conservation of the last Mediterranean native S. trutta populations.

     

Genetic introgression on freshwater fish populations caused by restocking programmes

The brown trout (Salmo trutta L.) is one of the best studied native salmonids of Europe. Genetic studies on this species suggest that a large proportion of the evolutionary diversity corresponds to southern European countries, including the Iberian Peninsula, where this study is focused. Stocking activities employing non-indigenous hatchery specimens together with the destruction and fragmentation of natural habitats are major factors causing a decrease of native brown trout populations, mostly in the Mediterranean basins of the Iberian Peninsula. The main aim of the present work is to examine the genetic structure of the brown trout populations of the East Cantabrian region, studying the consequences of the restocking activities with foreign hatchery brown trout specimens into the wild trout populations. We have based our study on the Polymerase Chain Reaction and Restriction Fragment Length Polymorphism technique conducted on a mitochondrial fragment of 2700 base pairs and on the lactate dehydrogenase locus of the nuclear DNA. Our results show higher introgression rates in the Ebro (Mediterranean) basin than in the Cantabrian rivers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Predator avoidance behaviour in wild and hatchery‐reared brown trout: the role of experience and domestication

Juvenile brown trout Salmo trutta from natural populations reacted to the presence of piscivorous brown trout by increasing the use of refuges. In contrast, second‐generation hatchery fish and the offspring of wild fish raised under hatchery conditions were insensitive to predation risk. The diel pattern of activity also differed between wild and hatchery brown trout. Second‐generation hatchery fish were predominantly active during daytime regardless of risk levels. Wild fish, however, showed a shift towards nocturnal activity in the presence of predators. These findings emphasize the potential role of domestication in weakening behavioural defences. They support the idea that the behavioural divergence between wild and domesticated individuals can arise from a process of direct or indirect selection on reduced responsiveness to predation risk, or as a lack of previous experience with predators.     

Effects of sea trout stocking on the population genetics of landlocked brown trout, Salmo trutta L., in the Conwy River system, North Wales, U.K.

The effect of the introduction of fry of anadromous sea trout, Salmo trutta L., on the genetic integrity of landlocked brown trout populations was evaluated. Samples were taken from six brown trout populations from streams above impassable waterfalls in the Conwy river system (North Wales, U.K.) in 1989 and 1990. Three of these streams had no known stocking history and three had been stocked with sea trout fry from the lower Conwy system over the last few years. Representatives of these sea trout were collected from two streams in the lower Conwy system and from a hatchery. Allele frequencies at 13 loci, six of which were polymorphic, were determined by starch gel electrophoresis.
The stocked populations were intermediate in their allele frequencies between unstocked brown trout and sea trout samples. A principal component analysis suggested significant numbers of hybrids in all of the stocked streams. This shows that some of the introduced sea trout did not migrate down the falls to the sea, but stayed in fresh water and hybridized with the local population. The significance of this finding for the conservation of the genetic resource of brown trout stocks is discussed.     

Genetic differentiation between freshwater resident and anadromous brown trout, Salmo trutta, within watercourses

Freshwater resident and anadromous life history forms of brown trout, Salmo trutta L., sampled from each of three watercourses were analysed for biochemical genetic variation. Ten enzyme systems encoded by a putative 30 loci were analysed. Significant genetic differences were found between the two life history forms in all three watercourses. Further, significant genetic differences were also found between the three anadromous populations analysed, although two of them are separated by a distance along the shore line by less than 2 km. The findings focus on the need to reconsider the conservation and management practice of wild and hatchery-reared stocks.     

Impact of live food on survival and growth of hatchery‐reared sea trout (Salmo trutta trutta L.) parr in the wild

Survival rates and growth parameters of hatchery‐reared sea trout (Salmo trutta trutta L.) fry were determined after stocking in the wild. The larvae were hatchery‐reared for 12 weeks in two groups: fry were fed either on live zooplankton and live chironomidae larvae (LFG), or fed a pellet diet (PFG). The survival rate and specific growth rates were higher in the LFG than in the PFG group. Most effective for hatchery‐reared fish intended for stocking was the natural, live feed. The mean number of chironomid larvae found in the stomachs of fish that were initially captured in the wild was significantly higher in the LFG than in the PFG group. The live diet supplied in the rearing period had a positive impact on the foraging skills of the sea trout fry and their survival in the wild after their release on 24 April 2010.     

Genetic structure of important resident brown trout breeding lines in Poland

The history of brown trout Salmo trutta L. stocking has long tradition in the European Union and other countries. Hundreds of hatchery facilities on continent have artificial broodstocks used for enhancement of neighbouring and also geographically far river basins. These practices have substantial effect on wild brown trout populations. To illuminate this phenomenon, eleven hatchery stocks and wild populations from northern Poland and Carpathian region were analysed using 13 microsatellite markers. Obtained results revealed high genetic diversity between studied stocks and clear differentiation between northern and southern populations and hybridization between these two major clads. As a recommendation, the principle of treating regions as metapopulations should be applied, which, in the case of Poland, means using the division of the northern and southern genetic lines that were revealed in the present study.     

Reproduction of interspecific hybrids of Atlantic salmon and brown trout in a stream environment

1. Reproduction between Atlantic salmon males and interspecific hybrid Salmo salar × Salmo trutta females was monitored in a controlled flow channel diverted from a south European river located at the edge of Atlantic salmon natural geographic distribution in Europe. 2. Post‐F1 hybrids were viable and survived in the wild, at least until dispersal from redds. After transfer to hatchery conditions, 67% survived into the second year. 3. The hybrids possessed 98 chromosomes: two sets of Atlantic salmon(2n = 58) and one set of brown trout (n = 40) chromosomes. 4. The existence of a low proportion of allotriploid individuals can be expected in rivers where Atlantic salmon and brown trout populations coexist.     

Genetic characterization of brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) populations from the Southern Balkans using mtDNA sequencing and RFLP analysis

In the present study, a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay was used to survey variation in a 1450-bp mitochondrial DNA (mtDNA) segment which comprises part of the cytochrome oxidase III (COIII) and ATPase subunit VI genes in 8 brown trout (Salmo trutta L) populations from the southern Balkans. In addition, a 300 bp fragment at the 5′ end of the control region was sequenced from representatives of the populations studied providing the opportunity to assign PCR-RFLP haplotypes into major phylogenetic lineages (i.e. Atlantic, Danubian, marmoratus, Adriatic and Mediterranean). The level of polymorphism found in the 1450 bp segment suggests that this PCR-RFLP assay may be useful for future diagnostic analyses of mitochondrial DNA in brown trout populations. A reduced within-population genetic variability but considerable among-population differentiation was observed. The results are in accordance with previous data on phylogeography of Mediterranean brown trout suggesting that mitochondrial DNA haplotypes are distributed in a mosaic pattern as a consequence of a complex evolutionary history. The present study shows that brown trout populations from the Southern Balkans are highly divergent and possess a unique genetic profile that should be taken into account when establishing conservation management programs. Handling editor: C. Sturmbauer     

Tales from scales: old DNA yields insights into contemporary evolutionary processes affecting fishes

Salmon and trout populations are suffering declines in abundance and diversity over much of their range around the Atlantic and Pacific rims as a consequence of many factors. One method of dealing with the decline has been to produce them in hatcheries but the wisdom of this approach has been hotly debated (e.g. Hilborn & Winton 1993 ; Waples 1999 ; Brannon et al. 2004 ). One concern is that domesticated hatchery strains will interbreed with locally adapted wild fish; but how do we study the genetic effects if the introgression might have occurred in the past? Hansen (2002 ) used DNA isolated from archived scales from brown trout, Salmo trutta ( Fig. 1 ), to show that domesticated trout had, to varying degrees, genetically introgressed with wild, native trout in two Danish rivers. Extending that study, Hansen et al. (2009 ) have examined DNA from brown trout scales in six Danish rivers collected during historical (1927–1956) and contemporary (2000–2006) periods and from two hatchery source populations, to assess the effects of stocking nonlocal strains of hatchery trout and declining abundance on genetic diversity. Using 21 microsatellite loci, they revealed that genetic change occurred between the historic and contemporary time periods. Many populations appeared to have some low level of introgression from hatchery stocks and two populations apparently experienced high levels of introgression. Hansen et al. (2009 ) also showed that population structure persists in contemporary populations despite apparent admixture and migration among populations, providing evidence that the locally adapted populations have struggled against and, to some extent, resisted being overwhelmed by repeated introductions of and interbreeding with non‐native, hatchery‐produced conspecifics.

Figure 1 Open in figure viewer PowerPoint Photograph of a brown trout, Salmo trutta, one of many species of salmon and trout (Family Salmonidae) that are widely produced in hatcheries to enhance recreational and commercial fisheries. Photo by Peter Westley, Memorial University of Newfoundland, St John's NFLD, Canada.     

Embryonic development of the endangered Caspian brown trout,Salmo trutta caspius (Kessler, 1877)

The Caspian brown trout, Salmo trutta caspius is an endangered species of economic value and better knowledge of its biology will contribute to conservation measures. In the present study we characterized its early development to identify critical embryonic stages, by observing embryos and larvae from a broodstock maintained in captivity incubated at 9 ± 1°C. Morphological analysis of landmark stages of embryonic development revealed that first cleavage occurred at 189 degree‐h, a many‐celled blastoderm formed at 62 degree‐days (dd), a germ ring at 72 dd, the neural keel stage at 88 dd, the blastopore closed at 137 dd, the eye became pigmented at 188 dd and the larvae hatched at 373 dd. The detailed timing of these landmark features will help development of strategies aiming at increased aquaculture production of this species.     

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.     

An assessment of the spatial scale of local adaptation in brown trout (Salmo trutta L.): footprints of selection at microsatellite DNA loci

Local adaptation is considered a paradigm in studies of salmonid fish populations. Yet, little is known about the geographical scale of local adaptation. Is adaptive divergence primarily evident at the scale of regions or individual populations? Also, many salmonid populations are subject to spawning intrusion by farmed conspecifics that experience selection regimes fundamentally different from wild populations. This prompts the question if adaptive differences between wild populations and hatchery strains are more pronounced than between different wild populations? We addressed these issues by analyzing variation at 74 microsatellite loci (including anonymous and expressed sequence tag- and quantitative trait locus-linked markers) in 15 anadromous wild brown trout (Salmo trutta L.) populations, representing five geographical regions, along with two lake populations and two hatchery strains used for stocking some of the populations. F_ST-based outlier tests revealed more outlier loci between different geographical regions separated by 522±228 km (mean±s.d.) than between populations within regions separated by 117±79 km (mean±s.d.). A significant association between geographical distance and number of outliers between regions was evident. There was no evidence for more outliers in comparisons involving hatchery trout, but the loci under putative selection generally were not the same as those found to be outliers between wild populations. Our study supports the notion of local adaption being increasingly important at the scale of regions as compared with individual populations, and suggests that loci involved in adaptation to captive environments are not necessarily the same as those involved in adaptive divergence among wild populations.