Swansea Salmon Research Group develops programme to identify hatchery fish

Over the last few years, the Swansea Salmon Research Group led by Dr Carlos Garcia de Leaniz and Dr Sofia Consuegra has worked closely with Natural Resources Wales and Welsh Water to develop a traceability programme for identifying hatchery fish in salmon stocking projects in Wales.

Swansea salmonWorking closely with Llyn-y-Fan hatchery and supported by funding from Welsh Government through SEACAMS, broodstock were sampled, genetically tagged and released last year as the first step in this ongoing programme of research. 

Stocking with hatchery-reared fish has been questioned as a mitigation tool due to the potential adverse genetic effects of hatchery fish on wild populations. However, data on the merits of stocking is still very scarce, as monitoring of hatchery fish through physical mark and recapture is expensive, time consuming, and limited to fish of a certain size. There are currently no compulsory marking systems for stocked fish in Wales (or anywhere else in  the UK) despite the fact that limits are being set to the maximum number of hatchery salmonids that can be stocked or, in some cases, the need to stock only sterile fish. 

Molecular tags can be used to assign fish to population of origin, determine paternity and distinguish between farmed and wild fish. The Salmon Research Group at Swansea is using molecular tags to develop a traceability system to enable the contribution of stocked fish to be determined, and to identify the fish from different stocks when these are being reared communally in hatcheries. In salmonid restoration programs, molecular tags can also be used to screen hatchery parents and to monitor the success of their offspring in stocking programmes according to a genetic equivalent of ‘mark-and-recapture’, thus dispensing with the need for costly and inefficient physical tagging programmes. 

All salmon broodstock available at Llyn-y-Fan hatchery from the Tywi, Teifi and East Cleddau rivers were sampled in 2012-13 and genetically screened for variation at 13 microsatellite loci. Total genomic DNA was extracted from fins, scales and mucus swabs from 32 samples (20 from the river Tywi, 7 from the Teifi and 5 from East Cleddau) and 13 di- and tetra-nucleotide microsatellites were PCR amplified in two multiplex reactions of 7 and 6 microsatellites respectively. PCR amplifications were carried out on the extracted DNA and fragment length was determined and scored. 

Genetic diversity and effective population size: Level of within-population genetic diversity was evaluated by considering population statistics: number of alleles, effective allele size, observed heterozygosity and unbiased expected heterozygosity. Results indicated that genetic diversity of the river Tywi - measured as heterozygosity and allelic diversity – are consistent with an acceptable large number of spawners in the population with no sign of high homozygosity or reduced genetic diversity. A larger sample would be needed to determine the genetic diversity of the other two populations from the Teifi and East Cleddau, although the observed heterozygosity of the few individuals sampled appears to be reasonably high.

Growth and relative survival of eggs planted in the River Tywi headwaters: A small scale investigation comparing the survival of eggs from different families at two different sites in the headwaters of the Tywi was carried out with the help of NRW and as part of an MRes thesis by Elizabeth Price (Price 2013). Eyed salmon eggs from 6 Tywi families were counted, split into equal groups and planted at sites along two impassable tributaries of the Tywi (Sawdde and Llechach, Figure 1) in February 2013. Electro-fishing surveys in May and July found significant differences in body size of alevins and eggs planted in the larger and less steep Llechach, resulting in larger fry than those planted in the smaller and steeper Sawdee. Differences in growth at the two sites became magnified over time. Ongoing parentage genetic analysis may reveal differences in relative survival between families.

Use of stable isotopes as trophic markers of hatchery fish: potential use of carbon and nitrogen isotopic signatures were tested as markers of hatchery-reared fish in stocking studies.  To this end it was necessary to assess for how long the enriched maternal component was detectable among the offspring of Atlantic salmon planted in the wild compared to fish kept in the hatchery as controls. The expectation was that juveniles in the wild would have high levels of δ15N and δ13C  whilst they were still feeding from the yolk sac, but that these would become diluted over time as they became less dependent on maternal reserves and began to forage for themselves.  In contrast, the isotopic signatures of fish kept in the hatchery were expected to remain high over time due to the enriched lipid content of fish food pellets.   The results of the study (Price 2013) indicate that enriched maternal signatures are still detectable in alevins from full yolk sac absorption to at least 95 days post-hatching.  Salmon fed a hatchery diet show significantly higher δ13C and δ15N values than those fed more natural diets, and tend thus to maintain, or even increase, the enriched baseline maternal signatures.  The results of this study also serve to highlight how stable isotope analysis can be used as a tool for distinguishing between naturally occurring and hatchery-reared juvenile salmon at the fry and parr life stages, and can hence help monitoring the success of hatchery–reared fish in salmonid conservation projects.