Using Stable Isotopes to Identify the Maternal Origin of Trout Fry
The 2011 Sea trout Workshop pointed out that ‘ Research into anadromy would be simpler if it was possible to assign the maternity of juveniles to the resident or migratory trout. There are microchemical and isotopic methods that work in the youngest fry stages, based on geochemical environments of the mothers.’ Stable isotopes of carbon and nitrogen in trout reflect the prey it has been feeding on, and the ratios of these chemicals will differ depending on whether the fish has been feeding in freshwater or at sea. During the early part of their life, the ratios in fry are determined by their mothers’ feeding regime, making it possible to identify their maternal origin.
The AST has been supporting two projects that use this technology to investigate the origins of trout fry in the Tweed and the Deveron.
In June 2012 the Tweed Foundation and Edinburgh Napier University began a joint study aimed to map the freshwater resident or marine migratory maternal origin of trout fry within the Tweed catchment. The aim of the study is to provide life history information that will improve the Tweed Foundation’s practical management of both brown trout and sea trout within the Tweed system. The whole issue of the connection between sea trout and brown-trout is fundamental to this, as without knowing how much of the catchment’s nursery areas are producing brown-trout or sea trout it is impossible to gauge the strength of these two resources nor know what effect different management regimes might have, especially given that they represent two life history types of a single species. Previous work on brown-trout and sea-trout in the Tweed catchment was based on the trapping of spawning runs, which is both time-limited and resource-consuming and could never cover the catchment as a whole nor even a single tributary. Stable Isotope analysis, however, offers a much more efficient and effective approach.
An initial validation study showed that the ratios of naturally occurring stable isotopes of Nitrogen and Carbon could be used to identify the maternal origin of Tweed trout fry (Briers et al. 2012).
Sampling began in 2012 and was completed in 2014. Sites were chosen based on Tweed Foundation trapping; electro-fishing and fish counter data. The Upper Tweed and Gala Water were sampled in 2012; the Teviot and Till catchments in 2013 and the Whiteadder catchment in 2014. Samples of 10 fry were taken from the chosen sites with the total number of sampling sites being determined by finance. One of the main findings of the validation study was that the maternal Stable Isotope “signature” was still detectable in fry even in August. Based on this, the samples were taken in August at the same time as regular electro-fishing surveys to save time and resources. The year in which each area was sampled was also chosen to fit in with the Foundation’s electro-fishing schedule.
A number of environmental variables were recorded for each site from which samples were taken so that these factors could be taken into account during analysis if needed.
The Deveron study was a one-year pilot project intended to test the methods and technology used in the Tweed in relation to the Deveron. It was undertaken by the same team at Edinburgh Napier University, in collaboration with the Moray Firth Sea Trout Initiative.
The results from these projects so far show that, as might be expected, sea trout tend to predominate in the lower parts of tributaries and resident trout in the upper, but this is not a consistent pattern and some upper tributaries appear to be dominated by sea trout; there are also tributaries which appear to have mixed populations. The results have also shown that the effect of freshwater feeding by fry on isotope levels is greater than expected, making results from fry sampled in August difficult to interpret. This is particularly the case in methane-rich waters (such as those flowing through peat), where the isotopic signal from freshwater can swamp signals from the maternal origin of the fry.
In order to resolve this issue, the Tweed and the Deveron are jointly undertaking, with AST support, a supplementary one year project. This involves resampling a range of sites, chosen to cover the spread of isotope results, earlier in the year. Samples from fresh run sea trout are also being tested to provide a stable isotope reference value for feeding at sea. It is hoped that these results will assist with the interpretation of previous results and provide important guidance for further work using these technologies.
Freshwater / Marine Origins of Trout Fry in the Tweed Catchment
AST Funding: £2,000 a year 2013,2014 and 2015
An exploratory study of Freshwater / Marine Origins of Trout Fry in the Deveron Catchment
AST Funding: £2,000 in 2013
Validation of Stable Isotope Analysis: Tweed and Deveron
AST Funding: £5,000 in 2016