Two genetic programs are being undertaken.
The Tweed Foundation is participating in the Living North Sea Program which, inter alia, aims to build a population genetic “map”, comprising information about the genetic profiles of all major Salmo trutta populations in the North Sea area. As many samples as possible are being collected for the major rivers draining into the North Sea, include the Tweed, and for adjacent rivers/populations that are likely to produce trout that migrate into the North Sea. More populations will be added to the database at later stages if more comprehensive sampling becomes possible. Tweed samples are being collected for this program.
The analysis of salmon genetics will be from samples collected from 36 sites on Tweed rivers. The analysis will principally support and inform ongoing improvement of local management practices but will also provide samples for analysis in the SALSEA-Merge project which is taking place across Scotland.
The most basic information needed on the Salmon of the Tweed is their stock structure: Is there just one interbreeding stock throughout the whole catchment, or are there stocks differentiable by their life-histories and / or genetics - essentially, is there a Tweed Salmon population or are there Tweed Salmon populations? Evidence from elsewhere is that there will be a number of genetically distinctive stocks within the Tweed catchment and the radio-tracking carried out from 1994-96 showed that particular runs of fish came from different parts of the catchment (Map 1)
Each such stock needs to be managed as a separate unit, as far as possible, to make sure that enough adults of each type of salmon are escaping to fully spawn their next generation and that the habitat of the nursery areas of each type of salmon are as in good condition as possible to maximise survival to smolting. The Tweed has a long, 10 month, fishing season, which is, presumably, based on a number of different populations that return to the river at different times. To keep this long season therefore, all the contributing populations need to be kept in good heath.
This requires that the home, nursery, areas of each stock of Salmon be defined and mapped, so that the juvenile abundances of each stock can be monitored to check on their state. At present, data from juvenile sampling sites cannot be assigned to any particular run of Salmon, which greatly limits its usefulness, as juvenile data cannot be matched up with adult data to see how populations as a whole are managing.
Knowledge of stock structure is therefore the most basic requirement for effective salmon management as can be seen from Diagram 1, which gives the structure of the fisheries management process for the Tweed.
This requirement is summed in the Fisheries Management Plan (FMP) for the Tweed and Eye Fisheries District as:
Policy 1.A (1) - Confirm and Refine Knowledge of the Salmon Stock Structure of the Tweed
(e) Devise a large scale genetics survey for the Tweed and Eye catchments to find out:
(i) How many different stocks of genetically distinctive Salmon there are?
(ii) What areas they occupy?
(iii) Map these areas and relate to the map produced for Policy 1.A (1) c
(iv) Determine whether the number of juvenile electric-fishing sampling sites within each population’s area is adequate to show trends in that population.
Policy 1.A (2) Continue collection of Salmon scales from the sample fisheries along the course of the main river and from the larger tributaries:-
(b) Analyse DNA from these scales to show which part of the catchment they came from and therefore which parts of the catchment produce the different types of adult fish (Spring, Summer, Autumn, Salmon, Grilse)
6 Depending on the results obtained from this genetics work, other parts of the Management Plan to do with working out the stock structure of Salmon will be superseded to a greater or lesser extent.
1.A (1) a – Tracking of fish back to their spawning areas
1.A (1)b – Mapping spawning times of salmon (early running fish spawn earlier, another way of finding out where Spring Salmon breed)
1.A (1)d – Microtagging of parr, so that when adults were caught, their area of origin within the system could be known.
A particular issue for genetic analyses of Tweed salmon not really encountered elsewhere is the number and extent of “new” populations that have naturally recolonised tributaries after obstacles were removed. The salmon of the Gala only date back to 1949 and those of the Leader to 1959. While some salmon were getting into the Whiteadder before its full opening up in the early 1990’s, electric-fishing results for juveniles show a great increase in both numbers and spread from the first survey of 1988 to subsequent surveys in the later 1990’s. Taken together, these three tributaries account for approximately 27% of the catchment where it might be expected that distinctive salmon populations will not be found.
It was to look at this issue that Exeter University were commissioned in 2006 to undertake a study of the Salmon of the Gala to see if a distinctive population had developed since 1949 (in 50 years there would be around 10-12 generations of salmon) and to see if the sources of the recolonising salmon could be identified. To do this, they analysed samples of juveniles from the Gala and nearby areas and a spread of samples from the catchment as a whole.
The method that was used to test how distinctive the fish from the different areas sampled were was to “assign back”. This involves using the genetic information from individual fish to “assign” them to one or other of the populations tested and see if they are identified back to their source population or not. If the populations are very distinct genetically, then individual fish from them will be assigned back in the blind test with a high level of accuracy. The less different the populations are from one another, the more likely individual fish are to be mis-placed.
The Gala fish tested assigned back to their neighbouring populations: 28% to Lower Ettrick; 18% to the Caddon, 16% to Upper Ettrick and 12% to Middle Tweed (74% of samples) indicating that these areas must be where the fish that recolonised the Gala came from. This means that it has not been possible to distinguish Ettrick and Gala fish genetically yet and that there may be something of a “smudge” in the Ettrick / Gala area on the genetic map.
Similar testing of the samples taken from the other parts of the catchment showed that while most fish assigned back to their source populations, there was considerable overlap with their neighbours. For example: 41% of Upper Ettrick fish assigned back to the Upper Ettrick and 25% to Lower Ettrick, making 66% identifiable as Ettrick fish. On the Teviot, 35% of Lower Teviot assigned back to Lower Teviot and 17% to Upper, making 52% testing back to the Teviot. While catchment accuracy is better than that of sub-catchment, it is still comparatively low; Tweed salmon may have relatively little genetic variation overall.
While the present survey will be more comprehensive and based on larger samples than the 2006 Gala one (and will utilise more genetic markers) it is likely that the resulting genetic map will be relatively “fuzzy” in that it will not be possible to identify adult fish as belonging to particular areas of the catchment with high levels of confidence.
However, it may be possible to sharpen up the picture of stock structure with information from other sources such as tracking. It might be found, for example, that all fish with “Upper Ettrick style” genetic ID in Spring did, indeed, head back to the Upper Ettrick but that others, later in the year, went back to different parts of the catchment.