Juvenile Atlantic salmon - mapping the early running component: report

Discussion

The proportion of the early run timing component varied among sites, but, generally across rivers, a higher proportion was found at higher altitudes further upstream, whilst a lower proportion was observed lower down the river. This was especially prominent in the rivers Spey, Dee, North Esk and South Esk but less so in the rivers Ness, Findhorn, Tay, Tweed and Ullapool.

The general pattern of an increasing proportion of the early running component with increasing distance to sea is in accordance with historical data obtained through tracking studies (Laughton and Smith 1992, Smith and Johnstone 1996; Smith et al 1998; Webb 1992; Webb and Campbell 2000). The fish considered part of the early run stock component mostly spawn higher up in the river systems (Laughton and Smith 1992; Smith and Johnstone 1996). In contrast, the late run stock component predominantly return to and spawn at locations lower down the rivers (Smith et al. 1998; Webb 1992). This pattern is most clearly seen in the majority of the relatively linear systems that have a source in the Grampian Mountains (rivers Spey, Dee, Don, North Esk and South Esk).

In the other two linear East coast rivers originating in the same area (Ness and Findhorn), there were tributaries (the Moriston on the Ness and the Divie on the Findhorn) relatively close to the river mouth characterised by a relatively high early running component.

Similar results were observed for the dendritic river Tweed, where some of the highest proportions of the early running component were found in the Whiteadder Water and Glen (Till) tributaries that flow into the River Tweed quite close to its mouth (Figure 9). In addition, there was also a high proportion of the late running component found at the top of the Tweed, in the Gala water (Figure 9). Although the results from the Tweed differ from those in other areas, they do accord with the spawning locations of early and late running radio-tracked salmon (Webb and Campbell 2000). This suggests that the genetics is accurately defining those areas used by the different run timing groups and that the factors determining those areas may be more complex than simply distance from the sea.

The only west coast river examined here, the River Ullapool, showed a high proportion of the early running component along its entire length. However, rod catches in the Ullapool are dominated by salmon taken later in the year, suggesting that the late running component should dominate. It is unclear why the salmon in the Ullapool are assigned to the early running group; possible explanations range from a failure of the markers outside the area that they were developed for (e.g. the East Coast of Scotland) to a uniqueness of the genetic makeup of the Ullapool. Further work would be required to fully understand the population dynamics in the river Ullapool and if the markers work for other rivers on the West Coast.

In summary, the early running components identified in the juvenile samples, based on the genetic marker panel, corresponded well with rod catch and tracking data. This, therefore, demonstrates that the panel of genetic markers can be used as a tool to map run timing. Given the observed relationship with distance to sea, the next step would be to validate this genetic method with published tracking studies, as well as examining the possible influence and predictive power of river characteristics, such as distance to sea, altitude, etc. on the early running component. The latter, if proven powerful, could then be developed into a predictive tool to identify areas dominated by early running populations. This, in turn, could then be used for management and conservation purposes (MacLean 2007).