Scottish Marine and Freshwater Science Vol 6 No 12: The demography of a phenotypically mixed Atlantic salmon (Salmo salar) population as discerned for an eastern Scottish river

Implications for Future Salmon Population Management

The key to progress in our sub-stock study was to focus on the fewest sub-stock phenotypes (so as not to overwhelm the available data) that were biologically realistic and also of management interest.

In the absence of correction, the mis-reporting by rod-anglers (in Scotland) of some grilse as being the less-common late MSW salmon (MacLean et al. 1996) can result in appreciable over-estimation of the uncommon, but important late MSW sub-stock. Recording of the sizes and dates-of-capture of individual Atlantic salmon (Bacon et al. 2011) could greatly reduce grilse-error, which will vary over space and time depending on fishery practices and staff (Maclean et al. 1996).

Of some 2,000 rivers that support Atlantic salmon populations world-wide (Chaput, 2012) only 15, around 1%, have suitably detailed population data for SR parameter estimation (Crozier et al. 2003a). However, around 25%, have "annual river-specific stock assessments" (Chaput 2012), comprising monitoring that may be assessed against a stock-recruitment relationship transferred from another, donor, river. So it becomes important to try and generalise the understanding gained from the few data-rich (parameter donor) rivers like the North Esk, in order to allow transfer of appropriate information to other (recipient) rivers (eg Chaput 2012; Crozier et al., 2003a; ICES, 2011; MacLean 2007). However, as the composition of salmon populations (sea age, run timing) often varies between rivers it would seem inherently more useful and appropriate to transfer information for sub-stock components (assuming they behave differently). Such allocation could be done in proportion to sub-stock abundances if utilised spawning areas were unknown. Otherwise, transferring 'single-stock' estimates (which are amalgamated over all sub-stocks) from a 'donor' river which may well have a different sub-stock composition to the recipient river, risks inaccuracy.

Scottish salmon managers took early pre-emptive action to reduce exploitation of early MSW stocks, which, based on an assessment of trends in rod catches (assumed to be strongly related to adult returns), were then in decline. Many practical ameliorative actions are already underway for Scottish early MSW phenotypes ( e.g. catch-and-release, delayed starts to fishing seasons; see http://www.gov.scot/Topics/marine/Salmon-Trout-Coarse/game/conservationreg [Feb-2015] ) . However, grilse and late MSW phenotypes are still both widely confounded and poorly documented, making it hard to evaluate the sustainability of their (sub-)stocks, even in catchments where they probably predominate.

Our present analysis further emphasised the need for a sub-stock approach to Atlantic salmon rivers having diverse salmon phenotypes and spatially partially-segregated sub-stocks. It suggests that current information is inadequate to permit detailed modelling of such situations and draws attention to the considerations and types of new information [detailed population monitoring, genetic sub-stock-identification and spawning-zone delineation] which are most likely to lead to further progress and better management.