This paper from the Tay DSFB provides some interesting thoughts regarding the lack of grilse in 2014 http://www.tdsfb.org/documents/FAQsWhathappenedtothe2014grilse.pdf . On the Spey the grilse have been in decline for a few years now, probably as suggested in the paper a response to changing oceanic conditions which may favour one species, and even one salmon life strategy, over another.

Spey Fishery Board