A two-way fish trap has been operated on the river Shieldaig since 1999. This system allows the capture of sea trout as they migrate to sea and as they return to the river. Individually marking the fish (using VI tags in 1999 and PIT tags since 2000) enables calculation of the proportion of tagged emigrants that subsequently return to the river, providing an index of marine conditions.
Due to the small number of wild fish migrating from the system, and the requirement for larger numbers to provide a more accurate measure of return rates, the Shieldaig system has been stocked since 1998. From 1997 to 2005, and from 2013, non-native trout were stocked into the river as fry and were identifiable by the presence of a clipped adipose fin1. During 2006-2008 stocking of eggs from a mix of native and non-native fish was undertaken, with eggs from known native origin fish being stocked from 2009 onwards. The return rates are therefore presented as two separate time series, one for clipped fish, which originated from fry stocking, and one for unclipped fish, which originated from natural spawning and stocked eggs. The return rates compared to the production cycles of the local farms are given below.
The proportion of marked sea trout of different origins returning to the River Shieldaig in relation to fish farm production cycles. Green bars are those in the first year of production, blue are in the second year. Data for clipped fish span 1999-2009 and 2015, unclipped 1999-2015.
1Hay, D.W. & Hatton-Ellis, M. (2006). Stocking sea trout (Salmo trutta L.) in the river Shieldaig, Scotland. Sea trout: biology, conservation and management, 349-355.