Salmon and recreational fisheries monitoring: traps

Autumn Emigrants (Parr)

Dataset available for download here: http://dx.doi.org/10.7489/1017-1

Autumn parr migrate downstream between September and December, with peak emigration occurring in October. They are typically aged 1-3 years old, but can emigrate as old as 5 years. Juvenile salmon emigrating in the autumn typically over-winter downstream before entering the sea the following summer (Youngson and Simpson, 1984). Counts of autumn migrants are shown in Figure 1.

Autumn parr can make up a substantial proportion of total production; however the importance of this component has been under-estimated or ignored by some studies (Wyatt and Barnard, 1997) despite having similar survival rates to spring emigrants (Youngson et al., 1994, Ibbotson et al., 2013). This is problematic when comparing stock-recruitment relationships between or within catchments, especially where the seasonal components also show trends.

There were a number of years during the 1980s where insufficient effort was deployed at the Girnock trap to ensure optimal efficiency. As such, emigrant numbers have been corrected for effort where possible (Fig.1: green line), or excluded in the case of extreme low effort (only a few trap visits). The data correction process is detailed in Bacon et al. (2015).

The Girnock and Baddoch data exhibit opposing trends in the numbers of autumn parr, with numbers generally decreasing at Girnock and increasing at Baddoch over their respective monitoring periods. In the case of the Girnock, the trends are thought to reflect changes in juvenile competition and growth that influence the time and age of emigration. A detailed analysis of trends in the number and age composition of migrants from the Girnock Burn is provided by Gurney et al. (2008).

Figure 1. Autumn emigrant numbers from the Girnock and Baddoch Burns. The green line shows corrected counts of emigrant parr in years where trap effort was estimated to be sub-optimal (See Bacon et al. [2015] for further information).