Dataset available for download here: http://dx.doi.org/10.7489/1607-1
Given a set of adult and emigrant traps it is possible to estimate a return rate that provides a combined metric of marine, freshwater and fisheries mortality. Return rates can be calculated in a number of ways depending on available data. Here they are illustrated by calculating the percentage of total emigrants that return as adult female spawners (Fig. 1). Emigrant numbers at the Girnock Burn have been corrected for historic periods of low trap efficiency in the autumn (Bacon et al, 2015).
Temporal trends in return rates are broadly similar to those for adult returns, with an overall downward trend between the 1970’s and mid-1990’s, followed by a small increase or stable conditions thereafter. Return rates from salmon emigrating to sea in 2013 and 2014 were 42% and 30% of the long-term mean for the Girnock respectively. In the case of the Baddoch, there are no records prior to 1988 when return rates were typically higher. Consequently, the estimates for 2013 were higher than observed at Girnock at 112% of the long-term mean. The return rate for 2014 was lower at 14%. Assuming an equal ratio of males and females, the overall return rate of 2014 emigrants from the Girnock and Baddoch was ca. 0.7% and 0.3% respectively. This compares to a maximum observed return rate of ca. 6.8% from fish emigrating in1971, despite major reductions in high seas, coastal and in-river fisheries in recent decades.
Figure 1. Percentage of juvenile emigrants (autumn parr and smolts) that return to the
Girnock and Baddoch traps as adult females