Salmon and recreational fisheries monitoring: traps

Adult Returns and Population Status

Dataset available for download here: http://dx.doi.org/10.7489/1588-1

The total number of salmon caught at the Girnock and Baddoch traps is a useful indicator of the relative abundance of spring salmon. However, data on female fish numbers are often reported preferentially as they limit recruitment and adult males are thought to move more widely between catchments at spawning time. Validation and correction of data using historical paper records during 2014 has produced a corrected time series (Fig. 1). The number of adult females returning to the traps varied substantially between years. However, there was an overall declining trend in female spawners between the late 1960s and 1997, after which numbers have stabilised at a low level. In 2018, 15 females were caught in both the Baddoch and Girnock traps. This represents 54% and 29% of the long-term means for the Baddoch and Girnock respectively (Fig. 1)

Figure 1. Numbers of adult female salmon returning to the Girnock and Baddoch traps. The solid blue horizontal line represents the estimated stock level required to maximise production of emigrants (Smax) for each site, with horizontal blue dashed lines indicating the error around these estimates (95% C.L.s). In the case of the Girnock Burn, estimates of Smax assume no effect of the previous cohort on production. Note that conservation limits may be set at a range of levels and often at maximum sustainable yield, which is lower than Smax.

Maximum emigrant production is one of a number of possible targets used in salmon management and is typically considered where the objective of management is to maximise freshwater production rather than harvest opportunities. Smax is the number of females required to obtain maximum production and can be derived from stock-recruitment data where detailed information is available on the number of returning adult salmon and juvenile emigrants (with associated data on ages and fish sizes). In the case of the Girnock Burn, the estimate of Smax  (Fig. 1) assumes no effect of the previous cohort (after Bacon et al., 2015) and thus represents a minimum estimate of the numbers of females required to maintain maximum production. Any points falling within the 95% confidence limits of Smax have a reasonable expectation of attaining maximum production.

For recent years it is estimated that 49 (95% Confidence Limits: 35-67) and 40 (95% C.L: 20-72) females would be required to maximise production of emigrants (Smax) from the Girnock and Baddoch respectively. Returning female numbers were below the lower confidence limits for Smax at both the Girnock and Baddoch in 2018. This indicates that maximum emigrant production is unlikely to be attained.