5.4 Parr (age-≥1 salmon)
Parr densities exhibit complex spatial patterns of abundance over and above the effect of altitude that are thought to relate to substrate characteristics and over wintering habitat, though water quality and food availability are also likely secondary effects (Fig. 9). Electrofishing data over five decades show significant downward trends once the data are corrected for observation bias. The electrofishing data, combined with advanced population models, show that the survival of fry to one year old parr is strongly density dependent and this is where much of the natural population regulation has occurred historically at the Girnock (Fig. 10b). In recent years low numbers of returning females mean that ova deposition is insufficient to fully stock the river resulting in salmon production that is well-below carrying capacity for fry, parr and emigrants. In these circumstances any management action aimed at increasing juvenile production would need to reduce density independent rates of mortality rather than increase the carrying capacity of habitat. In river systems such as the Girnock where there is ample cover from large boulders and plentiful available habitat, making improvements to juvenile habitat quality without addressing poor adult returns and ova survival are unlikely to be successful. However, reducing early life stage mortality without adequate understanding of the constraints on parr survival will also likely be unsuccessful as shown through previous conservation stocking experiments (Glover et al., 2018).