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).