Stocking the Girnock Burn fails to increase salmon production

Fisheries enhancement’ (specifically mitigation stocking) has often been used to offset fisheries or environmental pressures in marine, diadromous, and freshwater fish species, despite relatively scant data to support its efficacy. When considering salmonids, there is a large (and growing) body of literature on experimental studies which points to substantial risks where enhancements are intended to protect, support, restore, or enhance wild fish populations through the introduction of cultured (farmed) or non-native (transferred between systems) fish. We outline some of the problems on our library page on trout stocking, here.

Within the UK, one such long-term, seminal study site is the Girnock Burn, part of an intensively studied research catchment on the Aberdeenshire Dee, in north eastern Scotland. Scientists from Marine Scotland published a paper last year which presents the findings of an experimental conservation stocking programme designed to increase the freshwater production from a declining population of Scottish spring’ Atlantic salmon. The study can be accessed, here.

The stocking programme was in response to a long-term (~30 year) decline in returning female numbers, culminating in three successive years when egg numbers were estimated to be considerably lower than the threshold considered necessary to sustain production. It was designed in line with best practice guidance to maximise egg survival, minimise juvenile competition, and increase overall production. Hence, protocols were developed to:

  1. retain genetic integrity and diversity through the use of indigenous (ie returning) fish and by allowing multiple mating attempts;
  2. minimise overwinter mortality and promote natural rates of embryo development; and
  3. minimise local density-dependent mortality by distributing eggs at uniform densities throughout the catchment.

The Marine Scotland team assessed the relationship between stock (egg deposition) and freshwater production (juvenile emigrants), and determine whether the combination of incubation methods and stocking protocols used in the study resulted in detectable changes in emigrant production relative to natural conditions. They could do this using the wealth of long term data derived from adult and juvenile emigrant data collected over 34 years of natural spawning (1966 – 1999) in conjunction with 8 years of conservation stocking (2000 – 2007).

Despite stringent application of all the above, the results indicated that the stocking programme failed to increase salmon production, an important conclusion especially for a river where wild populations remain and where suitable habitat exists.