Mitchell, LTN; (2016) An assessment of rehabilitation gravels for Salmo trutta spawning: a case study from a small chalk stream, the River Stiffkey, Norfolk, UK. Doctoral thesis , UCL (University College London).

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Abstract

UK salmonid stocks have shown a sharp decline over the past 50 years. River channel modification, land-use intensification and change in agricultural practices are significant factors that have contributed towards this decline. An associated accumulation of fine grained sediment in spawning substrate inhibits population recruitment at the embryo stage of the life-cycle. Additions of rehabilitation gravel to the River Stiffkey, a small chalk stream in North Norfolk, in 2003 by the Wild Trout Trust (WTT) and again in 2009 as part of the Living North Sea project aimed to augment migratory Salmo trutta L. (sea trout) populations. Rehabilitation by means of gravel introduction has anecdotal short-term benefits but physical environmental constraints at various spatial scales over the medium- to long-term have not yet been adequately quantified. In order to better understand the role of rehabilitation gravel in the reproduction and recruitment of S. trutta populations, this study examined: the physical suitability and morphosedimentary nature of rehabilitation gravel as a spawning habitat, embryo survival within rehabilitation gravel and sedimentary constraints that limit recruitment at this early life-stage, catchment controls that define the physical character of the river, and the spatial relationship between key juvenile life-stage dependent habitat types. River Stiffkey rehabilitation gravel was installed to similar specifications in 2003 and in 2009 and as such provided a spatial and temporal assessment of physical and biological variability. Results indicate the importance of catchment controls and historic regulation in determining channel processes. Rehabilitation gravel underwent a sediment composition succession from an unstable well sorted gravel (40-10 mm) type to a poorly sorted stable deposit as fine sediment (<1 mm) was deposited and surface gravels eroded. Embryo survival declined as fine sediment (<1 mm) accrued and permeability decreased. Rehabilitation gravel was characterised by a net loss of small sized gravel (30>D50≥16 mm) required for spawning by non-migratory S. trutta populations and accrued an abundance of fine grained sediments (D<1 mm) over time. Rehabilitation gravel installed in 2003 had consistently poor embryo survival, whilst gravel installed in 2009 had a positive response to a reduced sediment load stress. Consequently, rehabilitation gravel in lowland chalk stream catchments characterised by high diffuse inputs of agricultural sediment may have a short (<10 years) lifespan. As such rehabilitation gravel is likely to have a limited role in S. trutta recruitment. Recruitment of S. trutta in the River Stiffkey was regulated by both poor abundances of key life-stage dependent habitat, and spatial connectivity between them. A river rehabilitation management approach based on a hierarchy of spatial scales that identifies and addresses ecological constraints to recovery in a systematic top-down approach from the catchment level to the macrohabitat is proposed.

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