Impact of sedimentation on the survival and availability of spawning grounds for brown trout (Salmo trutta L.) in the Central Otago streams

In the past few decades, rapid agriculturalization and urbanization have caused a massive increase in sediment concentrations within streams all over the world. This represents a potential threat to aquatic organisms. Imprudent deposition of sediments affects the stream biota by bringing about a change in the substratum conditions, affecting the spawning grounds for aquatic fauna. The lack of spawning grounds affects the life cycle and spawning activity of fishes. Brown trout, a species of salmonid native to Europe, has been introduced around the world from the late 1860’s. Brown trout have very specific requirements for spawning and early life history. The life cycle of brown trout requires cold, fast flowing streams, with abundant coarse gravel, in which they deposit large eggs. In catchments with extensive agricultural or urban development, fine sediment loads can increase considerably, smothering and filling interstitial spaces between coarse substrates. This prevents hyporheic (surface-subsurface) exchange of water, resulting in deoxygenation of the habitat in which the eggs develop. Large trout migrate from lower river reaches and the sea to the middle reaches of rivers for spawning. Excessive sedimentation could potentially render these sites along these middle reaches unsuitable for trout spawning, preventing migratory trout from spawning. My aim was to examine the physical and chemical quality of hyporheic habitat in streams affected by agricultural runoff and human activities with the aim of determining their suitability for trout spawning. I attempted to assess deposited sediments and hyporheic water exchange using the quorer technique and the minipiezometer respectively, at the sites along the middle reaches of rivers known to be used by migratory trout. Based on my results, I was unable to determine whether sedimentation was limiting the area available for trout spawning. In a subsequent experiment related to egg survival and mortality, I was unable to demonstrate measurable impacts on egg survival in relation to various physiochemical variables including dissolved oxygen, conductivity, vertical hydraulic gradient and sediment. I discuss challenges for assessing the impact of sedimentation on trout spawning, and suggest future strategies for addressing these challenges