Spatiotemporal variation in the density distribution of sprat (Sprattus sprattus) in Hardangerfjorden and Sognefjorden

Abstract

Sprat (Sprattus sprattus) is a pelagic fish species of considerable importance for the Norwegian fjord ecosystems, especially in Sognefjorden and Hardangerfjorden. Coastal habitats like fjords are heavily impacted by human stressors such as aquaculture, hydropower, fishing, industry, and climate change. Therefore, there is a need to better understand how sprat is affected by these stressors regarding spatiotemporal distribution, behavioural changes, and abundance. During this thesis, the density structure of sprat, both horizontally and vertically, was investigated in two Norwegian fjords, Sognefjorden and Hardangerfjorden, to look for potential seasonal differences. The data were collected with research vessels using an echosounder (38 kHz) during an annual survey in the period 2015-2021, covering both summer and winter, though in different years. It appears as if most of the sprat density in Sognefjorden shifts further into each fjord arm during winter while staying further out in summer. However, this pattern did not appear for Hardangerfjorden. As for the vertical distribution, sprat perform diel vertical migration (DVM) and stay close to the surface at night and deeper during daytime, probably triggered by light intensity changes. During summer, sprat stay closer to the surface than during winter at night-time and performs a longer DVM. No size-dependent pattern was found when comparing horizontal and vertical distribution between small and large sprat. This master thesis shows that there is an effect of season and time of day on sprat’s spatiotemporal distribution that might impact the abundance estimates from the surveys. Depending on the severity of human impact, the abundance of sprat might diminish further, ultimately affecting the entire fjord ecosystem.