Sammendrag
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.
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