Högskolan i Skövde

The burbot (Lota lota, Linnaeus 1758) is a benthic and predatory cod-relative that only occurs in fresh- and brackish water around the northern hemisphere. It has long been a common catch in Swedish lakes and rivers, but the species has seen a decline by over 20% since the mid 1980’s. This led to the burbot being red listed as nearly threatened (NT) in 2010. A decline of this magnitude might be alarming, the burbot is mostly declining in smaller lakes and rivers in southern Sweden and it’s still showing significant numbers in larger lakes such as Vänern – Sweden’s largest lake. Although recent monitoring of the burbot shows an erratic population that’s declining over several years, only to bounce back a couple of years later. This peculiar pattern made me interested in the historical aspects of the burbot population and its potential influencing factors in lake Vänern.

In this study I compare data from the oldest known gillnet sampling done in lake Vänern that also contain burbot with more contemporary samplings (2010, 2011, 2012, 2015 and 2018).

I manage to show that the number of large burbot caught were in line later years, but I also manage to show a decline by over 80% in the number of middle-sized burbot between 1973 and the later years. Furthermore, I produced results showing that the burbot seems to be aggregating deeper in later years compared to 1973.

Finally, I correlate the population changes to environmental data and get statistically significant correlations showing that the declining numbers of fish and their decent into deeper areas of the lake are connected to the rising water temperatures.

This result is in line with ecological studies that describes the burbot as cold-water dependent with a preferred temperature of around 10 – 14 °C. This study suggests that the burbot is affected by rising average water temperatures caused by climate change.