The decline and extinction of Noble crayfish in the river Ljungan, Mid-Sweden: A case of non-sustainable harvest

The Noble crayfish (Astacus astacus) is central to Swedish food culture and local societies. This species is now red listed and a treathened species due to mortality by an invasive disease and over exploitation.

Mid- and Northern Sweden is still free from the disease but many areas show a decline and extinction of the crayfish. The crayfish population in the river Ljungan crashed and went extinct in 1999 by unknown causes.

Here we present historical data of catches in the river Ljungan that suggests that overexploitation is the major reason for the crash. Analysing the data in a linear model we predict total extinction by the year 2003 due to over fishing. The harvest effect is largest on the adult and reproductive cohort of the population.

In 2004 a reintroduction of noble crayfish was made from populations remaining in surrounding areas. Until today 60 000 crayfishes had been reintroduced. The controlled reintroduction of noble crayfish to the river Ljungan provides a unique possibility to develop a sustainable harvest system that will maintain viable populations and for the conservation of the species in northern Sweden.