Massive Fish Kill After the Discharge of Artificial Fertilizer into a Species Rich River in Southwestern Germany: a Conservation Case Study

Abstract

Abstract In the summer of 2015, large amounts of artificial fertilizer containing ammonium nitrate were accidentally discharged into the Jagst River along with water used to extinguish a serious fire. The incident caused a massive fish kill the first 25 km downstream of the spill and impacted fish density along a 50 km stretch of this sensitive and important river. In this study, the long-term effects of the accident on the local fish fauna were investigated, and the implemented restoration measures were evaluated. A majority of fish surviving the immediate effects of the incident exhibited massive gill damage and weakness to infections shortly after the accident, but survival over the following winter appeared unimpaired. Two years after the accident, most survived fish appeared healthy. In 2016, about 9500 individuals (500 kg fish of 11 autochthonous species) were caught in unaffected sections of the river and distributed systematically into severely affected sections. Two control sections were left unstocked. Species diversity and fish density remained low over the first winter 2015/16, but increased in autumn 2016, most likely as a result of systematic stocking measures taken in response to the disaster. Stocking and natural migration were able to restore species diversity back to pre-accident levels in due time, i.e., 36 months, but density remains lower and shows no sign of further recovery. A positive consequence of the catastrophe has been the enactment in the ensuing years of various measures to improve the resilience of the Jagst River. However, connectivity is still lacking in relevant sections of the river and this, in combination with high predation from an increasing population of cormorants, has hampered the recovery of fish stocks. Generalizable conservation measures to mitigate the impact of similar catastrophes are developed and discussed.