Observation and quantification of two incidents of mass fish kill of Icelandic summer spawning herring (Clupea harengus) in the winter 2012/2013

Abstract

AbstractThe Icelandic summer‐spawning herring (Clupea harengus) stock overwinters in large, dense schools like other herring stocks. In the winter of 2012/2013 around 300 thousand tonnes, or ~70% of the spawning stock, overwintered in a fjord west of Iceland. The inner part of the fjord, where the herring was located, is separated from the outer part with a natural barrier and a built‐up road with a 210 m long bridge. This creates strong tidal currents under the bridge. On 14 December and again on 1 February mass mortalities of herring took place in this location, and the sea floor and the shores were covered with dead herring. Fieldwork, including camera and video recordings on the shore and on a small boat, was conducted 3 and 4 days after the incidents. Results from this indicated that a total of 55 thousand tonnes had died during these two incidents, an amount nearly equal to the total catch from the stock that year. Measurements of environmental conditions in the days following the incidents showed that the oxygen saturation was generally 20%–40% but was as low as 15% (1.1 ml/L). The most likely explanation for the mortalities was oxygen depletion resulting from respiration of the large herring biomass, limited atmospheric‐water gas exchange due to calm and cold weather prior to both incidents and sea ice on part of the fjord, and limited renewal of water coming in and out via tidal currents. Aerobic decomposition of dead herring came additionally in the latter incident.