Water exchange, circulation and oxygen in a small fjord in Iceland in relation to events of massive herring mortality

Abstract During the winter of 2012-2013 around 55 thousand tons of herring died due to lack of oxygen in two separate incidents in the small fjord Kolgrafafjor ð ur on the west coast of Iceland. In 2004, a bridge and a connecting road were built across the mouth of the fjord. A subsequent decrease in renewal of water inside the fjord was considered a possible factor contributing to lower oxygen levels in the fjord. After the mass mortalities, a research project was started to study possible effects of the bridge on the water exchange, circulation and oxygen levels in the fjord. Another purpose of this study was to investigate other possible factors that could have led to the observed mass mortality of herring. This involved measurements of currents, wind, sea-level, temperature, salinity and oxygen at various positions and depths. One factor contributing to the mass mortalities was the presence of large amounts of herring in the fjord. Results indicate that it is unlikely that the construction of the bridge was a significant factor in the chain of events contributing to the mass mortalities. Calm, cold weather during both incidents that reduced the exchange of oxygen through the ocean-atmosphere interface was likely more significant. It is also probable that ice formation prevented the herring from coming up to the surface to fill their swim bladders with air.