As was reported, about 30 thousand tonnes of herring died in Kolgrafafjord in December 2012 and specialists from the Marine Research Institute repeatedly visited the site this past December and January. On February 1 a second mass death was observed in Kolgrafjord. A group of specialists from the Marine Research Institute returned to Kolgrafafjord on Monday, February 4 aiming to study the environmental conditions, to estimate the amount of dead herring and the size of the area affected. The study was conducted onboard the boat Bolli SH. The study included oceanographic conditions (temperature, salinity, oxygen) and inspection of the bottom by underwater camera. In addition, the beaches were closely inspected and the amount of dead herring on the shore was estimated. The preliminary analysis is completed.
Temperature and salinity measurements were similar to those of the Marine Research Institute in mid-January. The institute also measured the dissolved oxygen at the surface and near bottom. Oxygen concentration was between 47-63 %, much lower at the bottom than near the surface. No samples had an oxygen concentration lower than that measured in December, but that was measured 5 days after the mass death on December 13. Interpretation of these results must consider that the three days between the mass death and the taking of measurements were very windy. Oxygen transferral from the air to seawater is strongly connected to wind speed and if the seawater is under saturated the concentration in the upper layers increases rapidly with increasing wind speed. For this reason, it is likely that the concentration of oxygen measured last Monday was higher than that on Friday, February 1.
Amount of dead herring
The area inside the bridge was inspected by undersea camera and photographs were taken on four transects in the fjord. A transect was taken from the bottom of the fjord in the direction of the bridge, which lies across the fjord. The depth ranges from 10-42 meters. In addition, three transects were photographed in the westernmost portion of the fjord where most of the dead herring were (Figure). Furthermore, the amount of dead herring on the beach was estimated from counts on 11 transects, from the top of the beach to sea level at each location. This estimate was conducted late in the day on February 5 to coincide with low tide. The results show that recently deceased herring was observed in the westernmost portion of the fjord but nowhere else in the fjord inside the bridge. The Marine Research Institute has analysed the data and concludes that about 22 thousand tonnes of herring died in Kolgrafafjord on February 1, 2013. There is considerable uncertainty in this estimate due to the distribution of the herring over a large area. Its likely that over 50 thousand tonnes of herring have died in the fjord since mid December. Only a negligent amount of live herring was observed inside the fjord on February 4, but fluctuations in the amount of herring inside the bridge have been observed since monitoring began following the last mass death. The expected rotting will maintain low oxygen levels in the coming weeks and therefore, herring could be in danger if they enter the area in great numbers. The Marine Research Institute will monitor the situation in the fjord closely. Kolgrafarfjord. Figure shows camera transects (yellow lines) as well as recently deceased herring on the bottom and on the beaches within the fjord on February 4.