During a SCUBA survey on herring spawning beds on and near Fisherman's Bank (NAFO Div.4T) conducted in September 1985, divers observed a spawning bed with heavy egg depositions, whichfrequently reached thicknesses between 20 to 30 layers. Continuous egg carpets of this thicknessextended over an area of approximately 200 × 500 m. Most of the egg mass on this spawning bed wasin a state of deterioration, showing mass mortality and extended fungi coverage.

Egg samples taken from various layers showed that almost all eggs were fertilized. Live egg batches were incubated for further observation. Embryos were found to be in various developmental stages. The rate of development was obviously related to the position within the egg mass. Many embryos from the surface appeared to be normal. All eggs located below the second layer exhibited embryoswith various malformations.

Egg samples did not show any evidence of the presence of Ichtyophonus, a known fungal disease in herring. None of the isolates obtained are known to be pathogenic to fish eggs. The observed mass mortality of herring eggs on the spawning bed was mainly caused by spontaneous intensive mass spawning leading to an unusually dense packing of the deposited eggs and therefore not permitting sufficient separation for water exchange and oxygen supply.

Environmental factors accompanying such spontaneous mass spawning are not well understood.Vertical temperature profiles from nearby locations showed sudden sub-surface temperature changescoinciding with the time of spawning. Meteorological conditions indicated wind-related water displacement near the spawning bed. While no drastic change in temperature was observed near the surface, changes of about 8 °C occurred in one day, at 20 m depth. These events may have triggered the mass spawning in a restricted area, later resulting in heavy mortalities when embryos reached advanced developmental stages requiring substantial oxygen supplies.