Mass mortality of herring eggs associated with a sedimenting diatom bloom

Abstract

This paper describes a mass mortality of herring (Clupea harengus L.) eggs laid on gravel and maerl ridges in a water depth of approximately 18 m in a site off the south coast of the Isle of Arran in the Firth of Clyde, Scotland in April 1990. When first examined the egg patch consisted mainly of healthy viable eggs (7% mortality), but within six days the mortality had increased to about 98%. This mortality was associated with a heavy precipitation of organic material largely composed of the diatom Skeletonema costatum (Greville) onto the egg layer. The oxygen level of 2.0 0.3 mg O21-1 measured in water taken from under the egg mat midway through the period of mortality indicated anoxia to be the likely cause of death. The implications of this event in relation to the survival of small populations of herring are discussed. © 1991 Conseil International pour l'Exploration de la Mer.