Types and frequency of malformations and mortality in eggs of Arcto-Norwegian cod: A field study
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Embryological studies on living cod eggs in NUNC-trays showed that spontaneous malformations during embryo development are lethal. They are called natural egg mortality in contrast to external lethal factors, as predation, extreme physical conditions and pollution. The natural mortality in cod eggs, both coastal and Arcto-Norwegian, is significantly reduced from first to second spawning, from laboratory experiments. The same embryological method was used during field studies on natural egg mortality on planktonic cod eggs in the spawning area in Lofoten, Northern Norway, in the March - April years 2000 and 2001. The types malformations which was found in a fild also corresponding to certain morphogenetic movements in embryodevelopment and equel discover in the experimental work. Most part of lehal malformations (more than 50 %) is identified after hatching only. The pattern of types malformations differ between spawning grounds and to be consistant during two years. The frequencies of malformations to the end spawning period significantly reduced of 29,7 to 9,8 % for Vestfjord. To demonstrate the higher natural egg mortality in eggs from first time spawners, two strategies were followed: 1. Comparing natural egg mortality in egg samples from 2000 and 2001, collected during same period and from the same areas. 2.On the basis of the acoustic survey of the spawning population of Arcto-Norwegian cod in Lofoten, areas of high frequency of first time spawners were compared. Luckily, in the year 2001 the proportion of first time 2 spawners was specially high. A comparison between the years 2000 and 2001 demonstrated an increase in natural mortality from 23 to 31 %. In Vesterålen, the area of highest frequency of first time spawners, natural egg mortality increased from 31 to 46 %. Because of two systematic errors the numbers are minimum values.