Årsaker til rike og fattige årganger av sild

Abstract

Causes of rich and poor Year-classes of Herring.

This paper continues RUNNSTRØMS investigation on herring larvæ during 1932-35 and is based on material obtained by hatching artificially fertilized eggs as well as on plankton material from the West Coast during late winters. In a preliminary account of the work (1940) the author pointed out that the artificially bred larvæ as well as those found in the sea went through a "critical stage" when the yolk was consumed, and that the greater number of the larvæ were unable to survive this dangerous epoch. This stage was described in detail and the cause of the great mortality ascribed to lack of suitable food. It was furthermore pointed out from experience gained by tow-netting that the herring larvæ were not encountered at the surface in sunshine but only at some depth. By measuring many hundreds of larvæ it was found that the critical stage occurred about the end of March. In the present paper the experiments with fertilizing and hatching of the eggs and feeding of the larvæ are treated in more detail. The salinity was found to be unimportant within the range 22,17 -33,57 per mille. The influence of temperature investigated more closely and it was found that the biological zero-point of the herring egg is situated at approx. ÷ 1.2° C, and that the product of the time and the average temperature (counted from the biol. 0-point) was nearly constant in all hatching experiments, the latest as well as those made in earlier years. It was furthermore found that light is of great importance for the act of feeding of the larvæ. Contrary to earlier conceptions it was found that a vegetarian diet alone was unable to keep the larvæ alive in early life. It was observed that the larvæ, provided suitable illumination, were catching and eating barnacle-nauplii (Balanus balanoides) and those of the "brine shrimp" (Artemia salina) even before the yolk was completely used up. Such food enable the larvæ to be kept a long time beyond the critical stage. Tow-nettings in the later years corroborate the findings mentioned above, that the larvæ avoid direct sunshine and keep to a twilight zone. In the period succeeding the great reduction in the number of larvæ found in the plankton, great quantities of dead and semidissolved herring larvæ appeared in the catches and in these place the sea was exceptionally poor in nauplii. In other catches it appeared that eggs and nauplii of copepods (above all of Calanus finmarchicus) most commonly are found as the first gut-content of the herring larvæ, along with remains of diatoms. The later spreading and transport of the herring fry takes place both northwards along the coast with the coastal stream and into the fjords by means of the tidal currents. Investigations in the little landlocked fjord Nordåsvannet (near Bergen) from spring to autumn 1941 showed the occurrence of herring fry but only after it had passed through its metamorphosis into "whitebait" which must have been carried into the fjord by the very strong tidal current through the narrow entrance.