Larvae of Ruegen spring herring Clupea harengus L. were reared from naturally fertilzed eggs In 1990, 1991, and 1992, in order to examine which larval size parameters (length, mouth width, mouth opening and oesophagus diameter) correlated with the time of the external feeding start and the size of prey ingested. The experimental conditions were similar to those occurring naturally at this time
(at Greifswalder Bodden, Germany). First feeding took place during or at the end of the period when the oesophagus had significantly opened. The increase of the oesophagus diameter was related to the decrease of the yolk sac diameter, rather than to the increase in standard length. In addition to physiological causes, these results imply that the increase of the oesophagus diameter has mechanical causes,
such as the decrease of yolk sac pressure on the oesophagus as yolk is resorbed. A mean decrease of 2.0 ± 0.6 µm in yolk sac diameter implies a 1.0 µm increase in oesophagus diameter At the first feeding time point and later, the ratio between the oesophagus diameter and the width of eaten prey was closer to 1.1 than the corresponding ratios for mouth sizes. Whereas the ratio of the oesophagus diameter
and the width of the eaten prey was between 0.5 and 1.4, it varied between 1.4 and 7.4 for the corresponding mouth size parameters and the prey width. The results of this investigation show that external feeding starts when the yolk is exhausted sufficiently to allow the oesophagus to open so that prey organisms can pass through it. Thus, external feeding can be prevented, especially for young larvae, not only by prey limitation, but also by a restriction of the oesophagus opening - even if sufficient prey is available.