Sand gobies in the Dutch Wadden Sea (pomatoschistus, gobiidae, pisces)

Abstract

The seasonal fluctuation in numbers of sand gobies was investigated in the Dutch Wadden Sea by means of trawling surveys in 1962 and 1964. Fluctuation in numbers appeared to be mainly related to offshore migration of the gobies in winter and spring, and their short life span (2 years). The distribution of eggs shows that sand gobies spawn mainly in the North Sea at 10 to 25 m depth. Sand gobies may spawn in estuarine lagoons, and eggs were also found occasionally in the Wadden Sea, but there is no evidence that sand gobies preferably reproduce in shallow parts of estuaries. They generally disappear from the inshore areas at the onset of the reproductive season. Long term fluctuations in population numbers were investigated by comparison of total numbers of sand gobies in the Wadden Sea in the autumns of 1961, 1962, 1963 and 1964. A marked increase in numbers in 1964 followed the severe winter of 1962–1963. The related common goby ( Pomatoschistus microps) which is normally very abundant in the Wadden Sea, disappeared after this severe winter. The increase in numbers of sand gobies may have resulted from an increase in food supply and a decrease in competition and predation for the sand goby. Results of an investigation on the geographic variation indicated that 2 different subspecies of sand gobies occur in the Wadden Sea. They differ in size, outward appearance, arrangement of dermal papillae and vertebral number. The results of rearing experiments have demonstrated that the vertebral number is largely hereditary in these gobies. Besides, a slight phenotypic variation was found in relation to environmental conditions during the development of the eggs and larvae. Further investigation yielded more evidence of morphological differences, as well as differences in temperature requirements, reproductive season and food preference. Based on these differences and the sympatric occurrence in the North Sea, it is concluded that the 2 sand gobies represent different species: Pomatoschistus minutus Pallas and Pomatoschistus lozanoi de Buen. Their systematics are discussed.