The biology of the spotted stargazer Geniagnus monopterygius (Bloch and Schneider) in Wellington Harbour, New Zealand

Abstract

The major features of the biology of the spotted stargazer, Geniagnus monopterygius (Bloch and Schneider, 1801) (Uranoscopidae), in Wellington Harbour were determined from a study of 335 specimens collected at monthly intervals. The length-weight relationship follows the cube law with no significant difference between the sexes, and seasonal variation in its relative condition factor is correlated with gonad development and food intake. Otoliths of G. monopterygius give satisfactory age determination enabling growth studies to be made. The growth of the fish is expressed by von Bertalanffy growth equation fitted by means of a Walford plot. The sexes exhibit a difference in growth rate, that of the males being slower. A visual maturity scale with supporting evidence from its coefficient of maturity and relative condition factor establishes that the spawning period is relatively long, from June of one year till January of the following year. The fecundity of G. monopterygius is comparatively low, 157,000 at length of 44.5cm, but the species has relatively large mature ova, slightly over 2.0mm in diameter. The food spectrum, as established by an analysis of stomach contents by occurrence, numbers and volume (by points) methods shows that G. monopterygius feeds mainly on teleosts. Laboratory observation made on the preying habit of the fish reveals that the time of intake of a prey is less than one-eighteenth of a second. Parasites infecting the fish are a digenean (Neocreadium geniagnii Howell, 1966), an encysted nematode larva (Contracaecum(Thynnascaris) sp.) and an unrecorded trypanorhynchan cestode larva ((?)Gymnorhynchus sp.).