The biology of Grahamichthys radiatus (Cuvier and Valenciennes) (Pisces: Eleotridae) in Wellington Harbour

Abstract

Grahamichthys radiatus (Valenciennes) is the only marine eleotrid recorded from New Zealand waters. It represents the southern most extension to the distribution of the family Eleotridae, a widespread family throughout the tropical Indo-West Pacific region. A twelve month (July 1980 - June 1981) study of the Wellington Harbour population of Grahamichthys radiatus was undertaken. From the 12 monthly samples, biological aspects of the fish were analysed from 976 specimens. A 24 hour cruise was made where seven trawls were taken at five stations. The 24 hour sample allowed quantitative analysis of the population to be studied. Grahamichthys radiatus is a small gudgeon which has a bentho-pelagic habit. It's spawning activity is restricted to the narrow strip of neritic water because of confinement to areas of suitable substrate. Adult, pre-juvenile and larval specimens could be collected from the harbour all year round. Sexual dimorphism is highly characteristic of the Eleotridae and sexual differences were observed. Males attain greater maximum length (67 mm T.L.) compared to females (49 mm T.L.). The length-weight relationship conforms to the cube law with no observable difference in isometric growth between the sexes. The diet of Grahamichthys radiatus was composed of 43 different organisms with copepods constituting the bulk of prey consumed. The fish was shown to be an active predator with no differences in day and night feeding activity. Despite increase in body size, copepods were retained as the main component of the diet. The overall sex ratio derived from a sample of 976 specimens was .35 females:1 male. This result was biassed in favour of males because of the selectivity of the smaller otter trawl used. Grahamichthys radiatus was found to spawn all year round within Wellington Harbour with synchronous development of ova. Fecundity values ranged from 640 to 1600 ripe ova per spawning female. The fish is thought to deposit the eggs onto demersal material. Males displayed a stronger relationship of greater gonad maturity with increase in size compared to females. The ultrastructure of the spermatozoa from spawning males is described. Percentage frequency of parasite infection and parasite infection intensity (parasite burden) was high for each of the monthly gut samples examined especially for fish collected over spring. The cestode parasites found in the gut lumen were not identified. A descriptive, morphometric and meristic comparison is made between the two genera, Grahamichthys (Whitley 1956) and Fagasa (Schultz, 1943). Although both genera show similarities no support is given for the inclusion of both genera into either Fagasa or Grahamichthys. Clarification of the affinity between the two genera must await revision of the Gobioidea especially comparison with the eleotrid taxa of the tropical Indo-West-Pacific area.