Biology of the cardinalfishGlossamia gjellerupi (Perciformes: Apogonidae) from the Sepik-Ramu River basin, Papua New Guinea

Abstract

Glossamia gjellerupi forms a considerable part of the fish biomass (25%) and density (12%) in floodplain margin streams and lower foothill streams throughout the Sepik-Ramu basin where it occurs up to an altitude of 320 meters above sea level. It prefers habitats with slow flow rates: pools, alongside banks, under cover of vegetation, etc. Biomass and density decreased with increasing current speeds. Condition factor and fat deposits were negatively correlated to current speeds, with populations from floodplain margin streams having the highest averages for both these indices. Fluctuations in population size appeared to be related to the irregular occurrence of spates. No seasonality in reproduction, condition and fat deposits were noted although peaks occurred according to specific local conditions. Fecundity was found to be low, increasing with fish size. Eggs in a ripe ovary were all in the same stage of development, with sizes of around 3.3 mm.G. gjellerupi is a male mouth brooder. Mouth-breeding may restrict the species to well oxygenated waters. Reproductive activity takes place throughout the year but is not random since clear peaks in gonad development occurred within different populations. Reproductive activity and/or survival rate of juveniles are possibly directly related to floods. Prolonged periods of environmental stability, such as those occurring in periods of less rainfall, seem to be conditional for the survival of juveniles. Fish up to 30 mm length fed predominantly on small benthic insect larvae. Fish larger than 60 mm are carnivorous top-predators feeding predominantly on crabs, bottom dwelling fish and large terrestrial insects. Fish of all sizes, except the largest, fed on caridinid prawns. Feeding habits varied between different habitats depending on local environmental conditions.