The ecology of the butterfish Odax pullus around the Kaikoura Peninsula

Abstract

This study examines the abundance, distribution, reproduction and feeding behaviour of the Odax pullus (butterfish) around the Kaikoura Peninsula. O. pullus is one of a small number of species of herbivorous fish inhabiting temperate marine waters and is one of only two herbivorous species that are widely distributed in New Zealand and extend their ranges into cold southern latitudes. In this study, the abundance of butterfish across habitats and depths, and associations with other mobile reef fish species, were examined monthly for thirteen months. This enabled a determination of the spatial and temporal variability of butterfish populations. Visual underwater transects at nine sites showed that butterfish were associated exclusively with habitats dominated by large brown algae, particularly Lessonia variegata, Marginariella boryana and Carpophyllum spp. Abundance declined during winter, probably because larger fish migrated into deeper reef habitats. Recruitment occurred during summer. Butterfish are wide-ranging along the patch reefs around Kaikoura and there were no marked differences in the depth distribution of the different ontogenetic stages. Histological examination of gonads was used to determine sex, minimum size to reproduction, reproductive seasonality and length of the spawning season. Butterfish are reproductive from July to December, with peak gonad ripeness occurring in, September and October. There was a clear separation between sizes of female and male fish. No individuals < 370 mm SL, but all of those above this size, were males. This is suggestive that O. pullus is a protogynous hermaphrodite, which was further supported by the histological examination of gonads. Nine testes in fish> 380 mm SL contained both sperm and retained eggs. Nineteen female butterfish reached maturity at around 200 mm SL, which is smaller than found in earlier studies in the North Island. Scales and otoliths were used to estimate age. The number of distinct rings in these structures agreed well until the age six years, after which scales tended to under-estimate age. Despite the distinct size separation of female and male fish, the two sexes overlapped in ages over years 4 - 6. However, most of the oldest fish were males. Butterfish appear to be a fast-growing, relatively short-lived species that reaches a maximum age of around ten years. Very similar colour patterns of larger females and males indicated that visual sexing is often not accurate and, therefore, histological examination of the gonads is necessary to determine sex reliably, at least for larger individuals. Gut contents were analysed to determine the diet of butterfish and how it compared to findings of studies done in the North Island. The latitudinal change in the species mix of dominant large brown algae is reflected in the diet of O. pullus. Around Kaikoura, butterfish feed predominantly on the laminarian Lessonia variegata which is very abundant in subtidal habitats. Butterfish in northern New Zealand feed predominantly on Ecklonia radiata and Carpophyllum spp. This latitudinal diet shift indicates that butterfish are highly adaptable in their feeding habit. Furthermore, a small proportion of butterfish (6.5 %) targeted salps in their feeding. The presence of animal material in the gut contents of herbivorous fish has usually been ascribed to incidental feeding, but it is clear that this feeding is both opportunistic and selective in butterfish populations around Kaikoura. Spawning and its associated behaviours were not seen during this study, probably because they occur either in open water or else on deeper reefs. However, the sex distribution in Odax pullus indicates that this species has a complex social system and there is clear evidence of male territoriality during the reproductive period. There is considerable fishing pressure on butterfish populations around Kaikoura, which targets mostly larger fish. Despite this, the populations around Kaikoura seem to be robust. Butterfish have only a moderate life span for an inshore fish, but there are many juveniles in the local populations, indicating that there was successful recruitment for at least a few years,