Habitat change effects on settlement and survival on post-larval and juvenile Jasus edwardsii

Abstract

Changes in habitats on coastal reefs is a global phenomenon that is thought to be triggered by human influences such as fishing of top predators. On the northeastern coast of New Zealand extensive areas of coastal reef have changed from kelp to barren reef habitat, possibly as a result of fishing pressure on key predators of sea urchins. It is unknown to what extent these habitat changes affect coastal fish assemblages or the recruitment of reef-dwelling benthic invertebrates, such as spiny lobsters. This study assessed the relative importance of these two different habitats in relation to aspects of the settlement and subsequent survival of post-larval spiny lobsters (Jasus edwardsii). A field experiment with settling pueruli (post-larvae) of the Australasian red spiny lobster, Jasus edwardsii, indicated that the presence of two kelp species (Carpophyllum maschalocarpum and Ecklonia radiata) did not increase initial settlement of pueruli. However, laboratory experiments showed that the post-settlement development of pueruli was expedited by underwater sound from both kelp and barren reef habitats, and through direct contact with kelp and clean rock surfaces. Furthermore, a laboratory experiment indicated that most developmental stages of pueruli preferred to shelter in rock crevices associated with attached kelp versus those without attached kelp. The use of photographic tethering methods for early juvenile lobsters indicated that there were no differences in the relative predation between kelp and barren habitats on coastal reefs at Leigh in northeastern New Zealand. Some of the limitations of traditional tethering methods were overcome through the development of a novel method of digital video recording for observing the behaviour of would-be predators of a lobster housed in a transparent bottle and illuminated with infrared light to allow night time observations. The use of this method indicated that there were differences in the relative predation of juvenile lobsters between kelp and barren habitats during daytime, but not during dusk, night and dawn periods. Baited underwater digital video recording methods were used to identify possible differences between kelp and barren habitats in the assemblages of fish species, including potential predators of early juvenile lobsters. A greater abundance and diversity of potential predatory fish species were present in barren habitat versus kelp habitat, and these differences were consistent during three seasons (autumn, winter, spring) and at night (summer), despite there being overall differences in fish abundances among seasons and diurnal periods. Overall, the results of this research failed to find evidence that the continuing loss of kelp habitat from coastal reefs in northeastern New Zealand will lead to a decrease in settlement of pueruli of J. edwardsii and subsequent survival of juveniles due to differences in predation pressure between the two habitats. However, further research is required to more closely examine possible differences in predation of early juvenile lobsters between the two habitat types, especially during those diurnal periods when juveniles are emergent and more vulnerable to predation.