2 Abundance of early life history stages of the rock lobster
Red rock lobsters (Jasus edwardsii) support one of New Zealand’s most valuable fisheries. Understanding larval recruitment processes greatly assist management of this fishery.
Rock lobsters spend several months as phyllosoma larvae tens to hundreds of kilometres offshore. Phyllosomas can disperse large distances: advanced phyllosomas were taken to the seaward extent of east coast trawl transects during trawl surveys and are widespread in the south Tasman Sea. At least recently, advanced (mid- and late-stage) phyllosomas have been much more abundant off the east coast of the North Island south of East Cape than off the east coast of the South Island (Booth & Forman 1995). This pattern appears to be determined by factors that include levels of local rock lobster larval production and the oceanography. Surveys have been carried out since the 1970s using mainly fine-meshed mid-water trawls, but also bottom trawls and bongo nets. Survey designs have include both transects and strata area.
Rock lobsters return to the shore as pueruli. The puerulus stage is the settling stage: it resembles the juvenile in shape and is 9-13mm in carapace length, but is transparent. Puerulus settlement happens when pelagic pueruli cease extensive forward swimming and take up residence on the substrate or in a crevice collector. Some older pueruli and young juveniles, however, move into collectors after first settling elsewhere.
Key sites to follow levels of settlement on crevice collectors (see Booth et al. 1991 for collector design) have been set up in the main rock lobster fishing coasts of New Zealand. Collectors are set in groups of 3-6, with a minimum spacing of 2-3m between individual collectors. At each key site there is a core group; additional groups of collectors are set in both directions along the coast, as conditions allow, 0.1-25km from the core collectors. At most sites, collectors are checked monthly and all lobsters removed. These collectors provide a combined index of:
- the number of pelagic pueruli in the water column which are settling;
- the result of post-settlement migration, the net number of older animals (older pueruli, and less often, young juveniles) moving onto the collector after having lived on the surrounding sea floor, and animals of similar age moving from the collector to the surrounding sea floor.
The index of annual settlement is the average catch per collector of pueruli, plus post-pueruli up to and including 14.5mm carapace length combined, of the core collectors over the main settlement season. The main settlement season varies between 6 and 10 months according to site, so values of annual index are not always directly comparable between sites.
Knowing the abundance of early life history stages (phyllosomas, pueruli, and young juveniles) may lead researchers to the factors that drive fishery recruitment. It may be possible to relate changes in levels of settlement to changes in breeding stock abundance, abundance of advanced larvae, and to changes in the ocean climate. Information on year-to-year settlement levels may be used to predict trends in recruitment, provide early warning of over fishing, and indicate to what extent recruitment varies from year to year. A discussion of the abundance of early life history stages of lobsters and the implications to fishery management are detailed in Booth et al. 1998.