Barry Bennett of Halfway Bush asks :-
Why do crayfish go on walks around the South Island?
Alison MacDiarmid, a marine zoologist with the National Institute of Water and Atmospheric Research, responded.
Crayfish, or rock lobsters, Jasus edwardsii, probably walk at least part way around the southern half of the South Island to compensate for the fact that their larval offspring are carried down-current from where they are hatched in Fiordland, Southland and Stewart Island.
In cooler southern waters female crayfish carry their fertilised eggs, as many as half a million depending on female size, under their tail for 150 days. The eggs hatch in spring enabling the spider shaped phylosoma larvae to swim quickly towards the surface. The larvae spend the next 12 to 18 months feeding and growing in the surface 200m of offshore waters. During this time they can be swept by currents far from where they hatched. Larvae hatched in Fiordland, Stewart Island and Southland may be swept around the bottom of the South Island and up the east coast to the Otago Peninsular, to Banks Peninsular or well beyond to the Chatham Islands or off the east coast of the North Island.
Eventually some of the larvae grow big enough to metamorphose or change into a non-feeding transparent stage known as the puerulus. This stage swims strongly toward shore from about 200km off the coast though how it navigates is unknown. Once it reaches shore it finds a hole or crevice in a reef and settles permanently. It sheds or moults its skin and transforms into a juvenile crayfish, a tiny replica of the adult. If it is lucky it survives and grows and reaches a large juvenile/sub-adult size after 5-7 years. Tagging studies show that a proportion of these larger juveniles around the Otago coastline migrate upstream against the prevailing current, which takes them down around Stewart Island, and up into Fiordland.
The numbers of juveniles that undertake this migration vary from year to year but the reason why this occurs is uncertain. Similar migrations of this species have not been found around other parts of the New Zealand coast. Once crayfish reach sexual maturity and start breeding they no longer undertake long distance migrations. Most adults spend the rest of their lives living along a 2-3 km stretch of coast.