Otago Harbour has been bursting with life this summer due to a population explosion of crab larvae, called Munida gregaria. The shoaling animals have attracted thousands of seabirds to the harbour since November, and the feeding frenzy looks set to continue for a few more months. The assault on Munida is not only aerial – fishes, seals and sealions are also following the shoals from beneath. Outgoing tides are leaving Munida stranded on the shore, again offering an easy feast for birds such as red-billed and southern black-backed gulls, pied oystercatchers and spotted shags. Human residents of the harbour may not be quite as excited however as the stranded Munida are creating a stench like no other and the birds are painting the town red. Literally.

As these "post larvae" mature they go from swimming at the surface to sitting on the seafloor. Photo courtesy of Tomas Bird and the New Zealand Marine Studies Centre

Munida larvae are common off the coast of Otago in spring and early summer, and this summer have moved into the harbour en masse where they will mature and settle to the sea floor. There they may live for 2-3 years, possibly migrating to deeper offshore habitats – that is if they are not feasted upon first.

This summer’s Munida explosion is likely to bring hundreds of tonnes of food into Otago Harbour. And this is by no means unusual. Records from the Portobello Marine Laboratory, going back to the 1950’s show that large Munida aggregations in the harbour used to occur most years, although were quite variable and did not appear once every four or five years. However, this is the first time since 1999 that there has been a significant Munida event in the harbour. The cause of this long absence is up for debate.

The New Zealand Marine Studies Centre at Portobello currently has thousands of Munida on display. Get in quick though – they are sharing their tank with some rather hungry fish…

The display at the New Zealand Marine Studies Centre, Dunedin. Photo courtesy of Tomas Bird.