Otago Harbour in a red frenzy

By Rebecca McLeod 18/01/2010

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 a the surface to sitting on the seafloor
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.
The display at the New Zealand Marine Studies Centre, Dunedin. Photo courtesy of Tomas Bird.

0 Responses to “Otago Harbour in a red frenzy”

  • So, forgive my ignorance of things marine, but when you say that the absence since 1999 is up for debate, what are the proposed explanations?

    • To be honest I haven’t heard a lot of discussion about the possible drivers of the Munida aggregations. They are at the whim of the oceanographic conditions, so it is possible that changes in local currents, water temperature or nutrient conditions may be responsible. Perhaps it’s related to the El Nino Southern Ocean Oscillation? Does anyone else have any thoughts to add?

  • All very plausible, and alas, invertebrates are never very easy to model. Small shifts in birth or mortality rates can have big effects on the overall population.

  • Hi Rebecca,
    The munida swarming is a fascinating event and as I am going to be in the Dunedin area this week I would like to try and film them in the water, – can you suggest any particularly good spots within the harbour to do this? – ie reasonably sheltered, shallow/ OK vis?

    • Ooooo I’m not sure if they are around in great numbers at the moment or not – donning a wetsuit in my current condition to check it out isn’t really an option! I would suggest giving the NZ Marine Studies Centre a call (the Portobello Aquarium, phone 03 479 5826 ) – they should have a good idea what’s happening out there. As for good places to film, well the whole harbour is pretty shallow, but it can get quite murky after a big wind. A drive out towards the aquarium on the peninsula should allow you to find good spot that has easy access. The further out towards the harbour mouth you go the better the underwater scenery. Enjoy!