Voyage to discover secrets of marine food web

Scientists are beginning a voyage to the middle of the marine food web to find out more about one of the most complex networks on the planet.

NIWA’s flagship research vessel Tangaroa has embarked for the Chatham Rise where scientists will be taking samples of the huge variety of small organisms that live in the water column and near the sea bed there.

Voyage programme leader and NIWA marine ecologist Matt Pinkerton says the middle part of the marine food web is a crucial part of the marine ecosystem but vastly under-researched.

"At the bottom of the food-web are microorganisms and phytoplankton, which is where it all begins, and at the top are fish, seabirds and mammals. We need to understand the bit in the middle that links the two ends together."

The midsection includes small crustaceans, zooplankton, gelatinous organisms, small squids, amphipods, shrimps, rat-tail fish, lantern fish and other small fishes (most of which are less than 20cm long) and eels.

"They are not things we harvest and are not commercially important, which is why we don’t understand enough about their importance. We need to learn more about this varied resource on which commercial fish and other top-of-the food-chain organisms rely," Dr Pinkerton says.

The Chatham Rise is an ideal place for this kind of study because it includes some of the most valuable fisheries in New Zealand and is home to more phytoplankton than anywhere else in NZ’s EEZ. The mixing of cold subantarctic water from the south and warm subtropical water from the north gives rise to the highest rates of phytoplankton growth in the New Zealand region.

However, obtaining samples for scientific study requires specialist equipment that can catch organisms that are small, fast moving and sometimes delicate. A fine mesh net, known as "the ratcatcher trawl" (because of its ability to catch small rat-tail fish) will be towed across the sea bed to gather samples and small nets will also be used in the water column.

In addition Tangaroa will be stationary at various points during the voyage to sample and observe movement in the water column over 24 hours to estimate consumption rates of fish and invertebrates from examination of stomach contents.

Dr Pinkerton says many of the organisms spend daytime near the sea bed but rise up through the water column at night where they partake in a huge feeding frenzy.

"We want to observe the feeding relationships, what things are eating what and what’s eating them. There will be a lot of stomach examinations."

This voyage is the sister to a similar voyage carried out in August – together the voyages represent the first of their kind to undertake a survey of this magnitude of this part of the food web. On the August voyage more than 500kg of biological samples were collected from almost 7700 fish of 121 species and more than 30,000 individual organisms were measured.

Once analyses from both voyages are complete, the results will be built into a model to help understand how the entire system fits together and how resilient it is to changes in one or more links in the chain.

"This research will inform management of New Zealand’s marine estate especially in regards to ecosystem-based management of fisheries and the resilience of these organisms to commercial fishing and climate change," Dr Pinkerton said.

Also joining the Tangaroa voyage to gain some hands-on experience with scientists are two NIWA Blake Ambassadors, Carl Meyer and Fenna Beets, both marine science students. They are part of a group of five NIWA Blake ambassadors joining research teams over summer.

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