Waiter, there's something fishy in my soup

For a group of fisheries scientists in Wellington, the next three years will be dominated by fish guts.

In a ground-floor lab at the National Institute of Water & Atmospheric Research (NIWA), they're painstakingly slitting open the stomachs of thousands of fish and sifting through the contents.

All up, some 50,000 stomachs from about three dozen species caught on the Chatham Rise will be analysed by the end of 2007.

The project is funded by the Ministry of Fisheries (MFish). "We want to better understand how our fisheries fit into the bigger environmental picture," says MFish senior scientist Dr Ben Sharp.

"The Chatham Rise is one of New Zealand’s most productive fishing grounds, and is particularly important for our hoki fishery. We have a pretty good idea of target fish stocks themselves – for instance where they are found, and about how many are there. But until now, we haven't done much research on how the whole ecosystem works.

"This work will provide some of that information. With a greater understanding of who eats who, and how the species all inter-relate, we will be better able to predict what might happen in those fisheries if there were a change in climate, or if we were to change the catch quota for a certain species."

The team is starting with 8500 stomachs taken from fish caught by NIWA’s deepwater research vessel, Tangaroa, in January this year.

'We'll be going out again next January and again in 2007, and the Ministry of Fisheries observers on industry boats will also be sending in samples' says Dr Matt Dunn of NIWA. 'We're focusing on 25 key species, including commercial species like hoki, hake, and ling, along with about a dozen so-called higher order predators such as various sharks. It’s by far the biggest fish feeding study ever undertaken in New Zealand.'

In the lab, Darren Stevens of NIWA holds a small, plastic bottle containing specimens of policeman crab, named because of their two long eye stalks. 'These are rare. We only pick them out of stomachs of fish from specific parts of the Chatham Rise. Potentially, we should be able to create distribution maps for species like these crabs based on this work.'

While some items of a fish’s last meal are easy to identify, others take some detective work. For example, says Darren Stevens, 'the ghost shark feeds right on the sea floor. It has a parrot-like beak and chips things into very small pieces before swallowing them.'

The team uses clues like the presence or absence of spines, and the general shape of the legs and claws to work out what type of crab a ghost shark had been eating.

'We've got some plunket shark stomachs,' says Matt Dunn. 'They seem to feed by taking bite-sized chunks out of larger fish as they swim past, just like the so-called cookie cutter sharks do, so they won't be easy.'

Meanwhile, Amelia Connell is shifting through a milky soup on a petrie dish. 'That’s a fish eye,' she says, pointing to a small white ball, 'but this is too well digested to identify what fish it came from.'

And what about the smell? 'It gets bad with fish like stargazers, and anything that eats squid,' she says.

Contact

Dr Ben Sharp
Senior Scientist, Biodiversity
Ministry of Fisheries