A study is underway to try and find out why the West Coast fur seal population is declining.

Massey University researcher Laureline Meynier spent the first week of this year armed with giant butterfly nets and shields, gluing six transmitters onto the backs of Cape Foulwind seals.

All going to plan, the transmitters would tell her where the seals ranged to feed and at what depth, said Dr Meynier.

She hoped this would help show whether the seals were sharing their feeding grounds with hoki fishers and if this was affecting their population.

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Currently it was unclear whether the seals ranged beyond the 200m continental shelf to feed where the deep-sea hoki were fished.

It was also unknown if the seals and fishers hunted at different depths and didn't interact.

While seals were a known hoki fishery bycatch, it was not known if this affected their population. If it did, the fishers could move a little to the north or south of the colony's feeding grounds, said Dr Meynier.

Warming oceans affecting the seal's food source could be another cause of their decline.

The only snag in her research plan was that while the seals' location was transmitted by satellite, their diving depth data was stored on the transmitters themselves.

The seals had to be recaptured and the transmitters cut free in order to retrieve the depth data.

Even without the usefulness of this information, Dr Meynier was keen to get the transmitters back as they cost around $4000 each.

She wouldn't have been able to carry out her research without the help of student volunteers, most of whom were also doing seal research, local Department of Conservation staff and a representative from Sirtrack, the company that supplied the transmitters, she said.

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A team of six was required to safely catch a seal.

Some of the group had to sneak up on the female seal with a net, while others stood by with big wooden shields to block the more aggressive male seals.

Usually the female would stay still and hope they couldn't see her, but some males might rush to her defence. "If it feels too dangerous we back off."

Only female seals were tracked as they stayed permanently at the colony while raising their pups.

The transmitter was glued onto a piece of wetsuit material then glued to the seal's back. It came off when their fur moulted in autumn.

Recapturing the seals was trickier. The capturer had to wait until a seal with a transmitter on came back to land and the seals were wary of humans after being caught the first time.

Four out of the six transmitters had been retrieved.

Dr Meynier said she was also tracking an east coast seal colony in Kaikoura to see what was contributing to its growing population.

She had received funding for the three-year study in the last round of Foundation for Research, Science and Technology funding, before it was cut. She would finish in spring next year and hoped to do some more monitoring in June to see if the seals foraged further in winter.