News: Putting the physics in coastal biophysics - Listening for trout in large lakes

A light trap suspended from a NIWA GPS drifter. (Photo: Craig Stevens)

A number of track experiments starting at the southernmost extent of Kapiti Island.

Putting the physics in coastal biophysics

Marine reef organisms often live in highly fragmented populations that are interconnected by complex physical transport processes, such as tides, currents, eddies, etc. We must understand how and why such fragmented populations fluctuate in order to predict how they might respond to pressures of environmental change and harvest, and their risk of extinction. A new Marsden Fund project, ‘Maelstrom in the matrix’, is examining transport of fish larvae in coastal waters. The project is a collaboration between the Victoria University Coastal Ecology Laboratory, NIWA’s Marine Physics Group, and the Marine Ecology and Physiology Group at the University of Melbourne.

The first phase of the work has focused on the current flows and larval transport around Kapiti Island, north of Wellington. This island sits 5 km offshore, near the western entrance to Cook Strait, and supports a marine reserve. NIWA’s marine physicists are tracking free-floating drifters (buoys that drift with the current and report their position by radio) over one or two days. Measurements using vessel-based ADCP (acoustic Doppler current meter) and CTD (conductivity–temperature–depth profile) instruments provide the background picture. Drifters released in the same locations on successive days ended up in vastly different locations, demonstrating that there is a huge amount of variability in the day-to-day currents that cannot be explained by tides alone. In fact, tidal currents are only about half the story, with the rest of the transport in the region due to wind-driven flows and eddies spun off the ends of the islands.

Light traps hung under the drifters (see figure) were designed to attract fish larvae, in order to see if they are responding to the currents. So far, most of the larval catch has been krill and a few small squid, with the occasional fish larva.

Though the Marsden Fund supports ‘pure’ research, the project clearly has some applied outcomes. It suggests that present computer models used to track, for example, drifting divers, would have only a moderate likelihood of pinpointing their exact whereabouts. Understanding specific flows at such scales requires knowledge about large-scale ocean and weather processes as well as local phenomena like eddies and stratification.

For further information, contact:
Dr Steve Chiswell, 0-4-386 0351,
s.chiswell@niwa.co.nz
Dr Craig Stevens, 0-4-386 0476,
c.stevens@niwa.co.nz

Listening for trout in large lakes

The survey vessel Paraki on Lake Tekapo, showing the small transducer attached to a moveable pole mounted on the boat’s side. (Photo: Wilbert Knol)

Over the last few years, NIWA has been developing a robust and workable method for using acoustics to assess trout populations in large South Island lakes. This work has been funded jointly by the New Zealand Fish and Game Council and the NIWA Capability Fund.

Last year we conducted a series of experiments in Lake Coleridge to confirm that bottom-dwelling brown trout could satisfactorily be distinguished from plants on the lake bed. Following the success of this work, this past summer we extensively surveyed seven lakes – Coleridge, Tekapo, Benmore, Hawea, Wanaka, Wakatipu, and Te Anau – over three weeks in February, using the NIWA acoustics group’s SIMRAD 120kHz system with a new wide-angle transducer built by IRL to our specifications.

After much effort and some frustrations, we now have a system that we can use, jointly with Fish & Game, to begin a long-term monitoring programme in these lakes. This will determine trends in trout abundance over time and enable Fish & Game to more effectively manage these increasingly important recreational fisheries.

For further information, contact:
Gavin James, 0-3-343 7862,
g.james@niwa.co.nz