How much food is in the ocean?
Mid-water prey (known collectively as micronekton), form the core of the ocean food web, transferring energy from primary producers at the ocean surface to top predators such as tunas, billfish, sharks, seals and seabirds.
The mass and distribution of micronekton reflects broad-scale patterns in the structure and function of the ocean, as well as the dynamics of marine ecosystems. But observations of micronekton in the Southern Hemisphere are sparse. We're working to improve information about micronekton to enhance fisheries management.
Fishers provide vital link in ocean observing system
Skippers for Sealord NZ, Australian Longline, Austral Fisheries and Onwards Fishing are using their echo-sounders to record slices of life beneath their vessels while en route to fishing grounds across the Southern and Indian oceans and Tasman Sea.
A team of scientists led by Dr Rudy Kloser is studying the acoustic signals to understand how mid-water prey species such as small fish, squid, krill and jellyfish are distributed.
The mapping will complement established observing systems such as physical sampling of ocean currents, surveys of ocean chemistry and biology (plankton and zooplankton), and electronic tagging and tracking of large marine fish and mammals.
The combined information will greatly enhance the capacity of marine scientists to monitor shifts in food availability over time, assisting in the monitoring and modelling of oceanography, ecosystems, fisheries and climate change, and in understanding the behaviour of top predators.
Fishing vessels involved in the acoustic mapping are equipped with scientifically-calibrated 38 kHz digital echo-sounders that record a slice of acoustic backscatter to a depth of 1500 metres.
The primary area of interest is the more densely populated ‘deep scattering layer’ (400-600 metres depth).
A special system of multiple nets is used to collect biological samples at a range of depths. The samples are compared with the acoustic backscatter to help interpret its signal.
To interpret the acoustic signal, or backscatter, scientists conduct targeted experiments (from research or commercial vessels) at set locations.
The sampling techniques include fine-scale acoustics, cameras, and a towed net system used to catch prey samples at 200-metre depth intervals (to 1000 m).
These are deployed in concert with the echo-sounder to help determine what is making up the backscatter.
A special acoustic probe is used to ‘see’ the backscatter signal of individuals of particular species.
The acoustic-optical system attaches to the headline of the trawl net and records video, digital stills and acoustic reflectance of fish as they pass underneath.
The prey samples are analysed at the Hobart marine laboratories for their species composition, and individuals checked for body shape, length and weight, and the reflective properties of their swim bladder (if present). Is it filled with air, oil or wax? These factors influence the acoustic signature of each prey group.
The weight and acoustic reflectivity of the dominant species groups must be quantified in order to estimate the volume (wet weight biomass) of fish in the water. (Much of the acoustic signal comes from small-gasbladder fish known as myctophids.)
This information is then used to extrapolate this to produce estimates of micronekton biomass at an ocean basin scale.
Integrated Marine Observing System
The acoustic surveys from fishing vessels are part of a Ships of Opportunity Program initiated in 2010 under the Integrated Marine Observing System (IMOS).
The program extends beyond fishing vessels to include the research vessel Aurora Australis on transects to Antarctica and Heard Island, and the Marine National Facility Research Vessel Investigator on ad hoc voyages in Australian and Pacific waters.
The acoustic surveys will align with existing monitoring programs for ocean biochemistry and plankton and will provide baseline information and record long-term changes in the distribution, biomass and behaviour of micronekton.
Calibrated measures of acoustic backscatter generated from the surveys for 10 metre (vertical) by 1 kilometre (horizontal) ‘cells’ of the ocean will be made available to the research community.
Specific data products will estimate the biomass of micronekton fishes and the daily transfer of energy between the epi pelagic (0-200 metres) and meso pelagic (200 to 1000 metres) layers in the Tasman Sea.
The acoustic monitoring program relies on the expertise and goodwill of fishing companies Austral Fisheries Pty Ltd, Sealord NZ Pty Ltd and Onward Fishing Pty Ltd.
Austral Fisheries operates the vessels Southern Champion (Australia's largest fishing vessel at 87 metres) and Austral Leader 2 to catch Patagonian Toothfish and Mackerel Icefish in Australian Sub-Antarctic territories. These vessels will record acoustic data on transits between Mauritius, Heard and McDonald Islands and Perth.
The Sealord longliners Janas and Avro Chieftain are based at Dunedin, New Zealand. They fish for ling and for Patagonian Toothfish.
They have onboard factories for processing and packaging their catch. The Janus will record acoustic data on transits between New Zealand and the Ross Sea.
The Sealord factory trawlers Rehua and Independent 1 catch and process species such as Hoki, Orange Roughy and Oreo Dory, in and outside New Zealand. The Rehua crosses the Tasman Sea each winter to fish for Blue Grenadier off the west coast of Tasmania on behalf of Australian Longline and will record acoustic data on transits between Dunedin and Devonport in June and August.
Crew members aboard the fishing vessel Rehua pitch in to help scientists sort samples taken from the midwater net during a transit across the Tasman Sea.
The Onward Fishing vessel Saxon Onward is a 35-metre deepwater trawler that operates around the east and west coasts of Tasmania, and sometimes further afield.
The vessel targets Orange Roughy and Blue Grenadier during the winter spawning season. Saxon Onward will record acoustic data in June between Hobart and Cascade Plateau.
Acoustic mapping technology helps to refine estimates of fish population
The CSIRO research team has developed the acoustic mapping technique during annual transits across the Tasman Sea by the Sealord fishing vessel Rehua from 2004–07.
This has involved conducting fine-scale sampling at six locations along the four-day, 1000-kilometre transect.
CSIRO scientist Tim Ryan ensures the acoustic equipment is correctly calibrated.
The maps of acoustic backscatter along the Tasman transect record the transfer of energy through the trophic levels of the ocean via the daily vertical migration of mid-water prey species: up to the surface at night to predate on primary producers, and back down by day to avoid predation.
They also reveal the structure and patchiness, but quite dispersed nature of micronekton in the Tasman, with localised cold water regions stretching tens of kilometres associated with eddy currents that are devoid of life.
The resulting preliminary estimates of wet weight biomass for small fishes in the entire Tasman Sea, which has an area of 4.1 million km², are in the range of 64.3–119.2 million tonnes.
This is significantly higher than previous estimates from both net and modelled results from the same area.