Counting krill and other plankton

Learn why we count krill and other plankton, and how we count them using acoustics.

Uses of population estimates

Better understanding of krill distributions and biomass concentrations allows for better stock assessment and fisheries management.

If krill stocks should fall, finfish could be affected. Krill are a large dietary proportion of many finfish, such as:

hake
salmon
herring
rockfish

A commercial krill fishery exists on the British Columbia coast with a current annual limit of 500 tonnes. Canadian fishers harvest krill mainly as a feed supplement for both fish farms (to give salmon their pink colour) and aquariums.

Fisheries and Oceans Canada, as well as private industry participants, have conducted surveys in various inlets and coastal areas of British Columbia since 1990. However, we're only able to survey small semi-enclosed areas, such as inlets or fjords under 300 square kilometres with minimal water exchange. For open areas, such as the west coast of Vancouver Island, we'll need to develop more complex models to count the krill populations.

Our zooplankton database holds over 350,000 detailed species records from over 9,500 oceanographic sampling stations.

Biology

Zooplankton have swimming appendages too small to enable them to swim effectively against the currents. Many can dart for short distances in short bursts of energy. However, these bursts are mainly reserved for flight from danger or pursuit of prey.

They're typically microscopic, up to a maximum few centimetres long. Krill along the British Columbia coast range up to 25 millimetres (1 inch) in length, but average 16 millimetres in length.

To stay out of sight of their predators (mainly finfish and baleen whales), krill undergo a daily cycle. This cycle includes:

spending daylight hours in the twilight depths of the ocean (around 100 metres or 300 feet)
rising to the surface as the daylight decreases to feed in the dark on phytoplankton
returning to the twilight zone in the morning, as the sky gets brighter

A clear day may push the krill as deep as 150 metres (450 feet). On a cloudy day, the krill may be at depths of only 60 metres (180 feet).

Krill aren't distributed evenly within a body of water. In some areas, they form characteristic clouds or patches of high biomass while they're absent in other areas.

Detection of krill

Traditionally, krill and other plankton have been captured with net tows, such as:

bongo nets, which are towed vertically from some depth to the surface
the Bedford Institute of Oceanography, Net Environmental Sampling System (BIONESS), which is towed diagonally over a specific depth range

Net samples provide great information about species and age classes within the sampling area, but provide only limited information about biomass. They also take a lot of time and labour to conduct, and are bad for the krill.

We can use bioacoustics to detect plankton at a much greater rate than net sampling. However, bioacoustics don't provide any information about what species and what age classes are being sampled.

Bioacoustics use a scientific echosounder (a high-tech 'fish finder') that:

emits a high frequency sound pulse into the water
records the position and intensity of the returning echoes from fish and plankton within the pulses' beam
sends this information to a computer that displays the acoustic return information on a monitor

By conducting both net sampling and bioacoustics at the same time, we can obtain a biomass calibration coefficient to relate to the echo return.

Sampling

If we're able to place an echo sounding system on a ship, we can sample for plankton. We've used boats as small as 7 metres (21 inches) in length for sampling.

We usually plot a transect (a sampling path) on a chart, then collect acoustic samples during daylight hours along it. Next, we calibrate samples with local net tows or acoustic spheres to provide biomass estimates along the transects.

Processing

Although we now have biomass estimates along the transects, this only covers a thin line of the total area.

We can determine estimates of biomass for the remaining area by interpolation. This consists of building a model of the population in an area and applying it to an estimate of the overall area population.

One way of showing an overall estimate of krill concentration is with contour maps. These show varying concentrations based on the survey transects.

Contact us

For more information, contact:

Email: Moira.Galbraith@dfo-mpo.gc.ca
Phone: 250-363-6692
Fax: 250-363-6690
Mail:

Moira Galbraith
Zooplankton taxonomist
Institute of Ocean Sciences
PO Box 6000
9860 West Saanich Rd
Sidney BC V8L 4B2

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