Echo-Sounding to Count Pacific Fish

Conservation requires counting, for fish or other animals. Scientists and managers use population estimates to avoid overharvesting. But counting deer, bears, or other animals on the earth’s surface can be difficult enough. So how does one count fish in the hidden world under the sea?

Most researchers start with catch results from commercial fishing and research vessels, then use complex calculations to estimate populations. But some use sonar to count fish directly, by sending out sound pulses and measuring the echoes.

Today, new sonar techniques are providing new accuracy. On Canada’s west coast, echo-sounding helps to assess Pacific hake, the biggest fish stock by volume, and shows promise for salmon, traditionally the biggest fishery by value.

Research scientist Dr. John Holmes of the Pacific Biological Station (PBS) at Nanaimo, British Columbia, says that “in theory, a survey that’s independent of the fishery should give a better overall picture.” Among the reasons: commercial catch statistics can be misleading. The trawl-nets of research vessels can influence fish behaviour and skew measurements. Rough bottom makes some areas untrawlable.

Echo-sounding traditionally had its own problems. For one, sound pulses reflecting off fishes at different angles could send back disproportionate echoes.

But split-beam sonars, increasingly common over the last two decades, receive echoes on four different quadrants of the sonar transducer, providing more accurate data. Operating several split-beam systems together at different frequencies allows researchers to view multiple echoes from the same fish, yielding better information on species identification and mix.

PBS research technician Ken Cooke points out that split-beam sonars not only give better measurements of a fish school’s size and structure, “in some cases, they allow us to track and measure individual fishes.”

Hake and rockfish soundings at the continental slope. (Courtesy John Holmes)

Split-beam sonar has become vital for surveys of Pacific hake. Canadian fishermen in 2003 caught about 70,000 tonnes of hake, their single biggest Pacific fishery. The bulk of the stock occurs off the American coast. Since the early 1990’s, the two countries have worked together to survey hake abundance, which declined at the turn of the millennium.

On the last joint cruise in 2003, the Canadian Coast Guard’s 58-metre research trawler W.E. Ricker surveyed the stock from California to southeast Alaska. Every 10 nautical miles, the vessel ran transects from near shore out to the continental slope, where hake tend to congregate, and beyond into 1,500 metres depth.

How can one be sure which species are producing the echoes? Today’s sonars, and the interpretive skills of researchers and fishermen, can generally pick out the differences.

Hake tend to swim 150-300 metres deep and well off bottom. They show up on the screen in strong, hot colours, red or yellow. And, they often appear in a W shape. Other species have their own patterns.

Where there’s doubt about the species, researchers verify results with trawl-caught samples, which also give data on the size and age of the fish.

The 2003 survey suggested that the hake stock is gaining strength from a new, strong year-class. It also revealed surprising distribution patterns. Hake showed up both further north than expected, in southeast Alaska, and further offshore, well beyond the continental slope.

For the 2005 survey, Canadian researchers led by Ken Cooke are working with U.S. investigators aboard the American research vessel Miller Freeman. Besides assessing the coast-wide stock, the international team will look more closely at hake distribution in relation to climate change.

PBS researchers are also making more use of sonar for other species, such as herring and rockfish. The latter is a groundfish, like hake, but lives even closer to the bottom, often near bluffs or pinnacles which can confuse the sonar picture.

Ken Cooke says, “By first creating a detailed map of the bottom and then viewing it in 3D from many different perspectives, we were able to decide what echoes were more likely from ‘high-rise’ shadows rather than dense schools of fish. That helped show fishermen why their nets often came back in pieces and the ‘big ones’ got away.”

Commercial fishermen help support research through the Canadian Groundfish Research and Conservation Society. They also, Ken Cooke says, provide valuable expertise in interpreting sonar soundings.

Pacific salmon remain the foundation fishery of the west coast, vital to the commercial, sport, and Native fisheries, and part of British Columbia’s identity. New sonar techniques can help protect salmon.

For parts of the far-reaching Fraser River system, the world’s biggest salmon producer, the Department of Fisheries and Oceans uses a mark-recapture program (MRP) to assess runs. First, workers tag a large number of returning salmon near the mouth of the Adams, Horsefly, Shuswap, or other rivers stemming from the Fraser.

Then, as salmon approach the spawning beds, the salmon-counters collect large numbers and check them for tags. The ratio of tagged to untagged salmon, applied to the original number tagged, suggests the total number of salmon that entered the river in the first place.

But each MRP estimate involves work by several people over months, at high costs. A new type of sonar developed at the University of Washington, the DIDSON (Dual-frequency IDentification SONar), promises equal data with less effort.

DIDSON sonar image of salmon (courtesy John Holmes).

A form of multi-beam sonar, the DIDSON uses multiple high-frequency sound beams, together with lenses, to provide sonar images of unprecedented detail, even in turbulent waters. Researchers can see individual fish, almost like watching television, while the machine counts their numbers. Small, portable, and easy to use by field crews, the DIDSON does not require specialized skills to operate or to interpret its images.

The high-frequency mode (1.8 MHz) of the DIDSON operates best where the river is narrow or a barrier channels salmon close to the bank. Last year, John Holmes and colleagues showed that DIDSON would work well for sockeye salmon on six major tributaries of the Fraser, including the legendary Adams River run. The new technology promises to reduce costs, freeing up funds for other salmon work.

“We’re finding new ways to apply acoustic assessment,” John Holmes says. “The method still has a ways to go, but it’s coming on strong, and it’ll help us more and more in future.”

Last updated : 2006-01-13

Important Notices