Exclusive Economic Zone (EEZ) Acoustic Surveys
by George A. Rose Fisheries and Marine Institute Memorial University of Newfoundland St. John’s, NF Canada A1C 5R3
Acoustics has been used to observe fish underwater since at least 1935, when the Norwegian scientist Oscar Sund reported observing echoes from cod in Vestfjorden near Lofoten in northern Norway. This startling finding was exploited with vigor following the Second World War, when recent advances in sonar and radio technology were applied to fishery echo sounders. Scientists began conducting experiments to determine fish target strength in the 1950s. This work led to development of quantitative methods to measure fish density in the 1960s, specifically to the echo counting and echo integration methods described briefly in the previous section.
The echogram in Fig. 1 is illustrative of the standard visual display of echo data on the basis of which judgments are made about fish species and size. In this case, the echogram shows a 10-mile portion of a 30-mile-long school of northern cod, one of the stocks of Atlantic cod. While echograms often take very different forms, depending on the fish species, behavior, and location, this echogram, recorded in 1990 on the northeast Newfoundland Shelf, is uniquely associated with cod. Extensive sampling by trawling confirmed the essential purity of this massive aggregation of cod, in fact, one of the pieces of a stock en route to commercial extinction.
Today, numerous groundfish stocks are being managed at least partly on the basis of acoustic data collected during organized surveys. Examples are arranged here by geographical region: Norway: Atlantic cod, haddock, pollock, redfish; U.S.A.: walleye pollock, Pacific hake; New Zealand: hoki, orange roughy, southern blue whiting; Australia: orange roughy; Canada: Atlantic cod, Pacific hake, rockfishes (Sebastes spp.).
Many lessons can be learned from particular groundfish stocks currently being surveyed acoustically to contribute to stock assessment and management. A number of examples is provided by New Zealand fishes, including black and smooth oreos in addition to those listed above.
The example of hoki (Macruronus novaezelandiae) is briefly reviewed. The location of the main fishing grounds is shown in Fig. 2. A representative echogram of a hoki spawning aggregation is shown in Fig. 3. An associated, also representative echogram of hoki "fuzz" is shown in Fig. 4. The nature of this is under active investigation to determine the species composition, including possible species mix and respective size and age distributions. Indices of hoki abundance derived from acoustic data collected during the spawning period are presented in Fig. 5 together with indices derived from catch-per-unit-effort (CPUE) data and trawl surveys conducted outside of the spawning period. Models are applied to these three sources of data to estimate the stock biomass. The consequences of harvesting the stock at different fishing levels are then assessed by means of other models. Ultimately quotas are recommended for management of the stock.
Currently, the hoki catch is averaging 200,000 metric tons per year. Hoki is the only large whitefish fishery in the world to be certified as a sustainable and environmentally responsible fishery by the Marine Stewardship Council. [The Atlantic cod stock managed by Iceland is probably an imminent candidate for such certification given its management according to ecosystem-based principles.]
In Atlantic Canada, the potential of acoustics for contributing worthwhile data to stock assessment has been recognized for both Atlantic cod and redfish. In the case of northern cod, a series of echo registrations of the last big school is shown in Fig. 6. The aggregation is observed to evolve from a school-like form to one of increasing dispersion, where individuals are migrating 20-25 miles per day. Some acoustic discrimination of size is possible, nominally to within a doubling in length, but not to the desired 4-cm resolution for mature animals.
A stock of coastal cod, which over-winters in Smith Sound along the northeast coast of Newfoundland, is also being studied. It is 100% pure cod. Confinement of the stock to a narrow fjord (Fig. 9) allows repeated surveying to be performed. Characteristic echograms of the cod in Smith Sound are shown in Figs. 10-14. Biomass estimates are shown in Fig. 15. Particular attention is called to the stability of the estimates from the acoustic surveys performed in January, beginning in 1999, which witnesses to the reliability of the survey method.
Distributions of raw age and length data are shown in Fig. 16. These are derived on the basis of trawl data. Such data are used in the interpretation and analysis of acoustic data in order to estimate the overall biomass of the spawning stock, as shown by the open circles in Fig. 15.
Admittedly, there are sources of uncertainty in acoustic surveys of groundfish stock abundance. These include fish behavior, especially as it influences detection of fish near the bottom, identification of species, and target strength. In addition, there is uncertainty from survey design including timing and interpolation of data between transects, and determination of the age-length relationship. A typical pattern well known from the echo record is illustrated in Fig.17. Groundfish tend to cluster in dense schools near the bottom during the day, then rise off the bottom at dusk, dispersing in the layer at night. However, the pattern can be reversed too, as in Fig. 18, which is considered atypical.
The myth that acoustic abundance surveying can be performed only for pelagic fish stocks has been dispelled. Acoustic surveying may be no less applicable to groundfish stocks. Issues of detectability of groundfish in the semi-pelagic state, target strength, and species identification, among others, may be approached successfully in ways analogous to those used for the more familiar pelagic case.
The author wishes to thank Richard O’Driscoll for allowing images on hoki to be used.
The NOAA Alliance for Coastal Technologies (ACT) is thanked for their sponsorship of a workshop in February 2003, hosted by Gulf of Maine Ocean Observing System (GoMOOS), where this material was first presented. This report has been excerpted from the original presentation entitled "Can acoustics be used to survey groundfish?" [author George Rose, Marine Institute of Memorial University, St. John's, Newfoundland].
[see GoMOOS February 26-28 2003]