Richard E. Thorne

Acoustical-optical assessment of Pacific herring and their predator assemblage in Prince William Sound, Alaska

Abstract The Pacific herring Clupea pallasi population in Prince William Sound (PWS), Alaska, is both a valuable commercial resource and an important forage species for marine fish and wildlife. Historically, the herring were managed by a combination of age-structured models and egg deposition estimates. When these methods predicted a large return for spring 1993 that failed to materialize, we began surveying with echointegration–purse seine methods. After a decade of acoustic surveys, we show the new approach yields highly precise biomass estimates, which are consistent with historical measures of the miles of beach spawning. When compared, we show the traditional methods overestimated stock biomass, which resulted in harvest rates approaching 40%. In contrast, the acoustic methods are most likely to underestimate biomass. Since the acoustic estimates can be quickly obtained, we recommend their use to set harvest quotas for the fishery in the spring just prior to harvest. The shift from the traditional preseason to inseason management practices for herring in PWS is consistent with the Precautionary Principle by the fact that protection of the spawning population does not rely on the ability of science to predict how the population is changing. Furthermore, synoptic infrared measurements on our night-time acoustic surveys revealed herring to be the most important winter forage to marine birds and wildlife in PWS, including the endangered Steller sea lion Eumetopias jubatus. Given the importance of forage to marine birds and wildlife in the North Pacific during the extended winter conditions (October–March), the implementation of inseason management for herring using echointegration–purse seine techniques may be the most effective method to restore depressed populations of marine birds and mammals in the North Pacific.

Distribution and abundance of pelagic fish off Spanish Sahara during CUEA expedition JOINT-I☆

Abstract The distribution and abundance of pelagic fish was investigated during Expedition JOINT-I of the Coastal Upwelling Ecosystem Analysis Program. The studies were conducted off Spanish Sahara, primarily along 21° 40′N between 17°05′ and 17°40′W from 31 March to 10 May, 1974. Data on fish abundance were collected with a hydroacoustic system consisting of 120- and 38-kHz echo sounders and a computerized signal processing system. Two congregations of fish were observed, one associated with the upper continental slope and the second at midshelf. The upper slope congregation was primarily horse mackerel (Trachurus sp.) and was associated with high abundance of large (over 500 μm) zooplankton. The second congregation was primarily sardine (Sardina pilchardus) and was distributed coincident with maximum abundance of small zooplankton and phytoplankton. The mean abundance of fish was estimated at 60 g m−2 wet weight over an area of about 4000 km2.

Application of Stationary Hydroacoustic Systems for Studies of Fish Abundance and Behavior

In most applications of hydroacoustic techniques for fish detection and abundance estimation, the data collection procedure involves echo sounding in a down-looking mode from a moving vessel. Stationary systems are virtually unused because of the reduced sampling power; however, in some cases stationary systems may offer several advantages. This study reports on two applications of stationary hydroacoustic systems: one at a coastal power plant intake in Southern California, and the second under the ice near Prudhoe Bay, Alaska. Several advantages of stationary systems were apparent, including more detailed information on fish behavior, capability for detection closer to boundaries, and better signal to noise characteristics.

RECENT APPLICATIONS OF HYDROACOUSTICS TO ASSESSMENT OF LIMNETIC FISH ABUNDANCE AND BEHAVIOR

ABSTRACT Since 1969, the authors have been involved in over 200 hydroacoustic surveys of fish populations in more than 25 lakes. These studies have included a variety of different species assemblages and objectives, although most, such as Lakes Washington and Ozette in Washington and Tustumena in Alaska, are sockeye salmon nursery lakes. The objectives of these studies have included fisheries management, evaluation of lake enhancement programs, or environmental impact. During the 14 years of these investigations, both the equipment and procedures have evolved and improved considerably. Earlier techniques were very limited in their ability to detect fish near surface or in shallow water and had very limited capabilty for size discrimination. Current technology has solved most of these problems. These developments and their capabilities are presented along with the results of surveys on lakes with a variety of biological and physical characteristics. The results include a considerable amount of “ground trut...

Assessment of Population Abundance by Hydroacoustics

AbstractThe extensive application of hydroacoustics to commercial fisheries provides a background for evaluation of its potential application to micronekton studies. Hydroacoustics has several advantages over other fishery resource assessment techniques, including high sampling power and capability for absolute population estimates, but also suffers from poor species discrimination, high complexity, and potential bias from target strength uncertainties. Application to micronekton encounters similar or even greater species identification problems and greater uncertainty associated with target characteristics, but even presently hydroacoustics offers a valuable tool for studies of patchiness and other behavioral characteristics.

Monitoring Pacific herring abundance with combined acoustic and optical technologies

The Prince William Sound Science Center has monitored the abundance of Pacific herring in Prince William Sound, Alaska, since 1993. The effort has expanded in the past three years because of the critical role of herring as winter-period forage for the endangered Steller sea lions. The new effort includes more seasonal coverage in Prince William Sound and expansion to herring populations around Kodiak Island. While acoustic applications for Pacific herring are well developed, species information has required expensive direct capture techniques. In the past three years, underwater cameras have been used for species identification. This application has become very effective both for identification and information on school structure and behavior.

Integrating passive and active acoustics for the assessment of fish stocks

The single most important information for the conservation of exploited marine fish stocks are precise measurements of their biomass so that harvest rates can be established that do not deplete the stock. However, the measurement of marine fish stocks is difficult due to the size, structure, and composition of the ocean, and the highly dynamic movements of the fish. Furthermore, traditional, discrete net sampling approaches have lacked sampling power to assess single fish stocks in time and space [2]. Without the ability to independently measure fish stock biomass with precision, managers have instead relied upon the commercial catch and deterministic indices as a primary source of empirical data. Also, without precise empirical data on stock biomass, the models used to make predictions are unverifiable and highly uncertain. Despite the severe management risks, this is the status quo, and it greatly confounds our efforts to sustain our fisheries, conserve exploited fish stocks and understand the dynamics of population response to natural and anthropogenic changes in the environment. High frequency active acoustics has been used to assess fish stocks for over four decades. When first introduced in the 1970s, there were hopes that acoustics would overcome the marine fish stock measurement problem because of a 105 increase in sampling power. However, prior to the introduction of acoustics the management agencies had already chosen large ocean areas to survey fish stocks, specifically in the summer months when the weather was good. In doing so, the agencies had assumed that surveys in large ocean areas would allow a representative assessment of single stocks of fish.

Some Impacts of Remote System Technology on Future Fisheries Management: Contribution no. 740, School of Fisheries, University of Washington, Seattle, Washington 98195

Abstract The fisheries profession is not immune to the pressures for increased efficiency which have characterized the history of human society. Currently, much of fisheries research and management suffers from inefficient and labor-intensive techniques. One of the highest potentials for increased efficiency is remote system technology. Examples are the application of remote, automated hydroacoustic sensing devices and remote operating vehicles (ROVs). Remote hydroacoustic systems are currently used to monitor salmonid escapements, measure downstream salmonid migration past hydroelectric dams, evaluate environmental impacts, and investigate long-term trends in fish abundance and behavior. Remote operating vehicles, especially with hydroacoustic as well as video sensors, allow for routine assessment of many fish stocks and for study of fish behavior in response to fishing gear.

Herring and Oil Don't Mix: A Lesson from the Exxon Valdez Oil Spill

Evidence of direct mortality to fishes exposed to oil is very limited. The 1989 Exxon Valdez oil spill had major impacts on marine mammals and seabirds, but was not implicated in the 1993 collapse of the Prince William Sound herring population because of the four year gap. However, we use several independent evidences, including changes in the predation behavior of Steller sea lions, to show that the collapse actually began in 1989. We show the failure to detect the actual timing of the collapse was due to deficiencies in the fishery model used to assess herring population abundance and lack of understanding about the vulnerability of herring. Finally, we show that the oil spill actually had greater impacts than originally believed as a result of catastrophic impacts on the ecosystem from the herring collapse

Acoustic assessment of trophic dominance in a marine ecosystem

High‐frequency acoustic surveys over the past 15 years show that the trophic structure in Prince William Sound (PWS), Alaska, functions as a wasp‐waist ecosystem. Three dominant biomasses in PWS are: (1) Pacific herring (2) walleye pollock and (3) the large‐bodied copepods (Neocalanus spps.) that dominate the spring zooplankton assemblage. The acoustic surveys and associated ecosystem observations suggest that the relative dominance of herring and pollock affects the composition of the apex predators. The near‐shore and near‐surface distribution of herring provides access by surface‐oriented marine mammal and seabird predators, such as sea lions, seals, murres and cormorants, to a crucial winter‐period food source. In contrast, the deep, off‐shore distribution of the pollock favors large benthic predators, such as demersal sharks, halibut, and flounder. The acoustic surveys also demonstrated that the abundance of large‐bodied copepods in PWS is critical to ecosystem productivity, including survival of juv...