Gary L. Thomas

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.

Sediment Phosphorus Loading Beneath Dense Canopies of Aquatic Macrophytes

ABSTRACT Dense surface canopies of aquatic macrophytes were associated with significant changes in the physical and chemical water quality of two shallow Pacific Northwest lakes. Internal loading of phosphorus (P) was observed at the sediment-water interface beneath canopies of Ceratophyllum demersum L. and Myriophtllum exalbescens (Fern.) Jeps. and in deep open-water areas when dissolved oxygen (DO) concentrations were ≤0.4 mg/L. Aerobic release of P was observed at sites with surface covers of the green filamentous algae Pithophora sp. where concentrations of DO were >20 mg/L and pH >9. An increase in surface P concentrations was also observed in sites dominated by the floating leafed Brasenia schreberi Gmel., and appeared to be associated with leaf decay within the surface canopy. There was an apparent net loss of phosphorus to the sediments beneath both submergent and floating leafed canopies when DO concentrations were ≥0.4 mg/L. The removal or reduction of the plant canopies could simultaneously red...

Mortality of Largemouth Bass (Micropterus salmoides) and Steelhead Trout (Oncorhynchus mykiss) in Densely Vegetated Littoral Areas Tested Using in situ Bioassay

ABSTRACT Concentrations of dissolved oxygen (DO) in dense beds of aquatic macrophytes in two western Washington lakes were routinely measured below reported lethal limits (≤1 mg L−1) for largemouth bass (Micropterus salmoides) and steel head trout (Oncorhynchus mykiss). The impact of the observed low DO concentrations on resident fish was examined both by field observation and by conducting 72 hour in situ cage bioassays. Replicate cages were placed in two Western Washington lakes at the surface, 1 m, and 2 m deep, in dense patches of aquatic macrophytes and open water. No steelhead trout mortalities occurred over 72 hours in the open water or surface water in dense patches of Myriophyllum spicatumia Lake Washington where DO concentrations were consistently > 4.0 mg L−1. No significant mortality of steelhead trout occurred in the surface cages of the floating leaf Nymphaea odorata, but all fish were dead within 12 hr at 1 m, where concentrations of DO were consistently < 4 mg L−1. No significant mortaliti...

The Application of Hydroacoustic Techniques to Determine the Spatial and Distribution and Abundance of Fishes in the Nearshore Area in the Vicinity of Thermal Generating Stations

Hydroacoustic surveys of the nearshore population of fishes along the Southern California coastline have been conducted in the vicinity of thermal generating stations since 1976. A digital echo integration system was used in conjunction with Simrad EK 120 and Ross 105 kHz echo sounders. The nature of the hydroacoustic data made it far superior to net measurements of fish density because of the quantity, the speed, and the accuracy of the information collected. The hydroacoustic data revealed that significant changes in the nearshore fish density occurred between days and diel periods. This finding indicates that conventional net sampling techniques are inadequate to describe nearshore fish densities because of the length of time required for deployment of the nets. The acoustic surveys were supported by data from lampara seine catches.

Modeling the sound levels produced by bubble release of individual herring.

Herring (Clupea pallasii and C. harengus) are known to release gas from their swim bladder to assist a number of complex behaviors, such as buoyancy adjustments and predator avoidance. The noise associated with the release has recently been reported in the literature and related to oscillating bubbles. Average source levels (SLs) of 73 dB with regard to microPa rms reference 1 m have been reported for bubbles produced by herring in the laboratory. A model is provided for predicting the SL in terms of the gas flow rate from the swim bladder into the bubbles. Based on these laboratory conditions, an inversion yields a rate of 0.9 (0.3-3.2) ml/min. Furthermore, the model predicts an acoustic SL of 89 (79-99) dB with regard to microPa rms reference 1 m for pulses emitted by herring in a natural shallow water environment at unknown distance corresponding to a flow rate of 2.5 ml/min. An analysis of published acoustic data suggests that herring is capable of controlling the gas flow and the corresponding acoustic levels over a wide range according to different behavioral needs. The proposed model allows an extrapolation of the laboratory results to situations that are relevant for bubble release of herring schools in the ocean.

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...

Density Dependence and Vulnerability of Fish to Entrapment by Offshore-Sited Cooling-Water Intakes

Simultaneous measurements of the offshore fish density and inplant fish entrapment were made during eight surveys at four electric-generating stations along the Southern California coastline in 1979. Hourly estimates of the fish biomass in the vicinity of the cooling-water intake sites with state-of-the-art precision were made possible with hydroacoustic techniques. Synchronous hourly measurements of fish entrapment were made in order to describe the density dependence of fish entrapment. An entrapment vulnerability statistic, the ratio of the weight (kg) of fish entrapped in the plant ( E ) to the weight (kg) of fish offshore in the vicinity of the intake ( B ), i.e., E/B , was employed to describe the effect of water transparency, the velocity-cap and volume of flow on the entrapment of fish.

Passive Acoustic Detection and Monitoring of Schools of Herring

Passive acoustic detection and monitoring of various marine fishes has recently received much attention in the literature. It has been recognized that passive acoustic techniques have the potential to complement traditional active acoustic surveys and to significantly increase their overall efficiency, if the acoustic signatures of the considered species are well understood. In this paper, the potential of passive acoustic techniques is explored for the specific case of Pacific herring (Clupea palassii). It is demonstrated that schools of herring can acoustically be detected by observing the sound of coordinated bubble release, triggered, e.g., by predator activity. This sound not only has identifiable features that can be exploited for determining the presence or absence by simple means, but could also carry abundance and size information. Work supported by ONR and the NMFS via the PWSSC.

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...

Acoustic monitoring of the juvenile pink salmon food supply and predators in Prince William Sound, Alaska

Previous research has indicated that juvenile pink salmon survival in Prince William Sound is positively correlated with the abundance of the large-bodied copepods of the genus Neocalanus. Neocalanus serves both as a valuable food supply for the juveniles and as a prey-sheltering mechanism. In spring 2000, the Prince William Sound Science Center initiated annual monitoring of the spring abundance and distribution of both macrozooplankton and fish predator populations. The monitoring includes multiple frequency acoustic systems and zooplankton net tows. The program has now completed five years of measurements, and there have been four associated adult pink salmon returns. Pink salmon returns were found to be positively correlated with average plankton net catches of both large copepods and euphausids in the nursery year. Some data gaps prevented correlation between acoustic scattering and pink salmon returns, but both 420 kHz and 120 kHz backscatter were positively correlated with the plankton net catches of large copepods. The acoustic data also allowed detailed examination of the spatial trends of the plankton distribution. Patchiness was relatively low, which may explain why the net catches seemed to provide a reasonable measure of overall abundance. Some changes in the monitoring procedures may be necessitated by the indication that euphausid abundance may also be an important factor in pink salmon survival.