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Title: Variability in Mobile Acoustic Fish Community Assessment
Author: Hateley J
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1224, Representation ID: 395, Object ID: 2439; Environment Agency Project ID:EAPRJOUT_1245, Representation ID: 409, Object ID: 2456
Hydroacoustics is a cost-effective tool for surveying fish communities in large lowland rivers and lakes and will be a key element in providing temporal and spatial data for the national fisheries monitoring programme and the Water Framework Directive. However, deployment of sonar in horizontal mobile surveys is still in the developmental stage, and successive surveys on the same fish populations can produce very variable results. In addition, three types of echosounder are currently in use within the Environment Agency and comparisons between results obtained from different systems are difficult to conduct. In order to improve robustness of information obtained from horizontal hydroacoustic fisheries surveys, this project was commissioned to account for the observed sampling variability. Two potential sources of variability were examined: 1) Differences between echosounders in operation. 2) Variability due to key abiotic factors affecting fish behaviour. One dual-beam (BioSonics model 102) and two split-beam (HTI model 241, Simrad EY500) echosounders were tested for variability in reported target strength (TS) and fish density. The objectives of these comparisons were: a A recommended standard echosounder system for Environment Agency mobile horizontal surveys. a To validate cross-calibration of echosounder outputs, enabling comparisons between rivers surveyed using different gears. The impact of key abiotic factors on fish behaviour was examined through a comprehensive literature review and an examination of existing acoustic data. Fish density results from longterm studies of the Rivers Thames, Trent and Ouse were examined in relation to three environmental factors; water temperature, river discharge and moon phase. These two studies had the following objectives: a An assessment of the role of different fish behaviours in influencing the vulnerability of fish to acoustic detection. a An indication of environmental sources of variability in acoustic data. All studies were also expected to contribute recommendations to the design of a future programme to establish the influence of key abiotic factors on acoustic assessments of fish communities. Echosounder configurations were individually standardised for echo-counting. The nonuniform operation and echo-selection criteria of the echosounders generally restricted analyses to relative comparisons of density and TS, rather than absolute values. In tank tests, reported TS varied by range from the transducers and between replicate samples of standard targets. Higher levels of variability were noted for the dual-beam BioSonics. For comparisons of density outputs, two types of data were collected and compared; 1) by passing a known number of targets through the echosounder beams and calculating volume density for a fixed time-window; 2) by collecting fish abundance data by mobile horizontal survey of six reaches of the River Thames and one reach of the Manchester Ship Canal. In all studies, paired density estimates were significantly correlated, however HTI densities were often an order of magnitude higher than equivalent BioSonics or Simrad estimates. No R and D TECHNICAL REPORT W2-063/TR/2 i evidence was found for reporting of afalsea targets, therefore the differences were attributed to the HTI having superior single-target reporting performance over the ranges analysed. The literature review was conducted by Dr Martyn Lucas of Durham University, the output of which is a complementary report, aA Review of Fish Behaviours Likely to Influence Acoustic Fish Stock Assessment in Shallow Temperate Rivers and Lakesa (Lucas et al. 2001). The review of acoustic datasets failed to identify significant sources of variability in acoustic assessments. However, the short duration of the studies, asnapshota measurements of environmental factors and difficulties accounting for the interactive effects of key factors restricted the analyses. This study concluded that the variability in reported TS is of little significance in the context of mobile horizontal surveys. The HTI would be recommended as the standard Agency system for such surveys primarily based on single target reporting and user-definable pingrates, however other systems must be gradually phased out with substantial (but undefined) overlap periods to enable cross-calibration of acoustic outputs. Finally, recommendations were made for the design of a future programme to determine the influences of abiotic factors on acoustic estimates. These included; use of a single echosounder system, studies on a number of sites and rivers with differing properties, multiple measurements of environmental parameters and combined mobile / fixed location studies. R and D TECHNICAL REPORT W2-063/TR/2 ii CONTENTS EXECUTIVE SUMMARY i 1.
Publisher: Environment Agency
Subject Keywords: Stock assessmentRivers; Hydroacoustics; Freshwater fishes; Variability
Extent: 83
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