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.