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Fishery acoustic indices for assessing Atlantic herring populations Claytor, Ross Richardson

Abstract

A method for measuring the spatial and temporal distribution offish school densities and exploitation rates using fishery collected acoustic data and voronoi-natural neighborhood analysis is described. A herring purse seiner fishing on non-spawning feeding aggregations, and a herring gillnetter fishing on smaller, highly dense spawning aggregations, in the southern Gulf of St.Lawrence, Canada, collected acoustic data during regular fishing activity for this study. The relationship between gillnet catch rates (kg/net) estimated for assessment of this stock reached asymptotic values at lower than expected densities and was not useful for tracking daily trends in school density. Gillnet and purse seine catch per metre searched were linearly related to density, and likely are suitable abundance indices for stock assessment estimates. An individual boat with data collected in this manner was found to represent trends in the entire fleet. There was a threshold density beyond which exploitation rates remained low. This threshold provides managers with a method for identifying and eliminating spatial and temporal trends in high exploitation rates and preventing overfishing. A simulation model calibrated with data from the Pictou 1997 inshore gillnet fishery compared the properties of abundance indices derived from fishery acoustic data to those derived from survey indices. The indices were examined over five fish distribution types ranging from a single spike to a uniform flat distribution, four conditions of fishing and fish movement, and sixteen stock sizes for each of these distribution and conditions. These data are suitable for deriving abundance indices provided the searching covers the entire temporal and spatial distribution of the population. Fishery acoustic abundance indices provide a basis for adopting a decision rule management paradigm and allowing the metapopulation structure of Atlantic herring to become the basic management unit for this species. These results represent an important alternative to the current F₀.₁ management paradigm for Atlantic herring populations and offer an opportunity to develop a more transparent and responsive management system for the long term viability of Atlantic herring fisheries.

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