Prince William Sound Herring Assessment: A 19-Year Hydroacoustic Series

The first of a series of annual hydroacoustic surveys for Pacific herring in Prince William Sound (PWS), Alaska, was conducted in fall 1993 after fishers were unable to locate herring concentrations despite a forecast for high abundance.  The 1993 survey confirmed that the herring stock had collapsed.  Spring 2011 will mark the 19^th consecutive annual survey.  There are several other long-term data sets on the PWS herring abundance including aerial surveys of beach spawning that extend back to 1973 and age-structure data that extend back to 1980.  Comparisons of these long term data sets reveal important characteristics of various estimators.  For example, the ASA models that are typically used for stock assessment showed limited capability to detect abundance changes that result from acute mortalities, such as those from an oil spill, and lagged other estimators by 1-2 years.  The primary challenge to the hydroacoustic surveys is the complex and dynamic distribution of the fish.  The current approach is derived from years of experience including seasonal surveys.  The design is multi-stage and takes advantage of spawning migration behavior to concentrate effort.  Variance in the estimator is derived from repeated surveys.  A week of survey effort over the 10,000 km² area of PWS achieves confidence intervals of ±25%.  Abundance scaling, species composition and vessel avoidance are only minor issues for this stock.