Révision et standardisation de l'indice du relevé acoustique pour la série d'automne 2009-2021 du hareng de l'Atlantique (Clupea harengus) dans les divisions 4RSw de l'OPANO

Abstract

Hydroacoustic surveys provide the main input for the assessment of Atlantic herring (Clupea harengus) in Northwest Atlantic Fisheries Organization (NAFO) division 4R since 1991. While the time-series from 1991 to 2002 has been deemed acceptable based on available diagnostic tools, the series that began in 2009 encountered several methodological challenges which led to the rejection of its associated population model in 2020. Furthermore, the preliminary results of a recent tagging experiment suggest that herring in divisions 4R and 4Sw should be assessed together. Here, focusing on the surveys conducted in the combined divisions 4RSw in the fall from 2009 to 2021, we standardized three main aspects of the methods which are believed to have affected the comparability of estimates over time and among strata. Specifically, we (1) processed and analyzed the raw acoustic data files in a more consistent manner; (2) employed a target strength equation considered more representative for the present survey; and (3) revised the stratum surface areas, a key input for the estimation of the mean biomass by spawning component, stratum and survey. The largest deviations from original (unrevised) values occurred as a result of changing the target strength equation; this revision resulted in the density being reduced by a constant 65% across all strata and surveys. The effects of re analyzing acoustic data and strata surface areas were highly stratum- and survey-specific, but had an overall negligible impact on the interannual variability in abundance. Although the present revisions have allowed for many improvements in our understanding of the methods, there remains a perception that the “optimal” spatio-temporal window for conducting the survey – that corresponding to a peak in herring abundance – has been missed over many of the years considered herein. Therefore, the next steps for this work will focus on identifying a better timing for future surveys, as well as on predicting missing observations and optimizing the use of biological samples within each stratum.