Design and analytical methodology for the Atlantic herring (Clupea harengus) summer acoustic survey in NAFO Divisions 4RSw (2019-2023)

Abstract

Since 1991, hydroacoustic surveys have been conducted in the fall in Northwest Atlantic Fisheries Organization (NAFO) divisions 4RSw to assess the spring and fall spawning stock biomass of Atlantic herring (Clupea harengus). In 2019, a summer hydroacoustic survey was introduced, as it was hypothesized to better and more consistently cover the total herring abundance in the area. The fall hydroacoustic survey was discontinued in 2022, and the summer survey became the primary source of fishery-independent input to the herring stock assessment. The implementation of the summer hydroacoustic survey was accompanied by several changes to the methodology and survey design, including the addition of strata in the northern portion of the survey and the standardization and automation of the analytical workflow. Given the significant changes in methodology, survey coverage, and survey timing, the summer hydroacoustic survey is unlikely to yield directly comparable results to the historical fall hydroacoustic survey. In terms of methodology, the most impactful difference between the two approaches originates from the differentiation between fish with and without a swim bladder, which leads to a generally larger area backscattering coefficient (𝑠𝑠𝑎𝑎), a proxy for fish abundance, with the new method. We conclude that the updated method improves efficiency, reliability and replicability of the herring biomass estimates compared to the previous method applied to the fall hydroacoustic surveys.