Biomass indices of NAFO Division 4TVn fall spawning Atlantic Herring (Clupea harengus) from hydroacoustic surveys on spawning grounds

Abstract

Atlantic Herring in the southern Gulf of St. Lawrence (sGSL) are found in the area extending from the north shore of the Gaspé Peninsula to the northern tip of Cape Breton Island, including the Magdalen Islands. Fall spawning Herring in the sGSL are assessed using regionally disaggregated assessment models (North, Middle, South regions). Since 2015, hydroacoustic surveys were conducted annually on six major Herring fall spawning grounds in the sGSL. To account for missing samples, a predictive model of nightly Herring biomass was used to obtain a complete data grid and derive unbiased biomass indices. The covariates included in the negative-binomial model allowed to predict data for missing values in the observed data set. The trends in Herring biomass generated from this hydroacoustic survey closely match the trends observed in other population indices and stock status estimates from the population model. The North region biomass showed a decline to very low values in 2021. However, the Middle region decline in biomass seemed to be slower and with more interannual variation. Finally, a declining trend in the South region seemed to have reversed and biomass levels in 2021 were as high as in the beginning of the time series. The quality of the sampling and potential biases are discussed.