Since 1991, an annual fishery-independent acoustic survey of early fall (September-October) concentrations of Herring has been conducted in the sGSL. The standard annual survey area occurs in the 4Tmno areas where both NAFO Div. 4T Herring spawning components aggregate in the fall. The survey uses a random stratified design of parallel transects within predefined strata. Surveys are conducted at night and use two vessels: an acoustic vessel to quantify the fish schools biomass using a hull-mounted 120 KHz split-beam transducer, and a fishing vessel to sample aggregates of fish with a pelagic trawl (details in LeBlanc et al. 2015; see also LeBlanc and Dale 1996).

Trawl samples are used to separate the estimated biomass by spawning component and age, determine species composition, and size distribution for the estimation of the target strength (LeBlanc and Dale 1996; LeBlanc et al. 2015).

A standardized abundance index is generated from this acoustic survey. This index includes catch-at-age data since 1994.

This survey also provides the age-disaggregated acoustic abundance index for ages 2 to 10 for spring spawners and fall spawners.