Many scientific surveys aim to estimate the stock biomass or abundance of one or more marine species. The intention is generally to determine stock state to support stock management. However, when survey coverage is incomplete, there can be considerable bias in the biomass time-series when there are directional changes in the distribution of a stock relative to the survey area. This might occur under changing environmental conditions.
The biomass of the northern contingent of Northwest Atlantic mackerel is assessed by Canada and based mainly on a fishery-independent annual egg survey. The survey covers its known main spawning ground in the southern Gulf of Saint-Lawrence (sGSL), Canada. It is commonly assumed that the survey tracks well the relative changes in biomass. However, indications of spawning or the presence of young-of-the year mackerel in other regions has resulted in uncertainty about the validity of this assumption. Conducting a cross-region egg survey is logistically unrealistic. Therefore, only a synthesis of a diverse body of evidence can provide an evaluation of the key assumption of the northern mackerel contingent stock assessment: that the sGSL is its dominant spawning ground.
The latest Editor's Choice article provides an in-depth synthesis of all data and knowledge available on northern contingent West-Atlantic mackerel spawning for each Canadian region, to help determine each region's importance relative to the sGSL. The review follows a weight-of-evidence approach in which multiple lines of evidence are systematically presented and include: evidence on the presence of spawning adults in each region (based on landings and maturity information), evidence indicating whether the habitat is suitable for spawning (based on environmental data and an available habitat suitability model), and evidence of direct observations of eggs or larvae (based on all accessible ichthyoplankton records).
The general conclusion is that the sGSL has been and still is the dominant spawning area. On the Scotian Shelf, where mackerel starts its spring migration, there is, however, evidence of minor and patchy albeit consistent egg production. Spawning off Newfoundland, where mackerel can migrate to later in the year, appears to be sporadic and is likely highly variable in intensity.
The review demonstrates the use of a weight-of-evidence approach to compile and present a variety of data with the aim of answering a recurrent question that could otherwise not be addressed. By providing a comprehensive review of all available knowledge on mackerel spawning in eastern Canadian waters, the authors also highlight knowledge gaps in our understanding of the spatial spawning dynamics of this stock.
Read the full paper, A review of the importance of various areas for northern contingent West-Atlantic mackerel spawning, in ICES Journal of Marine Science.
Editor's Choice articles are always free to read in ICES Journal of Marine Science.
Sampling of mackerel eggs in the southern Gulf-of Saint-Lawrence with a bongo net. © Melanie Boudreau