Gaspereau assessment for the Gulf Region to 2019 : population dynamics, reference points and status

Abstract

This working paper was prepared in support of the “Assessment of the status of gaspereau stocks of the southern Gulf of St. Lawrence (sGSL)” Regional Advisory Process, April 20-21, 2021. Reference points and assessment approaches for gaspereau (Alewife and Blueback Herring) fisheries; and the status of the Margaree River Alewife population, are provided in this paper. Fisheries and Oceans’s (DFO) precautionary approach to fisheries management allows for assessment relative to biomass or abundance reference points and relative to removal reference levels. The spawner biomass that produces maximum sustainable yield (MSY) is proposed as the upper stock reference level, and the biomass at which the recruitment is reduced to one half the maximum recruitment as the limit reference point. Because of the inherent variability in gaspereau population size and fisheries, the removal reference level when the population is in the healthy zone is defined as a fully-exploited zone ranging from the exploitation rate that produces 90% of MSY up to the exploitation rate that produces MSY. Data availability is a limiting factor for assessing gaspereau populations and fisheries throughout the Maritime Provinces. Age-structured assessment models are appropriate for populations with sufficient data; currently the Margaree River Alewife population is the only gaspereau population in DFO’s Gulf Region where this approach can be applied. For populations without data, collection of age and previous spawning information provides a mechanism to assess fisheries relative to removal reference rates. This approach has been demonstrated to bring exploitation rates into the appropriate range when coupled with management actions. Five variants of age-structured models were fit to data for the Margaree River Alewife population. Although status determinations vary among models, all models place the spawner biomass near the critical cautious boundary or in the cautious zone, and the exploitation rate in the over-exploited zone for the majority of years from 1983 to 2019.