Multi-decadal trends in contingent mixing of Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic from otolith stable isotopes

Kohma Arai,Martin Castonguay,David H Secor

doi:10.1038/s41598-021-86116-2

Kohma Arai, Martin Castonguay + Show 1 more

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https://doi.org/10.1038/s41598-021-86116-2

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Abstract

The Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic is comprised of northern and southern components that have distinct spawning sites off Canada (northern contingent) and the US (southern contingent), and seasonally overlap in US fished regions. Thus, assessment and management of this population can be sensitive to levels of mixing between contingents, which remain unknown. Multi-decadal trends in contingent mixing levels within the US fisheries region were assessed, and the contingent composition across seasons, locations, ages, and size classes were characterized using archived otoliths and developing a classification baseline based on juvenile otolith carbon and oxygen stable isotopes (δ13C/δ18O values). Classification of age ≥ 2 adults demonstrated that northern contingent mixing was prevalent within the US continental shelf waters during the past 2 decades (2000–2019), providing an important seasonal subsidy to the US winter fishery despite substantial depletion in spawning stock biomass of the dominant northern contingent. While the majority of older fish were of the northern contingent during the early 2000s, the southern contingent contribution increased with age/size class during the recent period (2013–2019). Spatial mixing was most prevalent during February and March when the northern contingent occurred as far south as the Delmarva Peninsula, but were mostly absent from US waters in May. A positive relationship (albeit not significant; r = 0.60, p = 0.07) occurred between northern contingent mixing and US fisheries landings, which could imply that higher contingent mixing levels might be associated with greater landings for the US winter mackerel fishery. The yield of the Northwest Atlantic mackerel depends upon the status of the northern contingent, with the southern contingent possibly more prone to depletion. Spatially explicit stock assessment models are recommended to conserve both productivity and stability in this two-component population.

Highlights

Complex spatial structure is a common attribute in fish p opulations[1,2,3,4] that confers stability and resilience through asynchronous responses of population sub-components to the same environmental conditions
Defining the unit stock is crucial to fisheries management for the Northwest Atlantic mackerel, but a challenge given its highly dynamic migrations and broad spawning areas that transverse the border between the US and Canada, which separately manage jurisdictional fisheries
A total of 217 otolith samples were selected from the Northwestern population of the Atlantic mackerel caught in the US winter fishery from October to November and January to March during 2013–2019 in the Mid-Atlantic Bight (MAB), Southern New England (SNE), Georges Bank (GB), and Gulf of Maine (GOM; Fig. 1, Table S1)

Summary

Introduction

Complex spatial structure is a common attribute in fish p opulations[1,2,3,4] that confers stability and resilience through asynchronous responses of population sub-components to the same environmental conditions (i.e., portfolio effect[5,6,7]). Fishery stock assessment models and management units should reflect the spatial structure of biological p opulations[15]. The Northwest Atlantic mackerel fisheries have fluctuated greatly over time, showing relatively high landings during the early 2000s, the stock is currently in a depleted phase with spawning stock biomass and landings at historically low-levels[25,26]. Defining the unit stock is crucial to fisheries management for the Northwest Atlantic mackerel, but a challenge given its highly dynamic migrations and broad spawning areas that transverse the border between the US and Canada, which separately manage jurisdictional fisheries. The population of the Northwest Atlantic mackerel has been traditionally broken into northern and southern “contingents” or sub-groups with different spawning sites, nursery areas, and migration b ehaviors[16,27]. Contingent mixing levels and spatial structure of the Northwest Atlantic mackerel is highly uncertain and dynamic, and has only recently been quantified using otolith tracer a pproaches[18,33]

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