Could harvesting mesopelagic fish be sustainable? Results from population and ecosystem models in the Northeastern Atlantic

The mesopelagic zone, located between 20-100m in depth, represents 20% of the ocean’s volume. The substantial biomass of organisms inhabiting these zones, estimated at 5-15 billion tons, has recently attracted the attention of states, raising the question of the possibility of sustainable exploitation. In the Northeast Atlantic, Maurolicus muelleri and Benthosema glaciale, the most abundant and well-studied mesopelagic fish species, stand out. Nevertheless, these species are still largely unknown, and stock assessments remain riddled with considerable uncertainties.

As part of the European H2020 MEESO project, three models were compared to better evaluate these uncertainties. This presentation will focus mainly on the SEAPODYM-LMTL (Low and Mid Trophic Levels) model. It is a spatio-temporal population dynamics model that explicitly simulates the vertical migration behaviours of mesopelagic species, grouped into six categories. Physical (temperature, current) and biogeochemical (net primary production) forcings are integrated across three layers (epipelagic, upper mesopelagic, and lower mesopelagic). The thickness of these layers is determined by the depth of the euphotic zone. Horizontal passive transport movements, as well as random water mass movements, are modelled using advection-diffusion equations. The model was adapted for Maurolicus and Benthosema by spatially restricting the recruitment zone. Maurolicus performs diel vertical migrations between the surface and the upper mesopelagic layer, while Benthosema migrates deeper, to the lower mesopelagic layer. These species are not currently exploited by any fisheries, and their stocks are considered to be at the equilibrium. Two fishing scenarios were tested: a harvest rate of 5% and 25%.

The spatial distribution of the two species remains stable over the period 1998-2022, with a total biomass of around 9.106 tons for Maurolicus and 6.5.107 tons for Benthosema. Neither exploitation scenario leads to a population collapse. Comparisons with the two other models confirm this result. However, the nearly one-order-of-magnitude difference in total biomass between models calls for great caution should exploitation be considered.