Fish stock assessment and fisheries dynamic modelling investigating sustainability of potential mesopelagic resource exploitation: The application of complex models in an unexploited and data-limited system

This Ph.D. thesis evaluates the ecological sustainability of a potential mesopelagic fishery on a single species/stock level and the economic viability of a potential Danish fishery targeting Maurolicus muelleri and Benthosema glaciale in the Northeast Atlantic Ocean. Driven by the vast, but variable estimates of global mesopelagic fish biomass, there is increased interest in potential exploitation of mesopelagic species. Despite this commercial interest as well as their potentially vast biomass and important ecological role, mesopelagic fish remain one of the least studied components in the marine ecosystem and major knowledge gaps persist. In the Northeast Atlantic Ocean M. muelleri and B. glaciale are among the most abundant mesopelagic fish species, and commercially interesting due to their high fat and oil content. The species are to date largely unexploited, and ideally, the development of potential commercial fisheries should proceed no faster than the gathering of information necessary to manage potential new target stocks sustainably and scientifically robust. Though the species can be used for direct human consumption, they are mainly considered as resources for fishmeal, oil production, and the development of nutraceuticals. In this thesis, key life history parameters for growth and mortality and their uncertainties are estimated using length-based methods, revealing significant variability influenced by environmental and biological factors. The absence of a fishery challenges the use of common stock assessment methods since there is no conventional stock assessment monitoring and data available, such as age data or data from commercial catches. This requires the adaptation and/or development of new stock assessment methods, requiring precise knowledge of population dynamics. A size-based method for data-limited stock assessment was adapted to unexploited conditions to estimate biological sustainability reference levels according to maximum sustainable yield, considering the uncertainty in the life-history of the species and different scenarios of total stock biomass. The sustainability and long-term economic viability of the fishery also depend on the fishing dynamics and cost-structure of the potential mesopelagic fishery. This also calls for an evaluation of the potential return on investment to avoid financial loss or unsustainable depletion before commencement of potential commercial exploitation. Economic viability analyses with the DISPLACE individual-vessel based model, centred on the Danish large-scale pelagic fishing fleet, suggest potential profitability only with high fish prices to offset elevated costs associated with low resource densities and distant fishing grounds. This thesis establishes a framework for assessing potential mesopelagic exploitation and its stock and fisheries sustainability based on various modelling approaches, that were rigorously sensitivity tested, and identifies key knowledge gaps that should be clarified before initiating any large-scale exploitation.