The multiple dimensions of spatial ecology in fisheries management - How climate, economy, and connectivity shape Cod (Gadus morhua) dynamics in a changing North Sea

• Published in: Ecological Modelling
• Main Author: Romagnoni, Giovanni
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: 2021
• Subjects: atlantic cod; Gadus morhua
• Online Access: http://hdl.handle.net/10852/86434; http://urn.nb.no/URN:NBN:no-89080

This thesis is centred on the interactions between climate and spatial dynamics regulating the abundance and distribution of marine fish. In particular, the thesis focus on Atlantic cod (Gadus morhua) in the North Sea, and its ecology and management under climate change by adopting a multidisciplinary approach, where the boundaries of oceanography, ecology, economics and fisheries management meet. I explored this interface, proposing some answers toward adaptive fisheries management. North Sea cod has been among the main target species of commercial fisheries for centuries. Intense fishing pressure, in combination with environmental change, has resulted in a dramatic stock decline in the past decades. Management of the fisheries has allowed a gradual a recovery of North Sea cod stock since the mid 2000’s. However, in the past few years (since 2017) the North Sea cod stock has showed once again signs of decline. In addition to fishing pressure, recent declines in the North Sea cod may be due to global climate change (in particular, increase in sea surface temperature and related changes in the zooplankton community) that has been causally linked to reduced recruitment and increased predation of juvenile cod. Such effects are uneven across the area, seemingly more pronounced in the South and less in the North. This heterogeneous spatial response may be attributed to geographical and environmental factors such as the latitudinal gradient of temperature, differences in topography (i.e. depth) or oceanographic characteristics (e.g. current and tidal patterns). At the same time the presence of multiple populations within the North Sea stock may promote niche differentiation in response to climate change. These populations with independent dynamics may potentially require different management strategies. Such spatial heterogeneity is acknowledged but the stock is currently managed as a homogeneous unit. However, appropriate management should account for the spatial distribution of populations, their connectivity, their response to climate, and the effects of predator-prey interaction at the correct spatial scale. Throughout this thesis, I attempted to investigate how the interaction between spatial dynamics and the effects of climate impact cod ecology and population dynamics, and in turn how these emerging interactions may influence management. First, I explored spatial ecosystem dynamics (Paper 1), then the effects of larval behaviour on their distribution across spatial scales (Paper 2), and effects of inclusion of larval transport and of connectivity on estimates of recruitment (Paper 3). Finally, I explored through modelling the potential effects of including spatial population structure and climate change on the projected optimal management strategies (Paper 4). In particular, in Paper 1, my co-authors and I developed a spatial version of an existing (non-spatial) ecosystem model, using the Ecopath with Ecosim framework (a widely applied ecosystem modelling tool) and its spatial component Ecospace. We explored quantitatively the capability of the model to correctly reproduce known spatial patterns of fish biomass and fishing effort. Our results show a satisfactory capability to reproduce spatial distribution for fish biomass, but not for fishing effort. Moreover our study explored the sensitivity of model performance to variations in Ecospace parameters, identifying the most influential, and discussing the importance of accounting for parameter uncertainty. In Papers 2, 3 and 4 we addressed the issue of multiple populations and their spatial distribution. In Paper 2 we applied a coupled physical-biological model that simulates spatial distribution of particles representing cod eggs and larvae in the North Sea. We assessed the relative importance of three factors commonly considered highly relevant for modelling early life stages of marine organisms, namely spatio-temporal resolution of the model, explicit inclusion of larval vertical movement, and interannual variability. We found that the predicted spatial distribution of particles is moderately influenced by vertical movement and ocean model resolution. However, spatial distribution differs substantially between years. This implies that interannual variation in ocean dynamics plays a critical role in determining the degree of retention in the study area. We additionally observed that the effect of vertical movement strongly depends on the spatiotemporal scale of the analyses. In Paper 3 we applied a coupled physical-biological model to assess whether explicit inclusion of eggs and larvae transport processes outputs can improve the performance of stock-recruitment models. We thus paired a 44-year long time series of cod recruitment and spawning stock biomass data with larval transport anomaly, connectivity and sea surface temperature, both population-specific and at stock scale. We proposed a novel method to account for connectivity explicitly. This showed an effect of connectivity on recruitment, albeit small, and only at the population scale. Conversely, the traditional method detects a small effect of transport anomaly, and only at the stock scale. Moreover, we investigated the relationship between temperature and populations connectivity. We found a correlation between increasing temperature and larval drift from south to north, revealing potential effects of changing climate on population connectivity in the area. Finally, in Paper 4 we developed a bioeconomic model, based on an age-specific population dynamic model, to assess whether management that accounts for population structure could provide higher long-term economic returns. We explored alternative management strategies for the North Sea cod metapopulation, where two sub-populations are managed either independently or as unique stock unit. We tested the hypothesis that the advantage of managing populations separately increases under rising temperature, given different population sensitivity to temperature. Our results showed that, in the context of optimal management, moving from non-spatial management to population specific management was not economically advantageous under any climate scenario, likely due to the similar response to temperature of our modelled populations. The economic impacts caused by increasing temperature or by adopting a suboptimal constant harvest rate (irrespective of population scale) were larger than managing at the incorrect spatial scale. This thesis proposes that interactions between climate change, fish population structure and the spatial distribution of fish eggs and larvae influence the population dynamics and, therefore, the sustainability and profitability of the fishery. These interactions should be accounted for by management, despite the existing gaps in our understanding of the interrelationships between ecology, oceanography, economics and management.