The Celtic Sea Through Time and Space: Ecosystem Modeling to Unravel Fishing and Climate Change Impacts on Food-Web Structure and Dynamics

Abstract

Both trophic structure and biomass flow within marine food webs are influenced by the abiotic environment and anthropogenic stressors such as fishing. The abiotic environment has a large effect on species spatial distribution patterns and productivity and, consequently, spatial co-occurrence between predators and prey, while fishing alters species abundances and food-web structure. In order to disentangle the impacts of the abiotic environment and fishing in the Celtic Sea ecosystem, we developed a spatio-temporal trophic model, specifically an Ecopath with Ecosim with Ecospace model, for the period 1985–2016. In this model, particular attention was paid to the parameterization of the responses of all trophic levels to abiotic environmental changes. Satellite remote sensing data were employed to determine the spatial distribution and annual fluctuations of primary production (PP). Spatial and temporal changes in the habitat favorable for zooplankton were predicted with a novel ecological-niche approach using daily detection of productivity fronts from satellite ocean color. Finally, functional responses characterizing the effect of several abiotic environmental variables (including, among others, temperature, salinity and dissolved oxygen concentration, both at the surface and at the bottom) on fish species groups’ habitat suitability were produced from the predictions of statistical habitat models fitted to presence-absence data collected by multiple fisheries-independent surveys. The dynamic component of our model (Ecosim) was driven by time-series of fishing effort, PP, zooplankton habitat suitability and abiotic environmental variables, and was fitted to abundance and fisheries catch data. The spatial component of our model (Ecospace) was constructed, for specific years of the period 1985–2016 with contrasted abiotic environmental conditions, to predict the variable distribution of the biomass of all functional groups. We found that fishing was the main driver of observed ecosystem changes in the Celtic Sea over the period 1985–2016. However, the integration of the environmental variability into the model and the subsequent improvement of the fit of the dynamic Ecosim component highlighted (i) the control of the overall pelagic production by PP and (ii) the influence of temperature on the productivity of several trophic levels in the Celtic Sea, especially on trophic groups with warm and cold water affinities. In addition, Ecospace predictions indicated that the spatial distributions of commercial fish species may have substantially changed over the studied period. These spatial changes mainly appeared to be driven by temperature and may, therefore, largely impact future fisheries given the continuity of climatic changes.

Mesoscale productivity fronts and local fishing opportunities in the European Seas

“…The use of a generic spatial estimate of ocean productivity available to fish based on chlorophyll-a gradients and productivity fronts (OPFish) can allow advances in marine ecosystem science as (i) it provides a direct, observation-based and local estimate of secondary production in relative levels but comparable in space and time at the global scale (Druon et al, 2019(Druon et al, , 2021, (ii) it can identify pelagic feeding hotspots for the higher trophic levels in the last two decades and in real time and (iii) it can be used operationally to increase the robustness of species habitat and full ecosystem analyses (Hernvann et al, 2020). At this stage, the Harvest index relative to ocean productivity (H P index) may enhance awareness that local overfishing of the pelagic or demersal species community may be linked to excessive fishing pressure compared to local productivity.…”

Copernicus Marine Service Ocean State Report, Issue 5

“…The habitat analyses suggest that the horizontal gradient of chlorophyll-a is a better proxy than chlorophyll-a when seeking the spatial distribution of mesozooplankton and marine predators. We emphasise in this paper that, while this variable requires an advanced knowledge in ocean colour limitations, its availability would greatly help the marine biologist community to produce robust, observation-based, planktonto-fish indicators from local to global scale in nowcast and hindcast modelling, such as input data to develop more robust full ecosystem models (Hernvann et al 2020).…”

EcoDiet: A hierarchical Bayesian model to combine stomach, biotracer, and literature data into diet matrix estimation

“…This highlights the need for more regular trophic sampling as part of ecosystem surveys (e.g., Doray et al, 2018). However, this divergence in prey identified between the literature and SCA could also result from the sampling design, conducted during one season only and not covering the whole mosaic of environmental conditions and all species assemblages of the Celtic Sea (Hernvann et al, 2020). For instance, anglerfish's isotope signatures were consistent with substantial consumption of Norway lobster, whereas it was not identified as a putative prey in SCA data (Issac et al, 2017) but was observed in the predator's stomachs in a neighboring ecosystem at a different season (Crozier, 1985).…”

“…Because of the environmental diversity characterizing the Celtic Sea (Hernvann et al, 2020; Walters et al, 2021), the generalist feeding behavior and ontogenetic diet shifts of some species (Day et al, 2019; Issac et al, 2017; Rault et al, 2017), the functional redundancy of several groups (Mérillet et al, unpublished manuscript), and benthic–pelagic coupling (Hernvann et al, 2020), data exhibited high intragroup variability and among‐group overlap in isotope signatures. Therefore, posterior distributions of dietary proportions were often diffuse, pointing out the limits of bulk SIA for studying such complex ecosystems.…”