SEAwise report on the effects of fishing on food webs and community diversity aimed at populating the MSFD Descriptor 4 and based on food web and end-to-end modelling.

The SEAwise project works to deliver a fully operational tool that will allow fishers, managers, and policy makers to easily apply Ecosystem Based Fisheries Management (EBFM) in their fisheries. This SEAwise report describes work that aims to provide a strong evidence base supporting the use of ecological indicators in fisheries management through statistical studies and end to end modelling. Our aim is to support the use of ecological indicators to guide managers towards strategies that lead to an ecologically safe space for fisheries, which we define as a range of states that provide yields for sustainable fisheries, maintains ecosystem functions and leads to low risk of overexploitation of species and loss of diversity. We show from an analysis of the past, using stock assessment model products for the northeast Atlantic and observations from scientific survey data of the North Sea, that fishing has already impacted ecosystems. Fishing on commercial fish stocks altered the balance of functional groups within the ecosystem of the northeast Atlantic, with depletions in the biomass of fish during the 1980s followed by stock rebuilding since the early 2000s, such that the biomasses of benthivorous fish and planktivorous fish are now much greater than that of piscivorous fish, which may be a sign of an improving ecosystem. Within the North Sea, fishing also appeared to have depleted the biomass of fish during the 1980s (particularly in the southern North Sea) and led to a change in the composition of species and the proportion of large individuals in the ecosystem (particularly in the northern North Sea).We use comparative ecosystem modelling, employing multiple model types, to investigate the response of foodweb and community diversity indicators under a range of fishing strategies. We consider ‘no fishing’ scenarios, under prevailing environmental conditions, as a measure of the unimpacted state of the ecosystem i.e., its carrying capacity. Our modelled unimpacted state is our ‘yardstick’ - a standard from which we can measure the impact on the ecosystem due to current levels of fishing in the current conditions. We propose that the depletion of species, the difference in biomass of each species from their unimpacted levels under prevailing environmental conditions, can provide a basis from which to evaluate the risk of loss across impacted groups within the ecosystem.We investigate if any of our selected food web and community indicators, including those used within the regional sea convention OSPAR, can support the use of assessment thresholds that may help managers reduce the risk of depletion of species or degradation of ecosystems by fishing. We explore management scenarios, linked to changes in levels of fishing impact due to current fisheries (so not favouring one type of fishing over another), to demonstrate how reducing or increasing activity levels will alter the risk of depletion of species and demonstrate how ecological indicators are expected to change given alterations in the structure of the ecosystem. We contrast these management scenarios to additional scenarios in which seabed-impacting gears only are restricted to avoid damage to habitats. This latter scenario promotes fishing on pelagic fish, such as herring and sprat, in the future and inhibits trawling for demersal fish such as sole, plaice and cod.In each model, we find clear predictable relationships between fishing mortality overall (averaged across all model groups) and the depletion risk within the ecosystem. The biomass of apex predators typically decreases as fishing mortality increases. Similarly, as the size-structure of demersal fish communities decreases, risk within the communities increase. In contrast, the balance of trophic guilds in the ecosystem and the Shannon diversity overall or by group respond in differing ways (increases and decreases with increased fishing) dependent on the type fishing strategies modelled. The indicator targets for recovery and risk-based limits proposed here will be explored further in WP6 of the project.Read more about the project at www.seawiseproject.org</p

SEAwise Report on the key drivers of stock productivity and future environmental scenarios

An ecosystem approach to fisheries management requires the consideration of commercial species as components of an ecosystem and the acknowledgement of the links between their productivity and the surrounding environment. To provide a knowledge base for such links, SEAwise consulted stakeholders throughout Europe and conducted a systematic review of the scientific literature. The systematic review resulted in 2050 articles from the literature search that were screened for their tile and abstract. 516 of them were retained for data extraction. The majority of studies were conducted in the Baltic Sea and the North Sea, followed by the Western Waters, and with only a few dozen papers in the Mediterranean Sea. Cod and herring were the most studied species, temperature and more generally climate and hydrodynamics indicators were the main drivers investigated, and reproduction was the main productivity-related process. The output of the systematic review is a database of scientific articles organised by regions, species, environmental drivers and productivity-associated processes and where outcomes, but also spatial and time scales, analytical methods etc. are described in a standardised fashion. This database will be analysed in the coming months and used in the downstream tasks of WP3. The most frequently driver identified by stakeholders across regions was climate change followed by species interactions, cod, pollution, commercial fish/shellfish and plankton. Climate change effects on stocks through temperature and salinity are relatively well covered in the literature as are effects of plankton and species interaction. Studies of the effects of pollution do not occur frequently and as a consequence require a dedicated effort is made in SEAwise to remedy this. Species reported frequently by the stakeholders included cod, seabass, sardine, sole, crabs, flatfish, Norway lobster, octopus, shrimps, herring, sprat, anchovy, hake, new species (species increasing in abundance as a result of climate change as well as invasive species of commercial interest) and sandeel. Among these, more than 10 papers were retrieved for cod, sardine, sole, herring, sprat, selected flatfish, anchovy, hake and sandeel. For the remaining species, a dedicated effort must be made in SEAwise if they are to be included in stock models.  This report describes results of the SEAwise project. More information about the project can be found at https://seawiseproject.org/</p

SEAwise Report on the key species and habitats impacted by fishing

The implementation of ecosystem-based fisheries management requires knowledge on the ecological impact of fishing activities on species and their habitats – those both targeted and not targeted by fisheries. To identify which ecological impacts are key and what is known about them, SEAwise consulted stakeholders through European Advisory Councils and conducted a systematic review of the scientific literature to map the available knowledge and evidence. Specific reference was given to the bycatch of Protected, Endangered and Threatened (PET) species, benthic habitats, food webs and biodiversity, and impact from fisheries-related litter and ghost nets.  At the stakeholder consultations, sharks and/or elasmobranchs, turtles, species interactions, and seals or marine mammals were identified as top ranked in at least three out of the five regions. Other terms identified by at least two Case Study regions were: seabirds, sensitive species, benthic habitats, litter, PET species, invasive species and species interactions.  Relevant data were extracted from 549 retained papers. The majority of studies were conducted in the Mediterranean Sea, whereas only few papers reported on fishing impacts in the Baltic Sea (see figure below). Bony fish (teleosts) and benthos were the most studied ecosystem components in all Case Study regions, whereas marine mammals and cartilaginous fish were often studied in relation to bycatch of PET species.  Out of the 549 papers, most of them were related to fishing impacts on food webs and biodiversity and benthic habitats, followed by bycatch of PET species and other fishing impact studies (not related to any task). Fewest studies were related to the impact of fisheries-related litter and ghost nets. Demersal trawls were by far the most studied gear in studies on commercial fishing impacts. For recreational fisheries, hooks and lines, in particular angling, was the most studied fishing activity.  Among the items identified by the stakeholders, marine mammals, seabirds and reptiles were all covered in at least 25 papers each, indicating that there is a considerable body of knowledge even though not all areas may have information for all species. Litter was the key item that was least frequently reported on in the literature, especially outside the Mediterranean, where scientific papers were rare. As a consequence, areas outside the Mediterranean may lack information for further analysis unless a dedicated effort is made in SEAwise to remedy this. The regional differences in topics identified by stakeholder scoping did not reflect the regional amount of papers available.  This report describes results of the SEAwise project. More information about the project can be found at https://seawiseproject.org/</p

SEAwise Report on improved predictive models of growth, production and stock quality.

The SEAwise project works to deliver a fully operational tool that will allow fishers, managers, and policy makers to easily apply Ecosystem Based Fisheries Management (EBFM) in their fisheries and understanding how ecological drivers impact stock productivity through growth, condition and maturity is essential to this proces. In this SEAwise report, we present the predictive models of fish growth, condition and maturity obtained so far in each of the four regional case studies.The biological processes (fish growth, condition and maturity) were studied in terms of body size (weight-at-age, length-at-age), condition factor, otolith increments and size at first maturity. Underlying data were available at different levels, ranging from individual fish, to sampling haul or stock level. Accordingly, the methods employed varied across case studies to adapt to the specific features of the process under study and the available data.The methodology encompassed statistical models (linear models, generalised additive models, mixed models, Bayesian nested hierarchical models, changepoint models), otolith growth increment analyses and mechanistic models (DEB-IBM model coupled to the environment and mizer model). Some of these models were focused on detecting overall trends, including potential changepoints along the time series or identification of the main intrinsic factors. Other models explored the impact of ecological drivers such as temperature, salinity, food availability or density dependence.In the Baltic Sea, two regimes were identified in the weight-at-age time series of herring in the Gulf of Riga (1961-1988 and 1989-2020). During the first period the main driver of the individual annual growth of the fish was the abundance of the copepod L. macrurus macrurus, while the abundance of the adult stages of E. affinis affinis was the dominating explanatory variable affecting herring growth during the second period. Neither SSB nor summer temperature during the main feeding period were significant drivers of the individual growth in the two distinct ecosystem regimes.In the Mediterranean Sea, the analysis of the impact of the environmental variables on biological parameters like size at first maturity, condition factor and growth in South Adriatic Sea and North-West Ionian Sea showed some significant effects in relation to the different species/area. In most of the cases, the environmental driver was bottom temperature, although some relationships with bottom salinity and primary production were also found. The model outcomes suggested that temperatures prevailing in deeper waters were the most significant factor affecting gonad maturity of hakes, while those in the shallow zone had the main impact on the L50 of red mullets. Condition factor of hake and red mullet in the Eastern Ionian Sea were affected not only by temperature, but also by zooplankton abundance.In the North Sea, mediated length-based growth models, linear mixed models and state-space linear mixed models were applied to four gadoids, two flatfishes and one pelagic stock and their performances were assessed in terms of model fit and predictive capability. For the mediated length-based growth model approach, the best model differed across stocks, but density dependent mediation effects were significant for five out of the seven stocks. Regarding the linear mixed models, the two types of models and the different penalisation procedures led to different models across stocks. Among the additional ecological variables, surface temperature was the most frequently included in the final model, closely followed closely by SSB and to a lesser extent by NAO. Detailed otolith increment analysis was used in the development of multidecadal biochronologies of average annual growth of sole in the North Sea and in the Irish Sea. In the North Sea, the best extrinsic model of sole growth included sea bottom temperature, fishing mortality at age, and stock biomass at maturity stage, and their interactions with age and maturity stage, while in the Irish Sea, the best extrinsic model included sea bottom temperature and fishing mortality at maturity stage and its interaction with maturity stage. These results confirmed the expected positive effect of temperature on adult growth. However, in the North Sea, temperature showed unexpected negative effect on juvenile growth, which might be linked to changes in food availability and/or intraspecific competition and need to be further studied. The mizer model (package for size-spectrum ecological modelling) with environmental forcing was used to study whether warming in the North Sea is responsible for the failure of the cod stock. The simulated fish community response when recruitment and carrying capacity depended on surface temperature fitted better with the assessment data than when the environment was fixed. However, the qualitative differences remain, suggesting that temperature effects were not the main cause of the model-assessment disparity.In the Western Waters, the mediated length-based growth models developed for the North Sea case study were applied to 14 stocks in the Celtic Sea. The best model differed across stocks, but again SSB mediation was significant for most of the stocks. From visual inspection of the plots, however, it was noted that the raw data from certain stock objects showed a reduced growth compared to the model fits, requiring further analyses. The analysis on biological measurements of individuals collected at fish markets, observers at sea or during scientific cruises allowed to study temporal variations in body size and condition factor of benthic, pelagic and demersal species in the Celtic Sea and the Bay of Biscay. The linear models indicated a significant negative monotonic relationship of sizes at all ages for anchovy and pilchard, but variations in size at age were less clear and significant for benthic and demersal species. In contrast, the results of the body condition indices showed a moderate but significant decrease for all the studied 19 species over time. The in-depth analysis for anchovy in the Bay of Biscay based on research surveys confirmed the decline in the length and weight of anchovy in the Bay of Biscay and pointed to a decline in body condition toward slender body shapes. Detected associations between temperature and size became more apparent for adult age classes than for juveniles, whereas the association between anchovy size and the biomass of spawners was more important for juvenile than for adult age classes. Associations between anchovy size and chlorophyll-a concentration were in general weak. Finally, the DEB-IBM model coupled to the environment that is under development for the two main seabass stocks of the North East Atlantic will provide further insights on how growth, condition and maturation can affect the future dynamics and productivity of these stocks.Read more about the project at www.seawiseproject.org</p