Ecopath Research Articles

Lake Restoration Improved Ecosystem Maturity Through Regime Shifts—A Case Study of Lake Baiyangdian, China

Lake ecosystems are impacted by anthropogenic disturbances and have become vulnerable worldwide. Highly disturbed lake ecosystems are not well understood due to the lack of data on changes in the structures and functions of ecosystems. In this paper, we focus on Lake Baiyangdian (BYDL), the largest shallow lake in North China. Following the establishment of the Xiong’an New Area (XNA) in 2017, concerted efforts to restore BYDL’s aquatic environment have been undertaken, which has led to significant changes in the structures and functions of the ecosystems. We evaluated the biomass dynamics of main biological communities and detected the regime shifts of environmental factors in BYDL from 2016 to 2023. Further, we constructed a food web model for the BYDL ecosystem in 2023 by using Ecopath with Ecosim (EwE) and made a comparison with the reported results in 2018. The results showed significant changes in the ecosystem structure of BYDL over the last 6 years. In 2023, the submerged macrophytes biomass in the system increased by 4.2 times compared to 2018, leading to an increase in total system throughput. We found that BYDL changed from an algal-type lake to a macrophyte-dominated lake. In addition, we found TN, NH4+-N, and CODMn were significantly decreased in BYDL during the restoration. TN and NH4+-N had a change point in approximately 2021, indicating that a regime shift had occurred during restoration. Overall, the BYDL ecosystem was in an immature but developing state, as indicated by ecological network analysis indicators. Nutrient-loading reduction, hydrological regulation, and rational biomanipulation may be the potential driving factors of change in the BYDL ecosystem. We strongly recommend the timely harvesting of submerged macrophytes, the proliferation and release of herbivorous fishes, and the assessment of the ecological capacity of carnivorous fishes in the future ecological restoration of BYDL.

A trophic modelling framework: Key metrics for the ecological assessment of artificial structures

As the global population expands, marine coastal ecosystems face mounting pressures from human activities, that have led to habitat deterioration and dwindling fishery resources. In this context, Artificial Reefs (ARs) have emerged as one of the promising solutions. They are generally implemented to provide habitat, to create a protective, physical boundary, to support sustainable fisheries and to facilitate ecosystem rehabilitation. Evaluating their ecological performance is crucial to ensuring they meet their objectives. Initially, assessment relied on comparing ARs to natural reefs using mainly ecological metrics which focused on fish assemblage and dynamics. Despite there being more research and documentation on effectiveness today, assessing ARs remains challenging due to the number of environmental factors that can affect the ecological systems. Moreover, ecological studies mainly used metrics that investigated the reef fish populations or ecological metrics such as fish assemblages or trophic structure that are often overlooked in studies that primarily focus on commercial fishery dynamics. Therefore, new ways of assessing artificial reef performance and the set-up of comprehensive metrics which integrate this level of complexity are needed. In this study, we focused on the "Rade de Cherbourg" in the English Channel, employing a trophic modeling approach using Ecopath with Ecosim (EwE). The study emphasizes the importance of Ecological Network Analysis (ENA) metrics for evaluating changes in the systems’ properties—such as complexity, flow diversity, and recycling capacity— which result from AR implementation. Furthermore, we identified which metrics are suitable for assessing specific AR objectives. The proposed metrics serve as a command-and-control tool for AR site managers, enabling them to evaluate the performance of each AR objective effectively. With the anticipated increase in AR projects, especially those which compensate for human impact like the Cherbourg ARs, this research offers valuable insights and future perspectives to continuously improve the ecological performance of ARs.

Effects of climate and anthropogenic pressures on chemical warfare agent transfer in the Baltic Sea food web

The Baltic Sea is a severely disturbed marine ecosystem previously used as a dumping ground for chemical warfare agents (CWA), which are now known to enter its food web. We have performed a modelling exercise using a calibrated and validated Central Baltic Ecopath with Ecosim (EwE) model to recreate the potential environmental pathways of the infamous Clark I (diphenylchlorarsine). Observations from modelling timestamps covering recent times correspond with in situ detections in sediments and Atlantic cod (Gadus morhua). Under applied modelling conditions and scenarios, there is an active transfer of Clark I from sediments through the Baltic Sea food-web. According to our results, Clark I bioaccumulates within the Baltic Sea food web exclusively throughout the detritus-based food chain. The EwE model for the Central Baltic Sea also allows the simulation of changes in the food web under multiple anthropogenic stressors and management efforts, including recommendations from the Helsinki Commission Baltic Sea Action Plan (HELCOM BSAP). Among all investigated scentarios and factors, the commercial fishing is the most impactful on Clark I accumulation rate and contamination transfer within the Baltic Sea food web. The study indicates the need to extend the existing monitoring approach by adding additional species representing a broader range of ecological niches and tiers within the food chains. From the environmental perspective, the remediation of Chemical Weapons by removal should be considered as part of the integrated management of the Baltic Sea.

The combined effects of predation, fishing, and ocean productivity on salmon species targeted by marine mammals in the northeast Pacific.

Along the northeast Pacific coast, the salmon-eating southern resident killer whale population (SRKW, Orcinus orca) have been at very low levels since the 1970s. Previous research have suggested that reduction in food availability, especially of Chinook salmon (Oncorhynchus tshawytscha), could be the main limiting factor for the SRKW population. Using the ecosystem modelling platform Ecopath with Ecosim (EwE), this study evaluated if the decline of the Pacific salmon populations between 1979 and 2020 may have been impacted by a combination of factors, including marine mammal predation, fishing activities, and climatic patterns. We found that the total mortality of most Chinook salmon populations has been relatively stable for all mature returning fish despite strong reduction in fishing mortality since the 1990s. This mortality pattern was mainly driven by pinnipeds, with increases in predation between 1979 and 2020 mortality ranging by factors of 1.8 to 8.5 across the different Chinook salmon population groups. The predation mortality on fall-run Chinook salmon smolts originating from the Salish Sea increased 4.6 times from 1979 to 2020, whereas the predation mortality on coho salmon (Oncorhynchus kisutch) smolts increased by a factor of 7.3. The model also revealed that the north Pacific gyre oscillation (NPGO) was the most important large-scale climatic index affecting the stock productivity of Chinook salmon populations from California to northern British Columbia. Overall, the model provided evidence that multiple factors may have affected Chinook salmon populations between 1979 and 2020, and suggested that predation mortality by marine mammals could be an important driver of salmon population declines during that time.

Calibrating ecosystem models to support ecosystem-based management of marine systems

Abstract Ecosystem models, such as Ecopath with Ecosim (EwE), provide a platform to simulate intricate policy scenarios where multiple species, pressures, and ecosystem services interact. Complex questions often return complex answers, necessitating evidence and advice to be communicated in terms of trade-offs, risks, and uncertainty. Calibration procedures for EwE, which can act as a source of uncertainty and bias in model results, have yet to be explored in a comprehensive way that communicates how sensitive model outputs are to different calibration approaches. As the EwE community has grown, multiple divergent approaches have been applied to calibrate models through the estimation of vulnerability multipliers: parameters that augment the consumption rate limits of predators. Here we explore the underlying principles of vulnerability multipliers as well as existing calibration approaches and their justification. Two case studies are presented: the first explores how vulnerability multipliers emerge based on the chosen calibration approach using simulated data, while the second takes two operational EwE models (Irish Sea and Northwest Atlantic Continental Shelf) and compares their outputs when calibrated following alternate calibration approaches. We show how calibration approaches can impact model-derived advice and provide a list of best practice recommendations for EwE calibration.

Modelling microplastic bioaccumulation and biomagnification potential in the Galápagos penguin ecosystem using Ecopath and Ecosim (EwE) with Ecotracer.

Bioaccumulation and biomagnification of anthropogenic particles are crucial factors in assessing microplastic impacts to marine ecosystems. Microplastic pollution poses a significant threat to iconic and often endangered species but examining their tissues and gut contents for contaminant analysis via lethal sampling is challenging due to ethical concerns and animal care restrictions. Incorporating empirical data from prey items and fecal matter into models can help trace microplastic movement through food webs. In this study, the Galápagos penguin food web served as an indicator species to assess microplastic bioaccumulation and biomagnification potential using trophodynamic Ecopath with Ecosim (EwE) modelling with Ecotracer. Empirical data collected from surface seawater near Galápagos penguin colonies, zooplankton, penguin prey, and penguin scat in October 2021 were used to inform the ecosystem model. Multiple scenarios, including a 99% elimination rate, were employed to assess model sensitivity. Model predictions revealed that microplastics can bioaccumulate in all predator-prey relationships, but biomagnification is highly dependent on the elimination rate. It establishes the need for more research into elimination rates of different plastics, which is a critical missing gap in current microplastic ecotoxicological and bioaccumulation science. Compared to empirical data, modelling efforts underpredicted microplastic concentrations in zooplankton and over-predicted concentrations in fish. Ultimately, the ecosystem modelling provides novel insights into potential microplastics' bioaccumulation and biomagnification risks. These findings can support regional marine plastic pollution management efforts to conserve native and endemic species of the Galápagos Islands and the Galápagos Marine Reserve.

Using a food web model to predict the effects of Hazardous and Noxious Substances (HNS) accidental spills on deep-sea hydrothermal vents from the Mid-Atlantic Ridge (MAR) region

Deep-sea hydrothermal vents host unique ecosystems but face risks of incidents with Hazardous and Noxious Substances (HNS) along busy shipping lanes such as the transatlantic route. We developed an Ecopath with Ecosim (EwE) model of the Menez Gwen (MG) vent field (MG-EwE) (Mid-Atlantic Ridge) to simulate ecosystem effects of potential accidental spills of four different HNS, using a semi-Lagrangian Dispersion Model (sLDM) coupled with the Regional Ocean Modelling System (ROMS) calibrated for the study area. Food web modelling revealed a simplified trophic structure with low energy efficiency. The MG ecosystem was vulnerable to disruptions caused by all tested HNS, yet it revealed some long-term resilience. Understanding these impacts is vital for enhancing Spill Prevention, Control, and Countermeasure plans (SPCC) in remote marine areas and developing tools to assess stressors effects on these invaluable habitats.

Ecosystem Characteristics and Trophic Model of the Artificial Reef Ecosystem in the Sea of Oman, Sultanate of Oman

This study aimed to understand the structure and function of the artificial reef ecosystem of the Sea of Oman and its stability and maturity. For this study, the trophic model of the Sea of Oman’s artificial reef ecosystem was described using the Ecopath with Ecosim (EwE) ecosystem modeling software (Version 6.6.7). The essential characteristics of the aquatic system were identified using a total of 38 fish species/functional groups, spread across an area of 140 km2 of artificial reef farm. The mean trophic level of the artificial reef ecosystem of the Sea of Oman was 3.039. Sharks were the keystone species of the studied ecosystem. Heniochus acuminatus and Chaetodon gardneri were the species with the highest niche overlap, whereas Acanthurus sohal and other crustaceans, and Terapon puta and Saurida undosquamis were the species with the lowest niche overlap. It was found that the ratio of total primary production to total respiration of the ecosystem studied was more than one, indicating that the system produces more energy than it uses to respire, and the ecosystem of the Sea of Oman can be regarded as a developing system because of its low degree of stability and maturity. The omnivory index was 0.260, the connectance index was 0.159, the total biomass to total throughput ratio was 0.006, Finn’s cycling index was 5.41, the total primary production to total biomass ratio was 64.895, and the total primary production to total respiration ratio was 4.424. The results indicate that the artificial reef ecosystem in the Sea of Oman can be categorized as immature (in the early developmental stage). Further study is needed to improve the input data and track ecosystem health, as well as exploring other management strategies. Based on the outcomes of the study, it is suggested that environmental management of the reef ecosystem, along with the fish catch data, should be taken into consideration for future research.

Energy flow analysis of in Litopenaeus vannamei-Tegillarca granosa polyculture ponds based on EwE model

In order to characterize the energy flow and trophic structure of a polyculture pond that contained Litopenaeus vannamei and Tegillarca granosa, we used an Ecopath with Ecosim (EwE) model. We constructed the EwE model using 13 functional groups including L. vannamei, T. granosa, macrozooplankton, microzooplankton, microbenthos, bacterioplankton, benthic bacteria, micro-phytoplankton, nano-phytoplankton, pico-phytoplankton, artificial feed, detritus in water, and detritus in sediment. Our findings showed that this polyculture ecosystem contained five trophic levels and that two of them accounted for 99.41% of the total system throughput. L. vannamei, microbenthos, and detritus in sediment displayed high ecotrophic efficiency (EE) values, while artificial feed held relatively low EE implying that the artificial feed was used inefficiently. The bacteria including benthic bacteria and bacterioplankton was inefficiently used. The main energy flows were associated with artificial feed, phytoplankton, and detritus in this polyculture system. The compatibility of L. vannamei and T. granosa was high according to the mixed trophic impact analysis. In conclusion, the study showed that this polyculture ecosystem still needed to be improved in the aspects of energy consumption. It would be necessary for this polyculture ecosystem to perform further optimization and enhancement on the energy utilization efficiency and comprehensive benefits.

Evaluating the efficacy of fisheries management strategies in China for achieving multiple objectives under climate change

It is increasingly recognized that effective fisheries management needs to take into account not only the target species, but also the broader scope of biological, environmental, socio-economic and institutional factors pertaining to human utilization of fishery resources. This recognition has stimulated a global call for the Ecosystem-Based Fisheries Management (EBFM), with an increasing appreciation of its merits in the face of climate change. In this study, we applied the concept of EBFM to China's fisheries and tested the performance of various fisheries management strategies, including status quo management and alternative strategies with different objective priorities, under contrasting climate warming scenarios. We used the Ecopath with Ecosim (EwE) as an operating model, with an explicit consideration of trophic interactions, and specifically quantified the trade-offs across multiple metrics (i.e., sustainable fishery production, economic profitability, seafood security, and ecosystem health). No single strategy was superior across all the four dimensions. Importantly, the existing approach to fisheries management underperformed in most metrics, with the current fishing intensity exceeding the level that would maximize multispecies yields (fMSY) and total profits (fMEY). In contrast, an alternative harvest control rule (HCR) strategy, designed to respond to variations in biomass, provided inherent resilience of the ecosystem under climate warming, resulting in increased commercial fish biomass, improved profit and seafood supply, as well as enhanced ecosystem structure and function across various time-climate scenarios. From a tactic perspective, the multiple species management strategies of “fMSY” and “fMEY” can be better suited for the highly mixed fishery context in China. These alternatives also consistently demonstrated superior performance compared to the existing fisheries management strategy. Our work reveals potential trade-offs among diverse ecosystem services and provides a framework using EwE model for quantifying possible performance of management strategies under climate change, leading to more informed and transparent decision-making.

Trophic fingerprinting of a pristine but rapidly deteriorating downstream region of a Western Ghats River.

Chalakudy River is renowned for its pristine waters and rich ichthyofaunal biodiversity. The downstream area of the river is confronting a series of risks, including pollution, saline water ingression, sand mining, illegal and intensified fishing practices, and invasion of exotic and alien species. A mass balanced ecosystem model was constructed for the downstream region of Chalakudy River (DCR) using Ecopath with Ecosim (EWE), incorporating 12 functional groups to delineate the food web and network flow indices for the period 2020 to 2021. The trophic level (TL) of the ecosystem network ranged from TL-1 (detritus) to TL-3.4 (birds). High fishing pressure is one possible cause for the high ecotrophic efficiency values as evidenced by the fish groups. Both the grazing food chain and detritus food chain (detritivory: herbivory ratio 0.94) contributed more or less equal to the energy transfer between TL. Network analysis of the model indicated a mean transfer efficiency of 12%, with shares from primary producers (14%) and detritus (11%). A mixed trophic impact analysis demonstrated a strong positive impact of primary producers and detritus groups on most of the other ecological groups at higher trophic levels. The DCR model showed a high system throughput (32,464.7 t km-2 year-1), low system omnivory (0.09), low connectance index (0.36), low Finn's cycling index (4.9), and mean path length (2.8), low relative ascendency (37.5%), and high system overhead (62.5%). These indices propound that DCR is an immature and developing ecosystem with moderate strength in reserve to resist external perturbations.

Interactions between cage fish farms and fishing in Brazilian semiarid reservoirs: An ecosystemic approach

Knowledge of the trophic web structure and control mechanisms in reservoirs of the semiarid tropical regions can enable a better understanding and proper management of these aquatic environments. This paper used an ecosystem approach to two Brazilian semiarid reservoirs and evaluated the interactions between cage fish farms and fishing activity. The trophic web models of the reservoirs were elaborated using Ecopath with Ecosim (EwE) software. Over two years, the data were locally obtained, including information on fish farm, fishing activity, and functional groups (phytoplankton, aquatic macrophytes, benthic fauna, and fish). The modeling results indicated that most of the system energy is derived from the detritivory chain, which was more critical than the herbivory chain, with a Detritivory/Herbivory ratio of approximately 3.0 in both reservoirs. The key group of the ecosystem was the piscivorous fish, with Hoplias malabaricus and Cichla monoculus as the species with the greatest trophic impact, especially on fish at the intermediate trophic level. The benthic community and detritus had positive impacts on almost all other components of the trophic web. The reservoirs were considered immature, with low nutrient cycling and a high Total Primary Production/Total Respiration ratio. Systemic stability was deemed low for both reservoirs, which was indicated by the low overhead values. In both reservoirs, the presence of a well-developed detritus chain supports the trophic web and represents a source of stored energy in aquatic systems. The simulations demonstrated a positive relationship between fish farming and fishing activity, with all communities benefiting from the nutrient input from cage fish farms, with a biomass increase of aquatic organisms, either by direct or indirect effects.

Ecotrophic perspective in fisheries management: a review of Ecopath with Ecosim models in European marine ecosystems

The aim of this work is to present the food web models developed using the Ecopath with Ecosim (EwE) software tool to describe structure and functioning of various European marine ecosystems (eastern, central and western Mediterranean Sea; Black Sea; Bay of Biscay, Celtic Sea and Iberian coast; Baltic Sea; North Sea; English Channel, Irish Sea and west Scottish Sea; and Norwegian and Barents Seas). A total of 195 Ecopath models based on 168 scientific publications, which report original, updated and modified versions, were reviewed. Seventy models included Ecosim temporal simulations while 28 implemented Ecospace spatiotemporal dynamics. Most of the models and publications referred to the western Mediterranean Sea followed by the English Channel, Irish Sea and west Scottish Sea sub-regions. In the Mediterranean Sea, the western region had the largest number of models and publications, followed by the central and eastern regions; similar trends were observed in previous literature reviews. Most models addressed ecosystem functioning and fisheries-related hypotheses while several investigated the impact of climate change, the presence of alien species, aquaculture, chemical pollution, infrastructure, and energy production. Model complexity (i.e., number of functional groups) increased over time. Main forcing factors considered to run spatial and temporal simulations were trophic interactions, fishery, and primary production. Average scores of ecosystem indicators derived from the Ecopath summary statistics were compared. Uncertainty was also investigated based on the use of the Ecosampler plug-in and the Monte Carlo routine; only one third of the reviewed publications incorporated uncertainty analysis. Only a limited number of the models included the use of the ECOIND plug-in which provides the user with quantitative output of ecological indicators. We assert that the EwE modelling approach is a successful tool which provides a quantitative framework to analyse the structure and dynamics of ecosystems, and to evaluate the potential impacts of different management scenarios.

Ecological carrying capacity and carbon sequestration potential of bivalve shellfish in marine ranching: A case study in Bohai Bay, China

IntroductionShellfish play an important role in ecological restoration and as carbon (C) sinks, but studies on their ecological carrying capacity (ECC) and C sequestration potential are sparse.MethodsIn this study, we selected a 57-hectare artificial oyster reef in a typical marine ranching in Bohai Bay, China, to evaluate the ECC and their C sequestration potential of bivalve shellfish, and projecting their impact on functional groups in the system, with an Ecopath with Ecosim (EwE) food web model. We conducted four biological surveys to obtain the biomass measurements, with one conducted in each of the summer, autumn, and winter of 2019 and one in the spring of 2020; and the functional groups included in the surveys comprised fish, cephalopods, crustaceans, snails, bivalve shellfish, annelids, other macrobenthos, meiobenthos, starfish, sea cucumbers, zooplankton, phytoplankton, and detritus.Results and DiscussionThe EwE model prediction results showed that the ECC of bivalve shellfish was established to be 282.66 t/km2, far more than the existing quantity of 187.76 t/km2. Therefore, at present, the ecosystem of the study marine ranching is not yet mature. Moreover, our ecological network analysis parameters indicated that the marine ranching ecosystem will be mature and stable when the bivalve shellfish population reaches its ECC. However, the increase in bivalve shellfish biomass will result in a decrease in the population sizes of species competing for food resources with bivalve shellfish, mainly gobiid fish such as Tridentiger bifasciatus, Tridentiger trigonocephalus, Tridentiger barbatus. Simultaneously, when the bivalve shellfish reach their ECC, 29.23 t of CO2 can be sequestrated by bivalve shellfish, comprising 14.32 t being removed from the ecosystem as prey and 14.91 t being stored on the seafloor through biodeposition.ConclusionTherefore, the research demonstrated that, within the scope of ECC, the increasing bivalve shellfish can improve the C sequestration capacity of the marine ranch ecosystem, and effective management of bivalve shellfish in marine ranching can improve the economic benefits and C sink service functions of marine ranching.

It is past time to use ecosystem models tactically to support ecosystem‐based fisheries management: Case studies using Ecopath with Ecosim in an operational management context

AbstractThe implementation of ecosystem management requires ecosystem modelling within the context of a natural resource management process. Ecopath with Ecosim (EwE) is the most widely used modelling platform for investigating the dynamics of marine ecosystems, but has played a limited role in fisheries management and in multi‐sector resource decision‐making. We review 10 case studies that demonstrate the use of EwE to support operational resource management. EwE models are being used to inform tactical decision‐making in fisheries and other ocean use sectors, as well as to identify key trade‐offs, develop appropriate policy objectives, and reconcile conflicting legislative mandates in a variety of ecosystems. We suggest the following criteria to enhance the use of EwE and other ecosystem models in operational resource management: (1) a clear management objective that can be addressed through modelling; (2) an important trade‐off and a receptive policy context amenable to trade‐off evaluation; (3) an accessible and well‐documented model that follows best practices; (4) early and iterative engagement among scientists, stakeholders, and managers; (5) integration within a collaborative management process; (6) a multi‐model approach; and (7) a rigorous review process. Our review suggests that existing management frameworks are as much or more of a limitation to the operational use of EwE than technical issues related to data availability and model uncertainty. Ecosystem models are increasingly needed to facilitate more effective and transparent decision‐making. We assert that the requisite conditions currently exist for enhanced strategic and tactical use of EwE to support fisheries and natural resource management.

Trophic Modelling of the Cross River Estuary Ecosystem, Nigeria

A trophic modelling of the Cross River estuary ecosystem was undertaken using Ecopath with Ecosim (EwE). Data were obtained from peer reviewed journals, monographs and stock assessment reports and used to construct the Ecopath model used to investigate the structure, function and system throughput of the estuary. These data included biological data such as biomass, production/ biomass, consumption/biomass, fish catch and diet composition of each species or functional group included in the model. The network analysis routine was used to estimate trophic levels and trophic interactions of functional groups, health condition and developmental stage, and food web features. Results of Ecopath model revealed three apical predators: Shark, Senegalese tonguesole, and Sea catfish. The ratio of total primary production to total respiration value of 4.182 revealed that the Cross River estuary is an immature ecosystem void of biological and ecosystem pollution and is at its early developmental stage. The mean trophic level of the catch of 2.186 for the system and the high gross food conversion efficiency (GE) of 0.300 for Macrobrachium spp and 0.292 for Penaeid shrimp, revealed high fishing pressure on the herbivores. The estuary ecosystem has a short food chain and low connectance index of 0.1879 which has phytoplankton being grazed upon by zooplankton which in turn are taken up by bonga. Model results and physico-chemical parameters showed that the Cross River estuary is still relatively pristine and immature ecosystem, void of organic pollution and with relatively low primary productivity as evident in relatively low nutrient concentration. The high value of ecotrophic efficiency (EE) of 0.993 for bonga and 0.991 for Macrobrachium spp indicate that the population of these animals are heavily exploited and predated upon in the estuary. Despite that this is a preliminary trophic model, it is clear that fish populations in the estuary are experiencing excessive fishing pressure; thus necessitating management measures to reduce pressure, while relevant data especially on fish diet composition and fish landing should be collected in order to get improved model parameterization in the future.

Identifying trade-offs and reference points in support of ecosystem approaches to managing Gulf of Mexico menhaden

Gulf menhaden (Brevoortia patronus) support the largest fishery by yield in the Gulf of Mexico (GoM) and are a key forage species for many marine predators. While menhaden stock assessments indicated that overfishing was not likely to have occurred in the past, concerns have been raised regarding the possible effects of menhaden fishing on their predators. In this study, we used a US Gulfwide Ecopath with Ecosim (EwE) model to explore the predicted effects of increased menhaden harvest on the GoM ecosystem and focused our analyses on Gulf menhaden predators. Key menhaden predators identified included king mackerel (Scomberomorus cavalla), Spanish mackerel (Scomberomorus maculatus), sea trout (Cynoscion spp.), red drum (Sciaenops ocellatus), and pelagic coastal piscivores [e.g., bluefish (Pomatomus saltatrix)]. As expected, these predators exhibited reduced biomass in response to increased Gulf menhaden harvest, with a predicted 11% decrease in predator biomass at simulated fishing levels near historical highs. Our results indicate strong relationships between the effects of menhaden fishing and the predator fishing mortality for king mackerel and intermediate relationships for Spanish mackerel, blacktip shark (Carcharhinus limbatus), red drum, large coastal sharks, and pelagic coastal piscivores. Biomass of predator groups such as demersal coastal invertebrate feeders [e.g., drums and croakers (Sciaenidae)] are more affected by menhaden harvest (through trophodynamics interactions and bycatch removal) compared to the isolated effect of their fishing mortality. For almost all the groups examined in the trade-off analysis, with the exception of sea trout, current biomass (2016) was higher than their target biomass representing 75% of their biomass at maximum sustainable yield. In comparison to the time series of fishing mortality rates estimated by the most recent Gulf menhaden stock assessment, the mean ecological reference point (ERP) of 0.862 was exceeded in all but 1 year from 1977 to 2007; however, neither the target nor threshold upper ERP value has been exceeded since 2008. The observed Gulf menhaden landings from 2003 to the present were generally within the range of the projected equilibrium landings (i.e., within confidence intervals) at both the ERP target and threshold values except for three recent years.

Macroalgal blooms affect the food web of tropical coastal ecosystems impacted by fisheries

Macroalgal bloom events have been frequent in recent years. Eutrophication and overexploitation fishing may favor blooms through nutrient availability and capturing top predators. We aim to investigate the drivers of the macroalgae blooms and their consequences on the food web of the two tropical coastal ecosystems: Porto do Mangue (with high macroalgae production) and Baía Formosa (control environment, without macroalgae), both exploited by artisanal fisheries in northeastern Brazil. The food webs are modeled using the Ecopath with Ecosim (EwE) approach. Our results suggest that fishing did not favor macroalgae blooms but rather the high concentration of nutrients added to the semi-arid conditions. Furthermore, the macroalgae bloom showed low trophic impact, so much of their biomass is transferred into detritus. However, when it decomposes, this accumulation of matter alters the structure and functioning of the ecosystem, affecting its main fish resources: shrimp and piscivorous fish. Investigating blooms is key to management.

Exploration of fisheries management policies in the Gulf of Nicoya (Costa Rica) using ecosystem modelling

The Gulf of Nicoya is a highly productive estuary located at the Pacific coast of Costa Rica. Previous studies have used trophic models to examine changes in the biomass of key species and in the food web in the last 25 years, revealing an overfished, degraded system with decreasing biomass of valuable target species. The ecosystem degradation was mainly driven by intensive fishing, while climate variations affected resource productivity additionally. This study tested the effectiveness of alternative scenarios including combinations of the current top-down fishing policy and the ban on shrimp trawling, together with a participatory management scenario developed in a previously performed stakeholder workshop. In parallel, the automated fishing policy search of the Ecopath with Ecosim (EwE) software was used to explore an optimized alternative management scenario. The analysis indicates that the ban on trawling is an important measure to allow for the recovery of certain target species, such as shrimps, demersal fish and their predators. However, this ban would not suffice to substantially rebuild the biomass of all key species in the system. Thus, two possible alternative management scenarios are proposed: in the first one, the economic losses are minimized, ecosystem health increases by 10% (by rebuilding target species biomass) and employment provided by fishing decreases (−15%). In the second scenario, higher economic losses are accepted (mainly for the semi-industrial fisheries sector) which allows for a higher increase in ecosystem health and biodiversity. Both scenarios call for additional reductions in fishing efforts, mainly by the semi-industrial purse-seine fleet and the artisanal longline fleet. This study exemplifies how holistic ecosystem models can be used for management advice, future policymaking and how stakeholders can be engaged in this process.

Effect of fishing effort on the trophic functioning of tropical estuaries in Brazil

A trophic web is a network of complex interactions and energy links between species. These interactions can be simplified into trophodynamic models, such as Ecopath (EP) and EcoTroph (ET), important tools providing the holistic view needed for the ecosystem approach to fisheries. We described food web structure and trophic interactions by developing an EP model for the Santa Cruz Channel (SCC), a large tropical estuarine system in northeastern Brazil, surrounded by mangroves and highly subject to the impacts of domestic pollution, industry, artisanal fisheries, and aquaculture. In addition, considering ecological and fisheries perspectives, we developed ET models in three neighboring Brazilian estuaries (SCC; Sirinhaém – SIR and Mamanguape – MAM) to explore levels of exploitation that affect their trophic functioning. Our EP and ET models consisted of 32 compartments (three primary producers, six invertebrates, 22 fish, and detritus). Keystone Index and Mixed Trophic Impact analysis pointed that several groups of commercial relevance are also ecologically relevant and lack fishing regulations, such as Snooks (Centropomus spp.), Jacks (Caranx spp.) and Barracudas (Sphyraena spp). Fishery impacts across the trophic level spectrum differ between ecosystems, which causes top-down effects depending on the exploitation dynamics of each system. The fishing pressure affects mainly the low and intermediate Trophic Level (TLs) in MAM and SCC and high TLs in the SIR estuary. Consequently, a decrease of biomass for low and high TL was found with the increasing of fishing effort, respectively. These findings are an important contribution to the trophic modelling of tropical estuaries, indicating that both EP and ET approaches can be effective tools to improve the understanding of the trophic functioning and fishery effect on estuarine ecosystems. Additionally, increasing the knowledge of key ecosystem processes in estuarine systems may help to enhance conservation initiatives for sustainable use of the ecosystem, such as protected areas, temporal control of fishing, and the catch size limit.