New trophic indicators and target values for an ecosystem-based management of fisheries

In the present study, we tested five trophic indicators and we demonstrated their usefulness to assess the environmental status of marine ecosystems and to implement an ecosystem approach to fisheries management (EAFM). The tested indicators include the slope of the biomass spectrum, the mean trophic level (MTL), the marine trophic index (MTI) and two newly developed indicators, the high trophic level indicator (HTI) and the apex predator indicator (API). Indicators are compared between current state and potential reference situations, using as case studies: the Celtic Sea/Bay of Biscay, North Sea and English Channel ecosystems. Trophic spectra are obtained from Ecopath models while reference situations are estimated, simulating with EcoTroph and Ecosim different fishing pressures including three candidate scenarios for an EAFM. Inter-ecosystems assessments are done using Ecopath models, simulations outputs and scientific surveys data to assess the current states of the studied ecosystems, contrast the reference situations and analyze the responses of all indicators. Sensitivity analyses are also conducted on the main simulation parameters to test the robustness of the chosen indicators. Ecosystems specific targets for EAFM are proposed for the five trophic indicators estimated from whole-ecosystem models, while in the Celtic Sea/Bay of Biscay ecosystem targets are proposed for the MTL (=3.85) and HTI (48%) estimated from standard bottom-trawl surveys. The HTI is proposed to be relevant for survey data and the API is recommended using whole-ecosystem models. We conclude that HTI and API show trends in ecosystems health better than MTI.     

CONSEQUENCES OF ALTERNATIVE FUNCTIONAL RESPONSE FORMULATIONS IN MODELS EXPLORING WHALE-FISHERY INTERACTIONS

We evaluated the utility of Ecosim for exploring interactions between cetacean predators, their prey, and fisheries. We formulated six Ecosim parameterizations, representing alternative hypotheses of feeding interactions (functional response) between cetaceans and their main fish prey, and examined differences in the predicted responses to simulated harvesting regimes for minke whales and their prey. Regardless of the type of function response formulated, intense fishing on the main fish prey of minke whales had a longer-lasting negative impact on minke whales than when minke whale biomass was removed directly by harvesting. Consumption rate, biomass, feeding time and mortality of minke whales were all sensitive to the type of functional response specified. Inclusion of "handling time" limited minke whales consumption at high prey densities and predicted higher consumption at low prey densities; features characteristic of a type II functional response. Predicted decline and recovery rates of minke whales were slower than when consumption rates were not limited. Addition of "foraging time" adjustments resulted in more conservative estimates of decline and recovery. However, when "other mortality" was linked to time spent foraging, exposure to higher mortality at low prey densities, and reduced mortality at high prey densities resulted in dramatic differences in predicted biomass trajectory. Sensitivity to the "other mortality" assumption is important for cetaceans whose predation mortality is only a small proportion of total mortality. Differences in the feeding and biomass dynamics were also observed when prey availability to predators was represented by changes in prey vulnerability, confirming earlier reports that Ecosim predictions are sensitive to this parameter.     

Coral–algal phase shifts alter fish communities and reduce fisheries production

Anthropogenic stress has been shown to reduce coral coverage in ecosystems all over the world. A phase shift towards an algae‐dominated system may accompany coral loss. In this case, the composition of the reef‐associated fish assemblage will change and human communities relying on reef fisheries for income and food security may be negatively impacted. We present a case study based on the Raja Ampat Archipelago in Eastern Indonesia. Using a dynamic food web model, we simulate the loss of coral reefs with accompanied transition towards an algae‐dominated state and quantify the likely change in fish populations and fisheries productivity. One set of simulations represents extreme scenarios, including 100% loss of coral. In this experiment, ecosystem changes are driven by coral loss itself and a degree of habitat dependency by reef fish is assumed. An alternative simulation is presented without assumed habitat dependency, where changes to the ecosystem are driven by historical observations of reef fish communities when coral is lost. The coral–algal phase shift results in reduced biodiversity and ecosystem maturity. Relative increases in the biomass of small‐bodied fish species mean higher productivity on reefs overall, but much reduced landings of traditionally targeted species.     

Combined effects of global climate change and regional ecosystem drivers on an exploited marine food web

Changes in climate, in combination with intensive exploitation of marine resources, have caused large‐scale reorganizations in many of the world's marine ecosystems during the past decades. The Baltic Sea in Northern Europe is one of the systems most affected. In addition to being exposed to persistent eutrophication, intensive fishing, and one of the world's fastest rates of warming in the last two decades of the 20th century, accelerated climate change including atmospheric warming and changes in precipitation is projected for this region during the 21st century. Here, we used a new multimodel approach to project how the interaction of climate, nutrient loads, and cod fishing may affect the future of the open Central Baltic Sea food web. Regionally downscaled global climate scenarios were, in combination with three nutrient load scenarios, used to drive an ensemble of three regional biogeochemical models (BGMs). An Ecopath with Ecosim food web model was then forced with the BGM results from different nutrient‐climate scenarios in combination with two different cod fishing scenarios. The results showed that regional management is likely to play a major role in determining the future of the Baltic Sea ecosystem. By the end of the 21st century, for example, the combination of intensive cod fishing and high nutrient loads projected a strongly eutrophicated and sprat‐dominated ecosystem, whereas low cod fishing in combination with low nutrient loads resulted in a cod‐dominated ecosystem with eutrophication levels close to present. Also, nonlinearities were observed in the sensitivity of different trophic groups to nutrient loads or fishing depending on the combination of the two. Finally, many climate variables and species biomasses were projected to levels unseen in the past. Hence, the risk for ecological surprises needs to be addressed, particularly when the results are discussed in the ecosystem‐based management context.     

Assessing the trophic impacts of marine mammals: From metabolism to food web indices

Marine mammals are an important part of ecosystems, and their trophic role and potential impact have been increasingly studied. One key question is how these large animals interact with fisheries or compete for similar resources. Consequently, some models once used only for fisheries management are now including pinnipeds and cetaceans. However, fish and marine mammals do not share the same ecology and bioenergetics, and complex ecosystem models may not be the best way to assess the impact of pinnipeds or cetaceans in food webs. Indeed, simpler methods based on thermodynamics might give us reasonable answers with limited amounts of data. Here, we present an assessment of two different approaches to assess the trophic role of marine mammals in the northern Gulf of St. Lawrence (Canada): mixed trophic impacts (MTI) based on ecosystem modeling and surface index (SI) impact based on bioenergetics. Our results show that while modeling represents a good way of getting a holistic view of the role of marine mammals in ecosystems, trophic impact estimates based on fundamental thermodynamics principles can also give us answers requiring less data. The body surface area approach presented here might provide a practical tool for ecologists, who are not necessarily ecosystem modelers, to study this issue.     

竺山湾湖泊缓冲带湿地生态系统EWE模型构建与分析

湖泊缓冲带在湖泊流域空间布局中具有特殊地位,缓冲带内的湿地对于保障流域生态健康和湖泊水环境质量具有十分重要的意义.本研究以太湖竺山湾湖泊缓冲带内的竺山湖湿地生态系统为研究对象,将生物组分划分为16个功能组,构建了生态通道(EWE)模型,并分析了生态系统的特征、状态以及功能组之间的相互关系.结果表明: 竺山湖湿地生态系统的有效营养级范围在1~3.72,营养流动主要发生在前4个营养级,开始于沉水植物和有机碎屑的食物链较多.湿地生态系统的总的能量转换效率为5.1%,并未达到“1/10定律”,说明当前的能量转换效率较低.物质流量在生态系统中的平均传输效率为4.3%.系统的总生产量为2496.66 t·km^-2·a^-1,总流量为10145.2 t·km^-2·a^-1.生态系统的多种特征参数表明当前生态系统处于幼态化阶段.     

Estimating the carrying capacity of French Frigate Shoals for the endangered Hawaiian monk seal using Ecopath with Ecosim

The carrying capacity of the French Frigate Shoals (FFS) region for the endangered Hawaiian monk seal was appraised using an updated version of the original FFS Ecopath model ( Polovina 1984 ). Model parameters were updated using recent literature, and data from surveys of the seal population and its bottom‐associated prey. Together they produced a static mass balance model for 1998 when the prey surveys began. The Ecopath‐estimated monk seal biomass was 0.0045 t/km², which was in close agreement with the biomass calculated from monk seal field beach counts (0.0046 t/km²). Model simulations through time were done in Ecosim using the Ecopath balanced model and included fisheries data time series from 1998 to 2008. Monk seal biomass declined concurrently with decreases in benthic bottomfish biomass, which were influenced by large‐scale changes in the environment of the North Pacific. This model scenario was extended from 2010, when the last permitted fishery in the Northwestern Hawaiian Islands was closed, through to 2040, assuming a constant environmental signal. Model results for this time period did not show a recovery of monk seals that exceeded the initial 1998 model biomass levels, highlighting the importance of including environmental variability in estimates of monk seals recovery at FFS.     

An Ecosystem Model for Exploring Lake Restoration Effects on Fish Communities and Fisheries in Florida

Developing quantitative ecosystem–scale expectations of habitat restoration projects and examining trade‐offs associated with alternative approaches has been a challenge for restoration ecology. Many of the largest freshwater lake restoration projects have occurred in Florida to remediate degradation to vegetated littoral habitats resulting from stabilized water levels, but effects across lake food‐webs have not been assessed. We developed an ecosystem model using Ecopath with Ecosim and Ecospace for a generalized large, eutrophic Florida Lake to explore how simulated restoration activities could influence fish communities with emphasis on sport fish abundance. We modeled three habitat restoration scenarios: (1) “no control,” (2) a “10‐year control” that restored littoral habitat every 10 years, and (3) a “combined control” scenario that restored littoral habitat every 10 years with maintenance controls between 10‐year periods. Our “combined control” scenario provided the largest long‐term habitat restoration benefits for sport fish abundance and the fisheries they support. In Ecospace, we simulated a littoral habitat restoration project that reduced lake‐wide tussock coverage from 30 to 15%. Ecospace predicted positive benefits to sport fish and fisheries following the restoration simulation and highlighted the importance of habitat edge effects, spatial design of habitat restoration projects, and sampling designs for evaluating restoration projects.     

Sustainability of the Lake Superior Fish Community: Interactions in a Food Web Context

The restoration and rehabilitation of the native fish communities is a long-term goal for the Laurentian Great Lakes. In Lake Superior, the ongoing restoration of the native lake trout populations is now regarded as one of the major success stories in fisheries management. However, populations of the deepwater morphotype (siscowet lake trout) have increased much more substantially than those of the nearshore morphotype (lean lake trout), and the ecosystem now contains an assemblage of exotic species such as sea lamprey, rainbow smelt, and Pacific salmon (chinook, coho, and steelhead). Those species play an important role in defining the constraints and opportunities for ecosystem management. We combined an equilibrium mass balance model (Ecopath) with a dynamic food web model (Ecosim) to evaluate the ecological consequences of future alternative management strategies and the interaction of two different sets of life history characteristics for fishes at the top of the food web. Relatively rapid turnover rates occur among the exotic forage fish, rainbow smelt, and its primary predators, exotic Pacific salmonids. Slower turnover rates occur among the native lake trout and burbot and their primary prey—lake herring, smelt, deepwater cisco, and sculpins. The abundance of forage fish is a key constraint for all salmonids in Lake Superior. Smelt and Mysis play a prominent role in sustaining the current trophic structure. Competition between the native lake trout and the exotic salmonids is asymmetric. Reductions in the salmon population yield only a modest benefit for the stocks of lake trout, whereas increased fishing of lake trout produces substantial potential increases in the yields of Pacific salmon to recreational fisheries. The deepwater or siscowet morphotype of lake trout has become very abundant. Although it plays a major role in the structure of the food web it offers little potential for the restoration of a valuable commercial or recreational fishery. Even if a combination of strong management actions is implemented, the populations of lean (nearshore) lake trout cannot be restored to pre-fishery and pre-lamprey levels. Thus, management strategy must accept the ecological constraints due in part to the presence of exotics and choose alternatives that sustain public interest in the resources while continuing the gradual progress toward restoration. Received 10 December 1999; accepted 13 June 2000.