Kim de Mutsert

Reanalyses of Gulf of Mexico fisheries data: Landings can be misleading in assessments of fisheries and fisheries ecosystems

We used two high profile articles as cases to demonstrate that use of fishery landings data can lead to faulty interpretations about the condition of fishery ecosystems. One case uses the mean trophic level index and its changes, and the other uses estimates of fishery collapses. In earlier analyses by other authors, marine ecosystems in the Gulf of Mexico (GOM) and U.S. Atlantic Ocean south of Chesapeake Bay were deemed to be severely overfished and the food webs badly deteriorated using these criteria. In our reanalyses, the low mean trophic level index for the GOM actually resulted from large catches of two groups of low trophic level species, menhaden and shrimp, and the mean trophic level was slowly increasing rather than decreasing. Commercial targeting and high landings of shrimps and menhaden, especially in the GOM, drove the index as previously calculated. Reanalyses of fishery collapses incorporating criteria that included targeting, variability in fishing effort, and market forces discovered many false cases of collapse based simply upon a decline of catches to 10% of previous maximum levels. Consequently, we suggest that the low mean trophic level index calculated in the earlier article for the GOM did not reflect the overall condition of the fishery ecosystem, and that the 10% rule for collapse should not be interpreted out of context in the GOM or elsewhere. In both cases, problems lay in the assumption that commercial landings data alone adequately reflect the fish populations and communities.

Using Ecopath with Ecosim to Explore Nekton Community Response to Freshwater Diversion into a Louisiana Estuary

Abstract Current methods to restore Louisianas estuaries include the reintroduction of Mississippi River water through freshwater diversions to wetlands that are hydrologically isolated from the main channel. The reduced salinities associated with freshwater input are likely affecting estuarine nekton, but these effects are poorly described. Ecopath with Ecosim was used to simulate the effects of salinity changes caused by the Caernarvon freshwater diversion on species biomass distributions of estuarine nekton. A base model was first created in Ecopath from 5 years of monitoring data collected prior to the opening of the diversion (1986–1990). The effects of freshwater discharge on food web dynamics and community composition were simulated using a novel application of Ecosim that allows the input of salinity as a forcing function coupled with user-specified salinity tolerance ranges for each biomass pool. The salinity function in Ecosim not only reveals the direct effects of salinity (i.e., increases in species biomass at their optimum salinity and decreases outside the optimal range) but also the indirect effects resulting from trophic interactions. Through multivariate analyses we determined that the simulated species biomass distributions in the estuary change significantly with distance from the diversion. However, the simulations do not show a significant difference estuarywide between the modeled distributions (with pooling of the data from different distances) before and after the opening of the diversion. This indicates a redistribution of species in the estuary rather than the replacement of species within the estuarine nekton community. The salinity function is a useful addition to the Ecopath with Ecosim software for estuarine ecologists and fisheries managers.

Simulating Fish Movement Responses to and Potential Salinity Stress from Large-Scale River Diversions

AbstractRiver diversions are a major but controversial management approach to restoring coastal wetlands and mitigating offshore oil spills in the northern Gulf of Mexico. One of the controversies concerns the potential displacement of and salinity stress on commercially and recreationally important fish species in response to the widespread and prolonged freshening of habitat. We developed a coupled hydrodynamics–fish movement model and applied it to the Caernarvon diversion located in the Breton Sound estuary, Louisiana. Hydrodynamics model output was used as input to the individual-based fish movement model. The period of model simulation was from April 1 to July 1, 2010. We simulated three diversion scenarios: baseline, pulse, and oil spill mitigation. We first used field data from Bay Anchovy Anchoa mitchilli and showed that the model predicted downestuary shifts similar to those observed in field studies under large diversions. We then defined generic low- and intermediate-salinity fish species and ...

A Before-After-Control-Impact Analysis of the Effects of a Mississippi River Freshwater Diversion on Estuarine Nekton in Louisiana, USA

Since 1991, the Caernarvon Freshwater Diversion has been reintroducing Mississippi River water into a previously hydrologically isolated estuary in an effort to restore wetlands. To determine the effect of freshwater inflow on estuarine nekton community structure, a Before–After–Control–Impact study design was applied. As a result of the opening, salinities in the impact area decreased, and the nekton community structure in the estuary changed significantly. Species of economical or ecological importance either increased in biomass or exhibited no response to the opening of the diversion. Higher abundances of small fish were observed in the area receiving freshwater flow, which is an indication that the area serves as a refuge from large marine predators. Because a salinity gradient was established, as opposed to a uniform but lower salinity regime, aquatic habitat was available to nekton species from a wide spectrum of salinity tolerances.

Recommendations on the Use of Ecosystem Modeling for Informing Ecosystem‐Based Fisheries Management and Restoration Outcomes in the Gulf of Mexico

AbstractEcosystem-based fisheries management (EBFM) and ecosystem restoration are gaining momentum worldwide, including in U.S. waters of the Gulf of Mexico (GOM). Ecosystem models are valuable tools for informing EBFM and restoration activities. In this paper, we provide guidance and a roadmap for ecosystem modeling in the GOM region, with an emphasis on model development and use of model products to inform EBFM and the increasing investments in restoration. We propose eight “best practices” for ecosystem modeling efforts, including (1) identification of priority management questions, (2) scenarios as simulation experiments, (3) calibration and validation needs, (4) sensitivity and uncertainty analyses, (5) ensuring transparency, (6) improving communication between ecosystem modelers and the various stakeholders, (7) documentation of modeling efforts, and (8) maintaining the ecosystem models and codes. Fisheries management in the USA adheres to a prescriptive set of calculations. Therefore, the use of ec...

Abundance and Size of Gulf Shrimp in Louisiana's Coastal Estuaries following the Deepwater Horizon Oil Spill

The Deepwater Horizon oil spill impacted Louisianas coastal estuaries physically, chemically, and biologically. To better understand the ecological consequences of this oil spill on Louisiana estuaries, we compared the abundance and size of two Gulf shrimp species (Farfantepeneus aztecus and Litopeneus setiferus) in heavily affected and relatively unaffected estuaries, before and after the oil spill. Two datasets were used to conduct this study: data on shrimp abundance and size before the spill were available from Louisiana Department of Wildlife and Fisheries (LDWF). Data on shrimp abundance and size from after the spill were independently collected by the authors and by LDWF. Using a Before-After-Control-Impact with Paired sampling (BACIP) design with monthly samples of two selected basins, we found brown shrimp to become more abundant and the mean size of white shrimp to become smaller. Using a BACIP with data on successive shrimp year-classes of multiple basins, we found both species to become more abundant in basins that were affected by the spill, while mean shrimp size either not change after the spill, or increased in both affected and unaffected basins. We conclude that following the oil spill abundances of both species increased within affected estuaries, whereas mean size may have been unaffected. We propose two factors that may have caused these results: 1) exposure to polycyclic aromatic hydrocarbons (PAHs) may have reduced the growth rate of shrimp, resulting in a delayed movement of shrimp to offshore habitats, and an increase of within-estuary shrimp abundance, and 2) fishing closures established immediately after the spill, may have resulted in decreased fishing effort and an increase in shrimp abundance. This study accentuates the complexities in determining ecological effects of oil spills, and the need of studies on the organismal level to reveal cause-and-effect relationships of such events.

Pharmaceutical chemicals, steroids and xenoestrogens in water, sediments and fish from the tidal freshwater Potomac River (Virginia, USA)

ABSTRACT Selected pharmaceutical chemicals, steroids and xenoestrogens (PCSXs) consisting of 29 endocrine modulators, therapeutic drugs, pesticides, detergents, plastics, and active ingredients in household products were measured in water, riverbed sediments and fish collected in a tributary embayment of the Potomac River (Hunting Creek, Alexandria, VA, USA) in the vicinity of wastewater discharge. A total of 17 PCSXs were found in the Hunting Creek samples, with steroid hormones (e.g., progesterone and 17α-ethinylestradiol), triclosan, dextromethorphan and bisphenol A being the most prominent micropollutants detected.The geospatial distribution of the PCSXs in Hunting Creek indicated that the steroids correlated with wastewater treatment plant discharge in all matrices, but such an association is tentative in Hunting Creek given the complex nature of urban sources of PCSXs and hydrodynamics in an urban tidal river. The sediment PCSX concentrations correlated with sediment total organic carbon content at all sampling sites. For the most part, the PCSXs showed an enrichment in fish tissue relative to sediments when concentrations were normalized to lipids and sediment organic carbon contents, but the influence of endogenous steroids is also an important consideration for these chemicals.

Using the Ecopath with Ecosim Modeling Approach to Understand the Effects of Watershed-based Management Actions in Coastal Ecosystems

ABSTRACT Resource managers are often tasked with identifying and assessing the potential impacts of management actions on the biotic communities under their care. When the management activities directly influence a single species of interest, managers can turn to a variety of models to aid in their understanding of potential changes to that population. However, as ecosystem-based management becomes more widely accepted, managers will have to understand how proposed actions will impact entire biotic communities, through both direct and indirect mechanisms. Thus, there is a need for ecosystem models that account for both trophic and non-trophic interactions, and that can be relatively easily used to assess a variety of management scenarios. We reviewed the available literature regarding incorporation of eutrophication and other anthropogenic impacts into Ecopath with Ecosim (EwE), one of the more widely used trophic ecosystem modeling frameworks. We found that a number of mechanisms of varying complexity have been used to include these stressors in models, providing managers with a suite of options that can be used to complement their existing management options as they seek to understand the impacts of human interactions on the natural environment.

Using Ecosystem Modeling to Determine Hypoxia Effects on Fish and Fisheries

The effects of coastal hypoxia on fish biomass and fisheries landings in the northern Gulf of Mexico have been difficult to quantify. A main complicating factor is the fact that nutrient loading from freshwater discharge is not only the main contributor to the formation of the hypoxic zone, but also a driver of secondary productivity through bottom-up processes. Other complicating factors include food web interactions, movement of nekton to more suitable habitat, and temporal and spatial variability in hypoxic area. Through case studies using Ecopath with Ecosim and Ecospace, we show that ecosystem modeling can provide a tool to evaluate population-level effects on nekton biomass as well as changes in fisheries landings due to hypoxia. Fitting model simulations to time series (observations) in Ecosim reveals that including hypoxia improves the fit of the model to observations. These findings led to the development of a spatially and temporally dynamic Ecospace model, coupled to a physical-biological model with high skill in replicating dissolved oxygen and Chl a levels. The results of simulations with this coupled modeling approach suggest that, for most species, the positive effects of increased phytoplankton as a result of nutrient enrichment from the Mississippi River outweigh the negative effect of bottom hypoxia. Decoupling enrichment from hypoxia also showed that hypoxia does reduce biomass and landing as compared to enrichment alone, and that there are winners and losers: Some species such as red snapper decrease in biomass even with enrichment. Future directions include simulating nutrient reduction scenarios to inform management.