Evaluation of the trophic structure of the West Florida Shelf in the 2000s using the ecosystem model OSMOSE☆
Introduction
Ecosystem-based management (EBM) of marine systems has become a central paradigm in the United States (Lubchenco and Sutley, 2010, USNOC, 2013). EBM considers interactions between exploited marine species and their biotic and abiotic environment to define management strategies. One major strength of EBM is its ability to expose indirect impacts of fisheries and tradeoffs between fisheries management objectives and conservation issues (Levin et al., 2009, McLeod and Leslie, 2009, Pikitch et al., 2004). Integrated ecosystem assessments (IEAs) are increasingly developed to organize science in order to inform decisions in EBM (ICES, 2010, Levin et al., 2009, Levin et al., 2013, Möllmann et al., 2013). A large IEA program has been recently initiated in the Gulf of Mexico (GOM) by the National Oceanic and Atmospheric Administration (NOAA). One of the goals of the GOM IEA program is to regularly incorporate ecosystem considerations into single-species stock assessments and deliver estimates of parameters that are highly difficult to evaluate from empirical data (http://www.noaa.gov/iea/gulfofmexico.html). In particular, the GOM IEA program is committed to informing SEDAR (SouthEast Data, Assessment, and Review), a management council process designed to improve the reliability of single-species stock assessments in the GOM (http://www.sefsc.noaa.gov/sedar/). Several ecosystem simulation models have been used toward that goal.
The ecosystem simulation models used within the GOM IEA program include two Ecopath with Ecosim (EwE) models for the West Florida Shelf, one of the main regions of the GOM under high and increasing fishing and environmental pressures (Coleman et al., 2004, Karnauskas et al., 2013, Okey et al., 2004, Steidinger, 2009; Fig. 1). Ecopath is a widely-used trophic mass-balance model which explicitly considers major functional groups in a given ecosystem (fish, invertebrates, marine mammals, seabirds, plankton, etc.), and provides a snapshot of the trophic structure of this ecosystem (Christensen and Walters, 2004, Pauly et al., 2000). Chagaris (2013) constructed the ‘WFS Reef fish Ecopath’ model to analyze the trophic structure of the West Florida Shelf (WFS) over the period 2005–2009. Biomass, catch and productivity parameters of the WFS Reef fish Ecopath model were rescaled to obtain an Ecopath model for the early 1950s, from which Chagaris (2013) and Chagaris and Mahmoudi (2013) evaluated changes in biomasses, trophic interactions and mortalities in the West Florida Shelf over the period 1950–2009 using the Ecosim module (resulting in a EwE model). Gray et al. (2013) developed an EwE model similar to WFS Reef fish EwE, referred to as ‘WFS Red tide EwE’, in which they focused on the impacts of red tide (Karenia brevis) outbreaks for gag grouper (also simply referred to as gag; Mycteroperca microlepis), a socio-economically important species of the GOM, over the period 1980–2009.
Most single-species stock assessment models assume that natural mortality rates are constant across ages and over time due to difficulties to evaluate these rates empirically. Through the development of ecosystem models, we now have alternative means to estimate the natural mortality rates of different life stages of marine species in relation to changes in predator abundance and abiotic conditions (e.g., Fulton et al., 2003, Möllmann et al., 2013, Walters et al., 2006). In 2013, Chagaris and Mahmoudi (2013) and Gray et al. (2013) used, respectively, WFS Reef fish EwE and WFS Red tide EwE, to provide estimates of age- and time-specific natural mortality rates for gag to SEDAR. Chagaris and Mahmoudi (2013) evaluated the instantaneous natural mortality rates of three life stages (stanzas) of gag grouper (younger juveniles, older juveniles and adults) from 1950 to 2009, under alternate assumptions about compensatory survival and predation. The authors found interannual variability of gag natural mortality to decrease with age and compensatory survival during periods of low abundance. Gray et al. (2013) showed that mortality due to red tides was by far greater than predation mortality for adult gag over the period 2005–2009.
In addition to these EwE models, an OSMOSE model was also developed for the West Florida Shelf, referred to as ‘OSMOSE-WFS’ (Fig. 1). OSMOSE is a two-dimensional, individual-based and multispecies model explicitly representing major processes in the life cycle of high trophic level (HTL) groups of fish and invertebrate species (Shin and Cury, 2001, Shin and Cury, 2004). The application presented in this paper is a steady-state version of OSMOSE-WFS with a monthly time step, which describes trophic interactions in the West Florida Shelf in the 2000s. OSMOSE-WFS and WFS Reef fish Ecopath share a number of characteristics, such as the spatial domain, reference period and reference biomasses. However, OSMOSE-WFS and WFS Reef fish Ecopath differ greatly in both their structure and assumptions. In particular, diets reconstructed from empirical data are input into Ecopath, while they emerge from size-based processes in OSMOSE. The use of the OSMOSE-WFS, WFS Reef Fish Ecopath/EwE and WFS Red tide EwE models offers different perspectives on the functioning of the West Florida Shelf ecosystem, while being able to identify from where discrepancies between the different models may originate. Using a multi-model approach for the West Florida Shelf will allow us to evaluate uncertainties in our knowledge of the West Florida Shelf ecosystem, and help identify avenues for reducing these uncertainties.
Here, we introduce the OSMOSE-WFS model and describe the trophic structure of the West Florida Shelf in the 2000s with this model, with a focus on the diet patterns and natural mortality rates of gag grouper evaluated for SEDAR in 2013 (SEDAR 33; http://www.sefsc.noaa.gov/sedar/). In the following, we: (1) provide a brief overview of the OSMOSE modeling approach; (2) describe the structure and assumptions of OSMOSE-WFS; (3) detail the parameterization of OSMOSE-WFS; (4) present the methodology we implemented to calibrate OSMOSE-WFS to a reference state matching the mean observed conditions in the West Florida Shelf region over the period 2005–2009; (5) use the calibrated OSMOSE-WFS model to explore the trophic structure of the West Florida Shelf in the 2000s; and (6) discuss our results and describe how OSMOSE-WFS is being improved to provide more information to EBM in the GOM. The overview of the OSMOSE modeling approach we provide is helpful to understand the choices we made regarding the structure and assumption of OSMOSE-WFS. Our exploration of the trophic structure of the West Florida Shelf in the 2000s from OSMOSE-WFS predictions is accompanied by comparisons of OSMOSE-WFS outputs with empirical data and WFS Reef fish Ecopath outputs.
Section snippets
OSMOSE (Object-oriented Simulator of Marine ecOSystem Exploitation)
OSMOSE is a two-dimensional, individual-based and multispecies model which singularity lies on both size-based interactions and explicit implementation of the whole life cycle of modeled organisms (Shin and Cury, 2001, Shin and Cury, 2004). OSMOSE has been used to model trophic dynamics and the impacts of fishing management strategies in a variety of ecosystems, including the Southern Benguela (e.g., Shin et al., 2004, Travers et al., 2010, Yemane et al., 2009), the Humboldt (Marzloff et al.,
Calibration of OSMOSE-WFS
Using the EA, we obtained a fully calibrated OSMOSE-WFS model such that the biomasses of all HTL groups fell on average within valid intervals over the last 20 years of simulation, i.e., after 115 to 134 years of simulation (Fig. 2). Among the different simulation replicates, the biomasses of all HTL groups except amberjacks, red grouper and gag grouper were always within valid intervals from year 114 to year 134 (Fig. 2a). However, the biomasses of amberjacks, red grouper and gag grouper were
Discussion
In the present study, we introduced a steady state version of the OSMOSE-WFS model, describing trophic interactions in the West Florida Shelf ecosystem in the 2000s. We detailed the parameterization and calibration of this model, both of which proved to be challenging. We then validated the model by comparing the predicted diets to observed diets, and the predicted TLs to TLs from the WFS Reef fish Ecopath model. Finally, we used OSMOSE-WFS to evaluate the trophic structure of the West Florida
Acknowledgments
We are grateful to Behzad Mahmoudi, Wade Cooper, Daryl Parkyn, John Walter, Mark Grace, Gary Fitzhugh, Ted Switzer, Matt Campbell, Brandi Noble, Paul Carlson, Chris Kelble, Geoffrey Cook, Caitlin Cravey and Amanda Izaguirre for their help and/or advice at different levels of this study. We also would like to acknowledge the Southeast Area Monitoring and Assessment Program (SEAMAP) and National Marine Fisheries Service (NMFS) bottom longline (BLL) survey program for the routine sampling efforts
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Funding: AG was supported by NOAA's Integrated Ecosystem Assessment (IEA) program (http://www.noaa.gov/iea/). MDD and CHA were funded by Florida Sea Grant, University of Florida and C-IMAGE (Center for Integrated Modeling and Analysis of the Gulf ecosystem) consortium, and the National Marine Fisheries Service/USF College of Marine Science Marine Resource Assessment fellowship. YJS was supported by the French project EMIBIOS (FRB, contract no. APP-SCEN-2010-II). ROR was funded by the PhD grants program from IRD (Institut de Recherche pour le Développement). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.