Impacts of the oil spill on West Florida Shelf demersal fish resources

Steven Saul and Dr. David Die
Cooperative Institute for Marine and Atmospheric Studies

St. Petersburg Times, Photos by Bill Seme

To evaluate the impact of the NOAA fishing closures in the demersal fish stocks harvested in the West Florida Shelf an individual-based simulation model has been developed to generate spatially-explicit simulated catch data. The model contains the five most important species in the snapper-grouper complex: red grouper, gag grouper, red snapper, vermilion snapper, and mutton snapper and the fishing vessels that harvest them.

As a result of the recent Deepwater Horizon explosion and oil spill, the National Oceanic Atmospheric Administration (NOAA) has implemented emergency spatial closures to fishing activity in parts of the Gulf of Mexico.  These closures have significantly altered the fishing behavior of individuals whose livelihood are supported by this resource. On the West Florida Shelf, members of the reef fish fleet have incurred oil related spatial closures for some of their fishing grounds, while other grounds remain open at this time.  The primary two gear types in this fishery affected by the closures are vertical line (including manual and electric bandit reel) and longline (including buoy gear).

SnapGroup Model

Figure 1:  Spatial domain of individual-based simulation model.  Black solid line represents the 100m model limit along the shelf, with ten meter isobaths represented as gray lines.  The dashed blue line represents the 35 fathom inshore limit of the longline fleet while the hatched area corresponds to the June 28 NOAA fishery closure to prevent seafood contamination. 

The model is organized in sub-models including those for fleet and fish dynamics including movement of both vessels and fish. Space in the model is resolved to grids of one minute of latitude by one minute of longitude (approximately 1.15 by 1.15 miles). The time step for the model is one day and simulations are carried out in the order of tens of years to match the lifespan of the species in the model.  Geographical limits of the model are the state border between Alabama and Florida in the North, the Dry Tortugas and Florida Bay in the South and the 100 m isobath as the limit on the shelf.   Fish are distributed among these grids by using a combination of fishery-dependent spatial indices of abundance and geostatistical models generated from fishery-independent data for each species.

grouper biomass

Figure 2:  Example of simulated distribution of red grouper biomass at one minute grids from one of the stratum in the model.  Within each stratum, relative biomass estimates are derived from generalized linear models of catch per unit fishing effort data scaled to the total biomass of the stock assessed by NMFS.  Simulated biomass within each one minute grid is a single realization of a stochastic, spatially auto-correlated abundance process.  Parameters of this process were obtained from fits to fishery independent abundance data obtained from video collected by NOAA. 

Parameterization of life history traits and estimation of annual recruitment is obtained from those values used in the most recent stock assessments for these species conducted by NOAA.  Ontogenetic migration is modeled to represent the movement of individuals from inshore nursery habitat to offshore adult reef habitat and is parameterized with tagging data. 

Figure 3:  Example of red grouper biased random walk migration from inshore nursery area to offshore reef habitat.

Simulating Fish

Vessel behavior and decision making in the model is driven by a series of econometric discrete choice models. Econometric models are estimated based on surveys currently being completed by vertical line and longline vessel captains on the West Florida Shelf holding current commercial reef fish permits.

Simulating the Fleet

The simulation model will be used to evaluate effects of spatially explicit spatial fishing closures and recruitment failures that may result from the oil spill. The fishery closures investigated will be those imposed by NOAA to date and also a set of hypothetical future fishery closure scenarios developed in consultation with experts. The recruitment failure scenarios will be developed based on the predicted extent of oil mediated impacts on nursery habitat of snapper and groupers estimated using ocean basin circulation models.