1. Building an Ecopath with Ecosim Model
for the Galveston Bay Blue Crab Fishery
Glen Sutton1*, Zachary Olsen2, and Tom Wagner3
Texas Parks and Wildlife Department- Coastal Fisheries, 1Dickinson Marine Laboratory, Dickinson, Texas 77539, 2Upper Laguna Madre Field Office, Corpus Christi, Texas 78412, 3Rockport Marine Laboratory, Rockport Texas, 78382
Introduction
The Texas Blue Crab population has been in a state of decline for over twenty years (Martinez-Andrade et al. 2005) despite reduced fishing effort from TPWD’s license buyback program.
Morris (2015) recommended possible management actions to address this decline, including protection of non-ovigerous mature females with seasonal or area closures, increasing minimum size to
5 ½ inches, increasing escape ring size, or reducing trap limits.
There is a need to examine management options for the Blue Crab fishery in an ecosystem wide context.
What is Ecopath with Ecosim?
EwE is a mass balance modeling software that allows users to construct virtual food webs based heavily on local and published knowledge of trophic interactions (Pauly et al. 2000).
This ecosystem model is then used to generate management scenarios in which scenarios of population increase or decrease are examined.
Methods
Model Inputs (data sources given in parentheses)
Production and consumption estimates for key species groups (Literature Values).
Percent dietary breakdown of all consumers (Literature Values).
Fishery harvest and bycatch data for major fisheries including:
bay and nearshore shrimp trawl fishery (TPWD Trip Ticket Program)
blue crab trap fishery (TPWD Trip Ticket Program)
oyster dredge fishery (TPWD Trip Ticket Program)
commercial finfish fisheries (TPWD Trip Ticket Program)
recreational finfish fisheries (TPWD Harvest Monitoring Program)
Fishery-independent trend data used for verifying model performance (TPWD Fishery-Independent Monitoring Program)
Figure 4. Hypothetical output from Ecosim run reducing blue crab trap fishery effort by 100% at year nine of the time series, showing effects on selected groups.
Preliminary Results*
Figure 5. Hypothetical output from Ecosim run for adult blue crab for entire time series showing examples of metrics useful to fishery managers (i.e. predation mortality, catch, and fishing mortality.
Objectives
Construct a mass-balanced food web model of the Galveston Bay Blue Crab fishery, including predators, prey, and fisheries affecting the species, (ONGOING).
Evaluate ecosystem-wide impacts of management options to maximize benefits to Blue Crabs taking into account relevant trophic interactions (FUTURE).
Calculated Trophic Level
* Please note that model results are hypothetical and for presentation purposes only, as we are still in the process of gathering information and validating the Galveston Bay Blue Crab model with time series data. The general model structure was modified from that of Sutton and Guillen (2010).
Study Area .
Figure 2. The Galveston Bay food web output from Ecopath showing the many complex interactions between trophic levels. Some trophic guilds have been combined (e.g., Apex piscavores) due to dietary similarity and model simplicity.
Implications and Importance
This study highlights a need to move towards an ecosystem approach to fisheries management. Trophic models such as this are one tool for evaluating fisheries effects in the ecosystem context.
The model is currently being verified and updated with input from a number of subject matter experts.
By examining a variety of scenarios in this modeling context we can better understand what may be driving population dynamics of Blue Crabs in Galveston Bay and critically evaluate management strategies for this species.
Group name
Trophic level
Habitat area (fraction)
Biomass in habitat area (t/km²)
Biomass (t/km²)
Z (/year)
Production / biomass (/year)
Consumption / biomass (/year)
Ecotrophic efficiency
Production / consumption 1
Seabirds
3.531
1.000
0.0110
0.250
35.000
0.000
0.007 2
Apex piscavores
3.932
0.0148
0.200
3.000
0.118
0.067 3
Gars
3.911
0.0100
0.500
0.167 4
Rays
3.104
0.0820
0.066 5
Red Drum
3.463
0.1000
1.100
5.100
0.158
0.216 6
Spotted Seatrout
3.601
0.0720
4.110
0.221
0.243 7
BlackDrum
3.031
0.0200
5.000
0.186 8
Flounder
3.532
0.420
6.360
0.453 9
Sandtrout
3.455
1.827
0.284
0.287 10
Hardheads
3.247
0.4000
7.600
0.262
0.132 11
Gafftop
3.464
0.800
0.057
0.105 12
Sheepshead
3.085
0.0250
8.000
0.429
0.125 13
Croaker_adt
3.037
4.3440
1.500
2.000
0.004
0.750 14
Croaker_med
3.012
2.5407
3.913
0.015
0.511 15
Croaker_juv
2.989
0.2026
3.500
10.714
0.777
0.327 16
Benthivores
2.897
0.9000
9.000
0.485
0.122 17
Menhaden_adt
2.578
1.3000
6.774
0.162
0.148 18
Menhaden_juv
2.189
0.7290
12.000
0.246 19
Mullet_adt
2.062
3.0000
0.037 20
Mullet_juv
2.056
0.3739
1.900
18.725
0.564
0.101 21
Anchovy
2.889
2.0320
2.530
14.000
0.106
0.181 22
Blue Crab_adt
2.968
2.0000
1.250
8.500
0.102
0.147 23
Blue Crab_med
2.699
0.6815
10.368
0.470
0.193 24
Blue Crab_juv
2.559
0.4891
2.500
13.073
0.861
0.191 25
Blue Crab meg
2.333
0.3571
5.500
22.644
0.901 26
Brown shrimp
2.438
0.772
0.600 27
White shrimp
2.276
0.683 28
Gastropods
1.0000
4.500
22.000
0.639
0.205 29
Bivalves
4.0000
0.577 30
Meiobenthos
2.078
0.510 31
Oyster
2.222
0.633
0.045 32
Zooplankton
2.111
7.6420
36.000
89.000
0.519
0.404 33
Bethic algae
25.000
0.178 34
Plankton
76.000
0.393 35
Detritus
0.156
Figure 3. Basic estimates generated by Ecopath for model groups at the beginning of the time series (1990). Values in blue are calculated by the model.
Want to become involved?
The EwE modeling is a process that relies on extensive input from subject matter experts.
If you are interested in learning more about this model or would like to provide subject matter expertise to the modeling process feel free to talk to one of the authors or join our Break Out Session Focus Group Meeting on Thursday, February 25th from 3:30-4:10pm in the Main Conference Room.
Figure 1. The study area, including Galveston Bay, Trinity Bay, and the adjacent Texas Territorial Sea, comprise a total area of 3699 km2.
References
Martinez-Andrade, F., P. Campbell, and B. Fuls Trends in relative abundance and size of selected finfishes and shellfishes along the Texas coast: November 1975-December Texas Parks and Wildlife Department, Coastal Fisheries Division Management Data Series No 232.
Morris, A Crab Fishery Enhancement presentation at TPWD Coastal Fisheries Division Regulatory Proposal meeting held July 22, 2015 at Victoria, Texas.
Pauly, D., V. Christensen, and C. Walters Ecopath, Ecosim, and Ecospace as tools for evaluating ecosystem impact of fisheries. ICES Journal of Marine Science 57:
Sutton, G.R. and G. Guillen An Ecosystem Based Fisheries Model to analyze the Effects of Freshwater Inflow. Poster presented at Harte Institute Freshwater Inflow Conference, Corpus Christi, Texas, February 2010.
Acknowledgements
We would like to thank Brian Witt, Carey Gelpi, Christine Jensen, Claire Iseton, and Nicole Carrillo for their input and assistance with this ongoing project.