Modeling for Conservation and Management of Marine Resources

Modeling is used for describing and understanding physical systems using logical relationships and behaviors and mathematical equations,  based on our knowledge of those physical systems from observations and sampling.   Models provide a simplified  theoretical representation of a small and specific part of the real world that can be used for describing real systems and predicting their behavior.  Models are used for not only in physical sciences such as biology and thermal sciences but in social sciences such as economics and political science.  Weather forecasting is a familiar application of modeling.  The capabilities and usefulness of modeling have expanded greatly with the general availability of powerful computer systems.
Temporal variation in oil concentration in Gulf Menhaden (1963 to 1979). From Guillory, V. 1986.
Actual annual total oil yield from the Gulf Menhaden fishery (1963 to 1979) compared to estimated values from modeling. From Guillory, V. 1986.

Biologists find models particularly attractive because of the complexity of the world they study and the difficulty in creating experiments to test their ideas in the real world.  For example, it is impossible to directly test the response of a change in the regulations that govern their harvest, but a model built with an understanding of the life history of the fish and a history of annual harvest data can provide intelligent predictions.  Modeling is an elegant approach to scientific investigation because of the ability to continually test them and refine them using observations of the real systems they represent.  As we learn more about a physical system, our ability to model that system improves.

Models are being used at GCRL for several purposes.  This webpage describes how Dr. Robert Leaf and his graduate students are developing and using models in studying commercially significant fish species in the Gulf of Mexico and Atlantic Ocean.

Modeling Fisheries Resources

The assessment and management of the sustainable natural resources of the United States is complex challenge. Management of harvested biological resources (finfishes and shellfishes) in the federal waters of the U.S. is carried out under the guidelines of the Magnuson–Stevens Reauthorization Act (MRSA) of 2006. This statute mandates management at maximum sustainable yield (MSY), defined as “the largest average catch or yield that can continuously be taken from a stock under existing environmental conditions” (Ricker 1975). The regulations in state waters also have similar objectives – to maintain long-term sustainability at catch levels that promote use of the resource for their stakeholders. In order to effectively manage fisheries, managers must have detailed information about the status of the fishery and the state of the fish population.

Because of the complex nature of the processes that occur in populations: individuals grow, reproduce, suffer mortality, and some even migrate, a conceptual model of the population is a useful starting point for trying to understand these dynamics. Because it is impossible to know the demographic properties of every fish and the actions of every fisherman, scientists use simplifying assumptions about these dynamics. We do this using conceptual models, which are simplifications (though they can be complex!) about fish population and harvest dynamics. The utility of models is that they help us understand the dynamics of populations and also to help us make predictions about what will happen in the future. For example, we may be interested in forecasting how the long-term sustainability of a stock may be impacted by a change in the gear or a change in the minimum size limit. Because of the gravity of the biological and socioeconomic implications that such changes have, quantitative modeling methods have become a central tool in decision making.

A unifying principal of population dynamic models for fishery assessment, regardless of their complexity, is that they enable scientist to understand two important criteria about the fishery. The first aspect is whether the population is overfished such that it does not have the biomass to sustain the current amount of harvest. The second criteria is that the level of harvest be quantified that will result in long-term yield for a given stock, and the question is whether the past and future harvest is greater or less than this amount. Assessment, using quantitative models of fish populations, helps managers determine the levels of fishing effort that is appropriate for given stock, because managers to not manage the fish, but actually manage the fisherman.

Dr. Robert Leaf

Robert Leaf examining blue crabs from Biloxi Bay.
Dr. Leaf working on an artificial oyster reef in the Bronx, New York City

Dr. Leaf and his graduate students focus primarily on the use of mathematical modeling related to fisheries assessment of commercially harvested species and understanding the factors that drive population growth. 

Dr. Robert Leaf joined GCRL in September of 2012 and has expertise in quantitative methods and computer-intensive modeling approaches. The goals of these analyses are to understand population regulation and appropriate and effective conservation and management strategies. Dr. Leaf received his Ph.D. in Fishery and Wildlife Sciences from the Virginia Polytechnic Institute and State University in 2010, where he studied how phenology of individuals in harvested populations were altered under size-selective fishing. As a post-doctoral researcher in NOAA’s “Fisheries and the Environment” program, Dr. Leaf examined how phytoplankton bloom phenology determined recruitment patterns in northeast Atlantic ground fishes. His current work involves assessment of Gulf Menhaden, Gulf of Mexico Blue Crab, and Mississippi’s Red Drum stock.

Robert Leaf investigating new marine life on artificial reef rubble.
Dr. Leaf research diving in the kelp forests of Monterey, California..

Gulf Menhaden

The commercial fishery for Gulf Menhaden (Brevoortia patronus) is one of the most economically important fisheries in the Gulf of Mexico. The results of the most recent stock assessment for Gulf Menhaden (SEDAR 32A, August 2013) indicate that the stock is resilient to commercial harvest and is not currently overfished.

Though the Gulf of Mexico menhaden stock is not currently overfished it is desirable that it should remain so. Potential future regulatory actions must be made with a good understanding of both the natural variations in menhaden population and the effect of the fishery. One of the outstanding questions is to what extent bottom processes, temperature, and primary productivity regulate population growth.

Total Gulf Menhaden commerical landings from the reduction and bait fisheries in the U.S. Gulf of Mexico from 1948 to 2010.
Total Gulf Menhaden commercial landings from the reduction and bait fisheries in the U.S. Gulf of Mexico from 1948 to 2010.
Monthly maps of sea surface temperature (left panels) and chlorophyll a anomolies (right panels) in the northern Gulf of Mexico from October 2012 - April 2013. Positive (red and green) and negative (blue and brown) areas are in units of standard deviations from the long-term mean monthly values (1998 - 2013). Data is from the NASA Ocean Color website.
Monthly maps of sea surface temperature (left panels) and chlorophyll a anomalies (right panels) in the northern Gulf of Mexico from October 2012 - April 2013. Positive (red and green) and negative (blue and brown) areas are in units of standard deviations from the long-term mean monthly values (1998 - 2013). Data is from the NASA Ocean Color website.
Click for larger image.

 

This is not a new issue. Research has shown that oil content in fish was correlated to environmental conditions in the spring, and that condition was highly variable.

Because of the importance of the menhaden stock and evidence that condition is determined by environmental factors prior to the fishing season, a research focus of Dr. Leaf's work is the determination of what environmental factors influence stock resilience.


Stephanie Taylor

M.S. Student

Stephanie Taylor with a large dolphin caught during sampling in the Loop Current.
Stephanie Taylor with a large dolphin caught during sampling in the Loop Current.

Stephanie Taylor graduated with a B.S. in Marine Biology from Auburn University.  During her time at Auburn she came to GCRL as a Summer Field Program student studying Shark Biology, Marine Invertebrates, and Oceanography.  After graduation from Auburn, she began to work for the Marine Education Center as an instructor and then later became accepted to the graduate student program at GCRL Southern Mississippi.  She is currently working on her Masters in Coastal Sciences with a concentration in quantitative fisheries.  She is studying the ichthyoplankton community composition of both the Loop Current and Sargassum in the Gulf of Mexico and will graduate May 2014.  During her time at the lab she has been a graduate assistant working for Center for Fisheries Research and Development and later with her advisor Dr. Robert Leaf working on Black Sea Bass.  She has also been a teaching assistant for the Summer Field Program during summer 2013. 

Stephanie’s interest in ichthyoplankton in the Loop Current stems from the fact that many fish species are spawned there and the diversity of fish life within the current.  The Loop Current’s downwellings and upwellings provide food and increased chances of survival for young fish.  

Avoiding the expense of field collection efforts specific to this research, Stephanie worked with data collected during bluefin tuna studies aboard GCRL's R/V Tommy Munro in 2003 and 2004. Those studies included three transects perpendicular to the Loop Current flow using a Tucker trawl, which collects a depths of 1 meter, 10 meters, and 20 meters below the surface.  In each transect, three zones were targeted. These included an “outside” zone located west or north of the outer boundary of the Loop Current (LC), a “transition” zone located at the edge of the LC, and an “inside” zone located east or south of the inner boundary of the LC.  These results indicate four unique assemblages of fish:  an outside assemblage that occur in Gulf waters outside the Loop Current; two transition assemblages that reside within the current and at the edge of the frontal boundary; and an inside assemblage that occurs in water originating from the Caribbean.  

The figures below are non-metric multidimensional scaling results from Stephanie's thesis project that describe the changes in community assemblage across the Loop Current. Non-metric multidimensional scaling (NMS) spatially describes spatially how ichthyoplankton density in a sample relate to corresponding environmental variables. 

Unique assemblages of fish within and near the Gulf of Mexico Loop Current.

 

 

Three transects were mapped and then plotted beside each transect's non-metric multidimensional scaling (NMS) plot. A, B, and C are mapped stations and D and E are NMS plots representing stations in ordination space. Symbols are based on the NMS-designated groupings that represent each habitat (outside, transition, inside) sampled at the Loop Current.

- outside assemblage

- transition assemblage

- edge assemblage

- inside assemblage

The arrows indicates movement between stations along a transect. Maps were then coded with NMS-designated symbols to confirm transition between habitat along a transect. These results confirm transition between the assemblages across the Loop Current transects.

   Figure A, Plot of Transect One, East-West    Figure D, NMS Plot, Transect One
   Figure B, Plot of Transect Two, East-West    Figure E, NMS Plot, Transect Two
   Figure C, Plot of Transect Three, North-South    Figure E, NMS Plot, Transect Three

Robert Trigg

M.S. Student

Robert Trigg
Robert Trigg

Robert Trigg earned a B.S. in Natural Sciences with minors in Physics and German from Shimer College in Chicago. He came to GCRL in 2010 and worked as a Biological Technician until 2013, when be became a full-time M.S. student. Robert's work at GCRL has included the Mississippi Blue Crab CPUE Study, Mississippi’s Fishery-Independent Sampling Program, Directed NRDA Assessment Research regarding blue crabs, and the Gulf of Mexico Nutrient Criteria Development in the State of Mississippi. He is a member of the American Fisheries Society and the Marine and Estuarine Graduate Student Association.

Robert's master's thesis research focuses on an individual-based model (IBM) of the Gulf menhaden (Brevoortia patronus) fishing fleet. His work adapts modern ecological simulation tools normally used for studying the foraging behavior of animals. He also hopes to verify that vessel movements, like animal searching strategies, can be characterized by a Lévy flight. This distribution of movements is heavy-tailed, with many short movements and relatively rare long movements.

IBMs are mathmatical tools used to model agents whose behavior depends on simple rules. In Robert's work, menhaden fishing vessels are the agents evaluated. The interactions between these agents, and between the agents and the environments, gives rise to system-level patterns. By studying how these patterns change when the agents' rules are changed, we gain an understanding of the system that is not predictable from the properties of the agents alone.

The Gulf menhaden fishery is the second largest in the United States.Robert Trigg's work studies the fishing fleet using ecological simulation techniques originally developed for studying the foraging behavior of animals within an ecosystem.
The Gulf menhaden fishery is the second largest in the United States. Robert Trigg's work studies the fishing fleet using ecological simulation techniques originally developed for studying the foraging behavior of animals within an ecosystem. Photo by Gary Gray.

Robert hopes to use his IBM work to gain a better understanding of catch per unit effort (CPUE) in the Gulf menhaden fishery and the role optimal foraging theory can play in fisheries. Further planned extensions include modelling within-week patterns of catch and the relationships of primary production and Mississippi River discharge to catch rates. Historical data, including vessel captain's daily fishing reports, are used as inputs to the studies.

Robert is also assisting Stepanie Taylor in evaluating fish specimens from her sargassum work