Multispecies and Ecosystem Modeling for the Northeast Shelf Ecosystem
We use models to address a range of research and management needs. Some models generate short-term projections to support management advice. Other models simulate long-term evaluation of management strategies.
What Is a Model?
A model is a simple representation of an object or system. Models are used to develop a common understanding of the object or system, and to predict possible outcomes of different changes to the system. At the Northeast Fisheries Science Center, we use mathematical models for research and for management advice. The models vary in complexity, from simple equations to model primary productivity to single species assessment models to models with multiple species to complete ecosystems.
Most current fisheries management is based on short-term projections from models of the population dynamics of a single species. We are developing a range of multispecies models that could be used for projecting options for sustainable harvest. These models can also be used in complicated situations where predators and prey are both fished, or where many species of fish are caught together.
Management Strategy Evaluation
Models can be used to experiment with different fishery management measures by testing them on a computer before applying them in the real world. Multispecies and ecosystem models are useful for:
- Evaluating unintended side effects of fishery management.
- Testing proposed multispecies management measures
- Comparing management strategies under a range of potential future conditions such as changing climate or offshore wind energy development.
Right Tool for the Job
Northeast U.S. Models
The first step in developing a mathematical model is to outline key processes and relationships. A conceptual model depicts these relationships. It can be used in various ways:
- As a communication tool on its own.
- As a checklist to ensure that no key processes are missed in further quantitative modeling.
- To outline areas for further research.
- To relate information across multiple scientific fields.
Scientists have developed conceptual models for the social-ecological systems of Georges Bank, the Gulf of Maine, and the Mid-Atlantic regions of the Northeastern United States. Our scientists have collaborated on conceptual model development for the Grand Banks in Canada, the North Sea in Europe, and for the summer flounder fishery in the Mid-Atlantic. While we use all of these models as communication tools, several have also been used to specify further quantitative analysis.
Production models track the changes in a species’ population biomass due to fishing, human interactions, and natural causes of death. These models can easily incorporate environmental changes over time, such as temperature, which can influence animal growth and reproduction. Multispecies production models also include interactions between species, such as competition and predation. This makes them a more complete description of an ecosystem if species are interacting.
Production models are much easier to create than more complicated models used in stock assessments, as the data required are usually easier to collect. They don’t include some of the data used in stock assessment models, such as information on the numbers of fish of different ages in the population. Multispecies models require information on diet and possible overlaps of habitat. We also have visualization software that can be used to observe changes in population sizes over time if we adjust fishing on different species.
Age- and Size-Structured Models
Age- or size-structured population dynamics models track the changes in total numbers of animals by accounting for births, growth, and deaths for each age or size group. Many current fishery stock assessment models track populations by age group. Our scientists have developed both age- and size-structured multispecies models, which extend the single species models by adding information on predation. We continue to evaluate the capabilities of these multispecies models for short term projections and providing annual harvest advice to fisheries managers.
We can also use multispecies age- or size-structured population dynamics models for management strategy evaluation. The multispecies model Hydra models 10 different fish species based on the size (length) of the fish. It is used to test multispecies management strategies for Georges Bank in support of the New England Fishery Management Council’s Ecosystem Based Fishery Management efforts.
Food Web Models
Food web models track energy flow in an ecosystem from primary producers (phytoplankton) up to zooplankton, fish, top predators, and fisheries. They simultaneously account for deaths due to predation and fishing. These models may factor in information including:
- Changes in biomass of a species like production models.
- Track age groups like age structured models.
- Combine these functions for different groups in the food web.
Food web models are useful for management strategy evaluation that addresses fishery and predator-prey interactions. We have developed food web models for the Gulf of Maine, Georges Bank, Southern New England, and the Mid-Atlantic. Recently, we used an updated Gulf of Maine model to evaluate the potential impacts of changes in Atlantic herring fishery regulations, along with more detailed species models.
The Northeast and Alaska Fisheries Science Centers have collaborated to develop a flexible new food web modeling tool called Rpath. Rpath has the capability to run many food web management strategy simulations efficiently, and is being used by researchers around the world.