Fishing within food webs: Modeling for management advice

Sarah Gaichas from the Northeast Fisheries Science Center addresses the critical question: What does modeling have to do with management? Fishing within food webs involves complex interactions, and modeling for management advice is essential for understanding how ecosystem structure affects dynamics. Which models are best for management decisions? The most effective models are based on clearly specified questions from managers and stakeholders, using observations directly from the ecosystem with a clear description of uncertainties.

The Fundamentals of Food Web Modeling

What is a food web model? It is often a system of linear equations where for each group, i, one must specify parameters such as Biomass (B) and Ecotrophic Efficiency (EE). These ecosystem models help in distinguishing climate, fishing, and food web interactions. To ensure reliability, researchers use a model ensemble and grade inputs to manage uncertainty. By running scenarios in an ensemble, they can provide timely answers regarding how which species respond to the same perturbation.

Alaska Pollock: A Tale of Two Ecosystems

The Eastern Bering Sea (EBS) and the Gulf of Alaska (GOA) provide two examples of ecosystem modeling in action. Although these adjacent ecosystems have similar biological communities and fishery management, they exhibit different pollock trajectories and mortality sources.

• Eastern Bering Sea (EBS): In this system, EBS pollock dominate at TL 3.5, representing the highest biomass of any fish. It is characterized as a "beer belly" system where an influential group at mid-trophic levels absorbs signals, making the system more predictable and stable.
• Gulf of Alaska (GOA): This ecosystem has the highest biomass above TL 4, including species like halibut and arrowtooth. It contains influential groups at high trophic levels that may magnify bottom-up and top-down signals, potentially making it a less predictable system subject to radical change.

Energy Flow and Trophic Dynamics

Energy flow is the flow of energy through living things within an ecosystem. Each of the levels within the food chain is a trophic level. These food chains are organized into trophic pyramids to show the quantity of organisms at each level. Trophic dynamics relates to thermodynamics because it deals with the transfer and transformation of energy originating externally from the sun via solar radiation.

Producers are important because they convert energy from the sun into a storable and usable chemical form of energy, such as glucose. However, energy is lost as heat at each step along the way. In general, only 10% of net primary productivity goes to the next level, the primary consumers. Ecological efficiency may vary anywhere from 5% to 20% depending on the specific ecosystem.

Management Strategies: Top-Down and Bottom-Up Approaches

Bottom-up and top-down are strategies of composition and decomposition used to gain insight into subsystems. In practice, they can be seen as a style of thinking or leadership within management and organization. A top-down approach starts with the big picture and breaks the system down into smaller segments to gain insight into its compositional subsystems. Conversely, a bottom-up approach is the piecing together of systems to give rise to more complex systems, where individual base elements are first specified in great detail.

Structure of a food web may determine how predictable a system is under perturbation and how changes in primary production propagate. Consequently, whether a system is controlled by bottom-up or top-down forcing determines how pollock react to other groups and how signals are transmitted through the ecosystem.

Comparison of Ecosystem Trophic Data

• Eastern Bering Sea Dominance: Pollock at Trophic Level 3.5.
• Gulf of Alaska Dominance: Predators at Trophic Level 4.0 and above.
• Standard Energy Transfer: 10% efficiency between trophic levels.
• Ecological Efficiency Range: 5% to 20%.
• Carbon Fixation: 100 × 10^15 grams of carbon per year fixed by photosynthetic organisms.