Forage fish exploitation

Are there cascading ecological effects associated with forage fish exploitation?

Introduction

This session marks a transition – ecosystem modelling courses tend to be quite traditional, cover the theory, learn about the methods and implementation, show what's possible and discuss how. They seldom get as far as to actually implement ecosystem modelling for addressing policy and research questions. In this course, we'll break with that tradition.

The first three session have introduced the basic theory and implementation of the Ecopath with Ecosim approach and modelling framework. Now, we'll move directly to addressing policy/research questions related to various aspects of ecosystem based management.

The first topic is forage fish exploitation. This is a controversial topic and there is considerable discussion about whether to allow such fisheries or not. See for instance this story about herring roe fishery in Alaska. The debate reflects a difference between single species and ecosystem based management. Should management consider that forage fish play an important role in the food web, or is precautionary management based on traditional maximum sustainable yield sufficient? A notable study by Cury et al. (2011) One third for the birds indeed finds that for marine birds to thrive, the biomass of small pelagics must be above one-third of their maximum observed biomass.

From an ecosystem modelling perspective, we often find that exploitation of small pelagics tends to replace predation mortality. The impact of fisheries for small pelagics may indeed be stronger for their predators than for themselves. This does not mean that the "right" solution is to ban such fisheries, rather, it serves to emphasize that we need to manage ecosystem with consideration for all of the three pillars of sustainability: people, planet and profit. There are trade-offs between these pillars, and we need tools (including ecosystem models) to evaluate such trade-offs.

Key topics and concepts

Trophic cascading effects: Understanding how fishing pressure on forage fish propagates through marine food webs, affecting predator populations above and prey populations below, often with counterintuitive consequences where predators suffer more than the targeted forage species themselves.
Ecosystem-level trade-offs: Exploring the tension between direct economic benefits from forage fish harvesting versus broader ecosystem services, including support for marine birds, marine mammals, and other commercially valuable predator species.
Threshold dynamics and alternate stable states: Investigating critical biomass levels (such as the "one-third rule" for small pelagics) below which ecosystem structure may shift dramatically, potentially leading to irreversible changes in community composition and function.
Replacement of predation mortality: Examining the fundamental ecological principle that fishing mortality on forage fish often substitutes for natural predation mortality, fundamentally altering energy flow pathways and predator-prey dynamics within the ecosystem.
Multi-stanza population dynamics: Understanding how forage fish exploitation affects different life stages and age classes, with implications for recruitment, spawning stock biomass, and long-term population sustainability.
Ecosystem-based vs. single-species management: Contrasting traditional maximum sustainable yield approaches focused on individual stocks with holistic ecosystem considerations that account for species' ecological roles and interdependencies.
Spatial and temporal variability: Recognizing how forage fish exploitation effects vary across different marine ecosystems and temporal scales, influenced by local environmental conditions, predator assemblages, and fishing practices.
Sustainability pillars integration: Balancing the three pillars of sustainability (people, planet, profit) through quantitative assessment of ecological, economic, and social trade-offs in forage fish management decisions.

Learning outcomes

This session will provide an introduction to and experience with:

Analyzing trophic cascading effects - Quantifying and interpreting the direct and indirect impacts of forage fish exploitation on marine food webs, including effects on predator and prey populations using EwE.
Evaluating ecosystem-level consequences - Assessing changes in energy transfer efficiency, competitive relationships, and ecosystem structure resulting from different levels of forage fish fishing pressure through comparative scenario modeling.
Investigating ecosystem stability and thresholds - Designing and executing modeling experiments to identify potential alternate stable states and threshold biomass levels that could trigger ecosystem shifts, including multi-stanza population dynamics.
Applying ecosystem-based fisheries management principles - Synthesizing modeling results to evaluate trade-offs between direct fishery benefits and broader ecosystem services, informing sustainable harvest strategies for forage fish populations.
Integrating multiple stressors and spatial dynamics - Examining how forage fish exploitation interacts with climate change effects and varies across marine ecosystems, developing comprehensive management recommendations that account for ecosystem resilience and recovery potential.

Resources and preparation

Please prepare for class by reading the following.

Research and policy questions: an introduction. This chapter presents an exemplary list of research/policy questions that can be, indeed are, being addressed with ecosystem modelling using EwE.
Forage fish exploitation. An introduction to today's topic, which also suggests how you may go about addressing the topic with EwE. Please explore the tutorials in this chapter before class.

Presentations

Please look through lectures before class.

05: Forage fish. An introduction to managing trade-off fisheries with emphasis on trade-offs in management

Tutorials

Forage fish exploitation. The chapter referred to above. There are several options for tutorials described in the chapter. Feel free to explore any of these as time allows, though it may be a good idea to start with the first.

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