Balancing Model Complexity, Data Requirements, and Management Objectives in Developing Ecological Reference Points for Atlantic Menhaden

Abstract

Atlantic menhaden is an important forage fish and the target of the largest fishery along the US East Coast by volume. Since 1999, managers at the Atlantic States Marine Fisheries Commission, stakeholders, and scientists have been interested in developing ecological reference points (ERPs) that account for menhaden’s role as a forage species. To accomplish this, we developed a suite of modeling approaches that incorporated predation on menhaden and changes in productivity over time and allowed for evaluation of trade-offs between menhaden harvest and ecosystem management objectives. These approaches ranged in complexity, from models with minimal data requirements and few assumptions to approaches with extensive data needs and detailed assumptions. This included a surplus production model with a time-varying intrinsic growth rate, a Steele-Henderson surplus production model, a multispecies statistical catch-at-age model, an Ecopath with Ecosim (EwE) model with a limited predator and prey field, and a full EwE model. We evaluated how each model could address managers’ objectives and compared outputs across the approaches, highlighting their strengths, weaknesses, and management utility. All models produced estimates of age-1 + biomass and exploitation rate that were similar in trend and magnitude to the single-species statistical catch-at-age model, especially in recent years. While the less complex models were relativity easy to implement and update, they lacked key elements needed to manage multiple species simultaneously. More complex models required a wider array of data and were more difficult to update within the current management time-frames, but produced a more useful framework for managers. Ultimately, an EwE model of intermediate complexity coupled with the existing single-species assessment model was recommended for use in management.

A Case Study in Connecting Fisheries Management Challenges With Models and Analysis to Support Ecosystem-Based Management in the California Current Ecosystem

“…The ESR comments also show a clear desire on the part of managers and stakeholders to better assess the broader ecosystem impacts of management actions, particularly with regards to the tradeoffs between the forage needs of predators, fisheries for prey and predator species, and protections for non-target predator stocks. In light of the stakeholders' and managers' comments, ecosystem models have the potential to be used more routinely to assess the impact of changes in forage to dependent predators when linked to stock assessments (e.g., Drew et al, 2021) or MSE model output (e.g., Deroba et al, 2019), and to develop multispecies harvest control rules (HCRs) or ecosystem-level reference points (Link, 2018;Fulton et al, 2019;Holsman et al, 2020). This is in addition to their demonstrated utility in addressing specific strategic questions, such as the role of krill in the ecosystem (e.g., Pacific Fishery Management Council (PFMC), 2009) or the impact of climate change on PFMC-managed species (e.g., Marshall et al, 2017).…”

The Path to an Ecosystem Approach for Forage Fish Management: A Case Study of Atlantic Menhaden

“…The most complex model was an updated version of full ecosystem model, the NWACS EwE model (Buchheister et al, 2017a,b). These models were described fully, along with the species included in each model, how the parameters were estimated, the outputs, and a comparison of the results in Chagaris et al (2020) and Drew et al (2021) but are summarized here in Table 3. The suite of models was chosen to evaluate the trade-offs between complexity, realism, and data requirements, as well as to explore the impacts of model structure and assumptions on population trends and reference points.…”

“…Newer data were not available for all of the groups included in the full NWACS EwE model; as a result, inputs for those groups were extrapolated from the terminal year of 2013. Drew et al (2021).…”

“…The multi-model approach, rather than selecting a single model from the outset, allowed trade-offs between model complexity and data requirements to be evaluated. Also, it allowed the uncertainty due to model structure and assumptions to be better understood by comparing similarities and differences across model predictions and outputs (Drew et al, 2021). This approach led to the development of a tool that combined the strengths of two different models, using the ecosystem dynamics captured by the NWACS-MICE model to provide long-term strategic advice, while the more detailed population dynamics of the singlespecies model provided short-term tactical advice.…”

It’s Not the Destination, It’s the Journey: Multispecies Model Ensembles for Ecosystem Approaches to Fisheries Management

“…Ultimately, the models produced similar patterns in key indicators (e.g., menhaden biomass and fishing mortality rates), which led the workgroup to greater confidence that key ecological processes were captured in the models. More details on the models and model selection process for this example case are in Cieri et al (2020).…”