Fishscape: Complex Dynamics of the Fishery for Tropical Tunas in the Eastern Pacific Ocean

Fishscape is a multi-organization project funded by the NSF: Coupled Human and Natural Systems program. The primary objective is to build, test, and analyze a fully-coupled model of the fishery for tropical tunas (bigeye, skipjack, and yellowfin) in the Eastern Pacific Ocean (EPO), including spatial-temporal characterization of interactions between oceanographic conditions, fish biology, fisher decision making, markets for tuna products, and international management, which is negotiated by multiple countries (or technically “states”) at the Inter-American Tropical Tuna Commission. Implemented in the EASy GIS platform, model development is complete and we are currently tuning parameters in preparation for responsive governance analysis and robustness analysis. The latter will cover three meta-scenarios: climate change, technological innovation, and growing demand. Figure FS.1 below is a screen-shot of the model in action.

FS.1 Fishscape Model in Action

FS.1 Fishscape Model in Action

As shown in the figure, Fishscape consists of two agent-based components.  One component describes the movement of several thousand metapopulations of fish to changing environmental conditions (dots).  Metapopulations are categorized by two age groups, young and old, for each of the three species of tuna.  These metapopulations move according to ocean currents, directed swimming to preferred waters, and random walk.  As they move, metapopulations, merge with other metapopulations when they converge on the same area and can split apart if the metabolic demands of the populations exceed the carrying capacity of the region.  One of several original features of the model is that size distribution of the schools within the metapopulation is calculated by metapopulation size and swimming characteristics. This is a key component of the linkage between fish and fisher models because school size is one of the factors that determins the maximum catch per set for purse seine fishers. Biologically, the fish grow with age, reproduce, and die based on natural and fishing mortality.

The other component of the existing model describes the movement of each of the approximately 300 purse seine vessels that operate in the Eastern Pacific Ocean (oblong shapes in Figure FS.1).  These vessels leave their port with full tanks of fuel, search for fish, which includes both directed and random walk components, deploy their nets to catch fish, and return to sell their fish at current prices.  Such processes occur within a quasi-3-dimensional, spatial model at time steps that range between 3 hours to 1 month.  The strategies of the fleet operations vary with economic conditions such as fuel price and market value of the catch as well as climatic events such as El Niño and La Niña.   The movement of vessels and center of metapopulations are tracked to roughly a kilometer.   We also have an economic model that predicts changes in the level of fishing effort over time.

The model is driven by satellite imagery of sea surface height, water temperature, chlorophyll concentration, as well variables from NASA’s ECCO-2 global circulation model and climatological maps of subsurface water temperatures and oxygen concentrations.  Changes in the price of tuna drive changes in the level of fishing effort and policies such as a limit on the total number of vessels and time area closures can be implemented in the current model.

With a few key exceptions, preliminary simulations using the simplest version of the model yield spatial and temporal patterns that conform well with the IATTC’s time series of catch and effort. After some additional tuning, we plan to explore the policy implications of Fishscape through testing of responsive governance using the AC/SC framework and by conducting a robustness analysis for three meta-scenarios: climate change, technological innovation, and growing demand. By using the responsive governance lens we can identify likely future policy outcomes and then test these management measures against less feasible but possibly more robust regulations. Specifically, robust regulations are highly likely to lead to acceptable (if not completely optimal) outcomes even in the face of exceptionally high levels of uncertainty. This work will help to identify policies that could help to maintain the IATTC’s exemplary record as one of the most effective international fisheries management institutions in the world.

Participating Institutions: Dartmouth College (D.G. Webster, Lead-PI), University of Southern California (Dale Kieffer, Co-PI), Inter-American Tropical Tuna Commission (Michael Hinton, senior researcher), Gulf of Maine Research Institute (Jenny Sun, Co-PI), SCRIPPs Institute of Oceanography (Dale Squires, Co-PI), and the RAND Pardee Graduate School (Robert Lempert, Co-PI).

Acknowledgements: The PIs would like to thank the NSF Coupled Human and Natural Systems Project for funding and the Inter-American Tropical Tuna Commission for providing access to key datasets and expertise.