Describing the spatial dynamics of fish populations to investigate the impact of fishing and environmental variability for improved scientific and management advice

About a third of the world’s tuna catch comes from the exclusive economic zones of the 22 Pacific Island countries and territories (PICTs) of the western and central Pacific Ocean. Tunas are highly migratory top predators, and their preferred habitats, the abundance of food resources and movement patterns are known to be affected by climate. To ensure the sustainable economic growth and food security of these Pacific Island nations in the context of climate change, it is important to manage their tuna resources effectively. By modelling the dynamics of tuna populations (how and why the stock changes over time), we aim to understand and quantitatively describe the historical fluctuations, and to forecast future trends in the stock dynamics. This information is vital to support evidence-based management decisions.

The challenge is in how to simplify the many different components of an ecosystem and the multitude of interactions that take place between them so as to have a model which can be used in a meaningful way. A high level of information on many oceanic variables is needed to be able to make realistic, detailed models of the dynamical processes at play. FAME scientists have been working alongside international partners for many years to produce a numerical model that is capable of integrating a wide range of data to investigate these interactions and to provide advice for fisheries managers.

The first version having been developed in 1995, SEAPODYM (Spatial Ecosystem And Population Dynamics Model) is now a numerical modelling framework for investigating the population dynamics of tunas under the influence of environment and fishing. It was initially used to study the influence of El Niño–Southern Oscillation (ENSO) on the productivity and catch rates of skipjack tuna in the western and central Pacific and was shown to be an efficient tool to model climate-related variability of tuna populations. Through an international effort, SEAPODYM has been continuously evolving to provide a general framework that integrates massive observations with biological and ecological knowledge about different tuna species and other top predators.

Synthesising current knowledge on the biology, ecology, and population dynamics of the key life stages of tuna species, the SEAPODYM modelling framework can provide valuable services to the PICTs, given the high importance of tuna fishing to their socioeconomic development. Besides the profound effects of ENSO on tuna catch rates, which already called for cooperative management of the purse-seine fishery by ten Pacific states, the Pacific region faces the greater challenge of climate change. SEAPODYM modelling has been used to make projections on the redistribution of tuna in the Pacific by 2050 under different greenhouse gas emission scenarios. This in turn allows the evaluation of the risks to the sustainability of tuna-dependent Pacific Island economies.

The SEAPODYM modelling framework is designed to assist stock assessment needs. SEAPODYM provides more detailed information on fishing impacts due to its detailed spatial structure and is therefore an efficient numerical tool for spatial planning and spatial management strategies evaluation. Another major advantage of this model is that fishing impacts can be easily discriminated from the effects of environmental variability on the modelled population. For example, the model can provide an understanding of observed relationships, such as relations between changes in tuna catches and ENSO events, through quantitative descriptions of the habitats and population dynamics. Finally, SEAPODYM offers opportunities for seasonal and decadal forecasting, provided that reliable predictions of ocean variables are available.