Towards “glocalised” management of tuna stocks based on causation between a stock and its component belonging temporally to local Exclusive Economic Zones

The creation of Exclusive Economic Zones (EEZs) in 1982 gave coastal countries sovereignty over their local tuna resources, whereas the migratory nature of tuna calls for regional management. However, as regional tuna management by Regional Fisheries Management Organisations (RFMOs) does not always facilitate optimal management of local resources by coastal countries, there is a need for an approach that ensures both the achievement of regional management objectives and optimisation of local benefits, i.e., “glocalised” management. To promote “glocal” management measures for the local populations of highly migratory species, (1) the spatio-temporal VAST model was used to estimate abundance indices for the adult regional Eastern Atlantic yellowfin tuna population and its local component in Ivorian waters using catch and effort data and (2) convergent cross mapping was used to examine the causal relationship between changes in abundance at these different spatial scales over time. Convergent cross-mapping detected a causal relationship between general stock dynamics and local dynamics, in the direction from global stock abundances to local resource abundances. This implies that the success of local fishing was closely linked to the general state of the stock. Conversely, we found no evidence of a causal relationship in the direction from local resources to the regional stock. This suggests that local adult dynamics had little or no influence on overall stock dynamics. Based on these results, we propose several criteria to ensure fairer distribution of tuna resources between coastal states and distant water fleets (DWFs).