Changes in fishing power and fishing strategies driven by new technologies: The case of tropical tuna purse seiners in the eastern Atlantic Ocean

• Changes in fishing power and strategies are analyzed in different time–space scales.
• The increasing modernization of the fleet directly led to an increase in fishing power.
• The modernization of the fleet indirectly increased the range of fishing grounds.

Technological advancements can influence both the fishing power of a fleet and the fishing strategies it employs. To investigate these potential linkages, we examined almost three decades of data (1981–2008) from French tropical tuna purse seiners operating in the eastern Atlantic Ocean. Applying a sequence of statistical methods at different temporal and spatial scales, we analyzed two indicators of fishing power (sets per boat-day on fish aggregating devices (FADs) and sets per boat-day on free-swimming schools) each of which represent a distinct fishing mode. Our results show that the increasing modernization of this fleet has led to increases in both fishing power and the available number of fishing strategies to choose from. A key output of this analysis was the breakdown of fishing power time series (for each fishing mode) into separate periods of continuous years during which catchability was assumed to be constant, thus identifying regime shifts. This partitioning allowed us to identify when key changes occurred in the fishery. Changes in FAD-associated fishing were mostly driven by the introduction of radio beacons (early 1990s) which lead to an increase in fishing effort and an expansion of fishing grounds (direct effect) and the implementation of time-area management measures which resulted in a fragmentation of the traditional fishing grounds in the 2000s (indirect effect). During the same period, fishing on free-swimming schools also increased despite the biomass of stocks decreasing and fishing grounds remaining unchanged. This suggests these increases were driven by improvements in fish detection technology (e.g., bird radars, sonar). These identified increases are not entirely unexpected: indeed it is widely recognized that fishing power in the purse seine tuna fishery has increased over time. However, these increases do not necessarily occur linearly. Thus, understanding how fishing power is changing over time (such as determining when regime shifts occur) is critical to improving the CPUE standardization procedure in tropical tuna purse seine fisheries.