Drifting fish aggregation devices as a tool to study oceanic marine protected areas

To meet global conservation targets, there is a growing effort to establish oceanic (waters >200 m depth) marine protected areas (oMPAs). However, despite substantial evidence for benefits of coastal MPAs to fish and fisheries, the effectiveness of oMPAs has been challenging to assess robustly. This is mainly because targeted data collection is expensive, so research often relies on catch data restricted to areas outside oMPA boundaries. Here we explore the use of drifting fish aggregating devices (dFADs) as a novel method to assess the effectiveness of oMPAs. We used acoustic data from 902 dFADs deployed by the fishing industry that drifted across the US Pacific Islands Heritage Marine National Monument around Palmyra Atoll – providing data both inside and outside the oMPA – to study spatial variation in tuna biomass density. Using a generalised additive mixed model with a suite of environmental covariates, we found the relationship between tuna biomass density and many environmental covariates made intuitive ecological sense with respect to known tuna behaviour, providing confidence in the model. We found no measurable increase in tuna biomass density inside the oMPA. This finding could have been influenced by the low fishing pressure around this particular oMPA, and regions with greater contrast in fishing pressure might show different results. This research highlights the utility of dFADs as a cost-effective tool for future studies to assess tuna biomass, especially in regions difficult or costly to sample as oMPAs.