Aerial surveys to monitor bluefin tuna abundance and track efficiency of management measures

Abstract

Conservation and management measures for exploited fish species rely on our ability to monitor variations in population abundance. In the case of the eastern stock of Atlantic bluefin tuna (ABFT), recent changes in management policies have strongly affected the reliability of fishery-dependent indicators due to drastic changes in fishing season/area, fisheries selectivity and strategy. However, fishery-independent indices of abundance are rare for large pelagic fish, and obtaining them is often costly and labor intensive. Here, we show that scientific aerial surveys are an appropriate tool for monitoring juvenile bluefin tuna abundance in the Mediterranean. We present an abundance index based on 62 aerial surveys conducted since 2000, using 2 statistical approaches to deal with the sampling strategy: line and strip transects. Both approaches showed a significant increase in juvenile ABFT abundance in recent years, resulting from the recovery plan established in 2007. Nonetheless, the estimates from the line transect method appear to be more robust and stable. This study provides essential information for fisheries management. Expanding the spatial coverage to other nursery grounds would further increase the reliability and representativeness of this index.

Improved management facilitates return of an iconic fish species

“…Bluefin tuna appear to have re-discovered former foraging habitat in northern European waters, which they vacated about 40-50 years ago. The observations are identical to those reported in historical fishery reports, newspapers and scientific literature from the 1920s-1960s (Figure 2), when bluefin tuna were common in these waters (e. g., (23, 24)); the jumping and surface-breaking swimming behaviour is typical for bluefin tuna foraging on prey species (23, 25). Our observations indicate that bluefin tuna were abundant throughout the combined Skagerrak-Kattegat and coastal Norwegian Sea region.…”

“…We have tried to minimize such observer bias by making broad contact to the public and especially commercial fishermen (e. g., via their associations). Nevetheless, to obtain a more representative distribution in the area, alternative methods would need to be employed such as aerial surveying via airplanes (25, 34) or with drones (35) or tagging with electronic tags (36, 37) and subsequent modelling (30, 38). In addition, increased public awareness of the species in the area and the need for its documentation could also increase public reporting of bluefin tuna observations and the reliability of distributional maps.…”

Habitat use, vertical and horizontal behaviour of Atlantic bluefin tuna (Thunnus thynnus) in the Northwestern Mediterranean Sea in relation to oceanographic conditions

“…Such knowledge thus represents key information for fisheries and stock management, i.e. for survey programs that seek to assess ABFT abundance (Fromentin et al, 2014a;Bauer et al, 2015a). In fact, ABFT can rapidly change their vertical and horizontal behaviour from resident to highly migratory states, from periods of surface orientation to repeated or long lasting dives (Brill and Lutcavage, 2001;Walli et al, 2009).…”

Assessing the number of moored fishing aggregating devices through aerial surveys: A case study from Guadeloupe

“…Line transect surveys are a common technique for monitoring animal abundance in terrestrial and aquatic wildlife management (Buckland, 2001). Aerial line transect surveys have been applied to assess the abundance of marine mammals, sea turtles and surfacing fish, including whale sharks and Atlantic bluefin tuna (Rowat et al, 2009;Fortuna et al, 2014;Bauer et al, 2015).…”