Description of the aggregations around FADs
Doray (2007) defined the different types of fish aggregations observed around anchored FADs from acoustic observations coupled with identifications of fish made from cameras and experimental and professional fishing operations (« DOLPHIN » project). Four types of aggregations stand out near FADs at daytime:
- A surface layer (0 to 20 m deep) made of small tuna (about 20 cm at the fork): black fin tuna (Thunnus atlanticus), yellowfin tuna (Thunnus albacares), skipjack tuna (Katsuwonus pelamis), frigate tuna (Auxis thazard), little tunny (Euthynnus alletteratus), etc. Ill-assessed with the sounder, this layer seems little important regarding the results of fishing activities targeting these fishes used as baits. Theses aggregations are mobile and generally located upstream of the FAD regarding the current pattern.
- An aggregation of fish usually found under “drifting wood”. It’s dolphin fish (Coryphaena hyppurus), jacks, triggerfish. This aggregation is located very close to the FAD and isn’t very numerous.
- The main part of fish biomass aggregated near FADs is located in subsurface between 30 and 100 m deep, upstream of the FAD regarding the current pattern, as far as 400 m maximum from the device. This cone-shaped aggregation is mainly made of blackfin tuna, yellowfin tuna and skipjack tuna of about 50 cm fork-lengths. Its biomass estimated with an echo sounder allegedly represents about 95% of the fish biomass aggregated around anchored FADs. It was evaluated (Doray et al. 2009) to an average of 7 tons per day (standard deviation: 5.4, from 1 to 24 tons). This concentration of fish is characterized by a great variability of its biomass during the nycthemeral cycle, but also from one day to the other and all along the year. At day time, the concentration forms in the morning from 6, peeks in the middle of the day and starts decreasing around 3 pm. The observations made during the “DOLPHIN” campaign suggest that when two FADs are set close from each other (1-2 mile distance), the concentration of fish reassembles in the morning in a seemingly random manner. When a FAD is fitted with 2 heads set a few hundred meters away from each other, fish concentrate upstream of the head located the most upstream regarding the current pattern. The variations in abundance of subsurface fish observed along the year appear to be connected with the abundance of prey. (Doray et al. 2009)
(the depth meter at bottom right indicates the depth of the camera)
Observation with an echo sounder of the fish aggregated in subsurface in professional catches
- Large pelagic fish probably stay close to the aggregation of sub-surface tuna. They are mobile fishes mainly consisting of blue marlins and yellowfin tuna whose importance is ill-assessed by the echo sounder. Some of these species, such as blue marlin, seem attracted by the concentration of fish around FADs. Indeed, 89% of the prey retrieved from the stomach contents consist of species found around FADS, among which 80% of small surface tuna or fish found under drifting wood and 20% average-sized fish apparently coming from the subsurface tuna layer (Reynal et al. 2006).
Illustration of the distribution of fish aggregations around an anchored FAD (from observations made in Martinique, in 2003-2005)