The structure and dynamics of natural communities result from the interplay of abiotic and biotic factors. We used manipulative field experiments to determine the relative roles of abiotic conditions and biotic interactions in structuring deep-sea (2500 m depth) communities along environmental gradients around hydrothermal vents of the eastern tropical Pacific Ocean (East Pacific Rise, at 9Њ50Ј N). We tested (1) whether predation by crabs and fishes affects the recruitment of benthic species and subsequent community structure and (2) whether the effects of predation vary along the steep gradients of temperature, oxygen, sulfide, and metal concentrations near vents. Recruitment substrates (basalt cubic blocks, roughly 10 cm on a side), both uncaged and caged to exclude predators (crabs, fishes, whelks, and octopi), were deployed along a decreasing vent fluid-flux gradient. The exclusion of predators for 8 mo increased the abundance of small mobile gastropods and amphipod crustaceans but decreased the abundance of sessile invertebrates, including juvenile vestimentiferan worms, tubiculous polychaetes, and mussels. Effects of predation were strongest nearest to hydrothermal vents, where abiotic environmental conditions were most extreme but productivity and the overall abundances of benthic invertebrates and mobile predators were the greatest. Additional 5-mo experiments conducted at three different locations showed similar trends at all sites, indicating that these effects of predation on benthic community structure are repeatable. Stomach-content analyses of the most abundant predators found at vents indicated that the zoarcid fish (Thermarces cerberus) primarily feeds on the vent snail Cyathermia naticoides, the limpet Lepetodrilus elevatus, and the amphipod crustacean Ventiella sulfuris, the very species that showed the greatest increase following predator exclusion. In contrast, brachyuran (Bythograea thermydron) and galatheid (Munidopsis subsquamosa) crab stomachs did not contain small mobile grazers, and crabs presented with arrays of the most common vent invertebrate species preferred mussels and vestimentiferans over limpets. Our results indicate that predation by large mobile predators influences the structure of hydrothermal vent communities, directly by reducing the abundance of gastropod prey species, and indirectly by reducing gastropod grazing and by bulldozing of recruits of sessile invertebrates.
Purse seining for tropical tuna is one of the most technologically advanced fisheries in the world. The purpose of this study was to apply Local Ecological Knowledge (LEK) to assist in the planning of future in situ studies of fish behaviour around Drifting Fish Aggregating Devices (DFADs) by prioritizing research topics, thereby reducing the number of potential hypotheses to explore. Interviews of fishing masters of the purse seine fleets working in the Western Indian Ocean (WIO) provided an alternate, independent and previously unexplored source of behavioural information: specifically on the attraction, retention and departure behaviours of tuna schools in relation to DFADs. Most fishing masters agreed that the maximum attraction distance of a DFAD is approximately 10 km and generally agreed to the following statements: Tuna form distinct schools under FADs, commonly segregated by species and size. The main reasons for the departure of tuna aggregations from FADs are changes in currents or FAD movements and location in relation to physical or oceanographic features. The number of actively monitored DFADs at sea in the WIO was estimated at approximately 2100 drifting objects. Incorporating fishermen into the planning and design stages of future research projects will facilitate collaborative and integrated approaches.
The structure and dynamics of natural communities result from the interplay of abiotic and biotic factors. We used manipulative field experiments to determine the relative roles of abiotic conditions and biotic interactions in structuring deep‐sea (2500 m depth) communities along environmental gradients around hydrothermal vents of the eastern tropical Pacific Ocean (East Pacific Rise, at 9°50′ N). We tested (1) whether predation by crabs and fishes affects the recruitment of benthic species and subsequent community structure and (2) whether the effects of predation vary along the steep gradients of temperature, oxygen, sulfide, and metal concentrations near vents. Recruitment substrates (basalt cubic blocks, roughly 10 cm on a side), both uncaged and caged to exclude predators (crabs, fishes, whelks, and octopi), were deployed along a decreasing vent fluid‐flux gradient. The exclusion of predators for 8 mo increased the abundance of small mobile gastropods and amphipod crustaceans but decreased the abundance of sessile invertebrates, including juvenile vestimentiferan worms, tubiculous polychaetes, and mussels. Effects of predation were strongest nearest to hydrothermal vents, where abiotic environmental conditions were most extreme but productivity and the overall abundances of benthic invertebrates and mobile predators were the greatest. Additional 5‐mo experiments conducted at three different locations showed similar trends at all sites, indicating that these effects of predation on benthic community structure are repeatable. Stomach‐content analyses of the most abundant predators found at vents indicated that the zoarcid fish (Thermarces cerberus) primarily feeds on the vent snail Cyathermia naticoides, the limpet Lepetodrilus elevatus, and the amphipod crustacean Ventiella sulfuris, the very species that showed the greatest increase following predator exclusion. In contrast, brachyuran (Bythograea thermydron) and galatheid (Munidopsis subsquamosa) crab stomachs did not contain small mobile grazers, and crabs presented with arrays of the most common vent invertebrate species preferred mussels and vestimentiferans over limpets. Our results indicate that predation by large mobile predators influences the structure of hydrothermal vent communities, directly by reducing the abundance of gastropod prey species, and indirectly by reducing gastropod grazing and by bulldozing of recruits of sessile invertebrates.
We adapted a visual census method, mainly used in demersal and reef fish studies, to characterize fish communities associated to drifting fish aggregating devices (FADs) in the Western Indian Ocean. Drifting FAD associated fishes from both equatorial (Seychelles) and tropical waters (Reunion Island) were examined by divers. A total of 32 species (belonging to 16 families) were observed associated with drifting FADs in equatorial waters, and 24 species (14 families) were found around FADs in tropical waters. Twenty species were found in both regions. The highest number of species observed at a single FAD was 18 (12 ± 2, mean ± SD) in equatorial and 13 (10 ± 3) in tropical waters, not counting circumnatant species loosely associated with the FAD. Some species like Kyphosus vaigiensis, Canthidermis maculata, Elagatis bipinnulata, Acanthocybium solandri and Coryphaena hippurus were observed on all or most of the surveys. In this study, the contribution in biomass of the 18 common species associated with drifting FADs (but excluding circumnatant species), represents more than 98% of the biomass. The overall biomass values of closely associated species remains well below tuna biomass estimates for circumnatant tuna schools at FADs, estimated as high as 200 tons. The species that most significantly contribute to the by-catch in tuna purse-seines logically match those that showing the highest biomass values in our surveys (Carcharhinus spp., Elagatis bipinnulata, Coryphaena hippurus, Canthidermis maculata, and Acanthocybium solandri). One of the most abundant and ubiquitous species in our study was the spotted oceanic triggerfish Canthidermis maculata that sometimes formed massive schools of many thousands individuals around the drifting FADs. Future research is needed to explore the role of such non tuna species in the attraction and aggregation processes of tuna around drifting FADs.
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