Movement and predation: a catch-and-release study on the acoustic tracking of bonefish in the Indian Ocean

Tourism generated through bonefish (Albula spp.) fishing contributes to the economies of many isolated tropical islands and atolls. However, little research has been conducted on bonefish in the Indian Ocean. This study aimed to contribute to the understanding of bonefish ecology in the Indian Ocean by quantifying the spatial and temporal movements of Albula glossodonta at a near-pristine and predator-rich atoll in the Seychelles; however, to achieve this, an analysis to identify the occurrence of possible post-release predation bias was first necessary. An acoustic telemetry study was initiated at the remote St. Joseph Atoll, within an array of 88 automated data-logging acoustic receivers. Thirty bonefish were surgically implanted with Vemco V13 acoustic transmitters and tracked for one year. Only 10% of the tagged bonefish were detected for longer than two weeks. A comparison of the final 100 h of movement data from fish detected for less than two weeks to the movement data of the fish detected for longer periods revealed distinct differences in area use and significant differences in the average daily distance moved, speed of movement and frequency of detections. This suggested that mortality in the form of post-release predation was at least 43% of tagged fish. The three surviving bonefish were tracked for 210 to 367 days. These individuals remained in the atoll and showed high use of the marginal habitats between the shallow sand flats and the lagoon. A generalised linear mixed model identified that water temperature, diel cycle and tide were significant predictors of bonefish presence in the lagoon. The high post-release mortality highlights that catch-and-release is likely not as benign as previously believed and management and policy should be adjusted accordingly

Stingray Habitat Use Is Dynamically Influenced by Temperature and Tides

Abiotic factors often have a large influence on the habitat use of animals in shallow marine environments. Specifically, tides may alter the physical and biological characteristics of an ecosystem while changes in temperature can cause ectothermic species to behaviorally thermoregulate. Understanding the contextual and relative influences of these abiotic factors is important in prioritizing management plans, particularly for vulnerable faunal groups like stingrays. Passive acoustic telemetry was used to track the movements of 60 stingrays at a remote and environmentally heterogeneous atoll in Seychelles. This was to determine if habitat use varied over daily, diel and tidal cycles and to investigate the environmental drivers behind these potential temporal patterns. Individuals were detected in the atoll year-round, but the extent of their movement and use of multiple habitats increased in the warmer NW-monsoon season. Habitat use varied over the diel cycle, but was inconsistent between individuals. Temperature was also found to influence stingray movements, with individuals preferring the deeper and more thermally stable lagoon habitat when extreme (hot or cold) temperature events were observed on the flats. Habitat use also varied over the tidal cycle with stingrays spending a higher proportion of time in the lagoon during the lowest tides, when movement on the flats were constrained due to shallow waters. The interplay of tides and temperature, and how these varied across diel and daily scales, dynamically influenced stingray habitat use consistently between three species in an offshore atoll.</jats:p

Movement and predation: a catch-and-release study on the acoustic tracking of bonefish in the Indian Ocean

Tourism generated through bonefish (Albula spp.) fishing contributes to the economies of many isolated tropical islands and atolls. However, little research has been conducted on bonefish in the Indian Ocean. This study aimed to contribute to the understanding of bonefish ecology in the Indian Ocean by quantifying the spatial and temporal movements of Albula glossodonta at a near-pristine and predator-rich atoll in the Seychelles; however, to achieve this, an analysis to identify the occurrence of possible post-release predation bias was first necessary. An acoustic telemetry study was initiated at the remote St. Joseph Atoll, within an array of 88 automated data-logging acoustic receivers. Thirty bonefish were surgically implanted with Vemco V13 acoustic transmitters and tracked for one year. Only 10% of the tagged bonefish were detected for longer than two weeks. A comparison of the final 100 h of movement data from fish detected for less than two weeks to the movement data of the fish detected for longer periods revealed distinct differences in area use and significant differences in the average daily distance moved, speed of movement and frequency of detections. This suggested that mortality in the form of post-release predation was at least 43% of tagged fish. The three surviving bonefish were tracked for 210 to 367 days. These individuals remained in the atoll and showed high use of the marginal habitats between the shallow sand flats and the lagoon. A generalised linear mixed model identified that water temperature, diel cycle and tide were significant predictors of bonefish presence in the lagoon. The high post-release mortality highlights that catch-and-release is likely not as benign as previously believed and management and policy should be adjusted accordingly

Dietary niche differentiation in a mesopredatory dasyatid assemblage

Most batoids are mesopredators and are often hypothesised to play important ecological roles. However, a comprehensive understanding into these roles remains limited given the paucity of information of their trophic habits. St. Joseph Atoll (5.43∘S, 53.35∘E) is a remote ecosystem that hosts a resident assemblage of dasyatids (Pastinachus ater, Urogymnus granulatus, and U. asperrimus). Both stomach contents (SC) and stable isotope samples (SI) (δ15N and δ13C) were collected in 2015 and used to investigate the trophic niches of this dasyatid assemblage and whether these niches differed inter- and intra-specifically. Dasyatid muscle samples as well as baseline samples of potential prey species and primary producers were collected for SI analyses. SC data highlighted significant inter-specific differences in diet, U. granulatus juveniles mostly consumed decapod crustaceans (Callianassidae and Portunidae) and P. ater juveniles mostly consumed bivalve molluscs. The mean species trophic positions for juveniles of all three species ranged from 2.9 to 3.6 when calculated using δ15N and from 3.4 to 3.6 when calculated using stomach content data. Analysis of δ13C showed that all juveniles were reliant upon the same carbon resources (primarily derived from seagrass beds) at the base of the food web. Diet appeared to change with size as larger individuals displayed lower δ13C and higher δ15N compared to smaller juveniles. Additionally, isotope values of the smallest individuals were similar to the largest individuals, suggesting maternal meddling. The identified patterns of inter- and intra-specific trophic niche differentiation may be indicative of competitive effects and contributes to the understanding of mesopredators in community trophic dynamics

At the turn of the tide: Space use and habitat partitioning in two sympatric shark species is driven by tidal phase

Coexistence of ecologically similar species occupying the same geographic location (sympatry) poses questions regarding how their populations persist without leading to competitive exclusion. There is increasing evidence to show that micro-variations in habitat use may promote coexistence through minimizing direct competition for space and resources. We used two sympatric marine predators that show high fidelity to a small, remote coral atoll as a model to investigate how temporally dynamic partitioning of space use may promote coexistence. Using novel methods (difference network analysis and dynamic space occupancy analysis), we revealed that even though blacktip reef sharks Carcharhinus melanopterus and sicklefin lemon sharks Negaprion acutidens both show focused use of the same atoll habitats, the spatio-temporal dynamics of their use was partitioned such that they only shared the same microhabitats 26% of the time. Moreover, the degree of overlap was strongly influenced by the tidal cycle, peaking at ∼35% at higher tides as both species appear to target similar intertidal micro-habitats despite the increase in available space. Our work provides a rare example of how two marine predators with similar ecological roles and habitat preferences may coexist in the same place through dynamic segregation of habitat use in space and time, potentially reflecting adaptive behavioral traits for minimizing interactions. The strong influence of small tidal variation on species habitat use and partitioning also raises concerns over how atoll ecosystem dynamics may be influenced by sea level rises that could alter tidal dynamics