Spatial Distribution and Residency of Green and Loggerhead Sea Turtles Using Coastal Reef Habitats in Southern Mozambique

Abstract

Sea turtles spend the majority of their immature and adult lives in foraging grounds, yet few studies have examined their abundance and condition in these areas when compared to more accessible nesting beach habitats. Here, a 5-year dive log, photo-identification (photo-ID) and surface encounter datasets were used to investigate the abundance, individual movements and distribution of sea turtles along 40 km of coastal reefs in southern Mozambique. A generalized linear model (GLM) was constructed with turtle sightings as the response variable. Habitat type, year and day of the year, as well as underwater visibility, were significant predictors of turtle sightings. However, only 8% of the total variance was explained by the model, indicating that other variables have a significant influence on turtle movement and distribution. Photo-ID differentiated 22 individual green turtles Chelonia mydas and 42 loggerhead turtles Caretta caretta from 323 photo-ID encounters. A majority (64%) of the photos could be used to identify the individual. Although residency times of up to 1152 days were calculated for juvenile green turtles, a low overall resighting rate indicates that individual turtles either had large home ranges or were transient to the area. Surface encounter data revealed a preference for nearshore shallow waters and an increased abundance close to reef systems. Sea turtles' preferences for shallow, nearshore habitats are likely to increase the encounter risk with opportunistic and targeted artisanal fishers who catch sea turtles.

In-water methods reveal population dynamics of a green turtle Chelonia mydas foraging aggregation in the Philippines

“…Standora et al 1984). Perception and availability bias may also affect detectability of turtles as highlighted by Williams et al (2017). Future work should standardise data collection protocols, including visibility (to ensure unbiased detection of turtles in-water) and in situ temperature logging to fully elucidate the daily activity of turtles at the study site if accurate estimates of abundance are needed.…”

Modeling the emergence of migratory corridors and foraging hot spots of the green sea turtle

“…Other minor nesting sites are located on continental islands and shores on the coast of Madagascar and East Africa (Bourjea, Ciccione, & Ratsimbazafy, ; Garnier et al, ). Seagrass beds, the main component of adult green turtle diet (Bjorndal, ), extend almost continuously over the east African coast from Mozambique to Somalia and over the western coast of Madagascar (Figure ; Gullström et al, ), and foraging green turtles are observed in all countries of the SWIO hosting seagrass beds (Ballorain et al, ; Fulanda et al, ; Muir, ; Okemwa, Nzuki, & Mueni, ; Williams, Pierce, Rohner, Fuentes, & Hamann, ). A tracking study (Dalleau, ) demonstrated that (a) the northern part of the Mozambique Channel is a major oceanic migration corridor for postnesting green turtles capable to migrate thousands of kilometers, (b) coastal grounds of East Africa and West Madagascar are important foraging sites and migration corridors, (c) turtles from the SWIO nesting sites make extensive use of available foraging habitats of the whole region, and (d) foraging grounds are used by turtles originating from different rookeries of the SWIO.…”

“…Other minor nesting sites are located on continental islands and shores on the coast of Madagascar and East Africa (Bourjea, Ciccione, & Ratsimbazafy, 2006;Garnier et al, 2012). Seagrass beds, the main component of adult green turtle diet (Bjorndal, 1997), extend almost continuously over the east African coast from Mozambique to Somalia and over the western coast of Madagascar (Figure 2; Gullström et al, 2002), and foraging green turtles are observed in all countries of the SWIO hosting seagrass beds (Ballorain et al, 2010;Fulanda et al, 2007;Muir, 2005;Okemwa, Nzuki, & Mueni, 2004;Williams, Pierce, Rohner, Fuentes, & Hamann, 2017). A tracking study (Dalleau, 2013) Threats are highly variable in the SWIO region, which is bordered by countries and provinces of heterogeneous economic levels.…”

Untitled

“…sites!can!be!influenced!by!underwater!visibility! (Williams!et!al.,!2017),!so!observers!estimated! horizontal!visibility!on!each!dive.!Water!temperature!can!influence!mobulid!sightings!(Jaine!…”

“…need!to!swim!to!the!surface!to!breathe! (Williams!et!al.,!2017).!Manta!rays,!in!contrast,!are! generally!…”