Somatic growth rates for a hawksbill turtle population in coral reef habitat around Barbados

Abstract

A generalized additive mixed modelling approach was used to investigate the somatic growth of hawksbill turtles Eretmochelys imbricata (23.7 to 80 cm curved carapace length [CCL]) on nearshore coral reef sites around Barbados at depths of 12 to 35 m. The effects of body size, sex, sampling year, recapture interval and an indicator of foraging habitat quality on growth rates were investigated. The model accounted for about 60% of the variance in growth rates, but only mean size and sampling year were significant predictors. The growth rate function was nonmonotonic, with peak growth (3.4 cm yr -1 ) occurring in turtles with a CCL between 30 and 35 cm. The lower growth rates recorded for smaller turtles may reflect a period when sea turtles newly recruiting from pelagic to neritic habitats are adapting to a change in diet. The decline in growth rates with sampling year over the 10 yr study may reflect density-dependent effects on growth. Declining growth rates may prompt large juveniles to leave the Barbados foraging ground to settle elsewhere to grow to maturity.KEY WORDS: Eretmochelys imbricata · Growth rate modelling · Barbados · Developmental habitat · Foraging ground · Density-dependent effect · Generalized additive mixed model Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 432: [269][270][271][272][273][274][275][276] 2011 Measurement of somatic growth rates of juvenile sea turtles is a prerequisite for assessment of life-stage durations and age to maturity, and has inevitably focused on the more accessible benthic feeding-phase size classes (but see Bjorndal et al. 2003). To date, modelling of growth has focused on loggerheads (e.g. Bjorndal & Bolten 1988a, Bjorndal et al. 1994, 2003, Chaloupka et al. 2004, Braun-McNeill et al. 2008) and green turtles (e.g. Bjorndal & Bolten 1988b, Limpus & Chaloupka 1997, Bjorndal et al. 2000, Balazs & Chaloupka 2004, Kubis et al. 2009). Growth rate data for juvenile hawksbills have been reported from the US Virgin Islands (Boulon 1994), the Dominican Republic (León & Diez 1999), Puerto Rico (Diez & van Dam 2002), the Bahamas (Bjorndal & Bolten 2010), and the Cayman Islands (Blumenthal et al. 2009a) in the Caribbean region, as well as from Australia (Chaloupka & Limpus 1997, Whiting & Guinea 1998 and Aldabra (Indian Ocean) (Mortimer et al. 2003). Interpretation of growth patterns in several of these studies has, however, been hampered by small sample sizes and short recapture intervals (i.e. <1 yr).A number of factors affect growth rates of juvenile green and loggerhead turtles. Variability has been attributed to habitat type (Balazs & Chaloupka 2004, Kubis et al. 2009, Bjorndal & Bolten 2010, population density (Bjorndal et al. 2000, Balazs & Chaloupka 2004, Kubis et al. 2009), and water temperature (Gibbons et al. 1981), as well as to stock-specific (Seminoff et al. 2002), age-specific (Bjorndal et al. 2003) and sexspecific factors (Chaloupka et al. 2004). León & Diez (1999) and Diez & van Dam (2002) foun...

Spatial and temporal variability of immature green turtle abundance and somatic growth in Puerto Rico

“…Mean SCL was calculated as the arithmetic mean between the SCLs at time of capture and subsequent recapture. This covariate describes the size-specific growth function well (Bjorndal et al 2000b, Krueger et al 2011, assuming a linear growth function through the time interval between release and re-encounter. Growth interval was included in the model to account for possible biases associated with variable sampling intervals, in particular with longer intervals, which often lead to deviations in the linear growth (Casale et al 2009).…”

“…Estimates of size-specific somatic growth rates of green turtles at the juvenile neritic phase allows assessment of the duration of this life stage, and hence contributes to the estimation of age at maturity (Balazs & Chaloupka 2004, Krueger et al 2011. Spatial and temporal variability in somatic growth dynamics, particularly when coupled with abundance estimates, make good indicators of the quality of foraging sites, as lower growth rates may imply less food availability (Balazs & Chaloupka 2004, Kubis et al 2009, and declines in somatic growth rates coupled with higher abundance suggest a density-dependent effect (Bjorndal et al 2000a, Kubis et al 2009).…”

Use of a towed camera system to investigate benthic habitat use by inter-nesting female hawksbill sea turtles

“…The study site encompassed the nearshore marine environment of the south coast of Barbados. Characterised by a narrow insular shelf, with the 200 m isobath lying 2 to 3 km offshore (Lewis & Oxenford 1996, Krueger et al 2011), the shelf consists of different forms of coral habitats including coral rubble, patch reefs and a bank reef most seaward, extending along much of the coastline. Patch reefs range in depth from 6 to 15 m, and the bank reef from 15 to 40 m. Depths reach as much as 40 to 55 m between the 2 reefs, and this deep habitat consists mainly of sand (Lewis & Oxenford 1996).…”

“…hard corals, gorgonians and sponges) being higher on the bank than on patch reefs (UWI 2008). Patch reefs also have lower structural complexity and are dominated by encrusting weedy Krueger et al 2011) and elsewhere (León & Bjorndal 2002).…”

“…those that had already nested in multiple nesting seasons) occupied internesting habitats farthest from the nesting beach (see Walcott et al 2012), suggesting that these areas may offer some particularly important characteristics that outweigh the energetic costs of travel to reach them. Competition may also come from the foraging population of juvenile hawksbills that resides year-round on the reef (Krueger et al 2011). However, the smaller size of these animals would likely increase the range of possible sites available to them for rest and refuge, thereby reducing competition between them and the larger adult females that only occupy Barbados waters during the breeding season.…”