Abundance and conservation status of green sea turtles

Literature review of the abundance and conservation status of green sea turtles, Chelonia mydas, in Bahia de los Angeles, Baja California, Mexico

By Greg Joder - http://www.Bajatrekker.com

Abstract: The green sea turtle, (Chelonia mydas) is listed as globally endangered in the World Conservation Union (IUCN) Red Data Book and is included in Appendix 1 of the Convention on International Trade in Endangered Species (CITES). Due to its highly migratory nature and varied habitat use during different life history stages threats to its continued existence include habitat destruction, poaching of eggs at nesting beaches, illegal capture of juveniles and adults for sale on the black market, and as bycatch in commercial and artisanal fisheries. Resource managers and local communities have an interest in reversing the population decline of the green sea turtle for many reasons: Ecological, economic, and aesthetic. In this paper I review the literature on the home ranges and feeding preferences of green sea turtles studied in Bahia de los Angeles, Baja California, Mexico, an important foraging ground for juvenile and adult green sea turtles and also an area high in illegal anthropogenic mortality. Conservation and management practices are also considered along with practical application of such strategies.

Introduction

Research and conservation of green sea turtles has increased worldwide, yet their foraging area demography is still poorly understood (Seminoff, Jones et al., 2003). Foraging areas, home ranges, food preferences, and growth rates of the green sea turtle (Chelonia mydas) were studied between 1995 and 2002 in Bahia de los Angeles, Baja California, Mexico (Seminoff, Jones et al., 2003; Seminoff et al., 2002a; Seminoff et al., 2002b; Seminoff et al., 2002). Green sea turtles are also known as black sea turtles (Chelonia mydas agassizi) in the eastern Pacific Ocean (Seminoff et al. 2003).

Bahia de los Angeles (BLA) is both a town and a bay on the eastern shores of the Baja California peninsula located approximately 400 miles south of the Mexico/U.S. border. The town of Bahia de los Angeles has a population of nearly 1000 and is supported by tourism, sportfishing, and artisanal fisheries. The bay it self is approximately 60 square kilometers in size and has 17 islands and islets. Marine algae beds and productive benthic communities are supported by strong tidal mixing and upwelling (Seminoff, Jones et al., 2003). Because of its neritic quality and abundance of food types BLA is an ideal foraging area for juvenile and adult green sea turtles.

Anthropogenic impacts on green sea turtles in BLA persist and capture-mark-recapture and radio and sonic telemetry studies by (Seminoff et al. 2002a; Seminoff, Jones et al. 2003) have provided survival estimates for green sea turtles in north-western Mexico. Understanding green sea turtle population dynamics is critical when considering effective management and conservation strategies.

In Baja California many fishing communities turn a blind eye to the illegal capture of all five species of sea turtles that live in Mexican waters, including the green turtle. To be effective management of green sea turtle populations must involve not only the scientists and federal agencies but also the communities in direct contact with green sea turtle populations. In this paper I will review the research to date, evaluate its strengths and weaknesses, and offer suggestions for future research and management strategies.

Discussion of past reseach

Survival, population structure, and human impacts on green sea turtles were studied for 8 years, June through August, in BLA (Seminoff, Jones et al. 2003). Data from these studies provided vital information for planning management policies for green sea turtle populations in Northwestern Mexico. Bahia de los Angeles provides an idea location for the study of juvenile and adult green sea turtles because of its neritic quality and abundance of food types. The research on green sea turtles in BLA focused on foraging areas, home ranges, food preferences, and growth rates (Seminoff, Jones et al., 2003; Seminoff, Resendiz, and Nichols, 2002; Seminoff, Resendiz, and Nichols, 2002; Seminoff, Resendiz, Nichols et al., 2002).

Throughout the study period, green sea turtles were captured live in entanglement nets, often using local fishermen as guides. All tracking and capture efforts utilized a 12’ inflatable with 25hp motor to minimize disturbance. Nets were set and monitored at .5 to 12 hour intervals both day and night in the neritic waters of BLA (Seminoff, Jones et al., 2003). After capture, an assessment of the turtle’s general health was taken by noting scars, missing flippers or other visible anomalies (Seminoff, Jones et al., 2003). All turtles were weighed, measured, and tagged at the research facility then released within 24 - 48 hours at the site of capture. Using forester’s calipers Straight Carapace Length (SCL) was measured from the posterior-most portion of the rear marginals to the nuchal notch and curved carapace length (CCL) was taken from the same locations with a soft tape (Seminoff, Jones et al. 2003). Turtles were placed in one of two groups delineated by mean nesting size (MNS) of females from the Michoacan rookery (Figueroa et al. 1993) which is considered the primary source populations for green sea turtles migrating to BLA (Nichols 2003).

The nature of threats to sea turtles allows mortality to be quantified by the use of several indices: Tag returns, discarded carcasses, and stranding records (Seminoff, Jones et al., 2003). Throughout the course of the studies reviewed herein monthly beach, dump, and fish camp surveys were made to search for discarded carapaces or stranded carcasses and information gathered followed the same procedure as with live captured turtles (Seminoff, Jones et al., 2003).

Tracking home ranges of green sea turtles in BLA

After capture all turtles were weighed, measured, and tagged at the Centro Regional de Investigacion Pesquera (CRIP) Sea Turtle Research Station in Bahía de los Angeles. Straight Carapace Length (SCL) was measured from the posterior most portion of the rear marginals to the nuchal notch (Seminoff et al. 2002a). An identifying tag was placed on the first large proximal scale on each rear flipper (Seminoff, Resendiz, and Nichols, 2002). Additionally, tail length was measured with a flexible tape measure from the trailing edge of the carapace to the tip of the tail (Seminoff, Resendiz, and Nichols 2002). Males were classified with a tail length greater than 20 cm and sex was classified as undetermined for turtles with non-elongated tails (Seminoff, Resendiz, and Nichols, 2002).

Seminoff, Resendiz, and Nichols (2002) used VHF radio and sonic transmitters to track 12 green sea turtles in an effort to determine home ranges in BLA. In a similar study Godley et al. (2003) tracked eight green turtles with the use of satellite transmitters in Brazilian coastal waters.
Defining the home range of green sea turtles in BLA required repeated sightings of individuals over a long period of time. Seminoff, Resendiz, and Nichols (2002) achieved this by attaching VHF radio transmitters and sonic transmitters to green turtles. The VHF transmitter was placed on the crown of the carapace attached with fiberglass and low heat resin while the sonic transmitter was placed on the posterior trailing edge of the carapace and held in place by two tie-wraps fit through holes drilled in the marginal scutes (Seminoff, Resendiz, and Nichols, 2002).

Tracked turtles were released within 24 hours of their capture at their initial tracking site and tracking commenced after a period of 24 hours to allow the turtles to acclimatize (Seminoff, Resendiz, and Nichols, 2002). VHF radio tracking was used to determine the general location/direction of the turtle. For the VHF signal to be readable the turtle had to be on the surface of the water. The sonic transmitter was used in tandem with a directional hydrophone when the turtle was below the surface or had lost its VHF tag. Points of tracked turtles within BLA were recorded with a global positioning system (GPS) where the turtle had surfaced (or visual estimate of the surface point) or by estimate of location by sonic signal strength if the turtle had not surfaced (Seminoff, Resendiz, and Nichols, 2002).

Diet of green sea turtles in BLA

Management of the green sea turtle populations requires a fundamental understanding of their diet (Seminoff, Resendiz, and Nichols, 2002). Foraging habitats of green sea turtles on the western coast of the Baja Peninsula have been shown to be spatially distinct and such habitats should be considered on a regional rather than local approach when designing protected areas for sea turtles (Milagros et. al. 2005). To better manage discrete populations of green sea turtles an understanding of the diet composition at various green sea turtle feeding grounds is necessary (Seminoff, Resendiz, and Nichols, 2002).

The study area for Seminoff, Resendiz, and Nichols (2002) was in BLA, Baja California Mexico. The bay is characterized by a multitude of islands, strong tidal mixing, high nutrient levels, and abundant neritic habitat (Seminoff, Resendiz, and Nichols, 2002). Marine algae are the dominant marine vegetation with extensive algae pastures in the western and southern-most neritic margins of the bay. These algal pastures are dominated by Gracilariopsis lemaneiformis, a macro alga (Seminoff, Resendiz, and Nichols, 2002).

As with the foraging, home range, and growth rate studies all turtles were weighed, measured, and tagged at the research facility. Using forester’s calipers SCL was measured from the posterior most portion of the rear marginals to the nuchal notch (Seminoff, Resendiz, and Nichols, 2002). An identifying tag was placed on the first large proximal scale on each rear flipper and the turtle released at the site of capture within 48 hours (Seminoff, Resendiz, and Nichols, 2002).

Marine algae were the primary forage for green turtles in BLA during the summer period (Seminoff, Resendiz, and Nichols, 2002). It remains unclear how changing sea surface temperatures and thus shifting algae assemblages change the dietary composition of green sea turtles in BLA (Seminoff, Resendiz, and Nichols, 2002). Through lavage and fecal samples taken in BLA Seminoff, Resendiz, and Nichols (2002) identified several genus of marine algae preferred by green turtles. These include Gracilariopsis, Codium, and Ulva. Seminoff, Resendiz, and Nichols (2002) determined that green sea turtles in BLA augment their diet with sponges, tube worms, sea pens, and sea hares. Also recovered in diet samples where plastic bags and nylon cord (Seminoff, Resendiz, and Nichols, 2002).

Green sea turtles consumed a larger diversity of herbivorous foods compared to turtles feeding in a nearby seagrass ecosystem suggesting that the diet of green turtles in Bahia de los Angeles differs from other Gulf of California populations (Seminoff, Resendiz, and Nichols 2002). Green sea turtles in BLA also supplemented their diet with animal matter. Seminoff, Resendiz, and Nichols (2002) documented over 25 non-alga species that were consumed by green sea turtles in BLA.

Growth rates of C. mydas in BLA

Understanding the demographics and life history characteristics of green sea turtles can also help wildlife managers develop management strategies that take into account age at a particular size, time to maturity, and geographic variability of distinct feeding areas (Seminoff, Resendiz, Nichols et al., 2002).

In an effort to better understand these aspects of green sea turtle life history traits Seminoff, Resendiz, Nichols et al. (2002) performed the first capture-mark-recapture research on green sea turtles in the Gulf of California. The research was conducted in Bahia de los Angeles, Baja California, Mexico, an ideal location for the study of free ranging marine reptiles.

As with the foraging, home range, and diet studies all turtles were weighed, measured, and tagged at the research facility. Using forester’s calipers Straight Carapace Length (SCL) was measured from the posterior most portion of the rear marginals to the nuchal notch (Seminoff, Resendiz, Nichols et al., 2002). An identifying tag was placed on the first large proximal scale on each rear flipper and the turtle released at the site of capture within 24 hours (Seminoff, Resendiz, Nichols et al., 2002). A profile of each turtles capture history was recorded. This information included date of capture, straight carapace length at each capture, time interval in months between the first capture or prior recapture, and the annual growth rate (Seminoff, Resendiz, Nichols et al., 2002).

The results of Seminoff, Resendiz, Nichols et al. (2002) showed recapture intervals that ranged from 11.0 to 50.2 months with a total of 195 turtles captured with 42 of those recaptured 54 times. SCL ranged from 58.6 to 93.8 cm with a mean of 75.6 cm and a recapture interval range of 11.0 to 50.2 months (Seminoff, Resendiz, Nichols et al. 2002). The range of growth rates was 0.2 cm/yr for turtle number BLA305 to 3.4cm/yr for a turtle number BLA020 at large for 50.2 months (Seminoff, Resendiz, Nichols et al., 2002).

A generalized additive modeling (GAM) approach was used to analyze the growth data. Generalized additive models are used to maximize the quality of prediction of a dependent variable Y from various distributions by estimating unspecific functions of the predictor variables which are "connected" to the dependent variable (See: Hastie and Tibshirani, 1990). The GAM accounted for 74% of the variance in size-specific growth suggesting the model was a good fit for the data (Seminoff, Resendiz, Nichols et al., 2002). Year and size of green sea turtles affected growth rates unlike recapture interval which had no affect on growth rates (Seminoff, Resendiz, Nichols et al., 2002). The size-specific growth rate represented by the data in this study is the first substantial data set for green turtles in Eastern Pacific Ocean temperate regions (Seminoff, Resendiz, Nichols et al., 2002).

Green sea turtles smaller than 55cm SCL are rarely found near shore in Bahia de los Angeles because of their epipelagic post hatchling phase. The SCL of green turtles studied in Seminoff, Resendiz, Nichols et al. (2002) ranged from 58.6 to 93.8 suggesting that the range comprises juvenile and adult green sea turtles.

In this study growth rates for most turtles were greater than mean nesting size (77.3cm at the primary nesting beach in Michoacan) which suggests the possibility that these turtles had not yet reached sexual maturity (Seminoff, Resendiz, Nichols et al., 2002). Other studies have speculated that different foraging habitats played a role in the differences in size at maturity (See: Carr and Goodman 1970). Seminoff, Resendiz, Nichols et al. (2002) suggest that green sea turtles in BLA may also mature at sizes above mean nesting size and, if so, begin breeding only after a long time period. Seminoff, Resendiz, Nichols et al. (2002) report that for green sea turtle starting at 46.6 cm in BLA would need 21.9 years to reach the mean nesting size of female green sea turtles in Michoacan. The time a juvenile sea turtle needs to spend in neritic habitat may be seen in the sustained growth rates and significant linearity of the size-specific growth function detailed in Seminoff, Resendiz, Nichols et al. (2002).

Discussion

The four studies reviewed here represent a tremendous step forward for the understanding of the life history characteristics of juvenile and adult green sea turtles in an important feeding ground in the eastern Pacific Ocean. Taken as a whole, the studies (Seminoff, Jones et al., 2003; Seminoff, Resendiz, Nichols, 2002; Seminoff, Resendiz, Nichols, 2002; Seminoff, Resendiz, Nichols et al., 2002) reveal previously unknown details about the diets, growth rates, home ranges, and estimates of population status within Bahia de los Angeles.

However, these studies also raise a number of questions as to the difficulties encountered with the studies and the need to follow through with further long-term research in BLA. For example, research is needed to better understand how size and age class effect home ranges of green sea turtles within BLA. In Seminoff, Resendiz, Nichols (2002) only summer home ranges were tracked. Further studies are needed in order to define seasonal changes in home ranges for both adults and juveniles and well as seasonal changes in available food types and preferences (Seminoff, Resendiz, Nichols, 2002) for adults and juvenile green sea turtles.

Problems associated with describing the population status of green sea turtles in BLA are confounded by the low population density and difficulty in accurately determining human caused mortality. Seminoff, Jones et al. (2003) depended on beach strandings and flipper tag recoveries directly from the research itself or from the local artisanal fishermen in order to quantify population status based on tag-recapture data. Seminoff, Jones et al. (2003) report that fishermen do not always return flipper tags of green sea turtles accidentally or intentionally captured.

The thoroughness of surveys for turtle strandings on beaches as well as surveys of dumps and fish camps is problematical due to the simple factors of chance or human causes. Turtle carapaces are easily hidden or destroyed and beach strandings are rare and random events.
Understanding dispersal behavior at the onset of sexual maturity and for ontogenic migrations of green sea turtles in BLA and adjacent feeding grounds could provide a deeper understanding of the population structure and status of green sea turtles in this region.

Understanding growth rates of wild green sea turtles is difficult for several reasons. Growth rate studies of wild populations involve having the ability to capture, mark, and recapture individuals within that population. Head-starting programs for sea turtles enable researchers to know the age of the turtles being raised because the turtles are raised from eggs to be released in the wild. For adult or juvenile turtles captured in the wild there is no reliable way to know their age and therefore, in the case of Seminoff, Resendiz, Nichols et al. (2002), were unable to distinguish the effects of environmental variation and age effects on growth rates.

Factors that contribute to the general tendency of green sea turtles display both monotonic and nonmonotonic growth characteristics are not well understood (Seminoff, Resendiz, Nichols et al., 2002). Further studies that distinguish between intrinsic and environmental effects on green sea turtle growth rates are needed.

Conclusion

Sea turtle research often has a secondary purpose that can be applied beyond the study itself. Home ranges, growth rates, diet preferences, and population estimates can be interesting information in and of it self. However, it could be argued that most research into the life histories of sea turtles is driven primarily by the concern for the conservation of the turtles and their habitat.

The research reviewed here was supported by both conservation oriented agencies and resource management agencies. Specifically, by the Center for Field Research at Earthwatch Institute, the Wallace Research Foundation, the PADI Foundation, and the Secretaría del Medio Ambiente y Recursos Naturales y Pesca (The Secretariat of Environment and Natural Resources and Fisheries, Mexico).

Frazer (1991) suggests that the use of ‘halfway technologies,’ such as head start programs, only serve to put more turtles into degraded or dangerous environments. In other words, the research and field work that puts hundreds of thousands of baby sea turtles back into the sea does nothing to address the actual reason the numbers of sea turtles are declining. Head starting programs may help as a short term solution but cannot ensure the long term survival of sea turtles (Frazer 1991).

Balazs and Chaloupka (2003) recognize that capture-mark-recapture studies in foraging grounds are the preferred method for gathering age class specific demographic data for sea turtles but acknowledge that, given the size and mobility of sea turtles this is a difficult thing to achieve. However, these types of studies are important in order to gain understanding of the recovery process of long-lived organisms such as the green sea turtle.

Understanding the movements, diet preferences, and age classes of the green sea turtles of BLA described by the studies reviewed here is critical for understanding the threats they face. Future studies should build upon the research conducted thus far in order to further inform resource managers and local communities in their quest to protect the endangered population of green sea turtles.

Literature Cited

Balazs G.H., Chaloupka M. 2004. Thirty-year recovery trend in the once depleted
Hawaiian green sea turtle stock. Biological Conservation 117, pp. 491-498.

Campell C.L., Lagueux C.J. 2005. Survival probability estimates for large juvenile and
adult green turtles (Chelonia mydas) exposed to an artisanal marine turtle fishery
in the western Caribbean. Herpetologica 61, pp. 91-103.

Chaloupka M, Parker D, Balazs G. 2004. Modelling post-release mortality of
loggerhead sea turtles exposed to the Hawaii-based pelagic longline fishery.
Marine Ecology-Progress Series, pp. 285-293.

Craig P, Parker D, Brainard R, et al. 2004. Migrations of green turtles in the central South
Pacific. Biological Conservation 116, pp. 433-438.

FRAZER N.B. 1992. Sea-Turtle Conservation and Halfway Technology. Conservation
Biology, pp. 179-184.

Godley B.J., Lima E.H.S.M., Akesson S, et al. 2003. Movement patterns of green turtles
in Brazilian coastal waters described by satellite tracking and flipper tagging.
Marine Ecology-Progress Series, pp. 279-288.

Hays G.C. 2004. Good news for sea turtles. Trends in Ecology & Evolution 19, pp.
349-351.

Lopez-Mendilaharsu M, Gardner S.C., Seminoff J.A., et al. 2005. Identifying critical
foraging habitats of the green turtle (Chelonia mydas) along the Pacific coast of
the Baja California peninsula, Mexico. Aquatic Conservation-Marine and
Freshwater Ecosystems, pp. 259-269.

Seminoff J.A., Jones T.T., Resendiz A, et al. 2003. Monitoring green turtles (Chelonia
mydas) at a coastal foraging area in Baja California, Mexico: multiple indices to
describe population status. Journal of the Marine Biological Association of the
United Kingdom, pp. 1355-1362.

Seminoff J.A., Resendiz A, Nichols W.J. 2002a. Home range of green turtles Chelonia
mydas at a coastal foraging area in the Gulf of California, Mexico. Marine
Ecology-Progress Series, pp. 253-265.

Seminoff J.A., Resendiz A, Nichols W.J. 2002b. Diet of East Pacific green turtles
(Chelonia mydas) in the central Gulf of California, Mexico. Journal of
Herpetology, pp. 447-453.

Seminoff J.A., Resendiz A, Nichols W.J., et al. 2002. Growth rates of wild green turtles
(Chelonia mydas) at a temperate foraging area in the Gulf of California, Mexico.
Copeia, pp. 610-617.

Troeng S, Rankin E. 2005. Long-term conservation efforts contribute to positive green
turtle Chelonia mydas nesting trend at Tortuguero, Costa Rica. Biological
Conservation, pp. 111-116.

Photo copyright: greg joder

[Edited on 10-1-2007 by gringorio]

gringorio Thanks for the information.