Artificial Lighting and the Recovery of Sea Turtles

Michael Salmon, Blair E. Witherington, Christopher D. Elvidge

Abstract

Florida serves as an important developmental habitat for sea turtles, and hosts the second largest loggerhead rookery in the world. This paper reviews the historical decline of sea turtles in Florida, U.S.A., and the recent evidence that suggests a modest recovery. ASEAN countries and Florida have in common that they are experiencing rapid coastal development. In Florida artificial lighting from coastal communities is an important source of hatchling mortality, and probably constricts the location of major nesting sites to the few remaining dark beaches. ASEAN countries can plan now to avoid these problems by developing national light management plans. Central to such a plan is protection of the present major rookeries ('core areas'). At these sites, lighting should be entirely excluded not only from the beach, but also from areas behind the beach in the form of a buffer (no development) zone. Such areas should be established as preserves, perhaps supported by ecotourism. The facility at the Malaysian Turtk Islands Park in Sabah can serve as a good model. Smaller rookeries located near existing coastal communities can be protected by well-established light management techniques. By planning now, ASEAN countries can avoid Florida's mistakes, which necessitate expensive and labour-intensive efforts to protect rookeries. They will also be in a better position to both protect and sustain their sea turtle resources for future generations.

Introduction

 ASEAN continental and island nations are custodians of some of the richest marine and terrestrial biological communities known to mankind. In many instances they also must exploit those same resources to provide the material wealth necessary to improve the lives of their people. The problem, as always, is to determine what levels of exploitation can be sustained while still preserving each country's biological heritage. Increasingly these days, ecologists and conservation biologists tell us that exponential human population growth must be curbed; that the habitat modifications we impose must cease; and that current levels of ecosystem exploitation cannot be sustained. These are not uniquely ASEAN problems, but worldwide problems that will require difficult but long overdue political, social, and economic decisions.

In this chapter, we focus upon one serious threat (artificial lighting) that relates to marine turtle conservation. In the USA, coastal lighting is responsible for the death of thousands of hatchlings annually. It also results in reducing the area of shoreline available for nesting adults. Taken together, coastal lighting in the USA is a major threat to the recovery of sea turtle populations. We realise that in other countries, such as those in Southeast Asia, lighting may rank as a minor threat. Indeed, it is hoped that is the case! However, because most future development is likely to take place on the coastlines in these countries, artificial lighting is a serious potential threat to their sea turtle populations. Our goal here is to trace the development of the problem in Florida, and to reveal parallels between what happened and what might occur in other countries. We then suggest a course of action to reduce the impact of coastal lighting upon turtle populations before it becomes a major problem.

That course of action is the development of a national light management plan. Such a plan will, by its very nature, have many other benefits for sea turtles. Keeping rookery beaches dark is an essential element for enhancing sea turtle survival and reproductive success. Because sea turtles tend to nest in isolated locations, national plans can be developed without seriously compromising the growth of coastal communities in presently occupied areas. What is required is the foresight to set aside major rookeries as protected (permanently dark) areas, and to improve conditions at minor rookeries through established management techniques.

When a national light management plan is combined with other efforts (monitoring of nesting and nesting success at the beach; protection of nesting females and their nests), it then might become possible to permit limited exploitation (such as traditional harvesting of eggs) without unduly harming sea turtle populations.

Decline of Sea Turtles in Florida

The historical data provide no reliable estimates of sea (green) turtle abundance in the Caribbean or in Florida waters prior to European colonisation. Jackson (1997) cites two sources of general information. One is the early annual take from the Grand Cayman nesting colony. The numbers indicate the existence of a population of 6.5 million green turtles (Chelonia mydas) at that rookery alone. Assuming comparable numbers of turtles nested at the five other known historical rookeries (including Florida), the total estimate of green turtle abundance before the arrival of Europeans is 33-39 million. Another estimate, again for green turtles, is based upon carrying capacity of green turtle 'pastures' (areas of seagrass beds). These numbers yield an estimate about 20 times higher than the previous estimated from fisheries. Regardless of which estimate is closer to reality, the numbers involved suggest that present day populations of nesting turtles represent but a fraction of the total that once existed.

We also know where green turtles in the Caribbean used to nest (in Bermuda, Grand Cayman Island, Dry Tortugas, Dominican Republic, Costa Rica and Aves Island; sources: King 1982, Jackson 1997). We know where fisheries (at Bermuda, Cayman Islands, and the Florida Keys) exterminated entire rookery populations. By the end of the 18th Century, the Cayman Island fishery collapsed; by the end of the 19th Century, the Florida (Key West) green turtle fishery was in rapid decline though other fisheries in the state persisted through the early 1900's (Fig. 1). Declines initially were in green turtle populations but as these dwindled, fishers began taking loggerheads and hawksbills.

 
Fig. 1: Abundance of sea turtles in Florida was historically great (left), but populations were depleted by overfishing during the 17th and 18th centuries. Currently, numbers of loggerheads are increasing but it is likely that all sea turtle populations are well below historical levels.

A Modest Recovery in Florida

The primary sea turtle nesting sites in Florida before the arrival of Europeans are unknown. Several early European naturalists (Catesby 1731, Audubon 1826, True 1884, Murphy 1890) described abundant nesting of green turtles and loggerheads in South Florida, and on both the East and West coast of the state.

The Florida Marine Research Institute began in 1979 to monitor sea turtle nesting in the state. As the program continued, more beaches were included in the survey and specific methodologies evolved (Meylan et al. 1995). The numbers of nests for three species (loggerheads, green turtles and leatherbacks) increased annually, reflecting the addition of surveyed area as well as the refinement of monitoring techniques, but not necessarily an increase in the number of nesting turtles. A special program within the statewide survey, the Index Nesting Beach Survey (IINBS), began in 1989. Participants in this program monitored the same 27 beach sites, using identical methods. These data give more reliable estimates of any change in the number of nests. Unfortunately, the INBS is seasonally limited to the nesting season of loggerheads, and thus fails to include all nesting by green turtles (later in the season) and leatherbacks (earlier in the season).

Both monitoring programs have shown that Florida's Southeast Coast is where most nesting occurs (>90 %) (Conley and Hoffman 1986, Meylan et al. 1995). The INBS data suggest that nesting by loggerheads is gradually increasing. Reports from the state's largest rookery (Melbourne Beach) indicate that numbers of nests by green turtles and leatherbacks are also on the rise. Green turtles nesting in Florida are genetically distinct from those nesting in Tortuguero, C.R. (Allard et al. 1994), and may represent the remnants of the Florida rookery population. Florida leatherbacks may also be genetically distinct from other nesting populations. Present numbers for all species probably are well below their presumed historical levels. We know this is the case for green turtles.

Present Rookery Sites and Artificial Lighting

The present major rookeries on Florida's East Coast consist of five beach areas (Fig. 2). Nest densities at four of these usually exceed 500 nests/km for loggerheads, at Canaveral, they average about half that number. A minor rookery at a city beach (Boca Raton) contains the highest nest densities found at any urban beach in Florida.

The following points are worth emphasis. (i) All the major rookeries are located to the North of major cities (West Palm Beach, Fort Lauderdale, and Miami) and their high levels of artificial lighting (see Fig. 2). (ii) All are located on dark beaches (iii) No major rookeries are located North of Canaveral, even though beaches there are also dark. Finally, (iv) green turtles and leatherbacks, while nesting in lower numbers, prefer the same rookery sites as loggerheads. These observations suggest that a common feature(s) attracts all nesting turtles to these rookeries.

Fig 2: Satellite image of Florida (left), with red indicating regions of brightest artificial lighting (Cities of West Palm Beach, Fort Lauderdale and Miami). Right graphs show distribution of one minor (Boca Raton) and five major rookeries on the East Coast (arrows). All major rookeries are North of West Palm Beach. See text for further discussion.

Discussion and Conclusions

Impact ofArtificial Lighting - There is abundant evidence (summarised recently by Witherington & Martin 1996, Witherington 1997) that artificial lighting has a severe impact upon sea turtles. There are two primary effects. Nesting beaches that are otherwise attractive to turtles but which are exposed to artificial lighting receive few nests because the lighting repels females. The site is therefore degraded in quality. Hatchlings that emerge from the few clutches that get laid on illuminated beaches may then be unable to orient toward the ocean. Many crawl on circuitous paths while others, especially if exposed to a single bright light, crawl toward the light itself (away from the ocean) where they eventually die from exposure or are taken by predators. Thus artificial lighting leads to the decline of sea turtle rookeries in two ways: immediately, by inducing many females to nest elsewhere, and over long time periods by selecting against the offspring of females that nest at light-exposed sites.

The satellite data certainly suggest that most breeding females avoid nesting beaches near the most southern metropolitan areas. There are some exceptions. In some locations between West Palm Beach and Fort Lauderdale, local patches of dark beach receive more nests than adjacent areas exposed to more intense illumination. The beach at Boca Raton is a good example. There, public (beach-site) parks and tall condominiums (Fig. 3) act as light barriers, shadowing the beach from city lighting (Salmon et al. 1995). At condominium beach sites, most nests were placed directly in front of the buildings (in the shadow), and more nests were placed in front of the tallest buildings that cast the largest shadows (Fig. 4; Salmon et al. 1995). Variation in nesting density at this urban beach was strongly correlated with variation in 'shadowing', but not with other beach attributes (such as elevation, width, or underwater bottom profile in front of the beach).

While the presence of metropolitan lighting may limit the distribution of major rookeries South of West Palm Beach, it does not explain their absence North of Canaveral where dark beaches are abundant (Fig. 2). Collard and Ogren (1990) hypothesised that sea turtle nesting beaches should be located adjacent to oceanic currents that transport hatchlings to 'nursery' areas. The Gulf Stream current en- trains hatching loggerheads into the North Atlantic Gyre (their nursery habitat). Its presence near the coast of Florida probably explains why over 90 % of all nesting by this species occurs on the East coast. It may also indicate why nesting is most prevalent on the southern portion of that coast, where the Gulf Stream is closest to shore (Fig. 5). North of West Palm Beach, the coastline of Florida is oriented NNW while the Gulf Stream flows almost due N. Thus the northern limit of the major rookeries may reflect an increase in the mortality of hatchlings that must swim greater distances to their migratory goal.

Fig 3: Condominiums behind the beach at Boca Raton. At night these act as physical barriers to the transmission of artificial lighting from the city, located to the right.

If this hypothesis is correct, then historically the major nesting beaches in Florida were located where city lighting now repels most turtles (Fig. 5). These speculations suggest that the present location of major nesting beaches farther to the North (Fig. 2) may well be a consequence of coastal development.

Lessons from Florida for ASEAN Nations - In both Florida and in many ASEAN countries, efforts are underway to protect nesting turtles, their nests, and their hatchlings. But in Florida unplanned development of coastal areas continued unabated for many years before it was realised that Florida's beaches were major sea turtle rookeries. The present rookeries may be located too far North, with consequences that are difficult to predict. Many nests on Florida beaches must be relocated to dark sites, a time-consuming and expensive procedure that will by necessity continue well into the future. Thus, considered as a whole, management efforts in Florida are reactive rather than proactive.

Light management plans can be more successfully implemented if they are instituted before development occurs, and at sites where human coastal population densities are low. Protecting rookeries in Florida has met with partial, but not complete, success because none are bordered by a buffer zone. Thus, development behind the beach remains a threat. Continued protection requires constant vigilance to make sure that hatchlings orient normally, that new construction does not expose the area to additional lighting threats, and that existing communities comply with existing lighting regulations.

 
Fig 4. Elevation of the condominiums (top) in degrees above the beach, as measured from where turtles place their nests. Below, view of the same buildings and the beach from above, showing the distribution of loggerhead nests (dots). Most nests are placed directly in front of the buildings because these structures darken the beach by blocking city lighting

ASEAN nations could minimise these problems! A general framework applicable to any natural resource was proposed by Mast (1999) based upon five objectives: (1) possess a clear mission; (2) establish priorities; (3) know the animals; (4) understand the threats, and (5) protect the core (the major rookeries). The lesson from Florida is that we failed to appreciate these points until it was almost too late!

 
Fig. 5: Location of the Gulf Stream current on Florida s East coast. Note that the current is adjacent to the shore at the southern portion of the coastline, but passes progressively farther from the beach at more northern locations. Solid bar shoes the present location of the major rookeries; dashed bar shoes the presumed historical location. See text for details.

ASEAN nations can learn from our mistakes by acting now. For example locations where major rookeries exist can be established as national parks, set aside as facilities for training a new generation of scientists and for ecotourism that can support the site for perpetuity (such a facility already exists in Malaysia's Turtle Islands Park in Sabah, and could serve as a model). At areas where small communities already exist near minor rookeries, light management can actually reduce energy costs to local residents, without compromising either their livelihood or their natural resources. And last but not least, the biological and cultural heritage of each nation can be maintained, much to the envy of others who have failed to think (or plan) ahead.

Acknowledgements

M. Salmon thanks his co-authors for their insight and expertise, and Florida Atlantic University (Division of Sponsored Research) for a travel grant.

References

Allard, M. W., M.M. Miyamoto, K.A. Bjorndal, A.B. Bolten & B. Bowen, 1994. Support for natal homing in green turtles from mitochondrial DNA sequences. Copeia 1994(1): 34-41.

Audobon, J.J., 1926. The turtlers. Pp. In: Delineations of American scenery and character (H. Hobart, ed). GA. Baker and Co., New York: 194-202,

Catesby, M., 1731-43. The natural history of Carolina, Florida, and the Bahama Islands. (2 volumes) London.

Collard, S.B. & L.H. Ogren, 1990. Dispersal scenarios for pelagic post-hatchling sea turtles. Bulletin of Marine Science 47: 233-243.

Conley, W.J. & B.A. Hoffman, 1986. Florida sea turtle nesting activity: 1979-1985. Report to the permit holders. Unpublished report of the Florida Department of Natural Resources: 52pp.

Jackson, J.B.C., 1997. Reefs since Columbus. Coral Reefs 16: S23-S32.

King, F.W., 1982. Historical review of the decline of the green turtle and the hawksbill. In: Biology and Conservation of Sea Turtles (K. Bjorndal, ed). Smithsonian Institution Press, Washington, D.C.: 183-188.

Mast, R., 1999. Guest editorial: Common sense conservation. Marine Turtle Newsletter 83: 3-7.

Meyland, A., B. Schroeder & A. Mosier, 1995. Sea turtle nesting activity in the State of Florida 1979-1992. Florida Marine Research Publication 52pp.

Murphy, J.M., 1890. Turtling in Florida. Outing Magazine, November issue. Outing Publishing Co., Deposit, New Jersey, U.S.A.: 97-103

Salmon, M., R. Reiners, C. Lavin & J. Wyneken, 1995. Behaviour of loggerhead sea turtles on an urban beach. I. Correlates of nest placement. Journal of Herpetology 29: 560-567.

True, F.W., 1884. The fisheries and fishery industries of the United States, Sec. I, Pt. 2. The useful aquatic reptiles and batrachians. U. S. Commercial Fish Fisheries, 1893.

Witherington, BA., 1997. The problem of photopollution for sea turtles and other nocturnal animals. In: Behavioural Approaches to Conservation in the Wild (Clemmons, J.R. & R. Buchholz, eds.). Cambridge University Press, New York & London: 303-328.

Witherington, B.A. & R.E. Martin, 1996. Understanding, assessing, and resolving light- pollution problems on sea turtle nesting beaches. Florida Department of Environmental Protection Technical Report TR-2.