Why Haven’t Florida’s Adult Loggerheads Recovered?

Insights from Keewaydin Island, I.
Why Haven’t Florida’s Adult Loggerheads Recovered?

Mike Salmon, Research Professor
Biological Sciences, Florida Atlantic University

We know from historical records, some from the ship logs of the early Caribbean explorers only 500 years ago, that marine turtles existed in numbers 1000’s or more above those seen today. We also know that we are responsible for their decline by both direct exploitation (e.g., killing the animals for meat; consuming their eggs) and indirectly (by altering, polluting or destroying essential habitats such as their foraging areas and nesting beaches). We’ve also had a negative impact through accidental injury (via an increase in boat collisions and propeller injuries; entanglement and/or capture in fishing longlines, drowning in trawls or nets), and most recently by altering climate. Concerted efforts in recent times by managers and conservationists have reduced the severity of some of these impacts, in some world locations, but not in others. And, as one might expect, marine turtle populations in some areas are recovering but in others are either remaining stable at abnormally low numbers or continuing to decline.

Clearly, if our efforts to promote the recovery of these magnificent animals are to succeed, we need to know why these conservation outcomes are so mixed. Our area (North Atlantic and Caribbean) is representative. The evidence indicates that green turtle numbers are rising dramatically while leatherback populations are stable and hawksbills show a slow decline. For those of us working in Florida, however, a major concern is the status of the loggerhead population. That’s because our beaches serve as the world’s largest nesting site for this species and, accordingly, a considerable amount of effort has been devoted by state government workers and other conservation agencies in Florida to protect these animals, both on the beach and in the coastal water habitats essential for food and shelter. Yet, in spite of decades of effort, loggerhead populations over the last 30 or so years have not increased (see ref. 1), at least as measured by nest counts (Figure 1). Why should that be, especially given that a nearly identical effort to promote the recovery of Florida’s green turtles has apparently succeeded? The number of female green turtles nesting annually on our beaches is increasing dramatically. Additionally, populations of juvenile green turtles counted in our estuaries and shallow reefs are also on the rise, indicating that they soon will make a major contribution to the number of breeding adults in the population.

Figure 1. Nesting trends for loggerheads in Florida, 1989 - 2017.
The cause of the decline between 2001 and 2009 is unknown but
has been attributed to the death of juvenile turtles captured by longline fishers.
Introduced reforms have subsequently reduced that threat so the population
recovered.(from S. A. Ceriani et al., 2019, Ecosphere 10(11).

What these mixed outcomes show, unfortunately, is how little we understand about the variables that presently affect the recovery of our local marine turtle populations. It’s not for any lack of trying! The problem centers around the particular life history characteristics of marine turtles and specifically, on the difficulty of determining what factors affect a turtle’s survival under water, and over the 35 or so years that it takes a loggerhead to “grow up” to sexual maturity in that environment which is largely hidden from view. For example, changes in climate may have affected the quality and quantity of the food supply required by females for growth to maturity, and for reproduction. Could it be that females are simply taking longer to mature, or once they begin breeding are producing eggs that are deficient in essential nutrients required to produce vigorous hatchlings? Recent studies indicate some changes might be affecting the survival of the juveniles. Many are found with digestive tracts full of discarded plastic debris. Does this pollutant ultimately accumulate in sufficient quantities to block the tract and cause death? Finally, increasing numbers of adults are killed annually by boat strikes and by toxic algal blooms (“red tides”). Are those numbers sufficient to nullify annual increases in the adult female population? While the total mortality numbers are small, only a small proportion (~ 1 %) of the loggerhead turtle population consists of breeding females (Figure 2). Thus, the loss of just a few individuals can have a monumental effect.

An additional problem centers on our ability to accurately estimate the number of adult females actually present in the loggerhead population. Estimates have traditionally been based upon annual nest counts that are highly systematized and regulated by the state. But females do not breed every year, and different females have different breeding schedules that may be lengthened, or shortened, depending upon the quality and availability of their food supply. Recent studies also indicate that we have underestimated the number of clutches deposited by females during a nesting season, leading to an overestimation of the number of females responsible for those nests. Complex mathematical techniques can be used to correct for these problems. These increase the accuracy of our population estimates. But even then, we are left with widely divergent estimates that don’t tell us exactly how many loggerhead females nest in Florida, or why their numbers aren’t increasing (1).

Insights from Keewaydin Island

Figure 2. The presumed composition of a marine turtle population if all individuals
are available for census. It consists predominantly of many hatchlings (left), then
progressively fewer post-hatchlings and juveniles. Only a few turtles survive long
enough to become sexually mature adults (far right). In loggerheads,
mature females are estimated to consist of ~ 1 % of the total population.
These characteristics are typical of animals (like marine turtles) that produce many offspring,
each having only a small chance of survival. (modified from J. Mortimer, 1995,
Marine Turtle Newsletter 71:1-4).

Keewaydin is a 13 km-long barrier island along the southwest coast of Florida, about midway between Marco Island and Naples. It is isolated from coastal development and dominated by natural vegetation. During the summer months, it is primarily occupied by an aggressive population of mosquitoes. It is also an extensively used loggerhead nesting beach. Beginning in 1983, the Conservancy of Southwest Florida conducted regular nesting surveys including an intensive tagging program under the direction of Maura Kraus (1983-1984), Ron Mezich (1985- 1989) and David Addison (1990-present), a frequent contributor to this magazine. Each year, Dave hired a small group of field assistants to help conduct nightly surveys to mark females, record the location of their nests, and document how many hatchlings entered the sea from each of those nests. In 2016, he hired Shelby Hoover who soon afterward, entered our Masters program at Florida Atlantic University. For her thesis, she took on the ambitious project of analyzing 33 nesting seasons of the most reliably accumulated Keewaydin data (1986 - 2018), stored in many excel files. The numbers, after analyses, provide potential insights into an important, and hitherto unappreciated, variable that might account for why the population of loggerheads nesting in Florida hasn’t increased.

The findings show that the number of marked females nesting at Keewaydin over the last 33 breeding seasons has doubled (from ~ 40 to 80 turtles, based upon the statistical trend line) and the number of nests they’ve left on the beach has tripled (from 100 to ~ 300; Figure 3). That encouraging result, however, is not evidence that the state-wide loggerhead nesting population has increased; the changes at Keewaydin might very well have occurred because nesting at other, near-by locations has declined. Surveys encompassing the entire state have made clear that nesting increases at some locations have been accompanied by decreases at other locations, with the net result that the nesting population in Florida as a whole shows no definite sign of increasing.

Figure 3. Records from Keewaydin Island indicate that over
33 nesting seasons(1986 -2018), the number of nesting
loggerhead females has more than doubled(from 40 - 80)
and the number of nests has increased to more than 400 from an
initial count of ~ 100. Dashed lines in both graphs show statistical
(and conservative!) trends. Note that the dip in both graphs
reflects the declinein turtles for the entire state from 2001 to 2009
(see Figure 1).(Source: S. R. Hoover, 2019, Masters Thesis,
Florida Atlantic University)

Another significant trend is illustrated by the Keewaydin data. Over the last 33 seasons, there has been a gradual but persistent decline in hatchling production from those loggerhead nests. The magnitude of this effect can be highlighted by comparing hatchling production during the first 5 years of observations (1986 - 1990) to production during the last 5 years of observations (2014 - 2018). Between 1986 and 1990, nests on average contained 101 eggs, and about 79 % of those eggs completed development to produce hatchlings that left the nest (Figure 4, lower graph). Between 2014 and 2018, nests on average contained slightly fewer (97) eggs, but only about 60 % of those eggs developed into hatchlings that left the nest. The difference in average hatchling production (80 vs. 58 hatchlings/nest) represents a decline of ~ 28 %.

Even though the probability of any hatchling’s survival to adulthood is tiny, it nevertheless remains certain that a minimum number of hatchlings must enter the sea, perhaps annually, if there is to be an eventual increase in the number of breeding adults. Nobody knows with certainty what that “minimum number” must be. We do know that it fluctuates depending upon the favorability of many environmental conditions for marine turtle growth and survival. However, it seems unlikely that a 28 % reduction in hatchling production represents a situation that would favor an increase over time in the proportion of mature loggerheads in the nesting population.

One final point is worth mentioning. While the increase in annual nest numbers at Keewaydin over the years does not represent an increase in the nesting population of Florida loggerheads, the decline in hatchling production is an entirely different matter. A recent publication (see ref. 2), based upon a shorter observation period (11 years), indicates that this decline in hatchling production is a state-wide trend and so the Keewaydin data are not only representative, but also generally informative. The numbers don’t tell us why that decline is happening but they do point to hatchling production as a potential causal variable that is likely to affect population structure, especially the proportion of individuals that most immediately influence recovery trends - the adult females. The problem now centers on identifying why hatchling production is declining and what can be done to reverse that trend. Hopefully, and through more focused work and innovative research efforts, we’ll soon have on hand some effective solutions.

(The author thanks D. Addison, S. Hoover and J. Wyneken for comments that improved the manuscript).

Figure 4. Emergence success from the loggerhead nests at Keewaydin Island over 33 nesting seasons , excluding nests washed out by storms. An average value of 1.0 indicates that all of the eggs produced hatchlings that left the nest; 0.5, that half of the clutch produced hatchlings. Dashed lines in both graphs show that over the years, hatchling production has declined, even when nests attacked by predators and partially destroyed (lower graph) are excluded. (Source: S. R. Hoover, 2019, Masters Thesis, Florida Atlantic University)

Further Reading

(1) Ceriani, S. A., P. Casale, M. Brost, E. H. Leone, and B. E. Witherington. 2019. Conservation implications of sea turtle nesting trends: elusive recovery of a globally important loggerhead population. Ecosphere 10(11):e02936. 10.1002/ecs2.2936.

(2) Brost, B., B. E. Witherington, A. Meylan, E. Leone, L. Ehrhart, and D. Bagley. 2015, Sea turtle hatchling production from Florida (USA) beaches, 2002−2012, with recommendations for analyzing hatching success. Endangered Species Research 27: 53–68, doi: 10.3354/esr00653