How many sea turtles do we want? The case of Kyparissia bay, Greece.

 

The story of loggerhead sea turtle nesting area of Kyparissia bay is an interesting one. Several issues have been risen during the last years, both in a political and a conservation level. In this post, we will focus on the latter. Situated in the western part of Peloponnese, Greece, the whole bay has total length of 44 km with the majority of loggerhead nests (around 85%) concentrated in the south 9.5 km. The average nest number in this core area used to be, up a decade ago, around 550-600 nests (a bit more in the whole bay), see this paper. It was considered to be the second largest reproductive site for the species in the Mediterranean, after Laganas bay, Zakynthos, where typically double the amount of nests were laid each year. However, unlike Zakynthos, the Kyparissia nests were suffering from mammal predation (e.g. dogs, foxes) as well as severe flooding due to large waves. Hence intensive conservation efforts began in the early 90’s by Archelon, the Sea Turtle Protection Society of Greece, with the objective to reduce these effects. Metal grids were placed on top of nests to reduce predation and nests laid close to the sea were being relocated into a safer place. It goes without saying that this requires a tremendous effort every year, by dedicated young volunteers. The result: hatchling production was steadily increasing every year. Hence, 15 years after (around 2007) Kyparissia’s nests started increasing as well. Up to then, its nest numbers, as well as the ones of Zakynthos was pretty much stable. Given the fact that the minimum sexual maturation age for loggerheads is 14-15 years (check here the source), this increase, as expected, is naturally attributed to the increased hatchling production since the beginning of the 90’s. In fact Kyparissia’s nesting data itself could lead to a nice scientific paper regarding sexual maturation age and other general parameters of the demographics of the Mediterranean loggerheads.

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Two of the main conservation measures in Kyparissia bay: nest relocation in order to avoid inundations and protective metal grids to reduce predation.

 And this increase continues till today. Last summer (2016), the 9.5 core sector had around 2650 nests,  a truly unprecedented number. Projections elevate this number to more than 3000 nests for the whole bay. To compare with, the most nests that Zakynthos ever had was a bit more than 2000, in 1995. We should note here that 2016 was an exceptional year for the whole Mediterranean with record breaking nest numbers in many areas.

It is also worth noting that unless our assumptions on demographic parameters are wrong, this current increase has not hit yet its exponential phase. This is because the turtles that nest for the first time (neophytes) still belong to the first generation of hatchlings that were born after the conservation measures.  So very very roughly, until now, the gain in neophytes every year, should directly correlate to the number of hatchlings that were “saved” 15 years due to conservation measures. That should theoretically result in a linear increase these days, i.e., a fixed amount of “saved” neophytes is added every year. It could be the case that 15 years after the first increase of the nest numbers (i.e., around 2022), the second generation of hatchlings will be mature enough to start laying eggs as well.


Will the nest numbers explode then? If yes, is that necessarily something good? Is it always true that the more nests, the better? With how many nests will we be happy? And at which point, conservation stops and intervention begins?


The IUCN marine turtle specialist group, the world authority in sea turtle writes on its website:

We envision marine turtles fulfilling their ecological roles on a healthy Planet where all Peoples value and celebrate their continued survival.

While this statement does not give a straight answer, to how many sea turtles we would like to have out there, it implies that a healthy population is desirable, meaning that its dynamics are driven by natural powers only while the anthropogenic impact is absent. In order to find out these ideal population numbers we would have to use a time machine and go many years back in time. How many years back though? Scientists and conservationists often suffer from the baseline syndrome: The tendency to regard healthy population levels as the ones that one sees at the beginning of her/his career, see for instance here and here. Regarding Kyparissia bay, nobody can know what the correct baseline is (i.e., the population number in equilibrium where no anthropogenic effects have acted). Let us note that given the fact that Kyparissia’s beaches are pretty much pristine, the main anthropogenic effect is that of the fishing industry. In fact, it is estimated that (source), over 44000 sea turtles die every year because of the fishing industry. (Note: however, all these do not belong to the Mediterranean population, as the majority of sea turtles in the west part of the Mediterranean basin come from the Atlantic populations. Thus, always with regards to conservative estimations, the number of deaths of local Mediterranean turtles should be less). It could be the case that the average nest number in the 80’s, around 600 nests per year, was an equilibrium point driven by a combination of natural dynamics and interactions with fishing industry. Of course with such long lived species, as sea turtles are, we cannot even be certain if this number was actually an equilibrium or not.

In any case, current conservation measures in Kyparissia address only nest predation and inundation, which are both natural processes (though even that is still debatable; are predator numbers what they should be? where they always predating nests? does climate change increase inundations?). Moreover, it could be the case that even these measures do not come without a cost. In fact, given that low nest incubation temperatures produce male hatchlings, nest inundations could be a natural mechanism to boost male turtle production in, what seems to be, a female biased population, see here (page 67). Also, sea turtles lay their nest away from the surf, something that has resulted as a natural selection process. One could argue that by relocating a doomed nest, that would result in survival of hatchlings that will keep doing the same wrong choice of nest site. However in this interesting study for an Australian loggerhead population, it was shown that

…doomed-egg relocation does not substantially distort the gene pool in the eastern Australian loggerhead stock and should not be abandoned as a strategy for the conservation of marine turtle populations.

Still further research is required on this topic. Taking all the above into account, my opinion is:


The current conservation measures in Kyparissia are only justified if they are considered as counter-measures to sea turtle mortality in the sea. If the anthropogenic sea turtle mortality was zero then they shouldn’t exist in the first place.


Of course the above statement, implies that we are trying to solve a problem (anthropogenic mortality), not by dealing with the problem directly but sort of by patching it up. Of course, this is not big news in sea turtle conservation. Mortality at sea due to fishing industry is extremely difficult to control and hence reduce. Thus, it looks like that start of conservation measures in the beginning of the 90’s was well justified indeed.

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But what about in the future? Should we always celebrate after another record breaking year? Nest numbers are starting to blow up, making the protection of every single nest, an almost impossible task.  Unless any density effects come into play, introducing some negative dynamics, this increase is likely to continue (?).  We have seen cases where nesting beaches host a huge number of nesting females, like the case of Raine island in Australia (see this impressive video), the largest green sea turtle rookery in the world with tens of thousands females every year. In fact, Raine island is an example where, turtles are just too many. So many in fact, that they destroy each other’s nests, resulting in an extremely low nesting and hatching success (just Google “Raine island reproductive failure”).

Yet, the young conservationist in Kyparissia has a decision to make: protect or not the nest she/he just found? Depending on if none, half or all the nests are decided to be protected, the long term equilibrium nest number that corresponds to each decision will be different (and currently unknown).  None decision seems right or wrong at the moment, but we should definitely dream of a time where sea turtle population dynamics are only driven by natural processes.

This post is dedicated to those, who have walked thousands kilometres, poured hundreds kilos of sweat and spent countless sleepless nights at the beaches of Kyparissia.

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