A week of fieldwork in Baja: Photo diary of ingredients for a good sample size (n=30)

I remember when I first saw, in an oh-so-exciting statistics class, how there was something almost magical about a sample size of n=30.  It was like watching a birthday party magician pull a quarter from behind your friend's ear. Surely there was a simple trick to that. Obviously your friend didn't have a quarter behind their ear all this time - gravity alone would rule that out, not to mention, why would your friend put a quarter there in the first place? My point being that when I was first told that a sample size of 30 (n=30) would pretty much produce the same mathematical mean and resemble a normal distribution of a much much larger sample size - I was surprised. And I remembered it. And I knew there was much more to it than that - and my additional experience with statistics and data analysis has reinforced that this "rule" of n=30 is more of a "guideline", a general trend and has many, many exceptions.

All that being said, when I went to Guerrero Negro, Baja Sur California, Mexico last month to collect samples for my research, I was extremely pleased when we ended up with a final sample size of... you guessed it... n=30.

Now if you've been a follower of this blog long enough, you'll know that my samples, sadly, come from dead sea turtles. Yes, I use the humerus bone (upper flipper/arm bone) to learn about the turtle's life history, habitat use, and diet.

So, a big sample size for me - is not a good thing for our awesome turtles. Sadly, sea turtle mortality is all too common of a thing and is caused by natural causes such as predators when the turtles are small, water that gets too cold too quickly, and standard things like old age - but a lot of mortalities are also the result of interaction with human-pressures. This may include the ingestion of plastics or toxins, entanglement in our trash and pollution or fishing gear, hunting for turtle meat, eggs, and other "products", - just to name a few. So my approach, when I'm out there walking along a beach - looking for turtle carcasses - is that it is a reality that these animals are dying, but if we can still learn something from these animals, even after they have died, that will help us protect the turtles that are still living - then let that be my role in sea turtle conservation.

With this perspective, these were the ingredients necessary to get this "good" sample size:

- One week in Baja California Sur; the first part spent patrolling the water and shorelines of Laguna Ojo de Liebre for dead-stranded turtles. Funding graciously provided by the UCSD Jeanne Marie Messier Memorial Endowment Fund made this trip and fieldwork possible.

- Two INCREDIBLE partners who helped make this trip and fieldwork possible! (Juan Manuel & Andrea from GTC rock!!)

- A team of ~10 AMAZING local partners from Guerrero Negro who helped with all the hardest parts of the fieldwork. Never could have done it without them!

- One day spent on the water patrolling for in-water dead turtles - while counting breeding grey whales! (Our boat counted over 500 whales, the other boat over 300, and that wasn't even peak season yet!!) We found a total of 4 dead-stranded turtles floating in the water - usually with a bird perched on top it!)

- 1 cold-stunned live turtle in need of help. Once the turtle was pulled into the boat - standard measurements were taken and then it was taken to a sunny beach to warm up before being tagged and released.

- 1 grey whale calf, caught in shallow water, in need of help... OF COURSE we jumped in the water to help save the baby whale!!!!

- And to top it all off, a few days spent at the Grupo Tortuguero de las Californias 15th Annual Meeting in Loreto talking with and learning from the many partners in this incredible group!

The final result: a total of 30 bones from turtles ranging in size from 24 cm to 100 cm (n=30). And this smallest turtle's bone - is INCREDIBLY important for my research. In order to know and estimate the age of these turtles - one of my primary research goals - I need samples from small turtles (and yes, this is sad.) Notice the humerus bone in the sand in the lower right corner of the photo.

Yup, it's pretty safe to say this was one of the most amazing trips and experiences of my life - despite it being only 1 week. Just another reason why I love my job as a marine turtle conservation ecologist.

East Pacific Green Sea Turtles: It's all about location, location, location

While there are seven different species of sea turtles alive in the world today, there is only one species we expect to find in the San Diego Bay, and that is the green sea turtle, scientifically called Chelonia mydas. The three other species which occasionally are spotted off of California's shore are the loggerhead turtle (Caretta caretta), the olive ridley (Lepidochelys olivacea) and the giant leatherback turtle (Dermochelys coriacea). And most of the sea turtle species, including the four found in California waters, have different groups, or populations, that live in different parts of the world.  (The flatback (Natator depressus) of Australia, and the Kemp's Ridley (Lepidochelys kempii) in the Gulf of Mexico and the Atlantic are two notable exceptions.)

For example, there is a population of leatherback turtles that are found in the Caribbean and further north along the Atlantic coast, and there is a population of leatherbacks found in the Pacific.  These two groups of leatherbacks are considered different populations because they are genetically distinct and physically separated from one another.  Managers responsible for protecting and monitoring leatherback sea turtles recognize that saving leatherbacks in the Caribbean and Atlantic coast, will do nothing to help the declining population of Pacific leatherbacks.

The below maps, created using SWOT's Online Map Application, illustrate where different population groups of individual species are found. (SWOT stands for State of the World's Sea Turtles - and is a great resource to check out!)  The first two maps below show some of the different population groups of leatherbacks and loggerheads that are found in the Pacific ocean. The colored dots show nesting sites, and the grouped colored sections show the range of the different population groups.

Then notice the map for the Kemp's Ridley turtles, that map shows the only population group found in the world for that species!  They nest in the Gulf of Mexico, and visit parts of the Atlantic, but there are no other populations of this species.

Leatherback

The Indonesian nesting population crosses the entire Pacific ocean to forage off the coast of the US.
The East Pacific nesting stock does not make this trans-Pacific migration. 

 Loggerhead

The two populations of loggerheads in the Pacific - the North and South - each nest in the West Pacific, but migrate to the East Pacific to forage.

Kemp's Ridley

Kemp's Ridleys nest around the Gulf of Mexico, and have been found as far away as Europe in the East Atlantic.

Source of Maps: Read, A.J., Halpin, P.N., Crowder, L.B., Best, B.D., Fujioka, E.(Editors). 2011. OBIS-SEAMAP: mapping marine mammals, birds and turtles. World Wide Web electronic publication. http://seamap.env.duke.edu, Accessed on October 25, 2011.


Similar to leatherbacks and loggerheads, green turtles, like those found in the San Diego Bay, are found all over the world and there are multiple populations of this species. The population I study, is usually referred to as "East Pacific green turtles", as they are the population found on the east side of the Pacific ocean, along the coast of North, Central, and South America.  This population is also referred to as the "Mexican nesting population" which is one of two Endangered populations in the US as defined by the US Endangered Species Act.  The other Endangered population in the US is the Florida nesting population, while all other green turtle populations in the US are listed as Threatened under the US Endangered Species Act. (More on the US management of green turtles at the NOAA Fisheries website.)

Green Turtles

San Diego sea turtles are in the East Pacific population. Other key populations shown here include the Caribbean, Hawaiian, and various West Pacific groups.

As mentioned before in regards to the leatherbacks - management and conservation of green turtles requires different efforts and approaches for each different population. Conservation efforts focused on green turtles in the Caribbean, or Hawaii, for example, will not directly benefit or protect green turtles in the East Pacific. Different populations require different needs, because they faced very different types of threats.

Despite these different needs, often times similar activities will benefit turtles - no matter where they are found. For example, a brand new type of fishing hook that may have been invented by Hawaiian fishers, turns out to reduce the number of green turtles caught accidentally. This new hook could also be a great tool for fishers in the East Pacific to use and will then help protect the green turtles in that population. In these situations, the sharing of knowledge of ways to protect and conserve turtles - anywhere in the world - is a very important part of marine turtle research and conservation. (And this is just one of many reason why I love being part of this research and conservation community!)

So I invite you to check out these different links, including the SWOT map tool, and explore just how many different populations of sea turtles researchers have identified. By recognizing different groups, and how different geographic areas are connected through marine turtle migration patterns, managers of these animals know where they should focus their work,  and who else they should partner with!  Because our San Diego sea turtles are actually a very small fraction of this East Pacific population - and they all start their lives on beaches in Mexico - we work very closely with our partners in Mexico and throughout Central and South America in all of our efforts to protect this population of green turtles.

In my next post, I'll share some highlights from this year's International Sea Turtle Symposium, the annual meeting of sea turtle researchers and conservationist from around the world where great new ideas, success stories, biological findings, and good times are shared by all.  This year's symposium was hosted in Baltimore - and was an incredible event! More coming soon!