Unusual Encounters: Ridges in the Water
December 2, 2016
Before my Farallon internship, multiple former interns and visitors warned me about the boat rides out; they would talk endlessly about the ten foot swells and the seasickness, how much dramamine they had to take, and the absurd amount of flies that would land on you once you stepped foot on the island. I did not expect that it would come to be one of the highlights from my Farallon internship. The excitement and anticipation of what awaits on the island is palpable, the conversation with the fantastic skippers of the Farallon Patrol is fascinating, and the open water always holds the opportunity for surprise.
On October 22nd I returned to the island, after two weeks of shore leave, along with fellow intern Boo Curry and biologist Jim Tietz. Leaving the harbor, we passed pods of harbor porpoise and flocks of Heermann’s gulls, and further out humpback whales lobtailing, rafts of common murres, and the occasional pelagic specialty like a parasitic jaeger. It was fifteen miles out from San Francisco when we were greeted by a giant inky blue object with a large square green tag, complete with prominent antenna. It was so large and alien looking that my immediate impression was of something manmade. Soon, we noticed the head and huge flippers sticking out in front of the carapace, and we realized that we were looking at a leatherback turtle. This was certainly a special sighting, but Jim was particularly excited, since after almost 40 crossings in the past ten years, this was only the second time he’d seen a leatherback. Soon after, we passed a second turtle, this one without a tag. We felt extremely lucky to have seen two individuals of this increasingly rare species, and were especially excited to hear more from the researchers who tagged the first one.
The turtle was tagged in the nearshore waters off the coast of San Mateo county by a team of leatherback researchers at NOAA’s Southwest Fisheries Science Center lead by Scott Benson. Scott has been studying the species since 1985 and his studies have clarified the ecology, particularly the movements of the turtles that we see off of our coast. Scott was kind enough to speak with me about the leatherbacks we see and explained that they are part of a metapopulation which nest in the west Pacific at beaches in Indonesia, Papua New Guinea, Vanatu, and the Solomon Islands, and have all been shown to be part of a single genetic stock (Dutton et al. 2007). Breeding occurs year round; winter breeders and their offspring disperse to more southerly locations from Indonesia southeast through New Zealand, and summer breeders disperse to locations further north, including the South China Sea and the California Current. 8000 miles away from nesting sites, California represents the furthest foraging site from breeding grounds for leatherback turtles, but studies at the major Jamursba-Medi nesting area in Indonesia show that 50-60 percent of turtles are using the eastern north Pacific as a foraging ground. It was unclear why so many leatherbacks undertake such a long and potentially dangerous migration until a 2006 study answered this in a remarkably direct way: hatchlings are weak swimmers and are carried off by strong ocean currents (Gaspar 2006). This explains how the same genetic stock produces individuals that forage in such disparate locations. However, sea turtles do have the characteristic of natal homing, or returning to the beaches they were born on to breed. They also remember the places they successfully foraged in the winter, and as stronger swimming adults, individuals return to them after breeding. The California Current, with it’s annual upwelling of cool nutrient-rich water, has a substantial population of leatherback’s primary prey, jellyfish. Though it’s remarkably distant from the breeding grounds, the incredible productivity of our coastal waters allow for high success of those leatherbacks that do make the journey, and their natal rehoming and continual return to winter foraging areas mean that our waters play a large role in the population’s success.
Sadly, the west Pacific leatherback turtle population is listed by the IUCN as critically endangered. Long-term studies on their nesting grounds have shown massive declines. Once a major nesting beach for the population, Terengganu, Malaysia had approximately 10,000 nests per year, but the population was completely lost following a crash of the population in the 1970’s and 1980’s (Chan and Liew 1996). Thankfully, leatherback turtles have a unique habit of laying a small percentage of their clutches at random sites, the vast majority of which are likely to be completely unsuccessful, but thankfully the very lucky ones that do end up being successful allow the population to colonize new nesting sites. Currently, the remaining 75% nest along the north coast of the Bird’s Head peninsula in Papua Barat, Indonesia. Nesting at the largest beach there declined from 14,522 nests in 1984 to 1,596 nests in 2011 (a decline of 78.3%), and for the entire peninsula there has been a continual nesting decline of 5.9% per year since 1984 (Tapilatu et al. 2013). A major reason for the crash in the 1970’s and 1980’s was the harvest of eggs and adults, which increased to unsustainable levels with the rise in outboard motors in Indonesia which allowed eggs and meat to be harvested not just for local subsistence, but for large city markets. A number of factors at the nesting grounds threaten to continue the alarming rate of decline. One problem is predation by pigs, which is rampant in the summer (Tapilatu and Tiwari 2007). Development poses another threat: leatherback nesting frequency is 3.4 times lower on developed beaches, partly due to disturbance from careless humans and dogs, as well as changes in abiotic factors that can make the beach inhospitable to developing embryos (Patino 2012). Sadly, the projected effects of climate change will significantly decrease habitat availability, as sea level rise eliminates beach zones. On the remaining beach habitat, suitable conditions for embryo development are likely to degrade as with higher water temperature and salinity, oxygen solubility will decrease, and embryo mortality is very likely to increase with less oxygen. Additionally, rising sand temperatures and higher high tides have already been shown to destroy nests and lower breeding success (Tapilatu and Tiwari 2007).
Once off the breeding grounds, leatherbacks face a number of problems out in the oceans. Pollution is one concern, particularly items like plastic bags that can look remarkably like jellyfish. Sadly, ingested plastic causes blocks in the digestive system that lead to starvation. A still greater threat to leatherback turtles comes from commercial and artisanal fishing. Both longline fishing and drift gillnet fishing, often used to catch tuna and swordfish, frequently ensnare leatherbacks as bycatch. This is especially harmful to the species as it’s often the larger, more mature individuals caught, and because of the long time to maturity, this can mean a substantial reduction of the breeding population. It’s estimated that between 1,000 and 3,000 turtles a year in the Pacific Ocean alone meet their end as bycatch to the longline fishing industry (Lewison, Freeman and Crowder 2004). In conversation with Scott Benson, he mentioned another concern that is highly visible to us on the Farallones: the dungeness crab fishery. When looking out from the island, we see the buoys attached to the stationary crab pots peppered all over the ocean, stretching from the edge of the marine protected area here to the San Francisco Bay. A recent increase in the effort of the fishery and of recreational dungeness fishermen is likely the cause of a surge in recent whale entanglements. While the leatherback numbers off our coast are low enough that there haven’t been a huge number of entanglements, in the Atlantic where the population is much larger, entanglements with crab fishing gear are a well-documented problem, and just in the last year we have had two entanglements in California that were believed to have been from dungeness gear.
There are measures we can take now to help the species recover. California recently passed a referendum banning plastic bags, a huge step in reducing plastic pollution. A number of modifications to driftnets have been shown to reduce sea turtle bycatch rates, including reducing buoys, illuminating the nets with glowsticks, and shortening the height of the nets (Gilman, Gearhart and Price 2009). In the California swordfish fishery, managers are considering allowing innovative deep-set buoy gear, which places hooks deep into cold water and allows fishermen to immediately reel in catches, resulting in higher efficiency for shorter fishing time compared to the current driftnets and thus hopefully fewer entanglements. Monitoring turtle movement has been key in establishing protected areas, and by using the TurtleWatch predictive map, fishermen can avoid areas where turtles are likely to be. The Hawaii-based longline fishery is an excellent model for their practices to lower the impact on the east Pacific leatherback population: participating fishermen are trained on safe turtle handling and release, and all boats must have observers. The fishery is closed for the year once 26 bycatch incidents with leatherbacks have been reported. These regulations have been successful in the United States, but given the complexity of the 8000 mile journey the leatherbacks we see take over 10 to 12 months through the Pacific, and the reliance on a number of country’s waters by the same metapopulation for foraging and migration, regulations in the United States cannot be enough to protect the species. NOAA Fisheries and USFWS are working with several countries to reduce bycatch, but these projects need institutional support and funding to continue. Additionally, caps on leatherback entanglements have not been set for any fishery in California.
Leatherbacks have been around for around 100 million years due to some incredible evolutionary adaptations. Their dispersal to a broad range of waters and repeated migration to successful waters, as well as their natal honing and ability to colonize new breeding grounds has allowed the species to thrive. However, the catastrophic crashes have left the population at numbers that leave them struggling to continue, and climate change and fishing entanglements threaten to worsen the already unsustainable decline. The IUCN estimates that with current conditions 96% of the population will be lost by 2040, leaving about 260 adult females in the breeding population, hardly enough genetic diversity to allow evolution to match the changing conditions. There are measures we can take as individuals to slow their decline. Avoid unnecessary use of plastic bags– while it is a small step, the only cost to you is remembering to bring your reusable shopping bags with you to the store. Reconsider the fish you are eating, and look into sustainable options like those set forth by Seafood Watch and other organizations. Support nonprofits that are doing work to protect these turtles, and support funding for crucial USFWS and NOAA programs.
Many thanks to Scott Benson for all of his help while I was researching and writing this piece.