Abstract
Located on South Padre Island, TX, Sea Turtle, Inc. has recorded 2,486 stranding events since 1980. This stranding data was analyzed using a combination of spatial mapping and statistics to examine trends in sea turtle length, species, and mortality rate in relation to stranding cause, location, and date. The green sea turtle (Chelonia mydas) is the primary species that strands on South Padre Island, and is expected to double in annual strandings by 2023. The Kemp’s ridley sea turtle (Lepidochelys kempii), a critically endangered species brought back from the brink of extinction in the 1980s, experienced a major spike in sea turtle strandings during the 2010 Deepwater Horizon oil spill that was followed by a dramatic decrease in 2011. This reflects severe resulting population loss as a result of the spill, and the stranding data shows that the species has yet to recover. The green, Kemp’s ridley, and loggerhead sea turtle (Caretta caretta) experience similarly high mortality rates (60-70% stranding dead on arrival), the hawksbill sea turtle (Eretmochelys imbricata) is discovered dead on arrival (DOA) in only 15% of recorded strandings, displaying mortality significantly lower than the other three species (p < 0.001).
Rapidly dropping sea surface temperatures due to Laguna Madre’s hypersaline waters result in cold stunning as the primary cause of sea turtle stranding, with cold stun strandings (n = 1,312) totaling more than live (n = 468) and DOA (n = 706) strandings combined. For non-cold-stunned strandings, sea turtles are nearly twice as likely to strand around South Padre Island due to anthropogenic activity (e.g. fishing, boating) than natural causes (e.g. predator attacks, hurricanes). Entanglements, incidental capture with fishing gear, and boat strikes are the three most frequent anthropogenic stranding causes, and show a high correlation with increased coastal activity during tourist season (May through August). Though fishing gear and entanglement strandings occur far more frequently, boat strikes result in significantly higher mortality rates (p < 0.05). Future conservation efforts are therefore recommended to place a higher priority on mitigating boat strikes than fishing gear or entanglement strandings, to most efficiently increase species survival. Sea turtle-vessel interactions are most likely to occur within the Brazos Santiago Pass (stranding risk multiplier of 142), and mitigation actions should consequently be focused within this area.
Fibropapillomatosis (FP) is another major factor in sea turtle stranding, and is spreading among the island’s green sea turtle population at an exponential rate. The disease was first recorded on South Padre Island in 2010, and has increased from 5.8% of live green sea turtles afflicted in 2014, to 63.64% of greens in 2016 so far. This report strongly suggests analyzing Laguna Madre’s poor water quality in relation to the disease’s rapid proliferation, specifically for high levels of FP-promoting vectors such as the marine leech Ozobranchus brachiatus, the okadaic acid-producing dinoflagellate Prorocentrum spp., and the Lyngbya majuscule cyanobacterium. Effluents from the county’s four water and wastewater treatment plants are of particular concern, as each of these plants have been in steady violation of the EPA and CWA’s legal standards for the past several years, including violations a) operating without a permit for over two years, b) releasing effluent and drinking water with dangerously high levels of carcinogens and other contaminants, c) failing to notify consumers of drinking water violations, and d) failing to report results such as those of lead tap water monitoring. Slightly over 63% of green sea turtles stranded within 1.5 miles of these plants, often during winter months when immunosuppression is severe. Furthermore, many of these strandings are within the age range proven most susceptible to fibropapillomatosis.
The island’s green and Kemp’s ridley sea turtles are stranding at steadily increasing rates, mostly due to anthropogenic causes. Unless immediate steps are taken by the South Padre government to mitigate the deleterious effects increased coastal activity is causing to sea turtle populations – particularly that of dense vessel activity around the Brazos Santiago Pass and chemical pollutants entering Laguna Madre – then the island’s green and Kemp’s ridley populations could decline to precarious levels.