Green sea turtles stranding with derelict fishing lines: insights from a research collaboration in Hawai‘i

Through research stays at the Center for Marine Debris Research (CMDR) in Hawai‘i, Marthe and Francesca got the opportunity to collaborate with scientists from local and national institutions on a paper that led to new insights on green sea turtles (Chelonia mydas). The outcome of this collaboration is now published in Marine Pollution Bulletin, where entanglement in fishing lines was found to be the leading cause of stranding.

Stranded turtles, defined as individuals found dead, injured, or exhibiting signs of poor health, provide valuable information on wildlife and their contemporary threats. Over the course of this project, we looked at more than 10,000 stranding records from 1982 to 2024, made available through NOAA’s Pacific Islands Fisheries Science Center who collaborate with local partners such as Hawai‘i Marine Animal Response (HMAR).

Historically, fibropapillomatosis (a virus that causes tumors) has been the most common cause of stranding in Hawai‘i. However, we saw a growing number of strandings linked to fishing line, which became the most prevalent cause as of 2016. In the latest years, extra in-water searches from wildlife response organizations revealed an even higher number of turtles affected by hook-and-line in local waters. An important takeaway from this study is that many turtles survive fishing-related injuries if they receive such timely assistance. This underscores the value of coordinated stranding response and ongoing marine debris removal efforts. We also noted that the increase in entangled turtles is likely the result of a combination of a growing turtle population overlapping with nearshore fishing activities. Measures to avoid gear loss and interactions with turtles are thus warranted.

To enhance our understanding of the different stranding causes, we developed spatial models which highlighted areas where stranding due to each cause is more likely. For hook-and-line related stranding, the island of Oʻahu, with its high population density and accessible coastline, was identified as a hotspot. The turtles are often entangled in monofilament line, commonly used in recreational fishing. These are difficult to see underwater, accumulate on coral reefs and in nearshore habitats where turtles forage. Interestingly, we found that derelict fishing line was positively associated with coastal access points and proximity to boat harbors, but negatively associated with human population density. Mapping monofilament lines around Oʻahu was possible due to divers from HMAR that conduct clean-ups and record their findings. As such, these missions provide both direct efforts for the environment and a data foundation for research.

We are very grateful that we got to collaborate with the researchers based in Hawaiʻi on this project, and that we also had research stays that allowed us to dive into the topic and even see the issue with our own eyes.

To read the full article, click here.