Four Decades of Green Turtle (Chelonia mydas) Strandings on Hawai'i Island (1983-2022): Causes and Trends

© 2024 Academia Sinica, Taiwan Open Access Four Decades of Green Turtle (Chelonia mydas) Strandings on Hawai‘i Island (1983–2022): Causes and Trends Skylar Dentlinger1, Karla J. McDermid2,* , Grady Weyenberg3, Laura M. R. Jim4, Marc R. Rice5, and George H. Balazs6 1Department of Marine Science, University of Hawai‘i at Hilo, Hilo, Hawaii 96720, USA. Current address: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA. E-mail: [email protected] (Dentlinger) 2Department of Marine Science, University of Hawai‘i at Hilo, Hilo, Hawaii 96720, USA. *Correspondence: E-mail: [email protected] (McDermid) 3Department of Mathematics, University of Hawai‘i at Hilo, Hilo, Hawaii 96720, USA. *Correspondence: E-mail: [email protected] (Weyenberg) 4Sea Turtle Research Program, Hawai‘i Preparatory Academy, Kamuela, Hawaii 96743, USA. E-mail: [email protected] (Jim) 5Sea Turtle Research Program, Hawai‘i Preparatory Academy, Kamuela, Hawaii 96743, USA. E-mail: [email protected] (Rice) 6Golden Honu Services of Oceania, Honolulu 96821, Hawaii, USA. E-mail: [email protected] (Balazs) Received 11 January 2023 / Accepted 11 March 2024 / Published 26 June 2024 Communicated by Benny K.K. Chan The Hawaiian population of green turtles (Chelonia mydas) has increased since Federal and State protections were implemented in the mid 1970s, and reported stranding events have also increased. This study analyzed Hawai‘i Island data: stranding location, date, size, sex, presence/ absence of tumors, stranding status, and cause of stranding. A total of 754 stranded green turtles was reported from 1983–2022: 379 stranded on the east (windward) coast of Hawai‘i Island and 375 on the west (leeward) coast. Strandings peaked in 2011 and 2018 and were highest from March to August. The most common known cause of stranding was hook-and-line fishing gear (21.4% of total strandings), followed by fibropapillomatosis (7.2%), human take (4.4%), miscellaneous (3.7%), boat impact (3.3%), shark attack (3.2%), and net (2.1%); however, 54.8% of strandings had no known cause. Statistical modeling did not provide convincing evidence of temporal changes in the distribution of strandings across three consolidated cause categories: human-caused; predation, disease, and weather; and unknown. Stranded turtles on east Hawai‘i Island had a higher frequency of fibropapillomatosis, whereas west Hawai‘i stranded turtles showed higher incidence of shark attacks. These results provide the first comprehensive analyses of stranding data from Hawai‘i Island and provide information that can inform resource managers, policy makers, and the public about the various types and magnitudes of impacts, anthropogenic and natural, to green turtles so that mitigation measures can be put into practice. Our findings allow for comparison with other green turtle populations worldwide. Key words: Sea turtles, Fishing gear entanglement, Fibropapillomatosis, Hawaiian Islands, Marine reptile mortality, Pacific Ocean BACKGROUND Green turtles (Chelonia mydas) are the most abundant large marine herbivores found throughout the world and in the Hawaiian Islands. The Hawaiian population of green turtles that was once depleted has increased since its 1974 protection under Hawaiian Law and 1978 protection under the Endangered Species Act (Balazs and Chaloupka 2004). Green turtles migrate long distances during their lifetime, from nesting to foraging Citation: Dentlinger S, McDermid KJ, Weyenberg G, Jim LMR, Rice MR, Balazs GH. 2024. Four decades of green turtle (Chelonia mydas) strandings on Hawai‘i Island (1983–2022): causes and trends. Zool Stud 63:16. doi:10.6620/ZS.2024.63-16. Zoological Studies 63:16 (2024) doi:10.6620/ZS.2024.63-16 1 © 2024 Academia Sinica, Taiwan grounds (Balazs et al. 2015). In the Hawaiian Islands, 96% of nesting occurs on the sand islets at French Frigate Shoals, located in the Northwestern Hawaiian Islands (Marine Turtle Biology and Assessment Program 2022). Migration patterns and complicated life history patterns cause green turtles to occupy many habitats during their lifespans including pelagic environments during their early years and during migrations, as well coastal areas in their later years (Balazs 1980; Bolten 2003). Therefore, green turtles are susceptible to threats in both offshore and coastal environments (Bolten 2003). Green turtles have experienced a long history of exploitation. The species was used for meat by indigenous coastal people around the world, as well as by European royals in the 18th and 19th centuries (Witzell 1994). Hawaiian green turtles have been additionally impacted by hunting at foraging grounds, by harvesting of both eggs and females at nesting grounds, and by the destruction of their nesting habitat. Since protection began under the Endangered Species Act, a reduction in such exploitation has been observed (Balazs and Chaloupka 2004). However, large marine vertebrates, including green turtles, face other threats, and are often victims of bycatch, becoming accidentally entangled or hooked by commercial or recreational fisheries activities targeting other species (Lewison et al. 2004). Bycatch is harmful to green turtles because it can cause drowning, and internal/external injuries from hooks and line entanglements. Fibropapillomatosis (FP) is another major threat to sea turtle populations. FP is a debilitating neoplastic disease associated with a herpesvirus found in turtles worldwide (Jacobson et al. 1991; Herbst 1994). The disease was first described in green turtles in the Florida Keys in 1938 and affects mostly immature turtles (Herbst 1994). FP is indicated by the presence of internal, external, and oral tumors. Oral tumors are unique to Hawaiian green turtles and are often found in the glottis, making survival difficult (Work et al. 2004). The presence of these tumors can impact the turtles’ ability to breathe, swim, dive, forage, and see (Perrault et al. 2021). On O‘ahu, Maui, and Kauai from 1982–2003, FP was the most common cause of stranding, defined as a turtle that has been found dead, injured, or exhibits ill health or abnormal behavior (Chaloupka et al. 2008). A variety of factors, both natural and anthropogenic, can cause sea turtle strandings. The majority of strandings involve sea turtles that died at sea and washed ashore; however, most stranded turtles show no cause of death (Hart et al. 2006). An unknown number of deceased turtles never reach shore. They are eaten by scavengers, sink, and/or decompose while in currents or eddies (Crowder et al. 1995; Hart et al. 2006). Therefore, the number of sea turtle strandings that is recorded is likely a minimal estimate (Hart et al. 2006). Stranding response programs can provide important insight into the health, welfare, and conservation status of sea turtle populations. Analyses of the data collected by these programs provide valuable information on mortality patterns and can aid regulatory managers (Crowder et al. 1995). Although Chaloupka et al. (2008) mentioned that 6% of statewide strandings occurred on Hawai‘i Island from 1982–2003, long-term stranding data specifically from Hawai‘i Island have not been thoroughly analyzed previous to this study. Hawai‘i Island merits additional scientific scrutiny of its green turtle stranding patterns with the most up-to-date, inclusive data available because of the island’s large size (over half of total Hawaiian land area), southernmost location in the archipelago, lowest human population density, and important turtle foraging and resting areas—recently proposed as critical habitat by the US Fish and Wildlife Service and the National Oceanic and Atmospheric Administration (Endangered and Threatened Wildlife and Plants: Designation of Critical Habitat for Green Sea Turtle 2023; Endangered and Threatened Wildlife and Plants: Proposed Rule To Designate Marine Critical Habitat for Six Distinct Population Segments of Green Sea Turtles 2023). The knowledge gained from stranding patterns can be used to establish mitigation measures to reduce strandings and maintain healthy green turtle populations. In the present study, a comprehensive analysis of 39 years of Hawai‘i Island green turtle strandings is presented to (1) identify the causes of strandings affecting green turtles around Hawai‘i Island, (2) assess trends in strandings, and (3) identify differences and similarities between strandings in west and east Hawai‘i Island. MATERIALS AND METHODS Data Collection Data were collected on turtles stranded on Hawai‘i Island (19.6°N, 155.5°W, land area 10,430 km2 with coastal circumference of 428 km) from 1983–2022 by members of the Pacific Islands Fisheries Science Center under the US National Marine Fisheries Service, the University of Hawai‘i at Hilo Sea Turtle Stranding Response Team, and the Hawai‘i Preparatory Academy Sea Turtle Research Program. The database used in this study was compiled from records available at https:// georgehbalazs.com/wp-content/uploads/2023/10/1982page 2 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan 2018-Hawaii-Stranding-Data.pdf. The west and east coasts of Hawai‘i Island are different in terms of climate (the windward east coast receives much more rainfall than the leeward west coast), terrain, currents, and population, so the data used in this study were analyzed for the island as a whole, as well as by west and east coast. West Hawai‘i included locations from Miloli‘i north to Kawaihae, and east Hawai‘i included locations from South Point north to Hawi (Fig. 1). For each stranded turtle, the following information was collected: date of stranding, stranding location, stranding status (alive/dead), and cause of stranding. Data on species, sex, straight carapace length (SCL), curved carapace length (CCL), and the presence or absence of tumors indicative of fibropapillomatosis were also recorded. SCL was used in size analyses because it was reported more frequently than CCL. In cases where CCL was recorded, but not SCL, CCL was converted to SCL using the following linear regression function: SCL = 1.245 + 0.913 CCL (Chaloupka et al. 2008). Determination of size classes of turtles followed Balazs (1980): juvenile-post hatchling to 65 cm SCL; subadult-rom 65 to 81 cm SCL; adult-greater than 81 cm SCL. The primary cause of stranding was based on direct observation and/or necropsy when available. Causes of stranding were classified into eight categories used previously by Chaloupka et al. (2008): fibropapillomatosis (FP), hook-and-line fishing gear, net and gillnet fishing gear, boat impact, shark attack, human-take, miscellaneous, and unknown. FP strandings were turtles that had gross evidence of external tumors. Fishing gear strandings were identified by obvious signs of an interaction or entanglement with the particular gear (hook-and-line or net) (Boulon 2000; Chaloupka et al. 2008). Boat impact strandings were recognized by the presence of a crushed carapace or deep cuts originating from propellers or hulls of boats (Boulon 2000; Guimarães et al. 2021). Shark attack strandings included turtles with deep incisions or removal of soft tissue or body parts (Stacy et al. 2021). Humantake (take is defined under the Endangered Species Act as “to harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect, or to attempt to engage in any such conduct”) strandings were turtles with obvious evidence of having been butchered or poached, often accompanied with spear wounds (Boulon 2000). Miscellaneous strandings included turtles with natural, non-anthropogenic causes not fitting in any of the other categories (e.g., natural disasters, including weather and tsunami events; and internal diseases confirmed by necropsy), and unknown strandings were those for which no cause could be determined (Chaloupka et al. 2008). Statistical methods Chi-square goodness of fit tests were used to determine if there were equal proportions among months of stranding, stranding status, causes of stranding, and 156.0°W 155.5°W 155.0°W 19.0°N 19.5°N 20.0°N 0 10 20 30 40 km Coast East West Fig. 1. Stranding locations and the division into eastern (windward) and western (leeward) sides of Hawai‘i island. Coastline map courtesy of United States Geological Service (USGS) and Hawai‘i Statewide GIS Program. page 3 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan sex of stranded turtles for all of Hawai‘i Island. When comparing west and east Hawai‘i, contingency tables and Chi-square tests of independence were used. All analyses were performed using the statistical software R version 4.2 (R Core Team 2022). Statistical significance was accepted at p < 0.05. It is reasonable to model the occurrence of turtle stranding events as a Poisson process with a rate λ that potentially changes through time and space. If strandings are classified into groups, then there are two equivalent ways of modelling this: each class as an independent Poisson process with its own rate λi, or as a single overall process at rate λ that generates an event at time T, and this event is then distributed to a class by a categorical random draw from some class distribution π that potentially also depends on time T. Of particular interest is the case where the categorical distribution π does not change with time, which is equivalent to saying that the ratios between the class rates λi are also constant. Probabilistically, the class is independent of the rate of the Poisson process. While the overall rate λ at which turtle strandings are observed depends on population size and human reporting patterns, this model allows us to investigate potential changes in the cause distribution π over time. To this end, multinomial linear models with Poisson error structures were fit using the nnet package (Venables and Ripley 2002) and model selection was carried out using Akaike Information Criterion (Akaike 1974). These models produce a prediction function which may be interpreted as the class distribution π(t), allowing us to compare models with and without a dependence on time. RESULTS A total of 754 green turtles stranded on Hawai‘i Island from June 1983 to June 2022. Of those strandings, 375 (49.7%) were located on the leeward or west coast of Hawai‘i Island, while 379 (50.3%) were located on the windward side or east coast of Hawai‘i Island (Fig. 1, Table 2). Of the 754 stranded turtles in the records, slightly over half had no discernable cause that could be determined (the “unknown” cause). The most common known cause of stranding was hook-and-line fishing gear, accounting for about 1 in 5 strandings. The distribution of causes is significantly different between the east and west coasts of the island (Chi-square test, X7 = 69.5, p < 10-10), with the effect being driven most strongly by the FP and Miscellaneous categories (Table 1). Temporal trends The number of strandings on Hawai‘i Island has fluctuated over the years but shows an overall increase over time (Fig. 2). Across both sides of the island, strandings were less frequent in winter, November– February, than in other months (Table 2). The highest totals were observed between March and August. Raw counts of hook-and-line fishing gear strandings have steadily increased over the years, and while strandings with FP as the chief cause of stranding have remained low overall, the number of FP-caused strandings was higher after 2000 (Fig. 3). The second most common known cause of stranding in west Hawai‘i was miscellaneous, a category that includes a significant number of strandings associated with the 2011 Tōhoku tsunami, while in east Hawai‘i FP is the second leading cause (Table 1, Fig. 3). To investigate changes in the relative rates of stranding causes, multinomial log-linear models were fit using date of record as a predictor. To reduce the variance of the fitted model parameters, the causes as recorded were consolidated into three categories: Human-caused (hook-and-line, boat impact, human Table 1. Raw counts and proportions of stranding cause from 1983–2022 for Hawai‘i island, separated into east and west sides. Fibropapillomatosis is abbreviated FP East West Total Cause n%n%n% Boat impact 9 2.4 16 4.3 25 3.3 FP 53 14.0 1 0.3 54 7.2 Hook/line 85 22.4 76 20.3 161 21.4 Human take 19 5.0 14 3.7 33 4.4 Misc. 5 1.3 23 6.1 28 3.7 Net 7 1.8 9 2.4 16 2.1 Shark attack 8 2.1 16 4.3 24 3.2 Unknown 193 50.9 220 58.7 413 54.8 page 4 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan take, and net); predation, disease and weather (shark attack, FP, and Misc.); and the original unknown category. The 2011 Tōhoku tsunami-related strandings, as well as the records prior to 1985 were excluded from the model fit. The Akaike Information Criterion (AIC) is used to compare a series models of models using natural splines based on date of standing with increasing degrees of freedom. The AIC increases going from a null model (AIC 1300.9) with no dependence on year to a predictor function with 2 degrees of freedom (AIC 1304.7), and then slightly decreases again, so that a 4 degree of freedom model (AIC 1299.7) has an AIC 1.2 smaller than the null model. Figure 4 displays a 3-degree of freedom model (AIC 1301), with confidence bands constructed using the bootstrap. The null model is represented by dotted white lines, and apparently fits within the confidence bands of the model that includes dependence on year. These results show a lack of evidence that the date of stranding provides significant information about the relative rates of stranding among the three consolidated cause categories. Size and gender Stranded turtles in the records ranged from 19.8 cm to 99 cm straight carapace length (SCL), with a mean of 54.8 cm, across 381 juveniles, 88 subadults, and 19 adults. No carapace length measurement was recorded in 266 of the case reports. Turtles stranding in east Hawai‘i (µ ± SE = 58.7 ± 1 cm SCL, n = 227) were significantly larger (t-test, t378 = 6.29, p = 9 × 10-10) than those in west Hawai‘i (µ ± SE = 51.3 ± 0.6 cm SCL, n = 261) Figure 5 shows SCL distributions for each cause, and while the distribution of SCL is not independent of cause (ANOVA, F(7, 480) = 3.41, p = 0.0014), the differences between the groups are small compared to the within-group variances. The records contain 154 female, 145 male, and 455 gender undetermined cases, also with marginally different distributions between sides of the island (Chi-square, X2 = 6.4, p = 0.042). FP tumor presence/absence As shown in table 3, 460 records indicated the absence of FP tumors, 150 records noted the presence of a tumor, and 144 records are missing this observation. Note that the presence of a FP tumor does not necessarily mean that the primary cause of stranding was recorded as FP. Tumor presence/absence is significantly associated with side of the island, with turtles stranding in east Hawai‘i more likely to have Table 2. Strandings in each month for Hawai‘i island, separated into east and west sides Coast Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total East 29 31 32 38 33 42 37 30 25 33 25 24 379 West 24 27 45 36 42 40 42 39 16 30 16 18 375 Total 53 58 77 74 75 82 79 69 41 63 41 42 754 Fig. 2. Number of strandings from 1983–2022 for Hawai‘i island, separated into east and west sides. Data for 2020 and beyond are incomplete due to COVID-19 disruptions to data collection. 0 10 20 30 40 50 1990 2000 2010 2020 Year Number of strandings Coast East Hawaii West Hawaii page 5 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan Fig. 4. A multinomial regression fit using natural splines with 3 degrees of freedom. 95% confidence bands are constructed by bootstrapping. Records from years with asterisks (*) are excluded from the model. The dotted white lines correspond to a model with no dependence on year. Fig. 3. Number of strandings from each cause, separated into east and west sides. Fibropapillomatosis is abbreviated FP. Data for 2020 and beyond are incomplete due to COVID-19 disruptions to data collection. Shark attack Unknown Human take Misc. Net Boat impact FP Hook/line 1990 2000 2010 2020 1990 2000 2010 2020 1990 2000 2010 2020 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20 Year Number of strandings Coast East Hawaii West Hawaii * * * 0.00 0.25 0.50 0.75 1.00 1990 2000 2010 2020 Year Proportion of strandings Cause Human Unknown Predation, disease and weather page 6 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan tumors than those in west Hawai‘i (Chi-square test, X2 = 197, p < 10-10). Stranding status Of all the stranded turtles, 359 stranded alive, 381 stranded dead, and 14 turtles had no stranding status reported. Stranding status was found to be significantly associated with cause (Chi-square test, X7 = 93, p < 10-10). More turtles stranded alive than dead because of FP, hook-and-line, and miscellaneous, while boat impact, human take, shark attack, and unknown were causes more likely to result in dead stranded turtles. Net fishing gear strandings showed equal numbers of turtles that stranded alive and dead (Table 4). More turtles stranded alive than dead in the months of November– March, while more turtles stranded dead than alive in the months of April–October (Table 5). Stranding status was also found to be significantly associated with stranding location (Chi-square test, X1 = 21.5, p = 3.5 × 10-6). West Hawai‘i had 146 turtles strand alive and 221 strand dead, while east Hawai‘i had 213 turtles strand alive and 160 strand dead. DISCUSSION Seven hundred and fifty-four green turtles were recorded stranded on Hawai‘i Island in the period 1983–2022, which represents an unknown fraction of total strandings on Hawaiian shores in that time. Stranding programs rely on reports from the public, and are therefore dependent on the density of human activity at the shoreline as well as public knowledge of the reporting procedures. However, if a location is regularly accessed by more than a few people, a stranding is likely to be reported, and it is reasonable to believe that this will happen independently of the variables observed Fig. 5. Straight carapace length (SCL) was measured in 488 records, and plotted for each stranding cause. Fibropapillomatosis is abbreviated FP. Boxplot outliers begin at 1.5 times the inter-quartile distance. 20 40 60 80 100 Boat impact FP Hook/line Human take Misc. Net Shark attack Unknown Cause Carapace length (cm) Table 4. Survival status of stranded turtles by cause. Fibropapillomatosis is abbreviated FP Cause Alive Dead Not Recorded Boat impact 12 13 0 FP 38 16 0 Hook/line 115 45 1 Human take 6 27 0 Misc. 23 5 0 Net 8 8 0 Shark attack 8 16 0 Unknown 149 251 13 Total 359 381 14 Table 3. Fibropapillomatosis tumor presence in stranded turtles by side of Hawai‘i island Tumor Coast Present None Not Recorded East 141 143 95 West 9 317 49 Total 150 460 144 page 7 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan in these records. Strandings on Hawai‘i Island showed an overall increase in rate between 1983 and 2022. Green turtle strandings have also increased on the other main Hawaiian Islands since 1982 (Chaloupka et al. 2008). One important reason for this increase is a positive one: Green turtle populations in the Hawaiian Islands have recovered significantly since their 1974 protection by the State of Hawai‘i under Regulation 36 and their 1978 protection under the Endangered Species Act (Balazs and Chaloupka 2004; Bennett and Keuper-Bennett 2008). The increase in turtle population size will directly lead to additional observed stranding events, even if the risk to an individual turtle remains constant over time (Boulon 2000). Additionally, the human population increase on Hawai‘i Island and the rise in numbers of visitors at the shoreline increase the chance of encountering and reporting a stranding. In general, the locations of strandings shown in figure 1 reflect beaches and other shoreline areas with easy public access. Increased public awareness of strandings and response programs and the greater use of cell phones and the internet probably have led to more reporting over time. However, the increase in reported strandings appears to slow in the early 2000s (Fig. 2), stabilizing at approximately 25–30 per year. This trend was also noticed in studies covering the other main Hawaiian Islands (Chaloupka et al. 2008). In a turtle carcass drifter experiment along the shores of the Mississippi, public reporting of stranded carcasses was unexpectedly low: on popular mainland beaches, only 50% of the stranded turtles were reported; on accessible, but more remote barrier islands, only 11.1% of stranded carcasses were reported by citizens, and 0% of turtle carcasses that drifted into marshes were reported (Cook et al. 2021). These results send a strong message that remoteness of and public accessibility to stranding areas greatly influence the discovery of turtles, and that even structured stranding networks with established reporting mechanisms” may be overestimating the rate of reporting by the public, which influences the conclusions that can be drawn from citizen-derived data. There are two years post-2005 which show an unusually large number of green turtle strandings: 2011 and 2018. The peak in 2011 is associated with the March 2011 magnitude 9.0 Tohoku earthquake off the coast of Japan and the subsequent tsunami, large waves, and hazardous currents that it caused around Hawai‘i Island, and particularly its western shoreline (Cheung et al. 2013). The waves and currents associated with tsunamis bring marine life onshore with them and can wash turtles inland. Two hawksbill turtles were reported stranded in Hawai‘i as a result of the 2011 earthquake (Brunson et al. 2022), and a 2009 tsunami in Samoa similarly led to 52 turtles stranding on land (Bell et al. 2011). The apparent downward trend of strandings after 2018 is probably not because fewer turtles stranded, but is rather due to human behavioral changes caused by the COVID-19 pandemic. Throughout the pandemic, people in general spent much less time in public locations, and for some periods, Hawai‘i County and State beach parks were closed for recreational use by executive decree (County of Hawai‘i, Office of the Mayor 2020; State of Hawai‘i, Office of the Governor 2020). Similarly, tourism to the island and state was heavily restricted. All of these factors lead to a sharp drop in the number of people visiting Hawai‘i Island coasts, and subsequent to decreased reports of strandings. The highest rates of green turtle strandings occurred during the Hawaiian spring and summer months, from March–August. This is similar to the findings on O‘ahu where green turtle strandings were highest from March–June (Chaloupka et al. 2008), and for adult hawksbills in the Hawaiian Archipelago where strandings were highest from June–September (Brunson et al. 2022). Similarly, strandings of loggerhead, green, and leatherback turtles in Brazil were highest during the austral spring and summer seasons (Monteiro et al. 2016). Peak sea turtle stranding months during 20102019 in the Gulf of Mexico were also in the spring to summer (March to August) (Cook et al. 2021; Howell et al. 2021). Strandings on Hawai‘i Island were lowest during the months of September, November, and December, but a secondary peak in the month of October was seen. This same peak was observed in the 2022 green turtle strandings on Maui (Cutt et al. 2023); and O‘ahu showed a similar secondary peak of strandings in September (Chaloupka et al. 2008). Although the major Hawaiian green turtle nesting season is mid-April to September/October in the Table 5. Survival status of stranded turtles by month Month Alive Dead Not Recorded January 31 20 2 February 33 24 1 March 49 28 0 April 26 47 1 May 25 45 5 June 38 44 0 July 35 43 1 August 29 40 0 September 17 23 1 October 27 36 0 November 24 17 0 December 25 14 3 page 8 of 14Zoological Studies 63:16 (2024) © 2024 Academia Sinica, Taiwan Northwestern Hawaiian Islands (Niethammer et al. 1997), no seasonal variation in green turtle abundance within localized coastal Hawaiian foraging grounds has been documented (Balazs unpublished). The higher spring/summer stranding patterns seen on Hawai‘i Island may reflect seasonal differences in water temperature which affects carcass decomposition rates (Cook et al. 2021), periodic shifts in shoreline human activity and stranding reporting, or cyclical changes in surf, currents, and winds that can push carcasses to shore. In the Hawaiian Islands, northeasterly trade winds are the most common weather pattern, especially in the summer; however, other weather patterns could influence turtle carcass drift. Migratory mid-latitude low pressure systems are common October to April with about nine fronts passing over Hawai‘i Island in a season, during which winds shift from southwesterly to northerly. Kona Storms or cold-core lows, November to April, although rare with unpredictable paths, can cause waterspouts, torrential rain, and high surf; and tropical cyclones from June to midNovember, can bring high surf, storm surge, and strong onshore winds to Hawai‘i Island (Longman et al. 2021b; Nullet 2023). In addition, El Niño Southern Oscillation events sporadically impact the Hawaiian Islands and can cause weakened trade winds, less rainfall, and warmer ocean temperatures (El Niño phase) between November to April or stronger trade winds, greater rainfall, and cooler ocean temperatures (La Niña phase) (Longman et al. 2021a). Hook-and-line fishing gear was the most common known cause of stranding of green turtles on Hawai‘i Island as a whole. Fishing gear strandings show a similar qualitative pattern to the overall time series (Fig. 3), increasing from 1983 to the mid 2000s and then apparently leveling off. Chaloupka et al. (2008) found a similar increase of hook-and-line fishing gear strandings since 1982. It is difficult to untangle the effects of the increased population of Hawaiian green turtles from the risk of hazard from fishing activity and gear, as both factors directly affect the rate of strandings observed. Hawaiian green turtles are frequently reported with hooks intact and line entangled around their flippers and body. These interactions are often a result of lost and/ or discarded fishing gear or fishers cutting the line when accidental hooking occurred, which illustrates the need for stronger management and preventatives (Nitta and Henderson 1993). Hook-and-line fishing gear strandings were also prevalent on O‘ahu, Maui, and Kauai, making up the second most common cause of stranding of green turtles (Chaloupka et al. 2008). Similar to the findings of the present study, fishing gear was the foremost cause of stranding for green turtles on Maui in 2022, with 81% of the total strandings showing interactions (Cutt et al. 2023). The number of hook-and-line strandings may be even greater than estimated. Work et al. (2015) performed necropsies (postmortem autopsies) on stranded turtles throughout the Pacific and found that 48% of foreign body ingestion cases (mostly all associated with fishing gear) showed no external sign of fishing line interactions. Green turtle strandings resulting from interactions with fishing gear are prevalent around the world, including the U.S. Virgin Islands (Boulon 2000), Brazil (Guimarães et al. 2021), Taiwan (Cheng et al. 2019), New Caledonia (Read et al. 2023), and Greece (Panagopoulos et al. 2003). However, unlike the line/ hook entanglements on Hawai‘i Island, in Taiwan, pond nets were the most common fishing gear causing turtle strandings over 23 years (Cheng et al. 2019). In contrast, at Samandağ Beach on the eastern Mediterranean coast of southern Türkiye (Turkey), from 2002–2017, fishing activities caused only 7% of the green turtle standings, while marine pollution accounted for 56% of strandings (Sönmez 2018). Fibropapillomatosis was the second most common cause of stranding on Hawai‘i Island, whereas Chaloupka et al. (2008) found FP to be the main cause of stranding in green turtles in O‘ahu, Maui, and Kauai. The relative rates of strandings by cause over time is of particular interest for managers and conservationists because it can indicate particular sources of danger to turtle populations. The overall rate of observation depends on population size and human reporting behavior in a complex way that is difficult to disentangle, but by looking at the distribution of causes over time we may be able to identify structural changes in the cause of strandings. Although slightly over half of the stranded green turtles in this study were listed with “unknown” cause of mortality, these turtles still provide valuable temporal, geographic, and biological data. We share this predicament of unknown cause with others studying sea turtle strandings. For example, 50% of strandings in New Caledonia were unknown, defined as “no necropsies were done and no apparent cause of death by external examination” (Read et al. 2023). Chaloupka et al. (2008) also had high rates of strandings with unknown causes. In our study and others, given the circumstances of discovery (time, weather patterns, location, retrieval), condition of the animal (undetermined health and behavior prior to stranding, unspecified time of death, decomposition, or scavenging), and limited resources for extensive diagnostic procedures (necropsies, histopathology, toxicology, and microbiology), many stranded turtles remain forever in the category of unknown causes of mortality. However, the goal should be to increase reporting of strandings by the public, to encourage detailed observations at time of discovery, and develop page 9 of 14Zoological Studies 63:16 (2024)