The effectiveness of rehabilitation post catastrophic environmental events as a conservation strategy for marine turtles

Abstract

In-water monitoring of the health and well-being of marine vertebrates is usually expensive and therefore may not be undertaken by management agencies with financial constraints. However, the use of stranding data can provide a cost-effective alternative estimation of disease and mortality. Strandings for marine turtles in Queensland are recorded in a web based database (StrandNet) managed by the Queensland Government’s Department of Environment and Heritage Protection (EHP). Data recorded in StrandNet for marine turtles stranded from the entire east coast of Queensland between 1996 and 2013 were investigated for patterns of stranding in an attempt to identify which factors, such as extreme weather events, may cause stranding of marine turtle species and, further, use these patterns to predict stranding and the required responses to mitigate the negative impact mass mortalities have on endangered species such as marine turtles. Significant stranding trends in Queensland between 1996-2013 were: (i) an increase in the number of animals reported stranded within the study site; (ii) a species (loggerhead and green marine turtles) prevalence for stranding; (iii) a seasonal effect on different age classes stranding with most overall strandings occurring between August and November; and (iv) stranding hotspots (Moreton Bay, Hervey Bay, Rockhampton region and Cleveland Bay) persisting throughout the study timeframe. One strategy to mitigate the negative effects of marine turtle stranding is to provide medical care to those that strand alive in the hopes they can return to the functional population. Rehabilitation of marine turtles in Queensland is multifaceted. It treats individual animals, serves to educate the public, and contributes to conservation. Of 13854 marine turtles reported as stranded during the 18-year period of investigation, 5022 of these turtles stranded alive with the remainder verified as dead or of unknown condition. A total of 2970 (59%) of these live strandings were transported to a rehabilitation facility. The original cause of stranding has an impact on the success of rehabilitation and this may influence where treatment efforts are directed. For example, of the turtles admitted to rehabilitation exhibiting signs of disease (natural cause of illness) (18% of all animals admitted to rehabilitation), 88% of them died either unassisted or by euthanasia. Sixty-six percent of turtles admitted for unknown causes of stranding died either unassisted or by euthanasia. By contrast, all turtles recorded as having a buoyancy disorder with no other presenting problem or disorder recorded, were released alive. One hundred and one of the turtles released from rehabilitation were reencountered: 77 reported as restrandings (20 dead, 13 alive subsequently died, 11 alive subsequently euthanized, 33 alive) and 24 recaptured during normal marine turtle population monitoring or fishing activities. Considering the high mortality rate and low successful recapture rate, rehabilitation may not be economically viable in its present configuration. Not admitting marine turtles to rehabilitation centres and returning alive animals to sea after basic in-field triage may not address the presenting problem either. During this 18-year retrospective investigation, 1261 turtles were released back into the ocean without being admitted to rehabilitation. Of these, 67% of animals re-stranded for a second time with the same initially recorded reason. Being able to understand commonalities of marine turtle strandings is important for marine resources managers to permit better decision-making and allocation of resources following increased strandings. Several environmental factors influence the prevalence of marine turtle stranding. These factors are thought to be rainfall, freshwater discharge and temperature. There have been links established between seagrass die off and flooding events making these chosen factors good proxies of seagrass availability/viability. Increased rainfall leads to increased freshwater discharge into the marine environment bringing with it increased nutrient and sediment loads that smother sea-grasses and other food items, directly impacting marine turtles by removing their available food sources. Similarly, for multiple underlying reasons, more strandings occur during the warmer months. Using these foundations, we can predict when and how many strandings are likely to occur by the manipulation of environmental variables in a predictive model. Given the identification of stranding predisposition, hotspots, environmental triggers, the cost of individual treatment and the availability of alternative options, this study suggests that rehabilitation may not be viable to treat all stranded turtles, unless the cause and circumstances of stranding are historically treatable. Instead, efforts may be better used if mobile triage units are deployed to treat juvenile green turtles with unknown reasons for stranding in hotspots such as Moreton Bay, Hervey Bay, Rockhampton region and Townsville Region (Cleveland Bay) after a major flooding event has occurred. While this model is robust based on the preliminary available data, it may be adjusted to incorporate other influencing factors such as specific disease effects under catastrophic conditions and improve outcome (successful returns to the ocean) through more research into diseases and survival rates to produce more accurate predictions.