Sirenian Conservation: Issues and Strategies in Developing Countries

17. Sirenian Pathology and Mortality Assessment

17 Sirenian Pathology and Mortality Assessment Robert K. Bonde, Antonio A. Mignucci-Giannoni, and Gregory D. Bossart Previous chapters in this book have discussed how rapid technological development in recent history has accelerated human impact on sirenians. Determining the causes of the numerous threats to manatees, especially those that result in mortality or disease (tables 17.1 and 17.2) will enable implementation of appropriate protection measures. As a conservation tool, cause of death determination and interviews with the local population are vital steps for establishing successful research and conservation programs1. Such information can identify threats to the species and provides an opportunity to collect information on population distribution, diseases , parasites, diet, reproductive condition, genetics, and morphological anomalies. Anthropogenic Causes of Mortality Beyond being exploited for food2, manatees and dugongs are faced with additional human-originated threats. In Florida many manatees are killed each year as a result of collisions with water vessels (figure 17.1). Twenty-five percent of Florida manatee mortalities are due to such collisions3. From 1990 to 2006, 19.8% of all known manatee deaths (n = 121) in Puerto Rico were identified to be due to boat strikes4. Most boat strikes are single events, affecting one individual. However, both in Puerto Rico and Florida, scientists have documented boat strikes of up to five manatees at one time, all assumed to be part of a mating herd when struck. Manatees in mating herds are preoccupied with breeding and may be especially vulnerable to collisions. Most adult manatees in Florida, and some in Puerto Rico, bear scars as evidence of nonfatal encounters with boats, and sirenians with propeller scars are observed in other countries as well5. Many manatees do not appear to be permanently affected by moderate to light vessel strikes, but others suffer very serious injuries that may affect reproduction and migration or lead to death. In fatal cases the trauma can be acute or chronic in nature6. In acute cases, the injury is often massive, deep, or severe Table 17.1. Determined cause of death for 68 fresh manatee carcasses from Florida by age/size and sex in decreasing order of frequency, January 1996 to January 2004. Adult Subadult Perinatal Cause of Death Total M F M F M F Trauma 32 7 6 12 7 . . Cold stress syndrome 12 1 . 4 7 . . Inflammatory/infectious disease 8 2 0 3 2 . 1 Suspected brevetoxicosis 6 2 1 2 1 . . Cachexia 4 . . . . 2 2 Unknown 4 2 . 1 1 . . Intestinal foreign body 1 . . . 1 . . Cardiomyopathy 1 1 . . . . . Total 68 Source: Bossart et al. 2004, reprinted with permission. Table 17.2. Predominant cause of stranding for 121 West Indian manatees recovered from Puerto Rico between 1990 and 2006 with comparison to previously tabulated data. Prior to 1990 Total Total Causes Cases % Cases % Cases % Natural 6 10.9 53 43.8 59 33.5 Dependent calf 6 10.9 28 23.1 34 19.3 Illness 0 0 21 17.4 21 11.9 Stillborn 0 0 3 2.5 3 0.6 Predation 0 0 1 0.8 1 0.6 Human related 31 56.4 35 28.9 66 37.6 Drowning 0 0 1 0.8 1 0.6 Entanglement 1 1.8 1 0.8 2 1.2 Gunshot 2 3.6 2 1.7 4 2.3 Pollution 0 0 2 1.7 2 1.2 Capture 22 40 5 4.1 27 15.3 Watercraft 6 10.9 24 19.8 30 17 Undetermined 18 32.7 33 27.3 51 28.9 Total 55 121 176 Source: Adapted and updated from Mignucci-Giannoni et al. 2000, with permission. Figure 17.1. Fatally injured West Indian manatee in Florida with characteristic propeller lesions on dorsum. (Courtesy of U.S. Geological Survey.) 150 Section II: Research Strategies for Sirenians enough for the animal to succumb to shock or drowning , and death is rapid. In chronic cases some individuals develop bone lesions that can become systemic and eventually cause death7. Another form of human-related death in Florida is attributed to entrapment in the gates of canal locks or flood control structures resulting in crushing or drowning8 . Poor nutritional condition in manatees can also be anthropogenic in nature, as it is often related to habitat loss and possibly high contaminant exposure levels9. Entanglement in fishing gear or debris is usually not life threatening but can lead to severe tissue damage and sometimes self...