Wildlife managers tasked with understanding mortality patterns and survivorship for populations of concern can potentially glean much information from recovered carcasses.
Determination of cause of death and how mortalities are distributed across populations and life history classes can play a crucial role in directing conservation action.
One of the pressing questions for cetacean scientists and managers is how to interpret and apply the information arising from scientific examination of recovered carcasses. Managers need to be able to extrapolate an approximate mortality rate for population units from the number of actual recovered carcasses.
The loss of known resident animals from a well-known population can be indicative of mortality, emigration, or changed identification features. The ability to discriminate among these possibilities provides the basis for developing a precise and accurate multiplier for extrapolating carcass recoveries to population-level mortalities, potentially related to specific sources. One of the few situations where it is possible to determine when specific individual bottlenose dolphins are missing, whether due to emigration or mortality, is the multi-decadal, multi-generational resident dolphin community of Sarasota Bay, studied by the Sarasota Dolphin Research Program since 1970. We compared historical Sarasota Bay survey data from 1993-2012 on bottlenose dolphin presence and absence, and stranding data on recovered carcasses, to obtain estimates of the proportion of dead dolphins that are recovered as carcasses.
Only a fraction of dead dolphins are recovered as carcasses. For cetaceans inhabiting the open waters of the Gulf of Mexico, one study published in 2011 estimated that only 2% of dead animals were recovered. For Sarasota Bay, we determined that a third of confirmed and probable bottlenose dolphin mortalities were recovered as carcasses each year, on average. These two situations may define approximate upper and lower bounds for estimation of mortality rates from recovered carcasses. The Sarasota Bay bottlenose dolphin carcass-recovery rates are intended to serve as points of reference when considering mortality patterns for other populations, recognizing that there is likely much variability across sites. In the wake of recent large scale environmental catastrophes such as the Deepwater Horizon oil spill, and several Unusual Mortality Events involving the recovery of hundreds of cetacean carcasses, there is a strong and pressing need for being able to evaluate mortality numbers relative to impacts to populations.
This article was published on page 25 in the January 2014 Nicks n Notches.