Recovery rates of bottlenose dolphin (Tursiops truncatus) carcasses estimated from stranding and survival rate data

Abstract

Recovery of cetacean carcasses provides data on levels of human‐caused mortality, but represents only a minimum count of impacts. Counts of stranded carcasses are negatively biased by factors that include at‐sea scavenging, sinking, drift away from land, stranding in locations where detection is unlikely, and natural removal from beaches due to wave and tidal action prior to detection. We estimate the fraction of carcasses recovered for a population of coastal bottlenose dolphins (Tursiops truncatus), using abundance and survival rate data to estimate annual deaths in the population. Observed stranding numbers are compared to expected deaths to estimate the fraction of carcasses recovered. For the California coastal population of bottlenose dolphins, we estimate the fraction of carcasses recovered to be 0.25 (95% CI = 0.20–0.33). During a 12 yr period, 327 animals (95% CI = 253–413) were expected to have died and been available for recovery, but only 83 carcasses attributed to this population were documented. Given the coastal habits of California coastal bottlenose dolphins, it is likely that carcass recovery rates of this population greatly exceed recovery rates of more pelagic dolphin species in the region.

Where were they from? Modelling the source stock of dolphins stranded after the Deepwater Horizon oil spill using genetic and stable isotope data

“…1) primarily remained within the Bay -and the few that entered coastal waters re mained within 1.75 km of shore (Wells et al 2017, this Theme Section). Strandings of offshore dolphins, including the offshore morphotype of bottlenose dolphins, are also very rare (Peltier et al 2012, Carretta et al 2016, DHNRDAT 2016. For example, in the nGoM, 620 stranded bottlenose dolphins were examined genetically during the time of the DWH oil spill and only 2 were of the offshore morphotype.…”

Drift and beaching patterns of sea otter carcasses and car tire dummies

“…, Carretta et al . ). Recovering carcasses from anthropogenic mortality events, such as boat strikes, fisheries interactions, and oiling, are especially important to identify and investigate to support mitigation measures.…”

“…, Carretta et al . ), seabirds (Bibby and Lloyd ), and sea otters ( Enhydra lutris ; Garshelis , Estes et al . ).…”

“…Enumerating and examining marine animal carcasses can be used to quantify mortality rates and determine causes of mortality (Epperly et al 1996, Eguchi 2002, Williams et al 2011, Carretta et al 2016. Recovering carcasses from anthropogenic mortality events, such as boat strikes, fisheries interactions, and oiling, are especially important to identify and investigate to support mitigation measures.…”

“…It has been well-documented that beach strandings grossly underestimate total at-sea mortality of many marine species, including sea turtles (Epperly et al 1996, Koch et al 2013, cetaceans (Eguchi 2002, Williams et al 2011, Peltier et al 2012, Wells et al 2015, Carretta et al 2016, seabirds (Bibby and Lloyd 1977), and sea otters (Enhydra lutris; Garshelis 1997, Estes et al 2003. However, in cases where baseline population abundance estimates are unavailable, carcass recovery may be the most reliable method to estimate mortality.…”