Littles, Chanda J.

2 publications

Littles, Chanda J.; Pilyugin, S. Sergei; Frazer, Thomas K. (detail)
2015A combined inverse method and multivariate approach for exploring population trends of Florida manatees.
Mar. Mamm. Sci. DOI: 10.1111/mms.12247 Publ. online July 30, 2015.
—ABSTRACT: Given the expense and time required to monitor marine mammal populations effectively, approaches that fully exploit the resulting data certainly are warranted. We employed a two-step modeling approach to estimate key demographic parameters, including immigration, from aerial surveys of manatees (Trichechus manatus latirostris) in the Northwest management unit of Florida. Abundances of adults and calves were predicted by multivariate adaptive regression spline (MARS) models, after accounting for heterogeneous detection rates caused by variable environmental conditions. The resulting predictions were incorporated into a stage-structured, deterministic model that used an inverse method to estimate parameters with and without immigration. The model without immigration estimated mean survival probabilities of 0.966, 0.923, and 0.794 for adults, subadults and calves, respectively, with a per capita reproductive rate of 0.135. These parameter estimates yielded an overall mean population growth rate of approximately 1.037, which is comparable to rates from mark-recapture studies. When we added an immigration term that accounted for the greater slope in adult counts since 1999, as identified by the MARS model, the estimated per capita reproductive rate was 0.122, with survival probabilities for adults, subadults and calves of 0.926, 0.920, and 0.833, respectively. These rates were coupled with an estimated mean winter immigration rate corresponding to roughly 5.2% of the adult and subadult population. In this latter scenario, the number of manatees in the core population of the Northwest management unit was predicted to remain constant, with a population growth rate near one, and additional manatees counted during aerial surveys were deemed to be immigrants. While further studies could certainly expound on the potential effects of migrants on population indices, we present this first published immigration estimate for wintering manatees in northwest Florida.
Littles, Chanda J.; Bonde, Robert K.; Butler, Susan M.; Jacoby, Charles A.; Notestein, Skye K.; Reid, James P.; Slone, Daniel H.; Frazer, Thomas K. (detail)
2019Coastal habitat change and marine megafauna behavior: Florida manatees encountering reduced food provisions in a prominent winter refuge.
Endangered Species Research 38: 29-43. 3 tabs. 6 figs. DOI: https://doi.org/10.3354/esr00933 Jan. 24, 2019.
—ABSTRACT: A decline in submerged aquatic vegetation (SAV) within Florida's spring-fed, thermal refuges raises questions about how these systems support winter foraging of Florida manatees Trichechus manatus latirostris. We analyzed telemetry data for 12 manatees over 7 years to assess their use of Kings Bay, a winter refuge with diminished SAV. After accounting for the effect of water temperature, we hypothesized that the number of trips out of Kings Bay would increase and the time wintering manatees spent in Kings Bay would decrease. Trips out of and into Kings Bay also were compared to assess potential influences on exiting or entering. There were no detectable differences in the number of trips out of the bay or overall time manatees spent in Kings Bay across winters. The percentage of time water temperatures were below 20oC was the single best predictor of increased time spent in Kings Bay. Trips out of Kings Bay were more likely to occur after 12:00 h and during a high but ebbing tide, compared to trips into the bay. Nine manatees tracked for longer than 75 days in winter spent 7–57% of their time in the Gulf of Mexico, and 3 of these manatees spent 7–65% of the winter >80 km from the mouth of Kings Bay. Results suggest the low amount of SAV in Kings Bay does not obviate its use by manatees, though there are likely tradeoffs for manatees regularly foraging elsewhere. Accounting for movements of Florida manatees through a network of habitats may improve management strategies and facilitate desirable conservation outcomes. Note correction on website: "February 22, 2019: Substantial changes were made throughout the article after publication: to the text, Tables 1 & 2, and Fig. 4 (see .pdf 'Original text with edits')."