Monitoring bottlenose dolphin population status and health in the Sea of Abaco

A BMMRO field effort led by John Durban and Holly Fearnbach.

Dr. John Durban is a research biologist with the Center for Whale Research, WA, working primarily under contract to the U.S. National Oceanic and Atmospheric Administration on killer whale studies. John has worked with BMMRO since 1995 and he conducted his PhD studies on bottlenose dolphins on Little Bahama Bank.

Holly Fearnbach is a PhD student with the University of Aberdeen (U.K). She is working with BMMRO to study the influence of environmental variability on bottlenose dolphin social structure and population demographics.

Background
Changes in the marine environment are integrated up though trophic levels and exhibited in the status of marine top predators. As such, bottlenose dolphins are an iconic symbol of the health of the Bahamian marine environment, representing a visible "indicator" species that can be studied to monitor the effect of environmental change and coastal development. Since 1992, the Bahamas Marine Mammal Research Organisation (BMMRO) has been studying the dolphin population that inhabits the shallow waters of Little Bahama Bank. This "island" population is isolated by the deep oceanic waters that surround the bank system, and photographic identification studies (Figure 1) have been used to monitor abundance, residency, social structure and survival trends.

Figure 1: Photographic "re-sight" of an individual bottlenose dolphin (Tt13) in 1992 and 2000.

These studies have demonstrated how long-term studies can help track individual dolphins to make inference about both population and ecosystem health. For example, annual survival estimates produced from mark-recapture analysis of photographic identification data show a generally high survival trend punctuated by an acute drop in survival around the active hurricane year of 1999 (Figure 2). Further monitoring of the occurrence of individual dolphins is necessary to track the effects of environmental change on this marine ecosystem and its dolphin population. Specifically, this will allow us to elucidate the links between dolphin survival and key environmental covariates such as climate variability and coastal development. In October 2007 we began a renewed dolphin monitoring effort in collaboration with Friends of the Environment to extend this important time series. This field season was very successful with more than 1200 miles of boat-based survey effort documenting 35 distinct individual dolphins in 20 encounters (Figure 3). Of the 35 documented individuals, the majority (25) were re-sightings of previously-identified dolphins, extending the time series on these resident individuals.

Figure 3: Map displaying survey tracks (blue lines) and encounters with dolphins (red circles) in the Sea of Abaco study area, October 2007.

In addition to documenting individuals, we also began documenting individual size. We have developed a simple approach for obtaining morphometric measurements based on photographing two green laser dots that have been projected onto the body of a dolphin using two small laser-pointers. These laser-pointers are mounted in a parallel orientation on the camera to maintain a fixed and known separation distance, and the dots therefore provide a scale of known dimension on the image of the dolphin (Figure 4). This technique will be used to monitor individual growth, to infer health status, by obtaining repeat body measurements over time. Morphometric measurements will also be used to evaluate the taxonomic discreteness of this population - dolphins in this unique "island" environment may be the smallest bottlenose dolphins in the world.

Figure 4:Two green laser dots (10cm apart) projected onto the dorsal fin of a bottlenose dolphin in the Sea of Abaco. This technique will be used to measure the size and growth of individuals.