Challenges of implementing Mark-recapture studies on poorly marked gregarious delphinids

Area of Science:

• Marine Biology
• Ecology
• Conservation Science

Background:

• Estimating population parameters for poorly marked, gregarious species like common dolphins (Delphinus sp.) presents significant challenges due to low mark ratios and high mobility.
• Accurate population assessments are crucial for effective conservation and management strategies for marine mammals.

Purpose of the Study:

• To develop and validate reliable photo-identification protocols for common dolphins in the Hauraki Gulf, New Zealand.
• To estimate key population parameters, including apparent survival and abundance, using mark-recapture techniques.

Main Methods:

• Implemented a novel photo-identification protocol combining nicks, notches, and pigmentation patterns, with a distinctiveness threshold.
• Stratified identified individuals into 'highly distinctive' (D1) and 'distinctive' (D2) categories.
• Utilized a POPAN super-population model with photo-identification data collected from January 2010 to December 2013.

Main Results:

• Identified 2,083 unique adult common dolphins, with 51.3% classified as D1 and 48.7% as D2.
• Achieved a re-sighting rate of 34.3% for identified individuals over subsequent years.
• Estimated apparent survival at 0.767 for D1 individuals and 0.796 for combined D1 & D2. Seasonal abundance for D1 individuals varied significantly, ranging from 732 in autumn 2010 to 5,304 in spring 2013.

Conclusions:

• The developed photo-identification protocol and data stratification provide a robust and novel approach for estimating population parameters in common dolphins.
• Estimates derived from highly distinctive individuals (D1) were considered least biased, offering a reliable measure of population abundance.
• The methodology can be adapted for other large, gregarious animal populations with poor natural markings, aiding broader ecological research and conservation efforts.