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Breeding Density and Habitat Selection of the Grey-headed Fish-eagle in Noakhali District, Bangladesh

MOHSINK. MIRON
House 16/C, Flat 501, Tallabag, Sobhanbag, Dhaka 1207, Bangladesh

SAYAMU. CHOWDHURY
House 16/C, Flat 501, Tallabag, Sobhanbag, Dhaka 1207, Bangladesh

Abstract/ Executive Summary:

The Near Threatened Grey-headed Fish-Eagle (GHFE, Ichthyophaga icthyaetus) has undergone a moderately rapid population decline due to habitat degradation. We studied population density and nest-site characteristics of the GHFE at Noakhali, Bangladesh, between November 2015 and January 2016. We used distance sampling along eight-line transects to estimate GHFE breeding density. At each nest, we recorded the height and crown density of the nesting tree, and distances from the nearest human settlement, waterbodies with commercial fisheries (hereafter, commercial waterbodies), and water bodies without commercial fisheries(hereafter, noncommercial waterbodies). We estimated 0.27 (95% CI: 0.15–0.49) GHFE nests or 0.54individuals per km². We detected a total of 26 nests, with 85% built on siris (Albiziaspp.) trees. All nests were built on the tallest tree within the vicinity of the nesting site, with an average height of 12.561.5 m, and on trees with an open (69%,n=18) canopy structure. We found that 76.9% of nests were located within 100 m of human settlements (x²=4.13, df=1, P=0.04) and 73.1% were located closer to a commercial waterbody than to noncommercial waterbody (x²=13.4, df=1, P=0.0002). The total area of commercial waterbodies within 500m of nests was higher than that of noncommercial waterbodies (W=507, P=0.001). These results indicate that GHFEs do not require undisturbed or natural water bodies and can survive well in some human-modified landscapes if adequate food (e.g., commercial fisheries) and tall trees for nesting are available; thus the species may be less vulnerable to environmental changes than previously thought.

Keywords: Grey-headed Fish-Eagle; Ichthyophaga icthyaetus; Bangladesh; Distance sampling; Density; Fisheries

Location: Noakhali, Bangladesh

Start Date:

End Date:

Program Area: Conservation and Biodiversity

Commodity: Fish

Objectives:

We investigated whether the presence of GHFE was correlated with human-modified land-scapes, especially water bodies with commercial fisheries (commercial waterbodies), and whether those landscapes influenced the nest-site selection. We also examined whether GHFE in Bangladesh preferred tall nest trees with open crown structures, which appear to allow unencumbered parental access in Cambodia (Tingay et al. 2010).

Methodology:

Study Area. We conducted surveys for nesting GHFE at Companiganj sub-district (hereafter Companiganj) of Noakhali district, Chittagong Division in south-central Bangladesh. We chose Companiganj for our surveys because we discovered four GHFE nests during an opportunistic visit to the area in January 2015. We initiated the study during the following breeding season to understand factors for such a high density of GHFE in an area of 305km2with between 183,000 and 183,500 people (Karim et al. 2013). Our study area covered 104km2, located approximately 100 km north of the Bay of Bengal were approximately 3700 km2of water-bodies, agricultural cropland, homestead patches, and human settlements, and approximately 70,500ponds (Karim et al. 2013) dotted the landscape. The average annual precipitation in the study arearangedfrom6.6mmduringthedryseason(January) to 723.4 mm during the monsoon season(July) in 1999–2010, and the mean annual temperature ranged from 21 to 31⁰C (1981–2010; Khatun et al.2016). Breeding Density. We used distance sampling to estimate GHFE breeding-pair density within the study area, as distance sampling is a simple and widely used approach to estimate densities of biological populations in defined areas (Buckland et al. 2001, Thomas et al. 2010, Cornils et al. 2015, Buckland et al. 2016). We searched for nests between 15 November 2015 and 31 January 2016 along eight transect lines that we established following relatively straight roads through home-stead forest and village landscapes with no natural or artificial barriers. We conducted the surveys in winter (November and December), the primary breeding season of the GHFE (Naoroji 2006, Siddiqui et al. 2008). Transects were placed 1 km apart and transect length varied from 9.2–15.5 km. We surveyed each transect by walking slowly (3 km/hr) in a two-person team, while each person searched for GHFE nests on one side of the transect. We used a Garmin e-Trex (Garmin, KS, USA)handheld global positioning system unit to follow transects and to record nest locations. We recorded a nested structure as occupied when at least one GHFE was present, or if the size, structure, and height of a nest we observed were consistent with a GHFE nest but no GHFE was present. In the latter cases, we visited the nest location later for further verification. In all cases, we used 10342 binoculars to examine nests. We measured the perpendicular distance between the transect lines and each GHFE nest we found, and we calculated the total area surveyed for GHFE as a polygon with a 1-km buffer around our transect survey area.

Source of Information: Journal of Raptor Research, 53(2) : 134-141

Article Link (Online Journal): https://doi.org/10.3356/JRR-18-33

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Our study represents the most complete report on the density of GHFE in Bangladesh. We recommend long-term study in other areas of Bangladesh with high (Sylhet division) and low (Rajshahi division)densities of commercial fisheries (Shamsuzzaman et al. 2017). Moreover, we recommend investigating possible conflicts between the GHFE and local fish farmers. Detailed information on these aspects will allow us to further understand the relationship between local fisheries and the breeding density of GHFEs. In addition, our study demonstrates the importance of tall trees around local fisheries, which underscores the need to preserve nativeAlbiziaspp.around commercial fisheries to facilitate the long-term conservation of the GHFE.

Knowledge Management: Journal