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Fish-eating birds are disappearing along the Olifants River in Kruger Park
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The fourth ground-based survey of the Pel’s Fishing Owl (Scotopelia peli) and other fish-eating species along the Olifants River in the Kruger National Park was conducted in June 2010 by two teams of observers consisting of staff from South African National Parks (SANParks), the Endangered Wildlife Trust’s Birds of Prey Programme and volunteers.
The survey was conducted in the riparian vegetation along the Olifants River in the central area of the Kruger National Park, stretching from the western boundary fence of the Park, known as Mamba Picket to the eastern boundary fence beyond Bangu Gorge — a distance of some 93 km along the river.
The riparian vegetation in this area is dominated by tree species such as Jackalberry, Natal Mahogany, Sycamore Fig, Ana tree and Weeping Boer-bean. Shrubby vegetation of the lower stratum consists mainly of species such as Red Spike-thorn and Large Fever Berry. Vegetation suited to the specific requirements of Pel’s Fishing Owl occurs sporadically along the course of the river.
Survey methods
The survey was conducted over a period of four days by two separate teams, with preference being given to the early morning — from shortly after sunrise until the survey was completed for the day. Each team consisted of 6-10 observers walking downstream and recording what they saw. Avifauna were only recorded once they had flown or had passed in a easterly direction, thus avoiding duplication.
Due to the high levels of the river and the inaccessibility of certain sites on the southern bank in the western half of the survey area, use was made of a motorised inflatable boat to transport the team surveying the southern bank.
The teams walked along the riverbank, scouring suitable trees for the presence (flushed birds or confirmed sightings) or signs of Pel’s Fishing Owls being present. Tributaries with suitable vegetation were also covered for at least 100-500 metres from the main river depending on the nature of the vegetation.
Regular radio contact was maintained between the teams and all relevant information was recorded on a CyberTracker device or Dictaphone carried by each team leader. Standard information recorded for all sightings or signs of Pel’s Fishing Owls were
- time;
- GPS coordinates;
- north or south bank;
- species of tree/s (if relevant) and description of site;
- nature of sighting or signs (feathers, faeces, prey remains, roost, potential nesting site);
- and number of birds.
In addition to the Pel’s Fishing Owl, sightings of other water birds, raptors and hippopotami were also recorded along the survey area. All relevant species of birds flying or moving past the teams in an easterly direction were recorded. Field rangers accompanying the teams also took the opportunity of recording any sightings and spoor of certain game species, the presence of alien vegetation and other information.
Results
A total of three Pel’s Fishing Owls were seen and counted during the survey, while a further six sites were identified where positive signs of the birds’ presence were observed. With the exclusion of the known pair of birds just outside the western boundary of the Kruger National Park, it is estimated that there are at most three to five active breeding pairs of Pel’s Fishing Owls along the Olifants River in the Park at present.
This compares poorly with the findings of Begg (1992-1994) along the same river where a total of 18 birds were recorded and 15 potential areas of occurrence were located. The decline in sightings of prey remains and other signs of birds being present seems to confirm a decline in this species during this survey
The 2010 ground survey for Pel’s Fishing Owl seems to indicate a continued decline in the population of this species along the Olifants River in the Kruger National Park. This decline is notable compared to results in previous years since the start of the project in 2007 (Fig. 1). Factors such as extraction, pollution, sedimentation and decline in abundance and quality of available prey species most likely all contribute to this decline.
A noticeable feature of the river this year was the extensive mats of green algae that occurred throughout. These mats were not present in previous years. The presence of this plant material reduces the suitable areas available for the birds to hunt for their prey despite the relatively high water levels of the river.
Fig. 1. Graph showing numbers of Pel’s Fishing Owl encountered during annual ground surveys and indicating decline since 1992.
No evidence could to date be found that pansteatitis, the same disease that impacted on crocodile populations in the eastern part of the survey area, could have had a similar impact on this or other fish-eating bird species. However, this possibility should certainly not be discounted (Myburg & Botha 2009).
Other bird species
Ten species of diurnal raptors were recorded during the survey. Table 1 compares the numbers of species recorded during the current survey with numbers recorded by Begg (1991-1994, unpubl. data) while Table 4 reflects the numbers of additional species recorded by teams during the current survey.
Table 1. Summary of the other bird species recorded during the survey compared to Begg (1992) and previous years of survey.
Considering the distance of river covered, African Fish Eagle (Haliaeetus vocifer) numbers were again rather low with 10 birds in total being recorded along the entire river. This reflects a decline of 42% compared to the 21 birds recorded by Begg in 1992. However, numbers for this species seem to have remained fairly stable over the four years of the current survey.
As in 2009, four Saddle-billed Storks (Ephippiorhynchus senegalensis) were recorded this year, a marked decline in the number of this species compared to the 2007 and 2008 counts. Similarly, Goliath Heron (Ardea goliath) numbers were considerably lower this year than both the 1992 and 2007-2009 counts. White-crowned Lapwings (Vanellus albiceps) seem to have a healthy population along the Olifants River with 78 birds being recorded during the 2010 survey.
Piscivorous (fish-eating) bird species such as Great White Egret (Egretta alba), Yellow-billed Stork (Mycteria ibis), African Open-billed Stork (Anastomus lamelligerus), Little Egret (Egretta garzetta) and African Spoonbill (Platalea alba), which were recorded in low numbers in the previous three years of the current survey, were not recorded at all during the 2010 survey. This is quite worrying as these species were previously common. The reasons for the absence or low numbers of these species are worth investigating.
Table 2: Other species of interest recorded during the current survey:
At least 13 active African White-backed Vulture (Gyps africanus) and 11 Hooded Vulture (Necrosyrtes monachus) nests were recorded along the river. Most of the African White-backed Vulture nests were located in Sycamore Fig trees, while almost all the Hooded Vulture nests were located in Jackalberry trees. Two Hooded Vulture mortalities were also encountered, one below a previously active nest and the other a few metres from a nesting tree. The remains of both carcasses indicate that these were adult birds, but it was not possible to determine the reasons for the death of these birds.
Another sighting of interest occurred close to the mouth of the Mpfuvu spruit where two pairs of White-headed Vultures (Aegypus occipitalis) were seen flying fairly close to each other in what could be interpreted as a territorial display. The first Osprey (Pandeon haliaetus) seen in the four years of this survey was also recorded..
References:
Barnes, K. (ed.). 2000. The Eskom Red Data book of Birds of South Africa, Lesotho & Swaziland. BirdLife South Africa, Johannesburg.
Begg, K. 1992-1994. Unpublished data, Transvaal Museum.
Botha, A.J. 2007. Report on the Pel’s Fishing Owl Scotopelia peli ground survey of the Olifants river, Kruger National Park 24th-27th of June 2007. Unpublished report, Endangered Wildlife Trust.
Botha, A.J. 2008. Report on the Pel’s Fishing Owl Scotopelia peli ground survey of the Olifants river, Kruger National Park 22nd-25th of June 2008. Unpublished report, Endangered Wildlife Trust.
Botha, A.J. 2009. Report on the Pel’s Fishing Owl Scotopelia peli ground survey of the Olifants river, Kruger National Park 21st-24th of June 2007. Unpublished report, Endangered Wildlife Trust.
Hockey, P.A.R., Dean, W.R.J. & Ryan, P.G. (eds.) 2005. Roberts’Birds of Southern Africa. 7th ed. John Voelcker Bird Book Fund, Cape Town.
Myburg,J. & Botha, A.J. 2009. Decline in herons along the lower Olifants river – could pansteatitis be a contributing factor? Vetnews. March 2009.
Tarboton, W.R., Kemp, M.I. & Kemp, A.C. 1987. Birds of the Transvaal. Transvaal Museum, Pretoria.
Tarboton, W.R. & Erasmus, R. 1998. Sasol Owls & Owling. Struik, Cape Town.
