Monitoring of waterbirds on the Banc d'Arguin, Mauritania
Introduction: Tom van Spanje
The Parc National du Banc d' Arguin (PNBA) is an area with islands surrounded by tidal flats, shallow inshore waters bordering the Sahara Desert on the Atlantic coast of the Islamic Republic of Mauritania. The park was founded in 1976, and was designated a wetland site of international importance under the Ramsar Convention in 1982. The park is also a UNESCO World Heritage Site, and covers an area of nearly 12 000 km2. The Banc d'Arguin is particularly important for the high concentrations of non-breeding waterbirds it supports during the northern winter. This includes upwards of 30% of all wintering waders using the East Atlantic Flyway migration route. It is therefore essential to build a lasting mechanism for effectively monitoring migratory (and resident) waterbirds, particularly waders, of the Banc d' Arguin in the future and to provide new data within the time scale available to meet the next Wader Population Appraisal for RAMSAR in 1999. Change in wader numbers The 1997 WIWO-WSG count of the waders and waterbirds on the Banc d'Arguin (Mauritania) in (Zwarts et al ]998) confirmed the decline in numbers such as suggested by recent winter counts of Gowthorpe et al ( 1996), but the degree of decline was variable between species. The 1997 count found a total of two million of waders, a similar total to the 1979 and I 980 counts. The totals are compared to earlier counts in the Bijagos archipel of Guinea Bissau and show the following changes: A decline in numbers in both the PNBA and the Bijagos was shown by: Ringed Plover, Kentish Plover, Knot, Little Stint and Turnstone. A decline in numbers in the PNBA but increase in the Bijagos was shown by: Oystercatcher, Sanderling and Curlew. A decline in numbers in the PNBA but stable in the Bijagos was shown by: Bar-tailed Godwit An increase in the PNBA and a decrease in the Bijagos was shown by: Redshank and Greenshank. An increase in both the PNBA and the Bijagos in: Dunlin and Curlew Sandpipers Food stock availability Confronted with the results of the analyses of the wader counts, by the end of 1998, the importance was realised to start a quantitative macrobenthos monitoring program. The 1993 'Research master plan for the Banc d'Arguin' of the Scientific Council of the Banc d'Arguin mentions "A redefinition of the sampling methods and a detailed study over at least two years would be needed to answer the questions posed by the deficiency of primary consumers. There is little information on the fauna of the shallow subtidal zone; regular sampling campaigns are required here." Therefore a plan was set up to monitor at least some of the sampling stations of the 1988 benthic research by Wolff et al. Despite the need of such information this monitoring plan was not accepted by the CSBA. The reason given was that the CSBA, which meets once a year in February, could not judge this proposal on such short notice. The biologist involved decided to make the best out of his presence by studying the macrobenthos species present at various sites in order to contribute to a reference collection to facilitate future research. The material collected was conserved and stored in approximately 4 % buffered formaldehyde. Further he worked on the existing reference collection present at the Biological station in Iouik, The collection was partly dried out, due to ill fitting lids. Where necessary reference material was transferred to other jars and fresh formalin was used to replace that was evaporated. General observations Seagrass During a survey by air from Arel - Tinimorgawoi - Nair to Tidra it was clearly visible that the former homogenous sea grass beds covering the tidal flats in the eighties were now widely marked with barren sand areas and pock-pitted with numerous small round ponds. It is known that seagrasses are adapted to highly dynamic environments and can normally restore themselves rapidly. However, if the degradation is caused by a stronger sea swell by a change in the Northern oscillation, this might have a negative impact on the growth of seagrass over a longer period. Some islands are losing the Suaeda vegetation and large sebkha flood planes are nowadays inundated at a much more regular base (Lamarche pers. comm.), One village has been split by the sea and part of the inhabitants created a new village in a safer place. These phenomena took place mainly in the areas which are exposed to the north-western winds. Evidence of this relationship can be found by comparing aerial photos and satellite pictures of the mid eighties. The food stock available to waders and fish eating birds might also be influenced by a change in the upwelling of nutrients to the Banc d' Arguin ecosystem and/ or a degradation of the seagrass beds by higher waves. The increase of the European subspecies sinensis of up to 16 % per annum at freshwater lakes and only a few percent at the north Atlantic coasts, has been related to the decrease of hunting pressure and a strong eutrofication (Debout et al, van Eerden et al & Suter 1995). In Mauritania hunting of the Great Cormorant, presumably a resident non-migratory)subspecies lucidus, does not exist, Therefore it is possible that the food supplies in the Banc d'Arguin improved for this species- One possible reason is an increase in eutrofication by the up welling of nutri‰nt rich water caused by meteorological changes. Increased fishing intensity of Sharks Gealeiforme s and Rays Rajiforme s inside the park may have repercussions on the age and species composition of the fish stock of the park. However, it is common knowledge that the fish stock ofsome species on the Atlantic coast of W est Africa shows signs of over exploitation for several years now. This could have led to a smaller size fish in the PNBA which favours, at least for the moment, fish eating birds, A complication in interpreting the fluctuation in numbers of birds present in the PNBA is that nothing is known about movements of fish eating bird species to and from the surrounding W est African countries. Only a West African colour ringing or radio tagging program could reveal such migration patterns. Unclear is also the influence of the fishing by local fishermen (on mainly Sharks and Guitar Ray) on both the macro benthos stock available for waders and the density and erosion of the seagrass beds by presumably the feeding behaviour of the much more numerous smaller sized Guitar Rays. We investigated the stomach contents of two Guitar Rays. These contained calinectus and benthic invertebrates. We also observed this ray species feeds by digging in the seagrass and found traces of a sieving pattern in the upper layer of sandy flats. In this way the smaller sized but probably more numerous rays can also contribute to a higher pressure on the benthic stock, including bivalves. By digging for food the more numerous small rays can have coursed the pock-pitted seagrass beds with numerous small round ponds, Rays can also have diminished the predation by the remarkable abundant calinectus swimming crab on young fish. Could the Redshank, Greenshank and Spoonbill be favoured by these more numerous small ponds fi11ed with young fish and shrimps? Could the change in numbers and distribution of waders like Oystercatcher, Bar-tailed Godwit, Knot and Curlew, as noted by the analyse of Zwarts et al., be related to a competition for food with the Guitar Ray? 1. Monitoring waterbirds: Completing the last count and field survey for next complete count in 1999. An aerial survey of the coast north of Iouik including the Baie d' Arguin, Cap St. Anne in the north of the PNBA was carried out at the 4th of February., Also the coast outside the boundaries of the PNBA along the Baie de Lévrier and the Baie de l'Etoile was counted during this flight by Cessna. The scenic Baie de l'Étoile north of Nouadhibou was also visited. Wader roosts were counted and all Spoonbills were checked and any colour rings read. The lagoon of this bay still shows a good coverage of Spartina grasses. Between Dakhla and this bay, no other wetlands can be found along the coast of the Sahara, This lagoon deserves a special status as nature reserve, In some area' s the Spartina grasses are exploited as fodder for camels, apparently without negative effects. W aders behaved much more shy compared to the birds in the PNBA, a possible indication of occasional hunting. The satellite reserve of the park at the Cap Blanc peninsula was visited and colour rings were checked on all gull species . The dune vegetation, ,Suueda, was largely removed by fishermen to be used as firewood or the construction of shelters, No Monk Seals were observed to be present. The small fishing harbour La Cherka contained few waders compared to 1989. 2. Fish eating waterbirds: feeding ecology and collecting and analysing pellets/ droppings. The following fish eating waterbirds showed in 1997 chances in numbers: Strong increase : Great Cormorant, Little Egret, Slender-billed Gull, Lesser Black-backed Gull and Caspian Tern. More or less stable: Grey Heron, Spoonbill, Long-tailed Cormorant. Decrease : W estern Reef Heron Strong decrease : Flamingo (a filter feeder of micro organisms) At several locations pellets of Cormorants and Herons were collected and also Spoonbill droppings, Both were partly analysed at the IBN-DLO in W ageningen and were found to be filled with otoliths, The islands of Zira and Arel were covered with a thick layer of pellets of mainly Cormorants and Pelican. The hide on Zira needs to be repaired an cleaned before it can be used for observations. To be able to identify the otoliths it is recommended to establish a reference collection of small fish of the shallow waters of the tidal flats of the Banc d'Arguin. 3. Spoonbill: feeding ecology and colour-ring reading in January This mission has added important information concerning Spoonbill migration and survival by checking colour rings. The European Spoonbill population is estimated at about 5100 individuals. At least SO 0/0 was present at the PNBA. Of the 9496 Spoonbills Platalea leucorodia counted, 5108 were checked for colour-rings, subspecies, sex and age, The local endemic subspecies bals'aci was with 6.921 individuals 73% (82% in 1997) represented I The proportion of the European nominate leucorodia was 2.575 birds 27% ( 1 8%o in 1997). Signs of migration, also arrivals from the south ( Senegal delta ? .) was observed shortly after our arrival in the park on the 1 9th of January and continued at least to mid February when we left the Park. The percentage of nominates in the roosts declined rapidly after the first of February. Apparently an unknown part of the adult European Spoonbills had left the area before the beginning of the fieldwork before the end of January. Before the 1997 mission only 30 colour ring sightings were known from Mauritania. During the 1998 expedition there were 208 ( 1 18 in 1997) new observations. From colour-ringing research it appeared that juvenile birds stay for three to four years in W est Africa before returning to the breeding areas in Europe. Analyses ofthe survival data indicates an increase in the survival rate. Annual colour ring reading missions during January and preferable also in July - August in the PNBA are important to determine survival in order to follow the population developments of the European Spoonbill. This improves the insight of starvation along the stopping over sites along the migration route, On the island Arel and the region of Tinimorgawoi the largest concentrations of Spoonbills were observed, Due to lack oftime only small amount ofdata was gathered on time budget, feeding behaviour, inter and intra speci6c aggression. Site 6delity ofgroups is believed to be high. The density and roost pattern of the both the nominate and balsaci showed the same pattern as in January and February 1997. High proportions of balsaci were found around Arel, Nair, Kiji, T ouffat, Cheddid and Iwili-Gouchna. No pe11ets were found and apparently not produced. However many fresh droppings were collected of both subspecies and these contained many small otoliths, With the help of a reference collection ofotoliths from fish species ofthe PNBA these otoliths will reveal at least part ofthe Spoonbill diet, The droppings were analysed at the IBN-DLO in Wageningen. The determination of otoliths can only be carried out with the use of a reference collection. As far as known such a collection is only available in Brussels, Belgium. Numerouw abandoned or lost eggs and old nests, which must have been occupied during 1997, were found on Cheddid and Toufflat. On these islands traces of Jackal were seen and some predated eggs as well as some predated juvenile and adult spoonbills were found. An isolated old nest was found at Zbarat, just north of Iwili. On Nair no nests were found. |