Skip to main content

SEABIRDS, TUNA, CLIMATE CHANGE – FUNCTIONAL RELATIONSHIPS IN THE TROPICAL INDIAN OCEAN AND SUSTAINABLE USE OF ITS RESOURCES

Final Report Summary - SEAFUTURE (Seabirds, Tune, Climate Change - Functional Relationships in the Tropical Indian Ocean and Sustainable Use of its Resources)

Tropical oceans represent a major biome of the world and are of immense socio-economic importance. Despite their significance, there is only little knowledge on the functional relationships between their biotic and abiotic components and between different trophic levels of their food web. This knowledge is crucial to evaluate effects of human activities on these ecosystems, the most important ones currently being climate change and tuna fisheries. Thus, the objectives of the project were:

1) to establish seabirds as bio-monitors of the tropical Indian Ocean by determining the relationships between seabird ecology (foraging behaviour and physiology), bio-physical environment, and fisheries; and
2) to use the acquired knowledge to assess the impact of climate change and fisheries on this ecosystem.

To accomplish those objectives, focus seabirds (fregatidae and sulidae) were chosen to represent the utilisation of different habitats within the tropical Indian Ocean: costal / in-shore and oceanic / off-shore. According to their different spatial utilisation of the ocean, the species should be affected differently by oceanographic parameters and climate change. In addition, multi-year data sets on the seabird ecology were used to include oceanographic variability in the analyses to evaluate the effect of varying oceanographic conditions. Based on field studies carried out on various Indian Ocean islands, but especially on Christmas Island in the Eastern Indian Ocean, the project allowed a better understanding of the relationships between several top predator species (seabirds, large predatory fish) and their tropical environment. A wide variety of results, important for scientific and conservational purposes, was produced and disseminated in various ways to scientific and public audiences and governmental and non-governmental bodies.

In-shore foraging study species foraged relatively close to the breeding colony and foraging trips were short. Those trips allowed for frequent change-overs between breeding partners and hence high provisioning rates of the chicks. Dives were shallow and short stressing the importance of large predatory fish: Due to their limited diving capabilities, the seabirds depend on large subsurface predators, like tuna, to drive up prey to the water surface. Birds foraged in all directions from the colony, no specific foraging areas were chosen and the foraging habitat was not characterised by specific oceanographic features - all of which reflects highly opportunistic foraging behaviour. Different oceanographic conditions had no effect on the foraging behaviour: In years with poor conditions, the main foraging parameters like trip duration, distance, dive depth and diving activity, did not change. Apparently, prey availability was still good enough close to the breeding colony and birds did not have to adjust their foraging behaviour to compensate for unfavourable conditions. A high competitiveness and good prey availability close to the colony might be the reasons for successful in-shore foraging behaviour, somewhat independent of overall oceanographic conditions.

In contrast to in-shore foragers, purely off-shore/oceanic foraging species showed distinct responses in their foraging behaviour to unfavourable oceanographic conditions. The foraging areas of those seabirds were truly oceanic waters without pronounced up-welling, currents, or influx of nutrients by rivers or surface water, located at 100s of kilometres from the breeding grounds. Those marine areas are considered as being low in productivity, homogenous in prey distribution and relatively stable concerning inter-annual variability - all of which should manifest in specific foraging strategies and a foraging flexibility to buffer variation in the overall poor prey availability. According to the homogenous resource distribution, birds did not show preferences for specific marine areas or distinct oceanographic features. Instead, they used prevailing wind directions for choosing foraging trip bearings. By doing so, they were able to use gliding / soaring flight to reduce energetic costs. According to the overall low prey availability, diving activity was low and opportunistic. Like in in-shore foragers, dives were shallow and short stressing again the importance of large predatory fishes. In fact, the relationship with the subsurface predators was even stronger as the timing of the foraging trips and the diving activity were synchronised with the activity patterns of subsurface predators, i.e. concentrated in the early morning and late afternoon when these predatory fishes hunt. This synchronisation allowed both birds of a nest to increase foraging efficiency. Under varying oceanographic conditions, the birds showed adjustments in foraging behaviour. When marine conditions were unfavourable, trip duration and distance increased and dives got deeper. In addition, the birds switched to larger prey which might have helped to balance increased foraging costs due to longer trips and deeper dives. By those adjustments the birds were able to buffer the poorer prey conditions: body condition of adults and chicks remained stable despite the varying prey conditions. Remarkably, parameters like diving activity and frequency remained similar under poor conditions, suggesting that the spatial distribution and temporal occurrence of prey patches, and hence most likely also tuna, was similar under bad and good conditions. However, as foraging trip durations increased under poor conditions, predicted deterioration of marine conditions due to climate change will ultimately result in durations that might not allow both partners to forage on the same day taking advantage of the foraging activity of large predatory fish. In addition, fasting stint will get longer and foraging costs will further increase due to the likely necessity to dive deeper and travel further. Consequently, chick provisioning rates will decline and adults will have increasing problems to sustain themselves both of which might eventually affect population size.

Moreover, it has to be stressed that all seabirds were affected by local climatic conditions at their breeding grounds independent of their foraging behaviour. Especially rain severely affected the breeding success of all species: nests of ground breeders were washed away and larger chicks, even if independent of nests, were constantly wet and eventually died, most likely of hypothermia. In addition, strong swells caused by storms / cyclones hit the shore and destroyed nests and breeding habitat. Tree nesting species were also affected as their nests are often made of twigs 'glued' together by the birds' guano. The guano dissolved in heavy rains, the nest disintegrated and chicks and eggs fell to the ground. This is a crucial point as according the predictions on climate change, extreme weather events like storms and cyclones which are usually accompanied by heavy precipitation will increase in intensity, frequency and duration. Thus, even if seabirds are not affected by climate change or fisheries in their foraging ecology or are able to buffer poor prey conditions by adjusting their foraging behaviour, they might be substantially affected by other effects of climate change. Furthermore, other factors might confound the negative effects. For example, a new blood parasite was discovered and the results showed that infected birds had higher stress levels than uninfected birds. Elevated stress levels might negatively affect breeding performance and hence ultimately population dynamics. As immune competence is energetically costly, infection prevalence might be linked to foraging efficiency which is, as mentioned above, influenced by oceanographic conditions. In addition, harvest / killing of birds, especially of adults, will directly aggravate the struggle of the seabirds to cope with the envisaged impacts of climate change and over-fishing. Thus, factors like those should also be taken into account when using seabirds as bio-indicators to evaluate the effects of human activities on the tropical marine ecosystems.

By combining multi-year data on the ecology of tropical seabirds with information of environmental conditions of their habitat, it was possible to use seabirds as bio-indicators of the tropical Indian Ocean and to elucidate relationships between seabirds and their environment. The results of the study will help to understand:

1) in what parameters tropical seabirds are affected by oceanographic conditions;
2) how they respond to changing / extreme environmental conditions; and
3) give clues to help ecologists, demographers as well as wildlife and fisheries managers to predict how seabird populations will cope with ongoing and future anthropogenic changes in tropical oceans.