Body composition, field metabolic rate and feeding ecology of walrus (Odobenus rosmarus) in northeast Greenland

Mario Acquarone

PhD thesis

Summary

The quantification of food requirements is an important key for understanding the role of animals within their ecosystem, but it represents also essential information necessary to evaluate the responses of these organisms to changes in their natural environment and hence fluctuations in food availability.

It is beyond doubt that the climatic conditions in the North Atlantic have been variable throughout history and that the arctic regions in this area show local amplifications of climatic changes. The arctic ecosystems are simple compared to their temperate, subtropical and tropical counterparts. This makes them particularly vulnerable in case of rapid changes. For this reason the effects of any variation in the global climatic conditions are necessarily amplified in the Arctic.

The east coast of Greenland represents an interesting climatic gradient because of its north-south orientation. The ecosystems range from polar, to high arctic and to low arctic. It is in the inshore areas and the deep fjords that a part of the small east Greenland walrus population comes to feed and moult every summer. The sea surface above the rich bivalve mussel beds is covered with a thick layer of ice during most of the year, but during summer, for a short window of about three months, the ice melts away and allows access to this important walrus food resource. Therefore any change in the timing of the break-up and formation of the sea-ice has a profound impact on food availability for these animals.

It has long been known that walruses feed on the main bivalve species (e.g. Mya sp., Hiatella sp. and Serripes sp.) of these fjord's benthic communities. Analysis of the stomach content of the animals killed in occasion of the traditional Inuit hunt or by European sealers has provided strong evidence in this direction. However, recent studies have demonstrated that stomach contents only provide a partial picture of the feeding habits of these animals. Even for a walrus that has just been feeding, the action of the gastric secretions is extremely rapid and some benthic organisms, as polichetes and holoturians, are almost completely dissolved before dissection can begin, while the most resistant, as bivalve mollusks, are at best partially digested. For a complete quantification of the walrus diet it is therefore necessary to adopt other approaches.

The main focus of this thesis is the quantification of food requirements in free-living walruses. Two different methods have been adopted to attain this goal: (a) the direct underwater observations of walrus feeding on the mussel beds and (b) the estimate of energy consumption by the Doubly Labeled Water isotopic technique.

In case of the first approach, the observations of foraging walrus made by brave divers have allowed the retrieval of empty bivalve shells after feeding dive. By coupling this information with data on the frequency of the feeding dives obtained both by direct observations and by satellite telemetry has resulted in the estimate of the average daily bivalve food consumption (shell free, wet weight) at 4.7% of the animal's total body mass.

The second approach involved the calculation of the animal's Field Metabolic Rate from measures of the washout rate of two stable isotopes tracers (2H and 18O) that had been injected in the animals at the beginning of the study. From these estimates of the average energy utilized over a period ranging from five to nine days, and from the energy content of the bivalve diet it has been possible to calculate a corresponding bivalve food consumption of 6-7% (shell free, wet weight) of the animal's total body mass.

These data have been utilized for the calculation of the impact of walruses on the benthic bivalve community in the Young Sound area. The interest in this area is due to the intense climate-related research and monitoring effort both by Danish and international scientists in relation to the Zackenberg environmental baseline monitoring station. From the results of this study walrus predation amounts to 1.2-1.3% of the standing biomass in the estimated walrus feeding area of their two most important food items (Mya sp. and Hiatella sp.). These values correspond to 19-28% of the annual production of these bivalves. Therefore, a climatic change causing an extension of the open water season, presumably would not immediately bring this ecosystem to the limit of its carrying capacity.

In the course of this work it has been necessary to repeatedly immobilize the study animals and two papers fully account for these procedures. One paper describes for the first time multiple immobilizations of the same individual and the other presents a novel technique especially useful for prolonged restraint for up to 6.5 hr. Furthermore, in order to prepare the Doubly Labeled Water study and minimize its costs, it was decided to perform a study of the body water metabolism by isotope dilution. The paper derived from this study presents novel data on body water pool size and body water turnover rate, which were used to estimate body composition which does not present significant differences with data obtained from dissection (in other studies).

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