MODELS FOR PREDICTING NITROGEN TENSIONS AND DECOMPRESSION SICKNESS RISK IN DIVING BEAKED WHALES

摘要

The work presented in this paper constitutes the second stage of a long-term project that aims to develop a model to predict the risk of decompression sickness (DCS) occurring during any given dive by beaked whales, particularly Ziphius cavirostris or Mesoplodon densirostris. The motivation for this work comes from the need to quantify the relative hazards associated with dive behaviours exhibited by these species following their exposure to mid-frequency active sonar. During the first stage of this project, static diffusion was identified as the most likely mechanism of bubble growth that may give rise to the DCS-like, gas embolic disease observed in mass-stranded whales following sonar exposure. This finding highlighted the need for a model of how the nitrogen tensions in the tissues of diving odontocetes evolve during diving. In this paper, the results of a parameter sensitivity study conducted on such a model are presented. This model is to be modified to make its output specific to the two beaked whale species. As such, the aim of these tests was to identify the relative magnitude with which the parameters affect the output of the model, so as to prioritise the order in which they should be quantified empirically in these species. The results showed that of the parameters tested, the assumed depth of lung collapse was the strongest determinant of the model output. This was followed by the relative Ostwald solubility coefficients for nitrogen chosen for the various tissue types, and then the perfusion of both individual tissue 'types' and the tissues as a whole. It is intended that the tissue nitrogen tensions predicted by the current model will be modified to be specific to the beaked whales of interest in line with the results of the parameter sensitivity study presented here. The resulting model will then be combined with the theory of bubble growth via static diffusion to produce the final version of the model, which will be capable of predicting the risk of DCS occurring during any dive by either Ziphius cavirostris or Mesoplodon densirostris. This model will then be used to identify the behaviours theoretically conferring the highest risk of DCS, so that they may then be compared with those actually observed during controlled exposure experiments such as those currently underway in the Bahamas.