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UBC Theses and Dissertations

Reconstructing historical abundances of exploited marine mammals at the global scale Christensen, Line Bang


The relationship between humans and marine mammals extends back for centuries and covers a multitude of facets; literature and spirituality, necessities (food) and luxuries (corsets). Their exploitation history changed from small-scale hunting for foodlfur, to large commercial ventures for oil, to limited hunting as marine mammals became the poster child for environmental movements. The International Whaling Commission, national and regional bodies assess the current status of marine mammals. Information on stock size relative to carrying capacities is, however, hard to come by. Assessments to date have been limited to a relatively small number of stocks/species because data quality, availability and politics. To address concerns over stock status, the analyses have to be expanded to all exploited populations. Employing a Bayesian approach to stochastic stock reduction analysis, I construct probability distributions over historical stock sizes. The method allows me to use historical catch time series to estimate a distribution over population parameters, the intrinsic rate of growth and carrying capacity, that give rise to extant populations. I tested this stock reconstruction approach on simulated data sets generated from a reference model, and then applied it to available catch and abundance data. Simulation tests indicate that parameter estimates were unbiased. Globally, I find that aggregated information on all marine mammal populations indicate a decline of 22% (0-62%) in numbers, and 76% (58-86%) in biomass. The decline has been greatest for the great whales, with a 64% (40-79%) decline in numbers, and an 81% (69-89%) decline in biomass. My estimates are consistent with published estimates of carrying capacity. These estimates are intended to provide a status overview, rather than direct management advice, affording us the ability to begin calculating how humans have impacted these large fauna in their marine ecosystems. The International Convention on the Regulation of Whaling (ICRW) predicates that stocks must be maintained at their most productive levels. Also, a moratorium on whaling exists, awaiting better science. This represents the fundamental conflict in interest between pro-whaling nations and those with morallethical opposition to whaling, intent on upholding the moratorium. Results in this thesis indicate that the Japanese hunt of minke whales could increase and abide by the ICRW, yet the majority of western IWC signatories want to see commercial hunting banned altogether.

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