Marine Mammal Populations: Reconstructing historical abundances at the global scale
2006 | FCRR 14(9)
This Report presents reconstruction of the populations (at least back to 1950) of nearly half of the 115 species of marine mammals, i.e., of all those which are, or have been commercially exploited. This reconstruction uses a robust method and provides a first approximation of their populations. In many case, this is the first such reconstruction.
The data used for this are catches of different species of marine mammals, going back as far as possible into the past, in one case to 1530. These catches are of variable quality and hence the author emphasizes, quite rightly, that the reconstructed populations should not be viewed as 'assessment', and compared for example, with assessment by the International Whaling Commission.
Rather, this report should be viewed as the first exercise based on the first publicly available compilation of all catches of all marine mammals in the world, hence providing the basis for various generalizations, and a public dialog about marine mammals.
One such generalization is that it is the large whales which have been first hunted historically, with the result that the mean size of marine mammals killed by humans has been declining over time. We are now mainly killing dolphins and pinnipeds, though there is a move afoot to reinstitute large scale hunting of large whales. Another generalization is that overall the population of marine mammals has declined since 1950 by approximately 56% in terms of weight and by 11% in terms of numbers.
Although some single species of marine mammal have recovered spectacularly in the last decades, for example the North East Pacific gray whale, overall, marine mammals have still to recover from the great hunts of the past. This process is slowed down by the continuation of various fisheries which have marine mammals as 'by-catch' in various parts of the world.
The population estimates presented in this report will also allow adding a temporal dimension of the global overlap between marine mammals and fisheries. A study done at the Fisheries Centre, which pertained to the 1990s, suggested that this overlap is rather limited. The population trends presented here suggest that this overlap might have been even weaker in the early 20th century, because the fisheries then took smaller catches, from a smaller area than now. Using the reconstructed data in this report, we will now be able to investigate whether the large populations of marine mammals we had in the past are compatible with the extensive fishery we have now. The jury is still out on this scenario.
This report could also be used for comprehensive evaluation of the hypothesis presented by Ugo Bardi, who suggested that whales, because of their slow reproduction rates, can be treated as if they were a mineral resource, such as petroleum. This hypothesis, which was tested using a small data set (whale oil and bone from 19th century US whaling), suggests that whales can be treated that way, and hence can be fitted with Hubbert's 'Peak Oil' model . This could be verified with the larger dataset presented here.
Thus, I welcome Ms Line Bang Christensen's comprehensive analyses and the catch data they relied on, included here in an Appendix, though I also wish to re-iterate her warning that these catch data need to be verified and, if need be, expanded. They will then allow widespread analyses of the interactions between marine mammals and humans, so as to enable us to coexist on our ocean planet.
Director Fisheries Centre, UBC
TABLE OF CONTENTS
|History of marine mammal hunting||4|
|History of marine mammal management||6|
|MATERIALS AND METHODS||8|
|Stochastic stock reduction analysis||9|
|Application to simulated data||10|
|Population trajectories of exploited cetaceans||19|
|Population trajectories of exploited pinnipeds||58|
|The big picture||82|
|Appendix I: Estimated population sizes for unexploited portions of exploited marine mammal populations||120|
|Appendix II: Total population sizes of unexploited marine mammal populations||121|
|Appendix III: Species list||122|
|Appendix IV: Model input||125|
|Appendix V: R code||130|
|Appendix VI: Catch data||134|
|Appendix VII: Catch data sources||159|