Dwarf minke whales in the northern Great Barrier Reef and implications for the sustainable management of the swim-with whales industry

Sobtzick, Susan (2010) Dwarf minke whales in the northern Great Barrier Reef and implications for the sustainable management of the swim-with whales industry. PhD thesis, James Cook University.

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Abstract

The dwarf minke whale is an undescribed subspecies of the common minke whale (Balaenoptera acutorostrata). Dwarf minke whales aggregate in the northern Great Barrier Reef each austral winter. The predictability of the aggregations and the tendency of the whales to approach vessels and swimmers have led to the development of a swim-with whales industry which, since 2003, has been formally permitted by the Great Barrier Reef Marine Park Authority. Dwarf minke whale biology is not well understood and any impacts of the swim-with activities on the dwarf minke whale population are largely unknown and unquantified.

In order to address these knowledge gaps, I designed my study with three overarching aims (1) to improve our understanding of dwarf minke whales involved in swim-with programs; (2) to assess the potential for cumulative impacts of the swim-with whales activities on the whales and (3) to make recommendations to contribute towards sustainable management of the activity (including the evaluation of potential sustainability indicators).

I addressed these aims by using underwater photo-identification data of dwarf minke whales involved in swim-with activities. Between 2006-2008, over 45,000 photos and video footage were collected by myself, other Minke Whale Project researchers and non-scientists onboard platforms of opportunity forming the Dwarf Minke Whale Sightings Network. I evaluated the quality of the data and found it to be suitable for individual whale identification, with non-scientists contributing more than 40% of the high quality data. In 2006, I identified a minimum of 176 individuals (complete identifications that include both left and right sides of the individual plus the higher number of the partial identifications, either left or right side) and a maximum of 195 individuals (complete IDs and the sum of the partial IDs). In 2007, the minimum number of identified whales was 158 with a potential maximum of 171 individuals; in 2008, I identified a definite minimum of 204 and a potential maximum of 219 whales. I investigated the spatial distribution of interactions between dwarf minke whales and the vessels that form the Dwarf Minke Whale Sightings Network. My study found that such interactions occurred in three regions: 1) the Agincourt Reefs region, 2) Ribbon Reef #3-5 region and 3) Ribbon Reef #9/10 region. The Ribbon Reef #9/10 region consistently had the highest number of in-water interactions per unit effort, interaction duration per unit effort and whale group size per unit effort. Individual sightings of the same animal were generally <50 km apart suggesting that whales were not travelling but rather staying in the general area. Of the 27 whales that were observed with more than 50 km between individual sightings during the 2006 and 2007 season, 19 were re-sighted south of their previous location. This finding suggests that once animals started travelling, they did so predominantly in a southerly direction.

While most animals were sighted only once, around one third of all identified dwarf minke whales (29% in 2006 and 33% in 2007) repeatedly interacted with vessels and swimmers over the course of a season; the maximum of eight interactions with a single individual were recorded during a single season. These sighting frequencies raised concerns about cumulative impacts of the swim-with activities on individual animals. I investigated individual and cumulative interaction durations between whales and vessels as indicators for such potential cumulative impacts. Individual interaction durations varied considerably between different interactions and different whales, resulting in a wide range of cumulative interaction durations for the animals.

The interacting dwarf minke whale population showed several characteristics of an open population: 1) individual whales remained in the population for only a short time (mean interval between first and last sighting was less than ten days); 2) new whales were identified throughout the season with the discovery curve not levelling off towards the end of a season; 3) the predominance of smaller whales (<6 m) in the population indicated recruitment of younger animals.

Using open population models, I estimated the size of the interacting minke whale population using Program MARK. The high number of identified whales and the sparse individual sightings histories resulted in some very small estimates for individual parameters and I therefore investigated several pooling scenarios and calculated the most conservative estimates for the size of the interacting population (Ntotal). Ntotal + SE in 2006 was 449 + 68 whales; in 2007 was 342 + 62 whales and in 2008 was 789 + 216 whales. Although these results suggest large variations between years, they provide the first indications about the size of the interacting dwarf minke whale population and show that it consisted of several hundred animals each year.

I used underwater videogrammetry to estimate the lengths of interacting dwarf minke whales which, in field studies, can act as a proxy for age and the state of sexual maturity. I found that sexually immature, maturing and mature animals were present in the interacting population. The majority (63% in 2006 and 65% in 2007) of the interacting whales were < 6m long suggesting that they were immature. These findings are important when discussing the significance of the study area for the dwarf minke whale population. The results provide limited support for previous studies which, based on vocalisation (Gedamke, 2004) and limited observations of courtship behaviour (Birtles et al., 2002) suggested that the area might be a mating ground for dwarf minke whales. Nevertheless, sexually immature whales may have been in the area to practise mating behaviour or simply because they accompanied adults during their migration. The hypothesis that the study area is a mating ground is further supported by my findings that most of the interacting whales form loose associations (which is consistent with promiscuous mating strategies) and that several animals show long lasting site fidelity over several years. If the study area is indeed a mating ground, its significance for the dwarf minke whale population will be greatly enhanced.

The findings of my study have several implications for the sustainable management of the swim-with activities. The area seems to have a high significance for the population, and swim-with activities might therefore have a high potential to cause impacts at a population level. Nevertheless, the open population structure, the large number of animals in the interacting population and the highly variable cumulative interaction durations between individuals indicate that such population level impacts might be small. In conclusion, I provide specific recommendations to improve the future management of the swim-with whales industry and I outline future research needs to address still unresolved questions.

Item ID: 28199
Item Type: Thesis (PhD)
Keywords: Balaenoptera acutorostrata; common minke whales; dwarf minke whales; GBR; Great Barrier Reef; Minke Whale Project; MWP; North Queensland; Northern Australia; Nth QLD; photo-identification; sustainability; sustainable management; swimming-with-whales; swim-with-whales; tourism industry; tourism management; whale identification; whale sightings; whale watching cruises; whale watching tours
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Interpretive DVD titled "Meet the Minkes" (Appendix 3) is also available on YouTube at http://www.youtube.com/watch?feature=player_embedded&v=kFEsRDlycww

Publications arising from this thesis are available from the Related URLs field. The publications are:

Birtles, A., Arnold, P., Curnock, M., Salmon, S., Mangott, A., Sobtzick, S., Valentine, P., Caillaud, A., and Rumney, J. (2008) Code of Practice for dwarf minke whale interactions in the Great Barrier Reef World Heritage Area. Great Barrier Reef Marine Park Authority, Townsville, QLD, Australia.

Dunstan, A., Sobtzick, S., Birtles, A., and Arnold, P. (2007) Use of videogrammetry to estimate length to provide population demographics of dwarf minke whales in the northern Great Barrier Reef. Journal of Cetacean Research and Management, 9 (3). pp. 215-223.

Date Deposited: 31 Jul 2013 05:32
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