† Coastal-Marine Research Group, Institute of Natural Sciences, Massey University, Private Bag 102 904, North Shore MSC, Auckland, New Zealand ‡ New Zealand Wildlife Centre, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11–222, Palmerston North Current address: Faculty of Veterinary Science, University of Melbourne, 250 Princes Highway, Werribee, VIC 3030, Australia * Corresponding author e-mail: k.a.stockin@massey.ac.nz PACIFIC CONSERVATION BIOLOGY Vol. 15: 217–227. Surrey Beatty & Sons, Sydney. 2009. Causes of mortality in stranded Common Dolphin (Delphinus sp. ) from New Zealand waters between 1998 and 2008 KAREN A STOCKIN *† , PADRAIG J DUIGNAN ‡+” , WENDI D ROE ‡ , LAURELINE MEYNIER ‡ , MAURICE ALLEY ‡ and TICIANA FETTERMANN † Post-mortem examinations provide valuable information on sources of mortality for marine mammal populations. However, no published data exist to describe causes of death in the New Zealand population of Common Dolphin (Delphinus sp.). In order to examine the proportion of human and non-human induced mortality affecting this population, necropsies were conducted on 133 individuals that stranded around the New Zealand coastline between 1998 and 2008. Of these, 92.5% (n=123) were found as beach cast carcasses, with just 7.5% (n=10) as live strandings that subsequently died or that were euthanized on humane grounds. The sample included 54 males, 67 females and 12 animals of unknown sex from a range of age classes. Of the individuals for which cause of mortality could be established, 41.2% (n=35) were classified as human induced, with 28.2% (n=24) of carcasses exhibiting evidence of net entanglement. A further 10.6% and 32.9% of mortality was attributable to disease and natural (non-human related) causes, respectively. Few examples of disease were detected, but this may be at least partly a consequence of sampling constraints. Of the carcasses assessed, 68.6% of individuals exhibited some form of parasitism. Parasites identified were typical of the genus and considered to be present in low to moderate burdens. The proportion of beach cast carcasses exhibiting evidence of net entanglement suggests that fisheries-related mortality maybe higher than that previously considered for the New Zealand Common Dolphin population. Key words: Common Dolphin, Delphinus sp., Mortality, Set nets, Conservation, Management, New Zealand INTRODUCTION V ARIATION in mortality is a principal determinant of any population’s stability, especially for threatened species (Gaillard et al. 1998). Therefore, describing trends and causes of mortality is important to both conservation research and management. Mortality in marine mammals can result from natural events (e.g., birthing difficulties and/or neonatal separation) and disease (e.g., morbillivirus and epizootics, see Van Bressem et al. 1999; Van Bressem et al. 2007) and human activities (e.g., entanglement in fishing, see Jefferson and Curry 1994; Kuiken et al. 1994) and boat strike (e.g., Visser and Fertl 2000; Laist et al. 2001; Fèlix and Van Waerebeek 2005). In New Zealand, there is generally inadequate resourcing to routinely perform post- mortem examinations in all stranding and beach cast events. However, since the instigation of the New Zealand Common Dolphin Project (NZCDP) in 2002, Massey University has made concerted efforts to recover carcasses and/or undertake post-mortem sampling. Although studies examining cause of death in marine mammals are numerous (e.g., Baker and Martin 1992; Kuiken et al. 1994; Kirkwood et al. 1997; Silva and Sequeira 2003; Kemper et al. 2005), few data are available within the published literature to describe mortality in New Zealand marine mammals. Those available have primarily focused on threatened endemic species, such as Hector’s Dolphins Cephalorhynchus hectori hectori (e.g., Duignan et al. 2003; Duignan and Jones 2005; Roe 2007) and New Zealand Sea Lions Phocarctos hookeri (e.g., Wilkinson et al. 2003; Castinel et al. 2007; Chilvers 2008). Common Dolphins (Delphinus sp.) have one of the greatest stranding frequencies of all New Zealand marine mammals (Childerhouse 2002; 2004; 2005) and remain the most frequently bycaught cetacean species within the commercial fishery for Jack Mackerel (Trachurus novaezelandiae) (Rowe 2007). However, no abundance estimates are available for this population, despite Common Dolphins being classified as not threatened under the current New Zealand Threat Management System (Hitchmough et al. 2007). This is of concern considering the apparent threats (e.g., Stockin et al. 2007; Meynier et al. 2008; Stockin et al. 2008a) facing the New Zealand population. In order to better understand sources of mortality, data collected by Massey University veterinarians and researchers during systematic necropsies were analysed to assess trends and potential causes of death. We examined for evidence of human and non-human related pathology including disease, natural mortality and trauma and/or injuries typical of human- induced mortality, for example entanglement and boat strike. Such human interactions have been implicated in the decline of other New Zealand marine mammals, particularly the Hector’s Dolphin, Maui’s Dolphin Cephalorhynchus