Invasive history of NZ Daphnia populations
To infer the source of invasive Daphnia pulex/pulicaria in NZ, we checked the possible routes of Daphnia pulex/pulicaria invasion. It is thought that D. thomsoni (formerly D. carinata,Burns et al. 2017) and D. tewaipounamu are the only nativeDaphnia species in NZ, although there are several invasiveDaphnia . One recent invader, D. galeata, now common in lakes throughout the North Island of NZ, may have been introduced to the South Island during translocation of farmed Chinese grass carp (Ctenopharyngodon idella ) for the purpose of weed control in northern South Island waterways (Duggan and Pullan 2017). The presence of D. pulex in some shallow constructed ponds in suburban Auckland, North Island, has also been attributed to introductions of grass carp (Branford and Duggan 2017). However, Chinese grass carp have only been used for the weed control on the North Island and the northern tip of the South Island, as water temperatures in southern NZ are generally too cold for these fish. Therefore, it is highly unlikely that grass carp were a major vector for the introduction of D. pulicaria to the South Island.
Duggan et al. (2012) suggested introduced salmonids (trout, salmon) and associated recreational fishing equipment as potential vectors forD. pulicaria on the South Island. The most common salmonid in NZ, Chinook salmon (Oncorhynchus tshawytschca ), was initially introduced from the Sacramento River, California, USA to South Island, NZ, between 1901 and 1907. Salmon fishing in NZ has become popular since the 1970s and has attracted many international tourists. The D. pulicaria clone on the South Island has been known to produce diapausing ephippia, which float on the water surface for long periods, adhering to any surfaces they encounter, such as fishing equipment, skin, fur, feathers, and clothing (Burns 2013). The movement of these ephippia attached to recreational equipment, people, and wildlife may facilitate the spread of D. pulicaria in lakes in the South Island, NZ. In support of this, the invasion time estimated from the molecular data is roughly consistent with transport during this period. As noted above, the North Island Daphnia has only one nucleotide difference on the COI gene compared to the JPN 1 clone that invaded Japan (So et al. 2015), which suggests the possibility of a recent transfer from Japan.