Stratification based on pre-survey observations of siphon holes and exploratory sampling proved
useful in the 1999 and 2007 surveys of toheroa at Dargaville Beach (Akroyd et al. 2002, 2008), in the
2000 survey of toheroa at Ninety Mile Beach (Morrison & Parkinson 2001) and in Southland toheroa
surveys (e.g., Carbines & Breen 1999). This method is suitable only for surveying beaches that
contain dense beds of toheroa which can be positively identified by the nature of the siphon holes on
the surface of the sand, and was, therefore, considered appropriate for the Dargaville 2011 survey.
Stratification using this approach is potentially problematic because the presence of surface siphon
holes appears to vary depending on environmental conditions, but given that the majority of toheroa
in a reasonably abundant population will be found in aggregated beds, it is very important to stratify
the beach according to the bed and non-bed areas if possible. The attributes of toheroa and tuatua
siphon holes must be easily distinguished, although by all accounts tuatua are not found in great
numbers in the intertidal zone at Dargaville Beach, unlike the perhaps unusual situation sometimes
observed at Ninety Mile Beach. Equally importantly, the pre-survey stratification should immediately
precede the main survey sampling to avoid problems of toheroa movement over time (e.g., with wave
and storm action). In the 2007 surveys of toheroa at Dargaville Beach and Muriwai Beach, Akroyd et
al (2008) found that the location and density of toheroa beds identified by siphon holes in the presurvey
stratification had changed considerably by the time the full surveys were conducted.