2.1.1 Sampling strategies
In accordance with the NSSP, NCDMF routinely samples all shellfish
growing areas on a regular basis (6 times annually) using the systematic
random sampling strategy, meaning samples for each station are collected
at randomly scheduled timepoints throughout the year; however, there are
some constraints as to when sample collection is permissible.
Specifically, samples are only collected during conditions when the SGAs
are open for harvest or assumed to be unimpacted by unsafe FC levels,
resulting in the subsequent data not capturing peak FC concentrations.
Because freshwater input and runoff are tied to increased FC
concentrations in estuarine waters, precipitation intensity is used as a
management indicator for closures of conditionally approved waterways
(NSSP, 2019; Leight et al., 2016). In North Carolina, rainfall closure
thresholds in conditionally approved waters range from 1-3 inches of
rainfall within a 24-hour period (Figure 1c) and dictate if a managed
shellfish area will be closed for harvesting after a meteorological
event. The “emergency closure” of additional areas can occur after
higher rainfall amounts are noted and, for the purposes of this
analysis, those areas will be represented by a 4 inch threshold.
Rainfall thresholds are assigned to conditionally managed areas. To
reopen closed areas that have been temporarily closed following
exceedances of those rainfall thresholds, NCDMF tests the water, and
reopening will only occur after samples confirm safe harvest conditions,
which is defined by waters that do not exceed a FC density of 14 MPN per
100 mL of sample (NSSP, 2019); sampling for reopening is hereafter
referred to as “conditional sampling”. NCDMF balances its limited
resources with speed of reopening by only conditionally sampling when
the organization suspects the FC concentrations will be low enough to
support reopening. SGAs A, C, D, E, and the northern half of B are
characterized by moderate to low rainfall thresholds (i.e., 1-3 inches)
whereas SGAs F, G, H, I, and the southern half of B have high rainfall
thresholds (i.e., 4 inches; Figure 1c).
Under systematic random sampling, routine sampling in conditionally
approved waters must occur when the SGA is open and not during a
temporary closure. Accordingly, this stipulation creates bias in the
routine monitoring data. As such, areas with a 1 inch rainfall threshold
will close more often than areas with 4 inch thresholds, resulting in
more restrictive sampling conditions and more homogeneous water quality
samples in the long-term. Furthermore, NCDMF will change rainfall
thresholds or exclude stations for areas with persistently poor water
quality, resulting in the longest-standing monitoring stations typically
being located in growing areas that are known to be productive with
relatively good water quality, contributing another source of bias to
the data.