Shellfish growing areas in the United States are
basically classified into one of four types: approved, conditional,
and prohibited (learn more about the classification
types). These classifications are based on water
quality standards. The National Shellfish Sanitation Program (NSSP)
created a model ordinance that sets forth the bacteriological standards
be used for shellfish classification.
To determine what classification an area should have, States must
conduct sanitary surveys of the area to identify possible pollution
sources as well as determine potential risk factors and threats.
They must also conduct water quality monitoring. The NSSP ordinance
requires that a minimum of 30 water quality samples be collected
from an area in order to determine classification. Bacterial analysis
involves a multi-tube fermentation technique and results are documented
as MPN/100mL. MPN, which stands for most probable number, is not
a direct count of bacteria, but is an estimation of the most likely
number of bacteria present in a sample.
The bacteriological standard for shellfish growing area classification
is two-part: it consists of the calculation of the geometric mean
and the calculation of the estimated 90th percentile. The estimated
90th percentile is a statistical technique that accounts for variability
within a dataset (i.e. random high bacteria counts) and due to
this, is a tougher standard to achieve than is the geometric mean
standard. For a shellfish growing area to have an approved or conditionally
approved status, the geometric mean for fecal coliform bacteria
must be less than 14 MPN/100mL and the estimated 90th percentile
must be less than 43 MPN/100mL (based on a minimum of the last
30 samples collected).
Shellfish Growing Area Classification Types
There are generally four classifications for a shellfish growing
area: approved, conditional, restricted and prohibited. Within
these categories, there can be additional subsets of classification,
for example, conditionally approved or conditionally restricted.
Washington State uses the four following classifications when determining
the status of shellfish growing areas in the State: approved, conditionally
approved, restricted and prohibited.
An approved status means that water quality meets the standards
set forth by the NSSP (fecal coliform bacterial results from a
minimum of the last 30 samples have a geometric mean less than
14 MPN/100mL and have an estimated 90th percentile less than 43
MPN/100mL). Shellfish harvest is allowable in these areas.
A conditionally approved status means that there are specific,
predictable events (such as rainfall) that can cause an area to
exceed the water quality standards. The area is approved for shellfish
harvest unless such an event occurs, at which time it is closed
for harvest for a period of time pre-determined by the State. Areas
with conditionally approved status must meet the standards set
forth by the NSSP (see under approved status) outside of the specific,
predictable events. Shellfish harvest is allowable in these areas
when a closure is not in place.
A restricted status is given to an area that is not able to meet
the shellfish standards, but where pollution sources are limited
and the contamination is low enough that shellfish are able to “cleanse” themselves
of the harmful contaminants if moved to a clean area for a period
of time. Washington State further defines restricted areas by requiring
that the limited pollution source be non-human in origin (based
on sanitary survey analysis). Shellfish in a restricted area can
be “relayed” or moved to an approved area for a pre-set
period of time to allow the shellfish to “cleanse” themselves,
at which time the shellfish can be marketed.
A prohibited status means that an area is unable to meet the bacteriological
standards of the NSSP, known pollution sources exist within the
area, or pollution sources are present that are unpredictable.
The State also classifies any area that has not has a sanitary
survey conducted as prohibited, as there is not enough evidence
to adequately determine risk to human health.