Nath, 1997 fisheries. These applications are extended to a coastal marine inverte- brate, the hard clam Mercenaria spp., and the capabilities of GIS techniques in
providing a user-friendly approach to selecting individual lease sites that may be only several hectares in areal extent are emphasized.
2. Methods
In Florida, hard clam aquaculture originally developed in the Indian River lagoon Fig. 1A, on the east central coast of the state, during the mid-1980s Vaughan,
1988. That lagoon also supports an important commercial clam fishery, and the distribution pattern of clams throughout the lagoon has been mapped on several
occasions with particular emphasis on an area between Bennett Causeway and NASA Causeway known as Shellfish Harvesting Area SHA C Fig. 1B. This area
supports the most productive natural clam beds and the highest concentration of active clam aquaculture leases in the lagoon. The most recent and comprehensive
survey of SHA C was conducted during 1994, when 126 randomly selected and geographically referenced stations were sampled Fig. 2. Each sample station
consisted of a 1-m
2
quadrat from which a SCUBA diver hand-raked all legal-sized
Fig. 1. A Map of Florida, showing the location of the Indian River lagoon on the east central coast, the Cedar Keys region, and the Charlotte Harbor estuary. B Shellfish Harvesting Area SHA C
between NASA Causeway and Bennett Causeway of the Indian River lagoon, showing geographic features and the approximate location of extant hard clam aquaculture leases in the northeast corner of
Area C shaded L-shaped polygon. Total area considered in SHA C is 5405 ha.
Fig. 2. Distribution of the natural hard clam population in Indian River Shellfish Harvesting Area SHA C during 1994. Two ranges are reported: areas with densities less than five clams per m
2
are suitable for clam culture whereas those areas with densities of five or more clams per m
2
are reserved for access by the natural clam fishery. , locations of clam sampling stations.
clams minimum 25.4 mm across the shell valves and at which latitude and longitude coordinates were recorded using a Garmin GPS-75 hand-held Geographic
Positioning System GPS unit. Hard clam density information was entered into a desktop computer, along with
GPS-generated latitude and longitude coordinates that had been previously con- verted from the acquired degreesminutesseconds format to a decimal degree
format suitable for incorporation into the ArcView graphical software program Environmental Systems Research Institute, Redlands, CA. Clam density contours
were then generated within ArcView using an exponential inverse-distance matrix for weighting sample points, with an exponent of four and four nearest-neighbors.
The number of nearest-neighbors defines the domain for the integration among sample points, and the exponent of the inverse-distance matrix determines the rate
at which the influence of each point drops off with distance the higher the exponent, the less influence with distance Anonymous, 1996.
A digital map layer representing hard clam density was generated and output. Pertinent geographically-referenced digital map layers were then added, including
long-term water quality estimates for salinity and dissolved oxygen D.O., seagrass distribution, bathymetry, shellfish harvesting zone classifications, the location of
navigable channels, and the location of boat ramps. The resultant product was a composite map of SHA C upon which the Spatial Analyst component of the
ArcView software program could be applied to isolate areas that met certain preselected criteria. Spatial Analyst provides the capability to perform locational
analyses on both vector point-to-point and binary cell-based data layers Anony- mous, 1996. A cell size of 1 ha was chosen, equivalent to the minimum resolution
available from the seagrass maps. The composite database can be queried to identify the location of specific features of interest to the investigator e.g. Rubec et
al., 1998, 1999. For Indian River SHA C, the location of all sites that were less than 1.8 m deep, contained no seagrass, were a minimum of 30 m from the nearest
marked navigable channel, and supported fewer than five clams per m
2
were requested.
Shellfish harvesting classifications and boat ramps were also considered, but those criteria were ranked because their value is relative. Two shellfish harvesting
zones are suitable for hard clam aquaculture Berrigan, 1996, but operations located within Approved zones are optimal because shellfish can be harvested
essentially any time, whereas in Conditionally Approved zones there may be harvest restrictions at certain times or under certain conditions Table 1. Thus, these zones
were ranked with Approved zones being more desirable and Conditionally Ap- proved zones being less desirable according to their relative ease of access. Distance
to the nearest boat ramp also was ranked; sites within 500 m of a boat ramp were ranked lowest in desirability because of interference by boat traffic that could
disrupt operations and disturb the clams, whereas sites between 500 and 5000 m from the nearest boat ramp were ranked highest due to ease of access, and sites
greater than 5000 m from the nearest boat ramp were ranked intermediate due to greater transport distance of clams and materials.
Water quality parameters were also ranked. Water quality information was obtained from the Shellfish Environmental Assessment Section of the Florida
Department of Environmental Protection, whose responsibility it was to monitor the quality of shellfish growing waters in Florida. In Indian River Body C, data
from 42 monitoring stations was obtained at which information on salinity and D.O. were collected from a point 0.5 m above the sediment – water interface
Vanderbleek, 1993. For the period 1987 through 1998, the minimum recorded value was selected at each station for salinity and D.O., and the resultant data was
contoured following the procedures described above for contouring clam densities.
The resultant data sets were then categorized according to their suitability for supporting clam growth and survival.
Clams are able to survive salinities as low as 12.5 parts per thousand ppt, but shell growth slows below 20 ppt and essentially ceases below 17.5 ppt Castagna
and Chanley 1973. Thus, four salinity categories were applied, these being unac- ceptable B 12.5 ppt, marginal 12.5 – 17.5 ppt, acceptable 17.5 – 20 ppt, and
ideal \ 20 ppt. The upper limit of suitable salinity for hard clams generally exceeds 35 ppt Castagna and Chanley, 1973; Malouf and Bricelj, 1989, a value
that was not encountered at any Indian River station.
Hard clams are tolerant of low oxygen levels, but levels of 0.9 mgl or less may impair burrowing and possibly other physical and physiological functions Malouf
and Bricelj, 1989. Therefore a minimum acceptable D.O. of 1.0 mgl was applied, below which the area was considered unsuitable for hard clam aquaculture.
From its origins in the Indian River lagoon, the clam aquaculture industry has expanded to several other areas of the state, including the Cedar Keys area and
Charlotte Harbor Fig. 1A. Clam aquaculture has been extant in the Cedar Keys region for several years, but the industry is just developing in Charlotte Harbor and
it is in that estuary that these techniques have their most timely application. Charlotte Harbor was therefore used as an example of the application of the
methods for the selection of sites suitable for hard clam aquaculture and as a means of determining what additional information is required in order to make informed
lease-site selections within that estuary.
Information on seagrass distribution, long-term water quality estimates for salinity and D.O. collected at 79 stations Fig. 3 during 1984 through 1998 Seagle,
1996; Rodriguez, 1998; Rodriguez and Seagle, 1998; McMichael, unpub. data, bathymetry, the location of boat ramps and navigable channels, and shellfish
harvesting classifications was available for the Charlotte Harbor estuary. However, clam distribution surveys have not been conducted in Charlotte Harbor, so a clam
data layer was not included in this analysis. Using the same techniques that were applied to the Indian River SHA C analysis, the available data were used to
construct a composite GIS map of the estuary. Spatial Analyst was then used to query the database for the location of sites that contained no seagrass, occupied
water depths less than 1.8 m, were no closer than 30 m to the nearest navigable channel, and possessed suitable water quality characteristics as described above for
the Indian River. The relative suitability of the resultant sites was determined based upon location within an Approved or Conditionally Approved shellfish harvesting
zone and upon distance to the nearest boat ramp. The same ranking scheme was applied to the salinity, harvesting zone, and boat ramp criteria as was applied in the
Indian River.
3. Results
