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
Individual map layers for SHA C illustrate the shallow nature of the Indian River lagoon in that area Fig. 4, the abundance of seagrass along both the eastern
Fig. 3. Location of water quality salinity and dissolved oxygen, D.O. sampling stations in Charlotte Harbor, Florida. See Table 3 for specific information on each station.
and western shorelines Fig. 5, the distribution and abundance of legal-sized clams recorded during 1994 Fig. 2, historical minimum salinity Fig. 6 and D.O. Fig.
7 distribution patterns, the location of navigable channels Fig. 5, and shellfish harvesting zones Fig. 8. No Approved shellfish harvesting zones were located
within SHA C. Also, the status of each zone changes with season Vanderbleek, 1993, so the most restrictive classification was used winter; December – February
in the analyses to provide for the most conservative placement of leases. Thus, restrictions on clam harvest may be less stringent during certain times of the year,
but they have been modeled using the more stringent harvesting conditions.
Fig. 4. Water depth ranges m in Shellfish Harvesting Area SHA C of the Indian River lagoon, Florida.
Fig. 5. Location of seagrass beds, boat ramps, and navigation channels in Shellfish Harvesting Area SHA C of the Indian River lagoon, Florida.
Additionally, the entire expanse of SHA C is characterized by marginal salinity conditions 12.5 – 17.5 ppt, a situation which is discussed below. Results from
Spatial Analyst queries of the composite SHA C map, for the location of areas that met the classification criteria, identified 281 ha of appropriate submerged lands
Fig. 9. Most of the areas deemed not suitable for aquaculture were constrained by the presence of seagrass 622 ha, commercially exploitable clam densities 1737 ha,
low levels of D.O. 1607 ha, water depth or the proximity of navigable channels 341 ha, low salinity 49 ha, a Conditionally Restricted or Prohibited shellfish
harvesting classification 764 ha, or a boat ramp within 500 m 4 ha.
The analysis of Indian River SHA C was conducted after the allocation of 28 lease sites within that area see Fig. 1B for approximate locations. Although water
Fig. 6. Minimum salinity ppt contours in Shellfish Harvesting Area SHA C of the Indian River lagoon, Florida. Numbers denote sample station locations see Table 2.
Fig. 7. Minimum dissolved oxygen D.O., mgl contours in Shellfish Harvesting Area SHA C of the Indian River lagoon, Florida. Numbers denote sample station locations see Table 2.
depth, seagrass and clam abundance, location of navigable channels, and shellfish harvesting classifications were used as guidelines in the process of selecting sites
appropriate for hard clam aquaculture, at that time the Spatial Analyst program
Fig. 8. Shellfish harvesting classifications in Shellfish Harvesting Area SHA C of the Indian River lagoon, Florida, during winter December – February. Only the Conditionally Approved area is suitable
for hard clam aquaculture.
Fig. 9. Areas suitable for hard clam aquaculture leases in Shellfish Harvesting Area SHA C of the Indian River lagoon, Florida. Areas categorized as ‘unsuitable’ are not appropriate for hard clam
aquaculture due to the presence of seagrass, high density clam populations recorded during the 1994 survey, low levels of D.O. recorded between 1987 and 1998, excessive water depth or the proximity of
navigable channels, or low salinity conditions inimical to clam survival. Of the remaining area, those cells classified as ‘prohibited’ = Prohibited or Conditionally Restricted classification do not meet
shellfish harvesting water quality standards. In the legend, CA, Conditionally Approved shellfish harvesting area; D.O., dissolved oxygen mgl; Sal, salinity ppt; range in meters represents water depth
first or distance to the nearest boat ramp second.
was not available for a more objective determination process. Salinity and D.O. data were also not incorporated, the implications of which are discussed below.
Fig. 9.
Therefore, the results from the Indian River provide an a posteriori test of a GIS-based approach for selecting hard clam lease sites.
The State of Florida is still allocating lease sites in Charlotte Harbor and the results are applicable to that decision making process. There, available informa-
tion on bathymetry Fig. 10, seagrass distribution Fig. 11, historical minimum salinity Fig. 12 and D.O. Fig. 13 distribution patterns, and shellfish harvesting
zone classifications Fig. 14 were used to develop a composite map of sites that met the same criteria that had been established for lease site selection in the
Indian River. The result was a map identifying 1844 ha of submerged land that appears ideal for hard clam aquaculture Fig. 15, although the nearest boat ramp
is \ 5000 m distant. Another 4477 ha are suitable for clam aquaculture but occasionally experience salinity levels that, though not fatal to the clams, may
cause a slowing or cessation of growth Fig. 15. As in the Indian River lagoon, most of the areas deemed not suitable for aquaculture were constrained by the
presence of seagrass 17 455 ha, low levels of D.O. 227 ha, spoil areas or proximity to boat ramps 554 ha, see Fig. 9, low salinity 2881 ha, water depths
exceeding 1.8 m 11 361 ha, or a Conditionally Restricted or Prohibited shellfish harvesting classification 7956 ha.
4. Discussion
