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Federal Register / Vol. 86, No. 124 / Thursday, July 1, 2021 / Rules and Regulations
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For this rule, we delineated critical habitat unit boundaries using the following criteria:
1 We compiled all available occurrence data records.
2 We used confirmed presences between the years 2000 and 2016 as the foundation for identifying areas currently occupied.
3 We evaluated habitat suitability of stream segments currently occupied by the species and retained all occupied stream segments.
4 We evaluated unoccupied stream segments for suitability, connectivity, and expansion, and identified areas containing the components comprising the physical or biological features that may require special management considerations or protection.
5 We omitted some unoccupied areas that are highly degraded and are not likely restorable e.g., insufficient flowing water, channel destabilized, and, therefore, are not considered essential for the conservation of the species.
6 We delineated boundaries of critical habitat units based on the above information.
Specific criteria and methodology used to determine critical habitat unit boundaries are discussed below.
Sources of data for this critical habitat designation include multiple databases maintained by Florida Fish and Wildlife Conservation Commission, Dr. James D.
Williams, Florida Museum of Natural History, and U.S. Geological Survey;
verified museum records from multiple institutions see Methods in Johnson et al. 2016, pp. 164165; and a status report by BlalockHerod and Williams 2001, entire. Historical and recent occurrence data included records collected from May 1916 to March 2016.
Many surveys were conducted throughout the Suwannee River basin by Florida Fish and Wildlife Conservation Commission biologists during 20122016, and all sites with historical occurrences of Suwannee moccasinshell were sampled during this period. Sources of information pertaining to habitat requirements of the Suwannee moccasinshell include observations recorded during surveys and information contained in Blalock Herod and Williams 2001, entire and Williams et al. 2014, pp. 278280.
Areas Occupied at the Time of Listing We define currently occupied as river reaches with positive surveys from 2000 to 2016. In making these determinations, we recognized that known occurrences for some mussel species are extremely localized, and rare mussels can be difficult to locate. In
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addition, stream habitats are highly dependent upon upstream and downstream channel habitat conditions for their maintenance. Therefore, we considered the entire reach between the uppermost and lowermost currently occupied locations to delineate the probable upstream and downstream extent of the Suwannee moccasinshells distribution. Within the current range of the species, some habitats may or may not be actively utilized by individuals, but we consider these areas to be occupied at the scale of the geographic range of the species.
We are designating as critical habitat for the Suwannee moccasinshell one occupied unit in the Suwannee River and lower Santa Fe River. This area contains one or more of the physical or biological features essential to the conservation of the Suwannee moccasinshell, and those physical or biological features may require special management conditions or protections.
However, this single population provides little redundancy for the species, and a series of back-to-back stochastic events or a single catastrophic event could significantly reduce or extirpate this one population.
Consequently, we have determined that the occupied area is inadequate to ensure the conservation of the species.
Therefore, we have also identified, and are designating as critical habitat, unoccupied areas that are essential for the conservation of the species.
Areas Not Occupied at the Time of Listing We are designating two unoccupied units as critical habitat. The units have some of the physical or biological features essential to the conservation of the species, and we are reasonably certain that each will contribute to the conservation of the species. Our specific rationale for each unit can be found in the unit descriptions below.
An examination of all available collection data shows that the Suwannee moccasinshells range and numbers have declined over time see Distribution and Abundance discussion in the final listing rule. For example, despite considerable survey effort, the species has not been collected in the lower Suwannee River or Withlacoochee River sub-basins since the 1960s, and was last collected in the upper Santa Fe River sub-basin in 1996
Johnson et al. 2016, p. 170. There has also been a reduction in numbers, with fewer individuals encountered during recent surveys than were collected historically Johnson et al. 2016, pp.
166, 170.

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The Suwannee moccasinshells reduced range and small population size may increase its vulnerability to many threats. Aquatic species with small ranges, few populations, and small or declining population sizes are the most vulnerable to extinction Primack 2008, p. 137; Haag 2012, p. 336. The effects of certain environmental pressures, particularly habitat degradation and loss, catastrophic weather events, and introduced species, are greater when population size is small Soule 1980, pp. 33, 71; Primack 2008, pp. 133137, 152. Threats to the Suwannee moccasinshell are compounded by its reduced and linear distribution, with nearly the entire population presently distributed within the Suwannee River mainstem. A small population also occurs in the lower Santa Fe River;
however, only 5 recent collections 3 of which are relic shell have been reported in this sub-basin Johnson et al.
2016, p. 171.
A larger population of Suwannee moccasinshell occurring over a wide geographic area can have higher resilience. A large population is better able to return to pre-disturbance numbers after stochastic events, and also has increased availability of mates and reduced risk of genetic drift and inbreeding depression. The minimum viable population size needed to withstand stochastic events is not known for mussels. For species with complex life histories like freshwater mussels, maximizing the chances of viability over the long term, likely requires a population of considerable size Haag 2012, p. 371. Reestablishing viable populations in the Withlacoochee and upper Santa Fe River sub-basins increases Suwannee moccasinshell redundancy by expanding its range into historically occupied areas, potentially increasing population size, and providing refuge from catastrophic events for example, flooding and spills in the Suwannee River.
We determined the Withlacoochee and upper Santa Fe River sub-basins have the potential for future reoccupation by the species, provided that stressors are managed and mitigated. These specific areas encompass the minimum area of the species historical range within the critical habitat designation, while still providing ecological diversity so that the species has the ability to evolve and adapt over time representation to ensure that the species has an adequate level of redundancy to guard against future catastrophic events. These areas also represent the stream reaches within the historical range with the best potential for recovery of the species due
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