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Federal Register / Vol. 86, No. 131 / Tuesday, July 13, 2021 / Proposed Rules the existing physical or biological features essential to the conservation of the species. Collection occurrence patterns suggest that adult pearl darters migrate upstream to spawn in suitable gravel or bedrock reaches, with elevated spring river discharge dispersing larvae and juveniles into downstream reaches;
an alternative hypothesis considers that the pearl darter moves from shallow, easily collected spawning habitats into deeper habitats where it is more difficult to detect the fish see Sites for Breeding, Reproduction, or Rearing or Development of Offspring, above.
While both hypotheses are partially supported by data, we note that the disappearance of the species from the Pearl River drainage occurred fairly rapidly and simultaneously in all stream orders, suggesting some element of migration may be involved in the darters life history. To allow for potential seasonal movement between stream reaches, we propose to designate one continuous unit of occupied critical habitat within the Pascagoula River drainage. This unit includes portions of the Chunky, Bouie, Leaf, Chickasawhay, and Pascagoula Rivers, as well as reaches of Okatoma and Big Black Creeks, as described below under Proposed Critical Habitat Designation.
Since the 2017 listing of the species, there have been 71 site collections of pearl darter in the Pascagoula River drainage Wagner et al. 2019, pp. 818;
Schaefer et al. 2020, pp. 2627, 4344.
One of these collections in 2018
extended the known range approximately 60 mi 97 km in Black Creek, above its confluence with the occupied reach of Big Black Creek Schaefer et al. 2020, pp. 2627. We consider this additional mileage of stream reach to be occupied at the time of listing. This is because the reach between the previously identified population in Big Black Creek and the newly discovered population upstream has the physical or biological features essential to the conservation of the species, and the species potentially seasonally migrates. The potential for seasonal migration, the species small size and rarity, and the fact that surveys for the pearl darter are difficult and not always definitive of the species absence within a particular reach of an occupied stream also support considering this area occupied at the time of listing.
In making these determinations, we recognize that collection sites for the pearl darter occur at areas generally accessible to fish biologists and that occupied habitats within a river reach may vary depending upon life stage, stream size, and season. Additionally, stream habitats are highly dependent
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upon upstream and downstream channel habitat conditions for their maintenance. Therefore, we considered the areas occupied at the time of listing to extend from an identifiable landmark e.g., bridge crossing, tributary confluence, etc. nearest the uppermost records within second or third order streams, through their confluence with third and fourth order streams, downstream to an identifiable landmark near the lowermost areas of collection in the Pascagoula River i.e., forks of the East and West Pascagoula River. Within the current range of the pearl darter within the Pascagoula River drainage, some habitats may or may not be actively used at all times by individuals;
however, these areas are necessary for maintaining population connectivity, as well as other physical or biological features essential to the conservation of the species, and, therefore, are considered the geographic area occupied at the time of listing for the pearl darter. This area referred to below as proposed Unit 1 contains all of the physical or biological features essential to the conservation of the pearl darter and which may require special management conditions or protections.
Areas Unoccupied at the Time of Listing To consider for designation areas not occupied by the species at the time of listing, we must demonstrate that these areas are essential for the conservation of the pearl darter. The proposed occupied critical habitat does not include geographic areas within the Pearl River drainagethe only other area in which the pearl darter historically occurredas it has been extirpated from that drainage. In addition, because the Pascagoula River drainage population is the only extant population, that population provides no redundancy for the species. Based upon the species rapid and system-wide extirpation from the Pearl River drainage, a series of back-to-back stochastic events or a single catastrophic event could similarly significantly reduce resiliency or extirpate the Pascagoula River population. For these reasons, we determined we cannot conserve the species by designating only occupied habitat as it includes only a single population in a single drainage.
Thus, we determined that habitat in another historical drainage is needed for the long-term survival and recovery of the species. Therefore, because we determined that the one occupied area alone is not adequate for the conservation of the species, we have identified and are proposing for designation as critical habitat specific areas outside the geographical area
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occupied by the species at the time of listing that are essential for the conservation of the species. We used historical occurrence data and the physical or biological features described earlier to identify unoccupied habitat essential for the conservation of the pearl darter.
Based on our review, we determined that the lower Strong River, a major tributary of the Pearl River, has the potential for future reintroduction and reoccupation by the pearl darter, provided that stressors are managed and mitigated. Reestablishing a viable population in the Strong River will restore the species redundancy within the historical range and increase the species ecological representation. The specific area encompasses the minimum area of the species historical range within the Pearl River drainage, while still providing ecological diversity so that the species can evolve and adapt over time. This river reach also provides the potential for the pearl darter to expand its range into other historically occupied areas, which currently may be or may later become suitable, to ensure that the species has an adequate level of redundancy within the Pearl River drainage and guard against future catastrophic events. The lower Strong River also represents the stream reach within the historical range with the best potential for recovery of the species due to current conditions, suitability for reintroductions, and access for monitoring.
Accordingly, we propose to designate one unoccupied unit in the lower Strong River within the Pearl River drainage.
As described below in the individual unit descriptions, this unit contains all of the physical or biological features essential to the conservation of the species and is reasonably certain to contribute to the conservation of the species.
General Information on the Maps of the Proposed Critical Habitat Designation The areas proposed as critical habitat include only stream channels within the ordinary high-water line. There are no developed areas within the critical habitat boundaries except for transportation and pipeline crossings, which do not remove the suitability of these areas for the pearl darter. When determining proposed critical habitat boundaries, we made every effort to avoid including developed areas such as lands covered by buildings, pavement, and other structures because such lands lack physical or biological features necessary for the pearl darter. The scale of the maps we prepared under the parameters for publication within the
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