Summary The Pascagoula watershed likely offers the greatest possibility for the survival of the Gulf sturgeon, Acipenser oxyrinchus desotoi, within Mississippi. Thus, understanding and preserving the connectivity between distant habitats in this region plays a major role in protecting and managing such anadromous fish populations. The focus of this project was to determine the within‐river routes Gulf sturgeon take through the lower Pascagoula River downstream of the point where it splits (river kilometer 23) into two distinct distributaries. Sixty days were sampled throughout a two‐year period with a total effort of 81 947 net‐meter‐hours and eight Gulf sturgeon were captured, ranging from 74 to 189 cm FL and weighing from 3.6 to 52.6 kg. Using an array of automated telemetry receivers, acoustically tagged Gulf sturgeon movements were monitored within the lower river and associated estuary. Estimated residence times (days) suggest Gulf sturgeon appear to prefer the eastern distributary upriver from Bayou Chemise as the primary travel corridor between freshwater habitats and marine feeding grounds. The western distributary mouth was more highly used by Gulf sturgeon during both seasonal migrations between upriver and offshore habitats. Thus, the western distributary appears to represent the main entrance point utilized by Gulf sturgeon to the Pascagoula River watershed and should be protected as the eastern distributary mouth has been altered from a natural marsh edge to one of hardened surfaces.

Gulf sturgeon Acipenser oxyrinchus desotoi is listed as 'threatened' under the US Endangered Species Act throughout its range in the northern Gulf of Mexico, with Mobile Bay, Alabama, USA, as the recognized break between eastern and western populations. Population recovery requires protection of the species and its critical habitat. We examined Gulf sturgeon physical habitat attributes and infaunal macrobenthic prey density and composition both spatially and seasonally relative to acoustically tagged Gulf sturgeon occurrence in the Pascagoula River estuary. Gulf sturgeon occupancy patterns indicated that adults move quickly through the system during fall and spring compared to longer but more spatially and temporally variable occupancy for juveniles and sub-adults in both seasons; sub-adults exhibited a less spatially and temporally variable occupancy pattern. We found significant differences in physical habitat and macrobenthic density characteristics that partially explained Gulf sturgeon spatial and temporal occupancy patterns. Direct comparisons of physical drivers (% silt, depth, particulate organic carbon) and macrobenthic density patterns (BEST procedures) were significantly correlated (p < 0.01) but weak (global R = 0.277) and suggest alternate hypotheses to better explain the differential estuarine habitat use patterns. The most parsimonious explanation with multiple weights-of-evidence suggests reduced use of the eastern distributary habitat by Gulf sturgeon based on synergistic effects of urbanization and industrialization such as bulkheading, channelization, dredging and related maintenance activities, and beach re-nourishment, all of which occur almost exclusively in the east zone of the estuary.

The spatial and seasonal occupancy by Gulf Sturgeon Acipenser oxyrinchus desotoi was estimated using acoustically tagged fish in the Pascagoula River estuary over a 3-year period (2010-2013) and was based on 131,381 detections. Juveniles spent considerably more time in the estuary than subadults or adults, but that varied by year. Gulf Sturgeon of all size-classes entered the acoustic array earlier when stream discharge was high (2011, 2012) compared with when it was lower (2010). The difference was about 14-28 d earlier for adults and subadults but only 12-17 d for juveniles. Temporally, there were no significant differences in occupancy by year or season for adults or subadults or by year for juvenile Gulf Sturgeon, suggesting repeated patterns across the 3 years of this study. Significant and consistent spatial differences in occupancy occurred by array zone, with juveniles occupying the West zone more than the East and River zones of the acoustic array; however, occupancy in the River zone was greater than in the East zone. We found similar patterns for subadults, with the highest occupancy in the West zone compared with the East zone; use was higher in the River zone than in the East zone but there was no difference between the River and West zones. In contrast, no year, season, or zone patterns were identified for adult Gulf Sturgeon, suggesting annual movement along a defined travel corridor (from the River zone to the West zone to offshore islands), with little time spent in the estuary. Adults appear to move quickly through the system during emigration (fall) and immigration (spring) compared with longer but more variable occupancy for juveniles and subadults. Our long-term data illustrates the importance of habitat connectivity in the recovery of Gulf Sturgeon and enhances earlier work on estuarine zones used during migration periods and estuarine residency; both are important for a better understanding of critical habitat maintenance and the development of a robust recovery plan.

Environmental cues that are associated with individual movement of threatened Gulf Sturgeon from upriver areas to nearshore and offshore winter feeding areas have been described throughout much of their range in the Gulf of Mexico. In this study, we focus on small‐scale movement of Gulf Sturgeon between summer ‘holding’ areas and the fall staging area in the Pascagoula River system (Mississippi, USA). We evaluated a set of logistic regression models using Akaike's Information Criterion and found that relative changes in barometric pressure, time of day, and water temperature were cues for small‐scale Gulf Sturgeon movements during fall outmigration. Numerous environmental cues appear to drive the activity of Gulf Sturgeon in staging areas, indicating the complexity of abiotic factors affecting the observed staging patterns during emigration. The identification of the environmental drivers that are associated with Gulf Sturgeon movement is particularly important if these known saline transition zones change spatially annually with variable rainfall or due to water withdrawals and are used by Gulf Sturgeon making osmotic adjustments while moving downriver.

SummaryLarge sub-adult/adult western population Gulf Sturgeon move to barrier islands in winter to feed whereas eastern fish move offshore or to nearshore non-island environments; most small sub-adults/juveniles remain in the estuarine system during winter. To test this, we used an acoustic data set deployed around the Port of Gulfport (hereafter Port footprint, east gate, west gate) within Mississippi Sound. We documented between three and six fish on each receiver totaling 12,285 detections for all 19 receivers between September 2012-May 2013. Only 30% of fish had both a high number of overall detections and a high number of detection days. In contrast, from October 2013-May 2014, between three and nine fish were detected on each receiver but with only 2,371 detections. Five fish (29.4%) had a high number of detections but a reduced number of detection days in the acoustic array; all fish appeared to be transients. Adults, unexpectedly, had the most prevalent occurrence and number of detection days in these shallow, non-island beach environments contrary to our initial hypothesis. Our data suggests annually variable, regional-scale use of beach environments not associated with barrier islands that likely serve as a travel corridor between drainages/offshore barrier islands, or as feeding zones; these movements may enhance the potential for mortality in Mississippi Sound.

Summary The Gulf sturgeon, Acipenser oxyrinchus desotoi Mitchill 1815, is an anadromous fish native to the Gulf of Mexico (GOM). This threatened species ascend northern GOM rivers ranging from the Pearl River, Louisiana to the Suwannee River, Florida. The drainage fidelity seen within this species coupled with the complex migration patterns create many unknowns concerning the functionality between drainages and size classes. Being a threatened species that cannot be sacrificed, morphometric measurements are difficult to acquire due to minimized handling times and non‐lethal data collection methods. Seven morphometric measurements of Gulf sturgeon were compared between western and eastern GOM rivers and multiple size classes (juvenile, sub‐adult, and adult). Principal Component, Discriminant Function, and Maximum Chance Criterion analyses were conducted and identified differentiating morphometrics and classified groupings among size classes and rivers of Gulf sturgeon. This morphometric approach has the potential to be used in parallel with molecular findings and should allow managers to identify sub‐populations for a variety of threatened and endangered species.

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