Consistent SpatioTemporal Estuarine Habitat Use during Emigration or Immigration of a Western Population of Gulf Sturgeon

<p>The spatial and seasonal occupancy by Gulf Sturgeon <i>Acipenser oxyrinchus desotoi</i> 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.</p> <p>Received May 6, 2015; accepted September 2, 2015</p

Survival, Growth and Reproduction of Non-Native Nile Tilapia II: Fundamental Niche Projections and Invasion Potential in the Northern Gulf of Mexico

<div><p>Understanding the fundamental niche of invasive species facilitates our ability to predict both dispersal patterns and invasion success and therefore provides the basis for better-informed conservation and management policies. Here we focus on Nile tilapia (<em>Oreochromis niloticus</em> Linnaeus, 1758), one of the most widely cultured fish worldwide and a species that has escaped local aquaculture facilities to become established in a coastal-draining river in Mississippi (northern Gulf of Mexico). Using empirical physiological data, logistic regression models were developed to predict the probabilities of Nile tilapia survival, growth, and reproduction at different combinations of temperature (14 and 30°C) and salinity (0–60, by increments of 10). These predictive models were combined with kriged seasonal salinity data derived from multiple long-term data sets to project the species' fundamental niche in Mississippi coastal waters during normal salinity years (averaged across all years) and salinity patterns in extremely wet and dry years (which might emerge more frequently under scenarios of climate change). The derived fundamental niche projections showed that during the summer, Nile tilapia is capable of surviving throughout Mississippi's coastal waters but growth and reproduction were limited to river mouths (or upriver). Overwinter survival was also limited to river mouths. The areas where Nile tilapia could survive, grow, and reproduce increased during extremely wet years (2–368%) and decreased during extremely dry years (86–92%) in the summer with a similar pattern holding for overwinter survival. These results indicate that Nile tilapia is capable of 1) using saline waters to gain access to other watersheds throughout the region and 2) establishing populations in nearshore, low-salinity waters, particularly in the western portion of coastal Mississippi.</p></div

Areal coverage (km²) of survival, growth, and reproductive habitats for <i>Oreochromis niloticus</i> in the Mississippi Sound for normal salinity years, wet years, and dry years.

<p><b>All values are ×10⁴.</b></p

Universal kriging results for the predicted mean salinity across 14 years (normal), mean salinity for three extremely wet years (wet) and mean salinity for three extremely dry years (dry) during both summer and winter seasons.

<p>N is the number of long-term stations used in each analysis. Nugget represents discontinuity at the origin due to microscale effects or measurement error. Total sill is the variance estimate. Theoretical and Effective ranges are the distances at which sampling stations are no longer spatially autocorrelated.</p

Model estimates and associated statistics from Firth-logistic regression.

<p><b>SE = Standard Error, PML = penalized maximum likelihood, WALD = Wald score, LRT = log ratio test. </b><b><i>R²</i></b><b> = Nagelkerke's R².</b></p

Areas of the Mississippi Sound with the highest probability of <i>Oreochromis niloticus</i> survival (S), growth (G), and reproduction (R) during summer months.

<p>A) normal years, B) wet years, and C) dry years.</p

Empirical estimates of the relationships between <i>Oreochromis niloticus</i> biological response variables and salinity.

<p>A) summer survival, B) summer growth, C) summer reproduction, and D) winter survival. The black line indicates penalized maximum likelihood estimate of the logistic response function (Eq. 2); gray lines are the 95% confidence intervals based on the profile of the penalized likelihoods.</p

Map of Mississippi Sound and locations of long-term environmental stations used in for salinity projections.

<p>Closed circles represent stations that have both summer and winter data while closed triangles only have summer data. The gray stars represent the location of established populations of <i>Oreochromis niloticus</i>.</p

Projected areas of the Mississippi Sound that <i>Oreochromis niloticus</i> might use to establish a population (survive, grow, reproduce, and overwinter), pulse seasonally in abundance (survive, grow, reproduce, but not overwinter), or use as a salt bridge between adjacent freshwater systems (Survive and possibly grow, but reproduction and overwintering are unlikely).

<p>Each panel is for the following scenario: A) normal summer and normal winter, B) normal summer and wet winter, C) normal summer and dry winter, D) wet summer and wet winter, E) wet summer and dry winter, F) dry summer and wet winter, G) dry summer and dry winter, H) wet summer and normal winter, and I) dry summer and normal winter.</p