Habitat Utilization and the Effect of Hypoxia Across Different Life History Stages of Silver Perch (Bairdiella Chrysoura) in the St. Johns River, Florida

Silver perch (Bairdiella chrysoura) are a dominant estuarine resident in Northeast Florida with a protracted spawning from April- November and highest densities from May-July due to the addition of young of the year. Based on analysis of fisheries independent monitoring data from 2001-2005, no single abiotic factor was responsible for presence of silver perch in a habitat, but a pattern of high temperature, intermediate salinity and high dissolved oxygen were significant factors correlated with silver perch presence. There was a positive statistical relationship for the three rivers between high dissolved oxygen, high temperatures, decreasing depth, and high abundance of silver perch within a habitat. Dense aggregations “hotspots” of silver perch within these three estuaries were identified, and further analysis revealed that “hotspot” utilization differed by size class. The “hotspots” are identified as Essential Fish Habitats (EFH) and can aid in the conservation and management of silver perch. The meso/polyhaline portion of the lower St. Johns River extends from the Fuller Warren Bridge in Jacksonville, Florida to the Atlantic Ocean. This stretch of the river has an area that undergoes seasonal hypoxia in July and August. There has been very little work done to test estuarine fish’s ability to detect and avoid hypoxic waters across size class. Avoidance to hypoxia by silver perch across different size class was first conducted with normoxic conditions throughout a partitioned aquarium (control) and the second trial was with a gradient within the partitioned aquarium from high DO down to low DO waters (experimental). Silver perch showed a significant difference in the expected distribution of the control group compared to the observed group for the three smaller size classes. Fish in the largest size class showed no difference in the distribution of the expected and observed and further analysis showed that oxygen choice differed across size class. Different size classes of silver perch were tested in a swimming speed apparatus under varying current speeds and oxygen levels. Under normoxic conditions (\u3e5 mg/L) a trend of decreased swimming ability with increased speed was observed and under hypoxic conditions (2-1.4 mg/L) swimming ability was decreased at all current speeds. This swimming data was incorporated into a multicriteria relative least-cost path GIS model that showed that the largest size class of silver perch could not escape the hypoxic zone from three different starting locations in the lower St. Johns River. Since the fish with the greatest swimming ability could not escape the hypoxic zone, no other size classes of fish were modeled and under a fast moving, large scale hypoxic event there would be high silver perch mortality