Scott W. Raborn

Predicting and managing risk of unsuitable dissolved oxygen in a eutrophic lake

Eutrophication of lakes is often accompanied by wide diel fluctuations in dissolved oxygen (DO) concentrations that can influence fish assemblage composition and periodically result in highly visible fish kills. We conducted a probabilistic risk assessment to estimate the likelihood that a shallow eutrophic oxbow lake would be affected by critically low DO concentration at dawn during mid-summer. Monte-Carlo simulations with a respiration model that considered DO concentration at dusk, water respiration rate, sediment respiration rate, air–water diffusive exchange, and night duration were used to generate a depth-specific probability distribution of DO concentrations for the following dawn. This probability distribution was combined with a bathymetric model of the lake, constructed in a geographic information system using geo-referenced depth data, to predict the probability that a certain area of the lake would be affected by critically low DO concentration. The area affected was evaluated as a function of lake stage elevation. Results of the risk assessment pointed to the importance of shallow water (<1 m depth) in determining the area of unsuitable DO at dawn. Simulation results suggested that water level manipulation can be used to reduce the probability of frequent unsuitable DO events that affect small areas of the lake, and simultaneously reduce the area of infrequent unsuitable DO events that affect large areas of the lake. High-probability unsuitable DO events affecting small areas of the lake are not of major concern because fish are able to locate refugia of suitable DO in adjacent areas. However, water level management in shallow eutrophic lakes can have a major effect on reducing the risk of low-probability unsuitable DO events that affect large areas of the lake.

Effects of Simulated Removal of Striped Bass from a Southeastern Reservoir

Abstract Since the introduction of striped bass Morone saxatilis and hybrids of striped bass and white bass Morone chrysops into reservoirs, much concern has been directed at the possibility of these predators competing with other sport fishes for limited prey. If density of striped bass is reduced or eliminated through modifications of the stocking program, the prey not consumed by striped bass may be shifted to other sport fishes. The resulting increase in biomass of other sport fishes would be a function of the amount of added prey, the percent of this additional prey eaten by other sport fishes, and the efficiency with which the prey is converted into biomass. We used bioenergetics models to estimate annual striped bass prey consumption in Norris Reservoir, Tennessee. Total annual consumption was estimated at 52 kg/ha (estimated range = 17–100 kg/ha), clupeids accounting for the majority (94%), followed by lepomids (4%) and other food items (2%). Existing biomass of black basses Micropterus spp., crap...

Prey Supply and Predator Demand in a Reservoir of the Southeastern United States

Abstract We estimated annual prey supply and predator demand distributions for the fish assemblage of Norris Reservoir, Tennessee, to assess potential prey deficiencies. Prey supply was defined as the surplus biomass that could be removed without affecting future prey generations and was limited to cohorts consumed by predators. Demand was represented by the annual consumption by the predator community (piscivorous fishes) and was estimated with bioenergetics models. Our demand estimates were conservative because predators may have greater demand for prey than what they actually consume. Together, clupeids (gizzard shad Dorosoma cepedianum, threadfin shad D. petenense, and alewife Alosa pseudoharengus) and Lepomis spp. accounted for 80% of overall prey fishes consumed. Median annual demand for clupeids and Lepomis spp. combined was 271 kg/ha (90% confidence interval = 183–552 kg/ha) from August 1996 to July 1997 and 182 kg/ha (92–509 kg/ha) from August 1997 to July 1998. Median supply during these times w...

Modeling Predation as a Source of Mortality for Piscivorous Fishes in a Southeastern U.S. Reservoir

Abstract We investigated the possibility that the survival of piscivorous fishes in Norris Reservoir, Tennessee, was reduced due to individuals being consumed by other piscivorous fishes. Black basses (smallmouth bass Micropterus dolomieu, spotted bass M. punctulatus, and largemouth bass M. salmoides) were the only piscivores consumed and were eaten only by other black basses. The total prey consumption for black bass populations was estimated via bioenergetics models. The effect of density-dependent survival was incorporated by modeling survival–density relationships based on estimates of densities in consecutive years. Survival for black basses decreased with increasing density. As a result, predation was projected to be a compensatory source of mortality that failed to reduce overall survival, even though we estimated that 85% of black basses were consumed during one year. The projections from the survival–density models indicated that these species were resistant to decreases in overall survival when ...

Effects of Turtle Excluder Devices (TEDs) on the Bycatch of Three Small Coastal Sharks in the Gulf of Mexico Penaeid Shrimp Fishery

Abstract The stock of blacknose sharks Carcharhinus acronotus in the U.S. South Atlantic and the Gulf of Mexico is overfished, and according to the 2007 stock assessment conducted by the National Marine Fisheries Service overfishing continues to occur. Penaeid shrimp trawl bycatch rates in the Gulf of Mexico were modeled for this species as well as for the Atlantic sharpnose shark Rhizoprionodon terraenovae and bonnethead shark Sphyrna tiburo using a combination of research trawl and observer data. Research trawls have never used turtle excluder devices (TEDs), which are expected to exclude larger specimens of blacknose sharks. Most of the observer data that contain blacknose shark occurrences were collected during the pre-TED era when the two data sets tracked one another. Minimum observer data were available for the post-TED period (1990–present). As a consequence, the pre-TED (1972–1989) relationship between observer and research trawl catch per unit effort (CPUE) is driving the observer CPUE estimates...

An Assessment of Larval Fish Density and Assemblage Structure within Mid-Channel and Backwater Habitats in a Mississippi Stream

ABSTRACT We investigated larval fish density and assemblage structure in mid-channel and backwater areas of Luxapallila Creek, Mississippi to better understand larval fish distribution across these two stream habitats. Our results showed backwater and mid-channel habitats not to differ with respect to total larval fish density, but rather in how larval fish taxa were apportioned across habitats. This research suggests stream alterations that reduce backwater habitats, which has occurred in Luxapallila Creek, may affect fish assemblage structure by reducing resource requirements of some fish larvae.

Did Declining Carrying Capacity for the Kemp's Ridley Sea Turtle Population Within the Gulf of Mexico Contribute to the Nesting Setback in 2010−2017?

Abstract The Kemps ridley (Lepidochelys kempii) is the most endangered sea turtle species. During 1966–2017, an annual count of nests (i.e., clutches of eggs laid) has served as an annual index of Kemps ridley nesting female abundance on the Gulf of Mexico (GoM) index beach in Tamaulipas, Mexico. This index was increasing exponentially at 19% per year in 2009, but it dropped unexpectedly by more than a third in 2010 and through 2017 remained well below levels predicted. We hypothesize that pre-2010 declining carrying capacity for the Kemps ridley population within the GoM contributed to this nesting setback. We discuss pre-2010 factors that may have caused carrying capacity to decline, including degradation of the GoM ecosystem, the exponentially increasing Kemps ridley population, and declining per capita availability of neritic (i.e., postpelagic) Kemps ridley food, including natural prey and scavenged discarded bycatch from shrimp trawling. We encourage evaluations (especially those within a robust modeling framework) of this hypothesis and others put forth to explain the nesting setback to provide information needed to guide restoration of the populations progress toward recovery.