Browsing by Author "Targett, Timothy E."
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Item Ecophysiological responses of juvenile summer and winter flounder to hypoxia: experimental and modeling analyses of effects on estuarine nursery quality(Inter-Research, 2006-11-07) Stierhoff, Kevin L.; Targett, Timothy E.; Miller, KerriLynn; Kevin L. Stierhoff, Timothy E. Targett, KerriLynn Miller; Stierhoff, Kevin L.; Targett, Timothy E.; Miller, KerriLynnGrowth and feeding rates were measured in juvenile summer flounder Paralichthys dentatus and winter flounder Pseudopleuronectes americanus exposed to sub-lethal hypoxia (low dissolved oxygen, DO) over a range of temperatures, to determine its potential effects on nursery habitat quality for these 2 estuary-dependent flatfishes. Growth rates of both species were generally reduced as DO decreased, particularly at DO levels of 50 to 70% air saturation, and as temperature increased. Summer flounder were more tolerant of low DO than were winter flounder at both 20 and 25°C. At these temperatures, summer flounder growth was reduced by ~25% (compared to growth at normoxia [7.0 mg O2 l–1]) at 3.5 mg O2 l–1 and by 50 to 60% at 2.0 mg O2 l–1. In contrast, growth of winter flounder at 20°C was reduced by ~50% at both 3.5 and 5.0 mg O2 l–1, and growth was zero at 2.0 mg O2 l–1. At 25°C, winter flounder grew poorly in all DO treatments and lost weight at 2.0 mg O2 l–1. Summer flounder were also tested at 30°C. Growth was significantly reduced even at 5.0 mg O2 l–1, and was reduced by ~90% at 2.0 mg O2 l–1. A significant relationship between feeding rate and growth suggested reduced consumption to be a major cause of growth limitation under hypoxia. There was no evidence of growth acclimation for either species after 7 to 14 d exposure to hypoxia. The effect of hypoxia on growth of summer flounder was reduced at lower salinity (15 vs. 25‰) and was unaffected by the presence of a sand substrate. Similarity between modeled growth under hypoxic conditions, based on our laboratory results, and observed growth of summer flounder in a hypoxic estuarine tributary suggests growth limitation in the wild. These laboratory and field results demonstrate that even moderate hypoxia can adversely affect growth rates, and thus the quality of estuarine nursery habitats for juvenile flatfishes.Item Juvenile weakfish Cynoscion regalis distribution in relation to diel-cycling dissolved oxygen in an estuarine tributary(Inter-Research, 2007-03-12) Tyler, Robin M.; Targett, Timothy E.; Robin M. Tyler, Timothy E. Targett; Targett, Timothy E.Shallow estuarine waters that serve as nurseries for fishes along the Atlantic and Gulf coasts of the USA can undergo wide diel dissolved oxygen (DO) fluctuations (<2 to ~20 mg O2 l–1) during summer. In this study, the distribution of juvenile weakfish Cynoscion regalis was investigated in relation to diel-cycling DO during summer 2001 in a mesohaline tributary of Indian River Bay, Delaware, USA. Weakfish were collected at 3 sites (upper, middle, and lower) along the ~5 km length of Pepper Creek on 15 d using an otter trawl. Near-bottom DO was monitored continuously over the summer, every 15 min, using multi-parameter sondes. Peak abundance of weakfish coincided with the greatest frequency, intensity, and spatial extent of severe diel-cycling hypoxia events (<2 mg O2 l–1). Severe hypoxia first occurred in early June in the upper creek and recurred there almost daily for periods of 1 to 4 h until early September. Whenever bottom DO was >2.0 mg O2 l–1, weakfish were more abundant at the upper site than at the middle and lower sites, which also experienced severe hypoxia but at much lower frequency. However, under all environmental conditions they were absent from the upper site whenever bottom DO was <2 mg O2 l–1, and returned within 2 h of DO exceeding 2 mg O2 l–1. Daily up- and down-creek movement occurred over a distance of ~1 km. These findings indicate an avoidance threshold of ~2.0 mg O2 l–1 for juvenile weakfish and demonstrate very temporally dynamic DO-related movement. Their rapid return to these areas as DO conditions improve, and relatively high density in tidal tributary headwaters, suggests that these relatively small areas provide important habitat for fishes.Item Mitochondrial DNA analysis of population structure in the Atlantic croaker, Micropogonias undulatus (Perciformes: Sciaenidae)(United States. National Marine Fisheries Service., 1999) Lankford, Thomas E. Jr.; Targett, Timothy E.; Gaffney, Patrick M.; Thomas E. Lankford Jr., Timothy E. Targett, Patrick M. Gaffney; Lankford, Thomas E. Jr.; Targett, Timothy E.; Gaffney, Patrick M.Genetic population struc- ture in Atlantic croaker ( Micropogonias undulatus Linnaeus) was examined by using the polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis of mi- tochondrial DNA (mtDNA). Juvenile croaker from three U.S. Atlantic locali- ties (Delaware, North Carolina, and Florida) and one Gulf of Mexico local- ity (Louisiana) were screened to docu- ment the magnitude and spatial distri- bution of mtDNA variation in M. undulatus ; to evaluate the integrity of Cape Hatteras, North Carolina, as a genetic stock boundary; and to estimate levels of gene flow among Atlantic lo- calities to provide an improved basis for future decisions regarding coastwide management of this fishery resource. RFLP analysis of the ATPase 6 and D-loop mtDNA regions revealed a total of 15 composite haplotypes in 93 indi- viduals. Monte Carlo simulations re- vealed no geographic heterogeneity in mtDNA haplotype frequencies among Atlantic localities and no evidence that juveniles collected north and south of Cape Hatteras originated from sepa- rate gene pools (net sequence diver- gence=–0.002%). There was significant heterogeneity between Atlantic and Gulf of Mexico samples, suggesting re- stricted gene flow between these two re- gions. Analysis of molecular variance also indicated regional (Atlantic versus Gulf) population structure, but pro- vided no evidence that Cape Hatteras represents a genetic stock boundary. AMOVA indicated relatively high gene flow ( N e m U = 12–23 effective female mi- grants per generation) among Atlantic localities. These findings are consistent with 1) a single genetic stock of M . undulatus on the Atlantic coast and 2) separate, weakly differentiated stocks in the Atlantic and Gulf of Mexico.