Timothy E. Targett

SUITABILITY OF ESTUARINE NURSERY ZONES FOR JUVENILE WEAKFISH (CYNOSCION REGALIS) : EFFECTS OF TEMPERATURE AND SALINITY ON FEEDING, GROWTH AND SURVIVAL

Juvenile weakfish, Cynoscion regalis (Bloch and Schneider, 1801), exhibit significant spatial diffrences in growth rate and condition factor among estuarine nursery zones in Delaware Bay. The potential influence of temperature and salinity on the suitability of estuarine nursery areas for juvenile weakfish was investigated in laboratory experiments by measuring ad libitum feeding rate, growth rate and gross growth efficiency of juveniles collected in Delaware Bay in 1990 (40 to 50 mm standard length; 1.4 to 2.1 g) in 12 temperature/salinity treatments (temperatures: 20, 24, 28°C; salinities: 5, 12, 19, 26 ppt) representing conditions encountered in different estuarine zones during spring/summer. Feeding rates (FR) increased significantly with temperature at all salinities, ranging from 10 to 15% body wt d-1 at 20°C to 33–39% body wt d-1 at 28°C. Specific growth rates (SGR) ranged from 1.4 to 9.4% body wt d-1 (0.3 to 1.5 mm d-1) and gross growth efficiencies (K1) varied from 13.6 to 26.4% across temperature/salinity combinations. Based on nonlinear multiple regression models, predicted optimal temperatures for SGR and K1 were 29 and 27°C, respectively. Salinity effects on SGR and K1 were significant at 24 and 28°C where predicted optimal salinity was 20 ppt. At these warmer temperatures, SGR and K1 were significantly lower at 5 than at 19 ppt despite higher FR at 5 ppt. Therefore, maximum growth rate and growth efficiency occurred under conditions characteristic of mesohaline nurseries. This finding is consistent with spatial patterns of growth in Delaware Bay, implying that physicochemical gradients influence the value of particular estuarine zones as nurseries for juvenile weakfish by affecting the energetics of feeding and growth. Laboratory results indicate a seasonal shift in the location of physiologically optimal nurseries within estuaries. During late spring/early summer, warmer temperatures in oligohaline areas permit higher feeding rate and faster growth compared to mesohaline areas. By mid-late summer, spatial temperature gradients diminish and mesohaline areas provide more suitable physicochemical conditions for growth rate and growth efficiency whereas oligohaline areas become energetically stressful. Substantial mortality occurred at 5 ppt and 28°C, providing additional evidence that oligohaline conditions are stressful during late summer. Furthermore, juveniles provided a choice among salinities in laboratory trials preferred those salinities which promoted higher growth rates. The extensive use of oligohaline nurseries by juvenile weakfish despite the potential for reduced growth rate and growth efficiency suggests this estuarine zone may provide a substantial refuge from predation.

Tropical marine herbivore assimilation of phenolic-rich plants

Phenolics in marine brown algae have been thought to follow a latitudinal gradient with high phenolic species in high latitudes and low phenolic species in low latitudes. However, tropical brown algae from the western Caribbean have been shown to be high in phlorotannin concentration, indicating that latitude alone is not a reasonable predictor of marine plant phenolic concentrations. This study shows that the range of high phenolic phaeophytes is not limited to the western Caribbean but encompasses the western tropical Atlantic, including Bermuda and the Caribbean, where algal phlorotannin concentrations can be as high as 25% dry weight (DW). Assimilation efficiencies (AEs) of phenolic-rich and phenolic-poor plants were examined in three tropical marine herbivores (the parrotfish, Sparisoma radians, and the brachyuran crab, Mithrax sculptus, from Belize and the parrotfish, Sparisoma chrysopterum, from Bermuda). AEs of phenolic-rich food by each of the three herbivore species were uniformly high, suggesting that high plant phenolic concentrations did not affect AEs in these species. This is in contrast to some temperate marine herbivores where phenolic concentrations of 10% DW have been shown to drastically reduce AE. The apparent contradiction is discussed in light of the effects of specific herbivore gut characteristics on successful herbivory of high phenolic brown algae.

Nutrient enrichment and fisheries exploitation: interactive effects on estuarine living resources and their management

Both fisheries exploitation and increased nutrient loadings strongly affect fish and shellfish abundance and production in estuaries. These stressors do not act independently; instead, they jointly influence food webs, and each affects the sensitivity of species and ecosystems to the other. Nutrient enrichment and the habitat degradation it sometimes causes can affect sustainable yields of fisheries, and fisheries exploitation can affect the ability of estuarine systems to process nutrients. The total biomass of fisheries landings in estuaries and semi-enclosed seas tends to increase with nitrogen loadings in spite of hypoxia, but hypoxia and other negative effects of nutrient over-enrichment cause declines in individual species and in parts of systems most severely affected. More thoroughly integrated management of nutrients and fisheries will permit more effective management responses to systems affected by both stressors, including the application of fisheries regulations to rebuild stocks negatively affected by eutrophication. Reducing fishing mortality may lead to the recovery of depressed populations even when eutrophication contributes to population declines if actions are taken while the population retains sufficient reproductive potential. New advances in modeling, statistics, and technology promise to provide the information needed to improve the understanding and management of systems subject to both nutrient enrichment and fisheries exploitation.

Fish-Nursery Use in Georgia Salt-Marsh Estuaries: The Influence of Springtime Freshwater Conditions

Abstract The fish assemblage using shallow nursery habitats in the Ogeechee River-Ossabaw Sound salt-marsh estuary, Georgia, was investigated during the winter and spring of two successive years. High river discharges during these periods produced fully freshwater conditions (all tidal stages and amplitudes) in the upper portion of the study area for up to 4 months. Abundances of Atlantic croaker Micropogonias undulatus, southern flounder Paralichthys lethostigma, silver perch Bairdiella chrysoura, and hogchoker Trinectes maculatus recruits were highest in the upper estuary. Spots Leiostomus xanthurus were more evenly distributed, but continued to use upper-estuary nursery areas during periods of high river discharge. Although the recruitment of several species likely was inhibited during discharge peaks, only striped mullet Mugil cephalus avoided freshwater conditions. Spots, southern flounder, Atlantic menhaden, and silver perch utilized shallow nursery areas on a size-specific basis. Recruitment and ut...

Low-Temperature Tolerance of Age-0 Atlantic Croakers: Recruitment Implications for U.S. Mid-Atlantic Estuaries

Abstract Age-0 Atlantic croakers Micropogonias undulatus overwinter in Mid-Atlantic Bight (MAB) estuaries, where interannual variability in year-class strength is pronounced. Previous researchers have hypothesized that this variability is the result of cold-induced winter mortality of age-0 Atlantic croakers within estuarine nurseries. We tested this hypothesis by means of laboratory experiments designed to (1) estimate the effects of winter severity and duration on the probability of survival of age-0 fish, (2) determine the relationship between body size and cold tolerance, and (3) examine the effects of temperature decline rate and salinity on susceptibility to low-temperature mortality. Age-0 Atlantic croakers (15–65 mm standard length, SL) were collected from Delaware Bay during October, acclimated to 8°C in the laboratory, and then cooled (1.0°C/d) to treatment levels of 1, 3, 5, or 7°C that were designed to simulate varying levels of winter severity in MAB estuaries. Survival ranged from 0% at 1°C ...

Effect of macrophyte secondary metabolites on feeding preferences of the herbivorous parrotfish Sparisoma radians

Experiments were conducted to test the role of secondary metabolites in determining the natural feeding preference hierarchy of the bucktooth parrotfish Sparisoma radians. The two least preferred food genera of S. radians, Halimeda and Penicillus, both contain 1,4-diacetoxy-1,3-butadiene terpenes, while the most preferred species, Thalassia testudinum, does not. Experiments with agar cylinders containing macrophyte homogenates showed that macrophyte biteability was not a factor. Instead preference could be altered by the application of the diacetoxybutadiene containing terpenes 4,9-diacetoxyudoteal and caulerpenyne or fractions or extracts containing them at naturally occurring concentrations. Concentration of the active terpenes affected the intensity of the fishs preference for the control in pairwise comparisons. Extracts and fractions which did not contain 4,9-diacetoxyudoteal or caulerpenyne did not affect fish feeding preferences at naturally occurring concentrations. Experiments in which S. radians were given no plant choice showed that coating T. testudinum with H. incrassata organic crude extract reduced the number of bites consumed and the biomass consumed to a level equivalent to that obtained for H. incrassata plants.

Effects of Ration Limitation and Low Temperature on Growth, Biochemical Condition, and Survival of Juvenile Summer Flounder from Two Atlantic Coast Nurseries

Abstract Laboratory experiments were conducted on juvenile summer flounder Paralichthys dentatus (25–80 mm total length) from Delaware and North Carolina nurseries to compare their responses to temperature and to ration limitation. Ad libitum feeding rates, maximum growth rates, and growth efficiency were measured at temperatures from 2 to 20°C. Although ad libitum feeding rates did not differ between fish from the two locations, North Carolina juveniles had higher maximum growth rates and growth efficiencies than Delaware juveniles between 6 and 18°C. Growth rates and changes in biochemical condition were also measured for different rations at temperatures that persist during the months following settlement (10–16°C). Survival was high (>98%) after 10–14 d of starvation and suboptimal rations at 10–16°C, but growth rates were highly dependent on feeding rates. Maintenance rations remained constant between 10 and 16°C, but scope for growth increased with temperature, Scope for growth of North Carolina juv...

The use of RNA:DNA ratios to predict growth limitation of juvenile summer flounder (Paralichthys dentatus) from Delaware and North Carolina estuaries

Nutritional indices were used to develop biochemical correlates of feeding and growth rates for juvenile summer flounder, Paralichthys dentatus (Linnaeus), from North Carolina (NC) and Delaware (DE). Six parameters (Fultons condition K=104xweight/(length3), wet weight/dry weight, [protein], [RNA], [DNA], and RNA:DNA) were related to feeding and growth rates of fish from previously reported 10 to 14-d experiments at temperatures ranging from 2 to 20 °C with varying feeding levels (0 to 100% and libitum). RNA:DNA ratios were the best predictors of growth rates, but inclusion of a temperature term improved the relationship between RNA:DNA ratios and growth rate for Delaware fish. Feeding rates were poorly correlated with all parameters. RNA:DNA ratios of fish in the laboratory changed significantly within 1 d of starvation and refeeding at 16 °C. RNA:DNA of juvenile summer flounder collected from one site in Indian River Bay, DE and two sites in the Newport River Estuary, NC, between January and June 1992 were used to estimate in situ growth rates following settlement. Predicted growth rates in both estuaries were close to maximum (suggesting ad libitum feeding) until early May. Growth rates of juveniles from Delaware were <0% d-1 from December through early March, and were higher (0.6 to 3% d-1) from April through early June. However, growth rates of DE juveniles during May were <50% of maxinum. North Carolina juveniles had growth rates of 2 to 5% d-1 from February through early April. Juveniles from one of the Newport River sites (a marsh habitat) were also severely growth limited (<20% of maximum) after April. Prolonged periods of sub-optimal growth may be important to survival and recruitment of juvenile summer flounder in northern mid-Atlantic estuaries. A model is presented which illustrates the potential impact that small changes in temperature and growth limitation can have on recruitment success in both delaware and North Carolina estuaries.

Rhythmic structural and chemical patterns in otoliths of the antarctic fish notothenia larseni their application to age determination

SummarySagittal otoliths from Notothenia larseni contain microincremental growth rings which are distinctly visible in otolith sections using Scanning Electron Microscopy. These microincrements are similar to those deposited daily in the otoliths of fishes from temperate and tropical waters. Microincrements were easily enumerated and fish length was related to increment number by a logarithmic curve. Otolith microstructure analysis appears to provide a technique to accurately determine age and growth rates in these fish. Sr/Ca ratios in otolith aragonite were analyzed along a radius from the outside edge to the core of an otolith section using an electron microprobe. The strontium/calcium (Sr/Ca) ratios varied with a cyclic periodicity apparently related to seasonal water temperature cycles and the number of cycles agreed closely with age estimated from “daily” microincrement counts. Sr/ca cycles can potentially be used to determine age, validate growth rates determined by other methods, and establish thermal conditions experienced during the life of a fish. Microstructural and chemical analyses of otoliths demonstrate great potential in helping to answer many questions about the growth processes and ecology of Antarctic fishes.

Spatial Patterns in Condition and Feeding of Juvenile Weakfish in Delaware Bay

Abstract Juvenile weakfish Cynoscion regalis were collected during July 1986 in Delaware Bay. Fish, ranging in size from 40 to 84 mm standard length, were analyzed for weight at length as a measure of condition and for gut fullness as a measure of feeding success. These data were compared among collections from the head of the estuary, the midbay region, and the mouth of the estuary. Condition and gut fullness were generally highest in the midbay region, lowest in the upper bay, and intermediate in the lower bay. The proportion of the diet consisting of mysids was also compared among the upper, mid, and lower bay collections. The mysid shrimp Neomysis americana dominated the diet in all bay areas. Despite their ubiquitous distribution throughout the estuary, the proportion of the diet consisting of mysids was generally highest in the middle bay, lowest in the upper bay, and intermediate in the lower bay. The upper bay is characterized by high turbidity and low illumination. These turbidity levels frequent...