We investigated the ability of trace element and isotopic signatures in otoliths to record the nursery areas of juvenile (young-of-the-year) weakfish Cynoscion regalis from the east coast of the USA. Juvenile C. regalis were captured with otter trawls at multiple sites in Doboy Sound (Georgia), Panllico Sound (North Carolina), Chesapeake Bay (Virginia), Delaware Bay (Delaware) and Peconic Bay (New York), from July to September 1996. One sagittal otolith from each specimen was assayed for Mg/Ca, Mn/Ca, Sr/Ca and Ba/Ca ratios uslng inductively coupled plasma mass spectrometry (ICP-MS), while 6I3C and 6180 values from the other sagittal otohth in the pair were determined using lsotope ratio mass spectrometry (IR-MS). A multivanate analysis of variance determined that there were significant differences in trace element signatures among locations. Bootstrapped 95% confidence ellipses on canonical variates indicated that all 5 locations were significantly isolated in discriminant space. On the basis of these differences, linear &scrirninant function analysis (LDFA) and artificial neural network (ANN) models were used to classify inbvidual fish to their natal estuary with an overall error rate of 37 % for LDFA and 29.6% for ANN. Addtion of 6I3C and 6180 values to the LDFA and ANN models derived from the trace element data resulted in overall error around 10%. We d, therefore, be able to use chemical signatures from the juvenile portion of adult C, reyalis otoliths to accurately classify these fish to their natal estuary.

The distribution and intensity of hypoxia (low dissolved oxygen) in estuaries is increasing worldwide due to cultural eutrophication. This study quantifies the strength of associations between the duration of diel-cycling severe hypoxia (≤2 mg O 2 l −1 ) in bottom water (∼15 cm above bottom) of a shallow (<2 m) coastal lagoon estuary (Delaware, USA) and abiotic environmental variables (water temperature, insolation, tide, streamflow, and wind) and predicts the duration of severe hypoxia given different combinations of these variables. The intensity and spatial extent and dynamics of diel-cycling severe hypoxia events were defined. Vertical variability in dissolved oxygen (DO) concentration during the daytime was also determined. During the summers of 2001-2005, bottom DO data were collected for periods of weeks to months at multiple sites using automated sondes. Multiple linear regression (MLR) and regression tree analysis (RTA) were used to determine the relative importance of the environmental variables in predicting the number of hours of severe hypoxia per day. Key findings of the study were that severe hypoxia events of minutes to hours in duration occurred frequently in all four tributaries sampled, primarily between 0200 and 1000 hours. Severe hypoxia duration and diel-cycling amplitudes of DO concentration increased in the up-tributary direction. Hierarchically, the duration of severe hypoxia was influenced mostly by the mean daily water temperature, then by preceding days' total insolation, percentage of morning hours (02:00 to 10:00 A.M.) ebb tide, and daily streamflow. Collectively, the variables examined by the MLR and the RTA approaches accounted for 62% and 65% of the variability in the duration of severe hypoxia, respectively. RTA demonstrated that daily mean water temperature above 26.3°C and previous day's total insolation below 13.6 kW m −2 were associated with the longest lasting severe hypoxic events (9.56 h). The environmental variables and combinations of conditions that modulate or augment diel-cycling hypoxia presented in this paper enhance understanding of this widespread and growing phenomenon and provide additional insight regarding the extent to which it can impact food webs in very shallow estuarine waters that often serve as nursery habitat.

The trophic ecology and structure of 5 benthic Antarctic fish communities were studied. Two locations at South Georgia Island, one at the South Sandwich Islands, and two at the South Orkney Islands were sampled using a bottom trawl. Specles diversity was low with 13 or fewer species in each community and a pronounced dominance of 3 or fewer species per community. H' diversity values were < 1.3 in all cases and < 0.8 in 3; e evenness values were < 0.6 in all cases and < 0.3 in 3.

No abstract

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.

No 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 to 99.3% at 7°C, increasing dramatically between 3°C (1.3%) and 5°C (86.8%). Cold tolerance was size dependent, with smaller individuals surviving significantly longer than larger ones. The rate of temperature decline (1.0°C/d versus 0.2°C/d) did not affect survival at lower winter temperatures. Atlantic croakers remained intolerant of temperatures of 3°C or less even when these temperatures were approached gradually. Cold tolerance increased with increasing salinity, implying that the risk of overwinter mortality varies spatially along an estuarine salinity gradient. The mean survival duration at 3°C increased from 5.9 d at 2‰ to 14.7 d at 10‰ and to 17.1 d at 26‰. Our findings provide empirical support for the hypothesis that cold‐induced overwinter mortality regulates the year‐class strength and recruitment success of Atlantic croakers in the Mid‐Atlantic Bight.

Low dissolved oxygen (DO) had a significant effect on specific growth rate (GS), length increment (IL) and haematocrit (Hct) of the mummichog Fundulus heteroclitus. Regardless of access to the water surface, F. heteroclitus maintained high growth rates (GS and IL) at DO concentrations as low as 3 mg O2 l−1. With access to the water surface, both GS and IL of F. heteroclitus decreased by c. 60% at 1·0 mg O2 l−1 compared to all higher DO treatments. When denied access to the water surface, a further decrease in GS (c. 90%) and IL (c. 75%) was observed at 1 mg O2 l−1. There was no effect of diel‐cycling DO (1–11 mg O2 l−1) with or without surface access on GS, IL or Hct of F. heteroclitus. Similar trends between GS and faecal production across DO treatments suggest that decreased feeding contributed significantly to the observed decrease in growth rate. Haematocrit was significantly elevated at 1 mg O2 l−1 for fish with and without access to the water surface. Increased Hct, however, was not sufficient to maintain high GS or IL at severely low DO. When permitted to respire in the surface layer, however, F. heteroclitus was capable of maintaining moderate growth rates at DO concentrations of 1 mg O2 l−1 (c. 15% saturation). Although aquatic surface respiration (ASR) was not quantified in this study, F. heteroclitus routinely swam in contact with the water surface and performed ASR at DO concentrations ≤3 mg O2 l−1. No hypoxia‐related mortality was observed in any DO or surface access treatment for as long as 9 days. This study demonstrates that surface access, and thus potential for ASR, plays an important role in providing F. heteroclitus substantial independence of growth rate over a wide range of low DO conditions commonly encountered in shallow estuarine environments.