Sampling characteristics and biases of enclosure traps for sampling fishes in estuaries

We evaluated the sampling characteristics of enclosure traps in estuaries in southern California, USA. Using enclosure traps that sampled 0.25, 0.5, and 1-m² footprints, we found that enclosure trap size significantly affected estimates of fish density and the precision of these estimates. The highest estimates were produced by the 0.5-m² trap and the lowest by the 0.25-m² trap. Precision of the density estimates improved with increasing trap size, while the proportion of zero values in the data sets decreased and estimates of species richness increased. The largest trap was difficult to use in the field and often did not function properly; thus we concluded that intermediate enclosure trap sizes offered the best compromise between statistical and logistical considerations. By examination of burrows in sediment cores taken in fished out enclosure traps, we found no evidence to support the widely held view that burrow-dwelling fishes evaded capture by hiding in burrows. We also used mark-recapture techniques to estimate recovery efficiency in 0.43-m² enclosure traps. Recovery efficiency averaged 91% and did not differ significantly among estuaries or sampling stations within estuaries. Based on extensive netting within enclosure traps, we determined that in areas with dense fish populations (>90 fish 0.43-m⁻²), netting could be ceased after the first sweep that captured no fish with only a trivial effect on the estimate of density. In more sparsely populated areas, netting had to continue until 2–3 sweeps had captured no fish in order to obtain estimates of density that were within 90% of the actual values present. Overall, we found enclosure traps to be effective tools for sampling small, abundant fishes in shallow estuaries in southern California, but we recommend that care be taken when choosing trap size and sampling (netting) effort within traps in order to optimize their sampling characteristics.     

Estimating densities of small fishes and decapod crustaceans in shallow estuarine habitats: A review of sampling design with focus on gear selection

Shallow estuarine habitats often support large populations of small nekton (fishes and decapod crustaceans), but unique characteristics of these habitats make sampling these nekton populations difficult. We discuss development of sampling designs and evaluate some commonly used devices for quantitatively sampling nekton populations. Important considerations of the sampling design include the size and number of samples, their distribution in time and space, and control of tide level. High, stable catch efficiency should be the most important grear characteristic considered when selecting a sampling device to quantify nekton densities. However, the most commonly used gears in studies of estuarine habitats (trawls and seines) have low, variable catch efficiency. Problems with consistently low catch efficiency can be corrected, but large unpredictable variations in this gear characteristic pose a much more difficult challenge. Study results may be bised if the varibility in catch efficiency is related to the treatments or habitat characteristics being measured in the sampling design. Enclosure devices, such as throw traps and drop samplers, have fewer variables influencing catch efficiency than do towed nets (i.e., trawls and seines); and the catch efficiency of these enclosure samplers does not appear to vary substantially with habitat characteristics typical of shallow estuarine areas (e.g., presence of vegetation). The area enclosed by these samplers is often small, but increasing the sample number can generally compensate for this limitation. We recommend using enclosure samplers for estimating densities of small nekton in shallow estuarine habitats because these samplers provide the most reliable quantitative data, and the results of studies using these samplers should be comparable. Many kinds of enclosure samplers are now available, and specific requirements of a project will distate which gear should be selected.     

Habitat partitioning of fishes in an urban,estuarine Bayou

The physiochemical variability inherent to estuaries makes it difficult to identify those variables of importance to habitat selection by fishes. We used factor analysis to investigate habitat partitioning of fishes in Contraband Bayou, an oligohaline creek within the City of Lake Charles, in southwestern Louisiana. This bayou receives runoff and organic wastes from the City of Lake Charles and adjacent farmland (adding levels of complexity to an analysis of fish habitat preferences). Fishes were sampled from locations within the Contraband drainage in winter and summer of 1983 and 1984. At each location, 14 environmental variables were measured. Distribution of each common species in a season relative to these variables was characterized by calculating means of individuals for each environmental variable. A species' mean for a variable (the state of the variable where individuals of the species are most likely to be found) was interpreted as the species' “preference” for the variable. Factor analysis was used to identify trends in habitat partitioning among species, based on their preferences for all environmental variables. In all seasons, habitat partitioning separated typically freshwater forms from estuarine fishes, and separated forms more tolerant of organic pollution from less tolerant species. Some species entered the bayou only in summer, and in this season, additional factors were related to the preference of some of these species for areas with little cover that were normally shunned by resident species.     

A supplemental sampling method for estuarine ichthyoplankton with emphasis on the atherinidae

A three-stage sampler, filtering water from the 0–5 cm, 5–25 cm, and 25–45 cm microhorizons was tested during conventional ichthyoplankton studies. The sampler was as much as 240X more efficient than 0.5 m nets in the collection of larval atherinids. The atherinids were found most abundantly in the 0–5 cm microhorizon, probably due to a positive phototaxic response.     

Differential effects of salinity changes on two estuarine fishes,Leiostomus xanthurus andMicropogonias undulatus

Salinity fluctuation has been proposed as an important determinant of estuarine fish distribution. To test this idea, we compared distribution, behavioral preference and physiological sensitivity of two juvenile estuarine fishes, spot (Leiostomus xanthurus) and croaker (Micropogonias undulatus), with respect to salinity change. In field collections, spot: croaker ratios were positively correlated with salinity variation. Subsequent behavioral observations revealed that croaker tend to cross a 10‰ salinity gradient less often than spot. We proposed that energetic costs of salinity adaptation may be higher for croaker, resulting in the observed avoidance behavior. Oxygen consumption rates over rapid salinity fluctuations showed no significant differences in metabolic response between species, although there was some indication that sensitivity changes with fish size. Apparently, juvenile spot and croaker are well-equipped to withstand extreme changes in salinity. We conclude that environmental factors correlated with salinity change may be responsible for distribution differences between these two abundant species.     

Water quality associated with survival of submersed aquatic vegetation along an estuarine gradient

The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen <10 μM; mean dissolved phosphate <0.35 μM; mean suspended sediment <20 mg l^?1; mean chlorophylla in the water column <15 μg l^?1; and mean light attenuation coefficient (Kd) <2 m^?1. These values correspond well with those derived in other parts of the Chesapeake, particularly in the lower bay, and may provide managers with values that can be used as target concentrations for nutrient reduction strategies where SAV is an issue.     

An assessment of habitat use by larval fishes in a warm temperate estuarine creek using light traps

Habitat use by larval fishes was investigated in a tidal creek of the Swartkops estuary, South Africa, during a trial study using light traps. A total of 12 fish families represented by 28 species were recorded during October and November 2000. Fishes caught were predominantly in a postflexion stage of larval development although preflexion larvae and some early juveniles were also recorded. The distribution of larval fishes indicated that fishes were influenced by proximity to either channel or littoral regions of the creek rather than showing specific preferences for eelgrass covered areas. Estuary-dependent marine species were caught in higher numbers along the margins of the estuary. Length frequencies of both estuary-resident and estuary-dependent marine fishes showed that larger individuals were found in the littoral regions suggesting active migration of these fishes to the margins. Catches of straggler marine species were higher in the channel areas and showed no difference in size distribution between littoral and channel sites. Estuary-dependent species may be using marginal areas as a means of avoiding high flow in the channel and as a predator avoidance mechanism. Light traps were found to be a useful sampling device for localized studies on the early life stages of fishes in an estuary.     

An assessment of biases associated with caging,tethering, and trawl sampling of summer flounder (<Emphasis Type="Italic">Paralichthys dentatus</Emphasis>)

We provide an example of the type of bias assessment that should, but often is not, used in ecological studies using techniques such as caging, tethering, and trawl sampling. Growth rates of summer flounder (Paralichthys dentatus) from cage enclosures were compared to those from mark-recapture trials, and prey types (identified through stomach content analysis) compared between caged fish and wild fish collected from nursery sites over a 2-yr period. Site-specific growth rates estimated from the caging method were similar (± 15%) to those estimated from the mark-recapture trials. Prey types were nearly identical between caged and wild fish, although selectivity may have varied quantitatively. Caging summer flounder will generally be an appropriate tool with which to measure growth rates in the wild, but comparisons with an independent measurement method are necessary for validation. In tethering trials, predation was significantly greater on tethered than on untethered fish, indicating that tethering is not an appropriate tool with which to measure absolute rates of predation on juvenile summer flounder. The lack of effects of substrate (sand versus mud) and fish origin (hatchery-reared versus wild) on predation of tethered versus untethered fish indicates that tethering trials will not indicate treatment-specific differences when none exist. Tethering may be an acceptable method for comparing relative rates of predation on different substrates and between hatchery-reared and wild juvenile summer flounder in the field, although true differences in treatment levels could be masked by tethering. Beam trawl efficiency estimates for juvenile summer flounder were similar between beach and marsh habitats, but differed significantly between marsh sites, indicating that site-specific trawl efficiency estimates may be critical to accurately assess juvenile flounder the appropriateness of comparisons of size-frequency information between the sites and habitats used in this study. Caging, tethering, and beam trawl sampling are appropriate tools for measuring ecological parameters of juvenile summer flounder, but only if possible biases of each method are identified and compensated for when interpreting data collected using these methods.     

Preliminary investigation of the effects of purse seine size on estimates of density and species richness of estuarine fishes

Small purse seines are well suited to sampling open water habitats in estuaries, yet little is known about how variation in their design affects estimates of density and species richness of estuarine fishes. We tested whether purse seine size (length and depth) affected estimates of density or species richness of fishes in San Dieguito Lagoon, southern California, U.S.A. Twenty-one species were captured, with the open water speciesAtherinops affinis dominating the catch. The larger net (36.4 m long × 3.6 m deep) produced higher estimates of density than the smaller net (18.2 m long × 2.4 m deep). The average number of species captured per sample was lower for the smaller net than the larger net, but species accumulation curves for the small and large nets were similar, indicating that the difference in the number of species per sample was primarily caused by the larger area sampled by the larger purse seine. Sampling with the larger purse seine was more time efficient than the smaller seine. We found small purse seines to be useful tools for sampling fishes in open water habitats in a small estuary, but we recommend that care be taken in selecting the size of a purse seine.     

Experimental studies of predation on polychaetes associated with seagrass beds

Predation on benthic polychaetes associated with seagrass beds was examined in laboratory and field predator inclusion experiments with one fish and three invertebrate predators. Predation had differential effects on polychaete taxa which depended on their microhabitat utilization patterns. The magnitude of predation effects strongly depended on the predator species, with the shrimpPenaeus duorarum having the greatest impact and the pinfishLagodon rhomboides the least. Abundance of spionid polychaetes, which are located near the sediment-water interface, was reduced most often by predation. Nereid and capitellid polychaete abundances were reduced by some predators and not by others. The results emphasize the need for more detailed natural history information on polychaete species in order to improve the interpretation of results from predation experiments.     

Experimental evaluation of solubility of gypsum

Karstic rocks exist in different areas of the world having by case useful/harmful structural and environmental impacts. These rocks like gypsum and salt are dissolved against water.     

A rapid procedure for environmental sampling and evaluation of polluted sediments

A rapid procedure is described to evaluate the significance of inorganic and organic pollutants in fine-grained sediments. The air- (<30°C) or freeze-dried samples are screened within 30 min. in stainless steel sieves (<0.04 mm) prior to chemical analysis. This procedure is also suitable for analysis of such elements as Hg and many non-volatile organic pollutants. The sediment size fraction of <0.04 mm was chosen in order to separate the smallest grain-size obtainable by rapid dry-screening techniques; this is representative of the grain-size of suspended matter transported at low and medium river discharge (0.1–0.7 m/s). The screened fraction (<0.04 mm) of aquatic sediments is more homogeneous in grain-size and distribution, and the fine particles are more capable of adsorbing pollutants than coarser fractions. However, large differences are observed in grain-size distribution and adsorbable phases from one sample to another, even in the smallest size fractions (<0.04, <0.02, <0.002 mm). Clay minerals, Fe- and Mn-(oxy)hydrates, amorphous silica and humic substances all adsorb water at normal humidity. In order to estimate and standardise the adsorption capacity of the surface of these screened sediment fractions, the weight loss after heating at 105°C is used, instead of tedious quantification of all the above mentioned sediment phases. In addition, this procedure allows the rejection of sediment samples, unsuitable for monitoring purposes, which have very low adsorption capacities. In order to correct for the variable adsorption capacity of different sediment samples or size fractions, the weight loss (WL) at 105°C can be inserted in the widely used GeoIndex formula (Igeo_105°=log2 (C_measured/C_background1WL_105°C). This improves the evaluation of pollution in aquatic environments.     

An evaluation of trace element release associated with acid mine drainage

The determination of trace element release from geologic materials, such as oil shale and coal overburden, is important for proper solid waste management planning. The objective of this study was to determine a correlation between trace element residency and concentration to trace element release using the following methods: (1) sequential selective dissolution for determining trace element residencies, (2) toxicity characteristic leaching procedure (TCLP), and (3) humidity cell weathering study simulating maximum trace element release. Two eastern oil shales were used, a New albany shale that contains 4.6 percent pyrite, and a Chattanooga shale that contains 1.5 percent pyrite. Each shale was analyzed for elemental concentrations by soluble, adsorbed, organic, carbonate, and sulfide phases. All leachates were analyzed to determine total trace element concentrations. The results of the selective dissolution studies show that each trace element has a unique distribution between the various phases. Thus, it is possible to predict trace element release based on trace element residency. The TCLP results show that this method is suitable for assessing soluble trace element release but does not realistically assess potential hazards. The results of the humidity cell studies do demonstrate a more reasonable method for predicting trace element release and potential water quality hazards. The humidity cell methods, however, require months to obtain the required data with a large number of analytical measurements. When the selective dissolution data are compared to the trace element concentrations in the TCLP and humidity cell leachates, it is shown that leachate concentrations are predicted by the selective dissolution data. Therefore, selective dissolution may represent a rapid method to assess trace element release associated with acid mine drainage.     

Hydroacoustics as a tool for assessing fish biomass and size distribution associated with discrete shallow water estuarine habitats in Louisiana

We developed a relative index of fish biomass and size distribution in ultra-shallow waters (< 2 m) of Barataria Bay, Louisiana, based on the comparison of horizontal hydroacoustic data with gill net and push trawl catches in an effort to understand the role that habitat plays in both fish biomass and distribution. Exclosure net experiments indicated that the contribution of acoustic backscattering from sources other than fishes were negligible. Split-beam transducer, gill net, and push trawl sampling were conducted concurrently in Barataria Bay to provide information on fish composition and length distributions and for comparisons among gear types. Results suggest that acoustic fish biomass was generally higher in the low salinity stations and lower at the high salinity stations, at least in March 2004. We observed a greater mean length of fishes associated with oyster shell habitats when compared to adjacent sand-mud habitats. This paper demonstrates the utility of hydroacoustics as a tool to quantify relative acoustic fish biomass and size distribution associated with common estuarine habitats in ultra-shallow waters. This study also illustrates the potential of using acoustics for augmenting traditional sampling procedures.     

对地下水资源评价与管理现状有关问题的探讨

本文从系统论的观点出发，并结合实例阐述了地下水资源评价与地下水资源管理的关系。用可持续的观点和系统分析的方法建立了地下水资源管理的数学规划模型。同时指出地下水资源管理的根本前提在于节水。     

Experimental study of Tunis soft soil improved by vacuum consolidation associated with geodrains

An experimental investigation was carried out on the reconstituted Tunis soft soil (TSS) that was extracted from the centre of Tunis City at 35-m depth. Three series of consolidation tests were performed on TSS specimens. The first series included consolidation tests by preload. In the second series, vacuum consolidation tests were performed. The third series comprised a vacuum consolidation test combined with the preload. Excess pore water pressure and settlement were measured during the consolidation tests. Experimental results showed that for the same magnitude of preload and the vacuum pressure of 4, 8, 16 and 30 kPa, the settlement caused by the vacuum pressure is lower than that generated by the preload, while the settlements generated by preloads of 60 and 100 kPa are slightly lower than those caused by vacuum pressure of 60 and 100 kPa. A rapid dissipation of recorded excess pore water pressure around the geodrain was observed compared to that measured close to the cell border.     

The calculation of estuarine turnover times using freshwater fraction and tidal prism models: A critical evaluation

Freshwater fraction and tidal prism models are simple methods for estimating the turnover time of estuarine water. The freshwater fraction method prominently features flushing by freshwater inflow and has sometimes been criticized because it appears not to include flushing by seawater, but this is accounted for implicitly because the average estuary salinity used in the calculation reflects all the processes that bring seawater into the estuary, including gravitational circulation and tidal processes. The model relies on measurable salinity differences among water masses and so must be used for estuaries with substantial freshwater inflow. Tidal prism models are based on flushing by flood tide inflow and ignore seawater inflow due to gravitational circulation. These models should only be applied to estuaries with weak or nonexistent gravitational circulation, which are generally those with little freshwater inflow. Using a framework that is less ambioguous and more directly applicable to the estimation of turnover times than those used previously, this paper critically examines the application of tidal prism models in well-mixed estuaries with complete tidal exchange, partial ebb return, or incomplete flood mixing and in partially mixed estuaries. Problems with self-consistency in earlier versions of these models also apply to the budgeting procedure used by the LOICZ (Land-Ocean Interactions in the Coastal Zone) program. Although freshwater fraction and tidal prism models are different approaches to estimating turnover times in systems with very different characteristics, consistent derivation shows that these models have much in common with each other and that they yield equivalent values that can be used to make comparisons across systems.     

入井工作液顺序接触煤岩储层损害实验评价

致密煤岩具有高毛管压力和比表面积大等特点,在钻井、完井和增产改造过程中,流体侵入将造成储层损害,显著降低工程技术效果。以往文献仅报道了单项工作液对煤岩储层的损害,忽视了工作液顺序接触煤岩储层损害的叠加效应。以川南煤田古叙矿区上二叠统龙潭组煤样为研究对象,开展了流体之间配伍性实验、钻井完井液与压裂液顺序接触煤样对渗透率损害实验。单项工作液储层损害评价实验结果表明,压裂液对储层的损害最严重,钻井完井液次之;工作液配伍性和顺序接触损害评价实验结果表明,与钻井完井液接触后,瓜胶压裂液损害最严重,离子平衡压裂液次之,活性水压裂液较轻,清洁压裂液最轻。顺序损害实验反映了工程实际过程,为入井流体评价和优选提供理论依据,有利于促进煤层保护和煤层气高效开发。