Fish biodiversity changes in the Lower Rio Grande/Rio Bravo, 1953–1996

Clearly defined changes in the fishesof the Lower Rio Grande/Rio Bravo, Texas andMexico, have been documented since 1953. Morerecent surveys show that the originalfreshwater fish fauna has been retreating fromthe lower reaches and is being replaced bybrackish and marine invaders. A total of 13localities between Colombia (Nuevo León,México) and the delta (580 km) weresurveyed, with a total fish fauna of 142species, which include native, exotic, andintruding species. Changes were analysed bylocality. These changes show a loss of themajority of freshwater species, replacement oflow to higher salinity forms. This shift isvery clear in marine invaders, with somespecies penetrating the whole areainvestigated. Those changes are morenoticeable in the lower-most localities, anddiminish upstream. Data suggest that thechanges in the basin, reflecting shifts inwhole-basin ecology, include elevatedtemperature, salinity, turbidity, and lowerrunoff levels. There are indications of higherlevels of pollution as well. Informationobtained in this study is being incorporated inan Index of Biological Integrity to appearelsewhere.     

Morphology of the digestive tract and feeding habits of Loricaria lentiginosa Isbrücker, 1979 in a Brazilian reservoir

The present study describes some aspects of the morphology of the digestive tract and the feeding patterns of Loricaria lentiginosa. Sixty‐three specimens with gut contents were captured quarterly from November 2001 to August 2002 in Porto Colômbia reservoir, Rio Grande basin, southeastern Brazil. The species has bicuspid teeth inserted in the lips, unicuspid teeth in the dentary bone and pharyngeal teeth, besides having short, numerous and slightly separated gill rakers. The stomach is rudimentary and the intestine is relatively short (Intestinal Quotient = 1.32 ± 0.15). Molluscs, especially bivalves, represented the most important item on the diet of this species, followed by organic matter. The characteristics of the digestive tract as well as the dietary pattern, suggest that L. lentiginosa is able to explore bottom food resources, being classified as benthophagous with a tendency to malacophagy, and utilizing organic matter as an associated or additional food resource. Loricariidae species are able to use not only algae, sediment and detritus, but also plant and invertebrate items.     

Seasonal Variation in the Diets of Bluefish, Pomatomus saltatrix (Pomatomidae) and Striped Weakfish, Cynoscion guatucupa (Sciaenidae) in Southern Brazil: Implications of Food Partitioning

The stomach contents of 516 Pomatomus saltatrix and 506 Cynoscion guatucupa from southern Brazil were examined. Both species preyed primarily upon teleosts and crustaceans and occasionally squid. Seasonal variation in the diet was related to prey availability in the area, confirming the opportunistic feeding behaviour of both species. During the summer, P. saltatrix preyed upon four prey species (Loligo sanpaulensis, Artemesia longinaris, Mullus argentinae and Paralonchurus brasiliensis), whereas in the autumn and winter Engraulis anchoita was the predominant prey. C. guatucupa similarly preyed upon A. longinaris and P. brasiliensis during the summer, with juvenile C. guatucupa and E. anchoita also consumed. The latter species was an important dietary constituent during other seasons, as were A. longinaris (autumn), Umbrina canosai (winter), Trachurus latami and euphausids (spring). Although P. saltatrix had a broader diet than C. guatucupa during the summer, the latter species had a broader diet overall. Diet breadths were lower from autumn to spring, due to the predominance of E. anchoita in the diet of both species. This restricted diet resulted in significant intra- and inter-specific diet overlap during the winter but, because of the seasonal abundance of E. anchoita, the high levels of diet overlap do not necessarily indicate competition.     

Is tamarisk allelopathic?

Tamarisk (Tamarix ramosissima Ledeb., T. chinensis Lour.), a pernicious invader of riparian areas in the western U.S., is often considered to be allelopathic by virtue of an ability to exude salt from its leaves. However, there is no evidence demonstrating allelopathy or even salinization of soils under tamarisk. We collected soil samples from beneath and just outside of tamarisk canopies at 12 sites along a 110-km reach of Fort Peck Reservoir in northeastern Montana. Samples were analyzed for electrical conductivity (EC), pH and concentration of several nutrients. Plants of western wheatgrass (Agropyron smithii Rydb.), the dominant herbaceous native plant in habitats invaded by tamarisk, were grown in the soil samples as a bioassay. We used principal components analysis (PCA) to explore how intercorrelated soil variables were affected by a tamarisk canopy. EC averaged more than twice as high under a tamarisk canopy than in the open. However, western wheatgrass plants growing in tamarisk-affected soil were 73% larger than those in unaffected soil. PCA analysis indicated that soil beneath a tamarisk canopy had higher values along a gradient of increasing EC and total inorganic N and decreasing pH and NH ₄ ⁺ /NO ₃ ⁻ ratio. This same gradient was also associated with increasing growth of the bioassay. We hypothesize that this fertilization was due to leaf secretions and/or leaf fall, possibly by increased input of nitrogenous organic compounds or increased P solubility. Our study provided no support for an allelopathic effect of tamarisk.     

Heavy Metal Distribution in Open Canals and Drains in the Upper Rio Grande Basin

The inclusion of reclaimed effluents for irrigated agriculture may allow communities along the U.S.-Mexico border to use water resources more efficiently and reduce urban costs of wastewater treatment. However, metal inflow from industry and urbanization may threaten food safety. This study examines metal concentrations in sediments from open canal systems charged with flow from the Rio Grande and from effluents discharged from border communities. At the surface of canal beds, sediments were collected from six canal segments that began at the fringe of the El Paso/Juarez metroplex to rural areas downstream, and ranged from 9 to 24?km in length. Sediments were analyzed for Cd, Co, Cr, Cu, Ni, Pb, and Zn. These metals rarely exceeded 20?mg kg^?1. Drainage and effluent conveyance increased the variability of metal concentrations in sediments. Geostatistical models did not significantly account for spatial variability of metals, except in Mexico. This may implicate multiple rather than single inflow sources of metals. Peaks in metal concentrations often coincided with growing rural communities. However, most metal concentrations were within conventional global ranges and were not at levels high enough to threaten food safety. Future sampling strategies will require reduced sampling intervals from 1 to <0.13?km.     

Comparisons of benthic communities adjacent to riparian native eucalypt and introduced willow vegetation

1. Willows, Salix spp., have been widely introduced as a riparian species in temperate Australia and New Zealand. The present study was a broad-scale observational survey to document the differences between reaches of river lined with willows and native vegetation in the community structure of benthic invertebrates and the resources which these plants use.
2. Nine rivers in south-eastern Tasmania were examined on three occasions in autumn, spring and summer. Taxa were identified to family level, with the exception of Oligochaeta and Acarina, and benthic organic matter (CPOM and FPOM) and epilithic biomass were measured for each reach.
3. Taxon diversity and evenness were lower in willowed reaches in autumn, and total macroinvertebrate density and number of taxa were lowest in willowed reaches in summer. No differences in the fauna between willowed and native reaches were observed in spring. Measures of community similarity of the fauna in willowed and native reaches were significantly different in autumn and summer, but not in spring.
4. The taxa responsible for the significant differences seemed to be responding to differences in food availability and habitat quality in reaches of each vegetation type. Organic matter standing stock was higher in willowed reaches in autumn although the influence of these litter inputs on the fauna were not marked. Epilithon biomass was highest in autumn and spring in willowed reaches when shading in these reaches was least.
5.  The most marked differences between willowed and native reaches were during summer low flows, when the instream fauna appeared to be responding to changes to shading, water quality and the quality of the habitat.     

Evidence of food items used by <Emphasis Type=Italic>Fulmarus glacialoides</Emphasis> (Smith 1840) (Procellariiformes: Procellariidae) in Southern Brazil

Monitoring trips were conducted along the coast of the state of Rio Grande do Sul, Brazil from July 1997 to July 1998 to contribute toward knowledge on the feeding habits of Fulmarus glacialoides during migration. Five hundred and sixty two carcasses of F. glacialoides were found, 42 of which were in adequate conditions for collection and subsequent stomach content analysis. All stomach revealed the remains of cephalopods. The following were identified from the 60 lower beaks of cephalopods found: Sepidae, Histioteuthis sp., Discoteuthis sp., Chiroteuthis sp., Mesonychoteuthis hamiltoni, Lycoteuthis sp., Lycoteuthis lorigera, Ancistrocheirus lesueuri, Alluroteuthis antarctica, Loligo plei and Argonauta nodosa. Histioteuthis sp. was found in 70% of stomachs analyzed, with a mean biomass of 53 g. Fish were identified through otoliths and were found in the stomach contents of two specimens. Elytra from insects were found in the contents of eight stomachs, at least 20 of which were coleopterans. A Belostomatidae was found in the contents of one stomach.