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The geochemistry of fish otoliths is a valuable tool for reconstructing environmental conditions, migrations and life histories. In this study we validate the relationship between temperature, the water oxygen isotope composition (δ18O) and otolith δ18O for the endangered Totoaba macdonaldi, raised under controlled aquaculture conditions and in the field. This type of validation is instrumental for habitat reconstruction. By comparing δ18O values in the natal portion of totoaba otoliths from modern and pre-dam specimens ~1000–4500 yr BP, we test the hypothesis that the totoaba used the Colorado River estuary as a nursery site before the river was over-allocated. We found that otolith δ18O could be predicted in a laboratory setting as well as in the wild. Totoaba otoliths from before river diversion had drastically lower natal δ18O values than predicted values, indicating that these differences in δ18O values are the result of a change in the water δ18O, a consequence of diverting the isotopically negative Colorado River flow from the totoaba's nursery grounds. We conclude that the Colorado River flow was a major component of the totoaba's nursery habitat before river diversions. These results are pertinent to ongoing research on this endangered fish, using otoliths to piece together important ecological and life history information.

Entre 1999 y 2000 se determinaron la composición y distribución de la macrofauna bentónica (moluscos, equinodermos y braquiópodos) en la zona intermareal del Delta del Río Colorado, al norte del Golfo de California, México. Los muestreos se obtuvieron a lo largo de diez transectos de la zona mesomareal desde la parte superior a inferior, en dos épocas del año para determinar la variación estacional de la fauna. Fueron identificadas 26 especies en 112 muestras con un total de 1954 individuos. Dominó el gasterópodo epifaunal Nassarius moestus, bivalvos y equinoideos infaunales y el braquiópodo linguloide Glottidia palmeri. La densidad de la fauna fue mayor hacia la parte inferior de la zona intermareal. La riqueza de las especies no varió significativamente entre transectos, pero la diversidad (índice de Shannon) y equitatividad fueron mayores en los transectos del sur, los más alejados de la boca del río. La diversidad y equitatividad de los transectos del sur pueden explicarse por la variación en la región de la salinidad y tipo del sustrato. Actualmente la densidad de la fauna es de 3 a 7 ind m–2, mucho menor a la reportada antes de la construcción de las presas y que afectó los hábitat en el Delta del Río Colorado, principalmente por la disminución en la población del bivalvo máctrido Mulinia coloradoensis. Surveys were conducted in 1999 and 2000 to document the composition and distribution of the shelly (mollusk, echinoderm and brachiopod) fauna of the intertidal zone of the Colorado River Delta, northern Gulf of California, Mexico. Samples of the benthic fauna were taken along ten high-to-low intertidal transects, during two seasons to assess seasonal variation in the fauna. Twenty-six species were identified among 1954 individuals in 112 samples. The fauna was dominated by the epifaunal gastropod Nassarius moestus, infaunal bivalves, infaunal echinoids and the lingulid brachiopod Glottidia palmeri. Faunal density was generally greater toward the lower part of the intertidal zone. Species richness did not vary significantly among transects, although Shannon diversity and equitability were greater in the southern transects, those distant from the river mouth. The distinctive species composition, diversity and equitability of the southern transects may be the result of regional variation in salinity and substrate. Current faunal densities of 3?7 ind m–2 are much lower than the estimates before upstream dams and water diversions affected the habitats of the Colorado River Delta. The post-dam decrease in density is largely the result of the decline in the population of the mactrid bivalve Mulinia coloradoensis.

Experimental determination of sediment reworking rates in a subtropical intertidal flat environment yielded information about the amount, nature and implications of sediment reworking in nearshore deposits. Callianassid shrimp in Bahía La Choya, Sonora, Mexico, overturn the sediment in the inner flats at an average rate of 0.56 m3/m2/year. Elasmobranch rays overturn the sediment in the midflats at an average rate of 1.01 m3/m2/year. Resin castings indicate that the shrimp are capable of burrowing to a depth of at least 1.15 m and, where present, can completely rework this interval in Bahía La Choya in two years. The rays reach a maximum observed burrowing depth of 20 cm and, where present, can completely rework this interval in Bahía La Choya in 72 days. Reworking rates are high enough to preclude the preservation of most physical sedimentary structures under normal conditions. Only large-scale sedimentary structures or those buried deeply and rapidly are likely to escape reworking. Rates of biogenic sedimentation by callianassid shrimp are high enough to generate subsurface shell beds. Short-term biogenic sedimentation rates are higher than long-term rates, indicating that such intertidal sediments are not only thoroughly reworked, but are incomplete at time scales of weeks to months.

This report presents the results of comparative taphonomic and faunal analyses of shelly cheniers (mollusk-dominated lag concentrations) from the tidal flats of northeastem Baja California, Mexico. Three generations of cheniers that differ in age and in the duration and complexity of their taphonomic history were sampled for taphonomic and faunal analyses. The three generations, although all dominated by the bivalve mollusk Mulinia coloradoensis, differ signiticantly in their taxonomic composition due to taphonomic and/or biological differences. Taphofacies analysis of 4,334 shells of M. coloradoensis described by nine variables indicated that the shells accumulated in the cheniers are rarely affected by biological processes and moderately affected by physical processes. The shells collected from the chenier surface have poorer preservation than the subsurface ones, indicating that taphonomic degradation is mostly a surface phenomenon. Multivariate analyses discriminate the chenier generations even when the poorly preservable luster variables are excluded from the analysis. Chenier shelly assemblages are taphonomically distinct from assemblages formed in other marine environments. They are characterized by a very low intensity of bioerosion and encrustation.