Macroinvertebrate organisms were sampled at four sites on Tecate Creek (U.S.–Mexico) and quantitatively evaluated using the SIGNAL-w (Stream Invertebrate Grade Number—Average Level-weighted) index. A morphological assessment of the stream structure was also carried out. Bioindication by SIGNAL reflected a very low water quality in the upper three sampled stream reaches, but with a significant improvement by the last site on the Rio Alamar, but only to a grade of critical to high pollution over a flowing distance of 29 km. Levels of BOD and ammonium-N at the Rio Alamar (Toll Bridge) site remained quite high, 56 mg/L and 48 mg/L, respectively. Metal levels also generally decreased as the water flowed downstream to the the Rio Alamar. Despite the fact that Tecate Creek has a quite natural morphological structure, solid inorganic surfaces and aquatic macrophytes (as settlement area) are mostly absent in Tecate Creek. This lack of stable habitats prevents the development of an effective biofilm which would significantly enhance self-purification.

Spotted sand bass, Paralabrax maculatofasciatus, were collected from Bahia de Los Angeles, Baja California, Mexico covering as wide a size range as possible over four seasons (spring, summer, fall, and winter). Age was estimated and growth parameters calculated from growth zones counted in transverse otolith sections. An otolith edge analysis indicated an opaque growth zone was deposited once per year during the summer, validating the annual periodicity. Spotted sand bass from this region are fast growing with a relatively short life span of up to 11 years. Growth differs from the disjunct Pacific coast population by having a higher growth rate and a shorter longevity.

We investigated the feeding ecology of tilapia (Oreochromis spp) in the Salton Sea. Stomachs and intestines were sampled at each season. Fish foraged at the surface during the summer when dissolved oxygen was low. The proportion of plant materials in fish stomachs was higher at river mouth areas. Diatoms dominated the phytoplankton; rotifers were dominant during the spring and summer; copepods were dominant during the fall; barnacle larvae dominant during the winter. Pile worms (Neanthes succinea) were present throughout the year. Feeding activity was reduced in the summer, when dissolved oxygen was low and water temperature high.

I compared vegetation and substrate characteristics at capture locations of three syntopic species of pocket mice (Chaetodipus: Heteromyidae) to determine differences in microhabitat affinities of these ecologically similar rodents in coastal Baja California, México. Principal components analysis revealed that capture locations of C. spinatus had disproportionately higher cover of medium and large rocks, and tended to have higher plant species diversity than capture locations of the other two species. C. spinatus and C. arenarius, the two smallest species, differed most in their microhabitat affinities, suggesting spatial segregation that may minimize competition for similar food resources. C. arenarius was captured most often at sites with the finest-textured soils, whereas C. rudinoris, the largest species, was widespread and frequently associated with gravel and small rocks. No species showed any apparent preference for plant species or significant amounts of overhead cover, a conclusion supported by the high densities of C. rudinoris on small, barren islands in the Gulf nearby. My results represent the first quantitative descriptions of the habitat associations of these species, which are wholly or predominantly restricted in their distributions to Baja California.