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The life history and habitat interactions of southern Oncorhynchus mykiss populations have received less attention than their Pacific Northwest counterparts. In this article we create a conceptual model describing the factors affecting O. mykiss population dynamics in Topanga Creek, Los Angeles County, California to understand the process that led to extirpation following floods in 1980 and 1983, re-colonization in the late 1990's, and continued persistence. We conclude that key factors influencing population dynamics include life-history variability with both resident and anadromous individuals, population spatial structure connecting Topanga Creek with other watersheds within the metapopulation, exclusive distribution within the mainstem Topanga Creek, high-quality summer and winter rearing habit, and food availability sufficient to maintain growth at high temperatures. Protecting the population in Topanga Creek from future extirpation should include restoration of the lagoon, and preventing changes to the flow regime and water quality.
The germination of seed is critical in deserts where annual plants are abundant and rely on seed buried in the soil for sustaining populations. The exotic annuals Bromus rubens and Brassica tournefortii threaten arid indigenous ecosystems such as the Mojave Desert, but little is known about the potential effects on seed emergence of different burial depths and substrates that could enhance or reduce emergence. Using seed from Mojave Desert populations, we conducted a three-factor greenhouse experiment testing the effects of species (Bromus or Brassica), burial depth (0, 2, 5, or 10 cm), and substrate (none, gravel, or litter) on seed emergence. Species and substrate interacted significantly with burial depth. Both species displayed the greatest emergence when seeds were sown on the soil surface (70% emergence for Bromus and 52% for Brassica), but Bromus emergence declined less at a 2-cm depth than Brassica. Emergence of surface-sown seed did not differ significantly among substrate types, but emergence of buried seed was significantly reduced below gravel substrates compared to no substrate or litter substrates. This suggests that seed fates in the soil (such as seed mortality by germination but not emergence from the soil) can be altered by manipulating soil surface conditions.
Opaleye (Girella nigricans) and halfmoon (Medialuna californiensis) are herbivorous sea chubs (Perciformes: Kyphosidae) that occupy an ecologically important role in kelp forests off southern and Baja California. This study provides information on length-weight relationships, age, and growth of these two ecologically important species. Opaleye and halfmoon were collected from throughout the Southern California Bight to evaluate these life history characteristics. Length-weight relationships were described by the equations W = 0.00002L3.081 for opaleye and W = 0.000003L3.454 for halfmoon. Sagittal otoliths were used to age opaleye from ages 3–10 and halfmoon from ages 0–8. In addition, age classes 0-II for opaleye were determined from length frequency analysis of preserved specimens. Von Bertalanffy growth curves were fitted to mean standard length (mm) at age for each species. Opaleye were aged up to 10 years whereas halfmoon was recorded up to eight years of age. Standard length-at-age growth curves were typical of nearshore marine fishes with rapid growth in the first few years, reaching an asymptote quickly thereafter. This study demonstrates opaleye and halfmoon are short-lived, fast growing species, and this information combined with other life history characteristics shows the importance of opaleye and halfmoon and the need for ecosystem-based management in kelp forest communities.
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