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12Otolith microchemistry is a widely used technique for elucidating life history patterns in 13 fishes. This typically involves sectioning the otolith and collecting elemental signatures 14 via laser ablation. But this requires time-intensive handling that may influence results. 15As an alternative to traditional cut/polish/ablate techniques, we tested depth-profiling 16 laser ablation, which offers reduced handling and contamination risk. To validate depth-17 profiling as a robust method for collecting trace element otolith microchemistry data, 18 we constructed composite otoliths using otolith materials from fishes of different 19 origins (freshwater, seawater). Test ablations were conducted on composite 20 diadromous otoliths at a range of spot sizes and pit depths. We measured tailing and 21 fractionation effects in the following elements; Na, Mg, K, Mn, Zn, Rb, Sr, and Ba. Given 22 appropriate instrument parameters, depth-profiling can accurately collect elemental 23 concentration data both between and within top and bottom layers of an otolith 24 composite across a range of spot sizes and pit depths. Analytical power and lag effects 25 were dependent on spot size, highlighting the importance of optimizing spot size based 26 on sample morphology and instrument parameters. 27 28

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The arroyo southwestern toad is a specialized and federally endangered amphibian endemic to the coastal plains and mountains of central and southern California and northwestern Baja California. It is largely unknown how long these toads live in natural systems, how their population demographics vary across occupied drainages, and how hydrology affects age structure. We used skeletochronology to estimate the ages of adult arroyo toads in seven occupied drainages with varying surface water hydrology in southern California. We processed 179 adult toads with age estimates between 1 and 6 years. Comparisons between skeletochronological ages and known ages of PIT tagged toads showed that skeletochronology likely underestimated toad age by up to 2 years, indicating they may live to 7 or 8 years, but nonetheless major patterns were evident. Arroyo toads showed sexual size dimorphism with adult females reaching a maximum size of 12 mm greater than males. Population age structure varied among the sites. Age structure at sites with seasonally predictable surface water was biased toward younger individuals, which indicated stable recruitment for these populations. Age structures at the ephemeral sites were biased toward older individuals with cohorts roughly corresponding to higher rainfall years. These populations are driven by surface water availability, a stochastic process, and thus more unstable. Based on our estimates of toad ages, climate predictions of extreme and prolonged drought events could mean that the number of consecutive dry years could surpass the maximum life span of toads making them vulnerable to extirpation, especially in ephemeral freshwater systems. Understanding the relationship between population demographics and hydrology is essential for predicting species resilience to projected changes in weather and rainfall patterns. The arroyo toad serves as a model for understanding potential responses to climatic and hydrologic changes in Mediterranean stream systems. We recommend development of adaptive management strategies to address these threats.