The effects of early sandbar formation on the abundance and ecology of coho salmon (Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss) in a central California coastal lagoon

We investigated how extreme drought conditions influenced the abundance, growth, movement, and seawater readiness of juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss) in a small central California coastal lagoon. In 2015, the seasonal sandbar at the mouth of Scott Creek formed over 2 months earlier than average, effectively trapping fish in the lagoon for 7 additional months (mid-May through December) before outmigration opportunities eventually resumed. Monthly mark–recapture sampling demonstrated that juvenile coho salmon and steelhead were able to persist in the lagoon during extended periods of high water temperature and low dissolved oxygen concentration. Both salmonid species exhibited similar temporal trends in abundance, growth, and Na+-K+-ATPase activity levels during lagoon residence; however, abundance and growth rates were consistently higher for steelhead. Stationary passive integrated transponder tag antenna detections revealed recurrent movement of individuals between the warm lagoon and cooler lower mainstem creek, suggesting individuals regulated key physiological processes by moving between the adjacent habitats. Our study provides new insight concerning the consequences of drought for imperiled salmonid populations and underscores the importance of life-history diversity during extreme climatic events

Predator life history and prey ontogeny limit natural selection on the major armour gene, Eda, in threespine stickleback

Abstract: Natural selection shapes the evolution of antipredator traits in prey. However, selection in the wild depends on ecological context, including features of predator and prey populations, making field studies of selection critical to understanding how predators shape selection on prey defences. Threespine stickleback (Gasterosteus aculeatus) is a classic system to study the effects of predators on the natural selection of prey. In lakes and rivers, fish predators have been shown to impose selection against low plated adult stickleback phenotypes and genotypes. We directly measured selection by predatory salmonids on the Ectodysplasin‐A (Eda) gene in estuary stickleback from California. Despite previous studies showing a positive correlation between predator presence and frequency of the Eda “complete” allele in estuary populations, we found that Eda “low” genotypes were not significantly more frequent in salmonid predator diets. Further, we found no evidence of changes in Eda genotype frequencies across generations that would suggest directional selection driven by predators. Prior selection studies have examined the effects of large resident trout on adult stickleback. In contrast, predators in this study were juvenile anadromous salmonids, which only ate juvenile stickleback whose plate phenotypes had not fully developed. Thus, in this case, predator life history and stickleback ontogeny may preclude strong selection on stickleback armour. Our results underscore the importance of selection studies in the wild for understanding the context‐dependent nature of selection in natural populations

Spatiotemporal Variability in Environmental Conditions Influences the Performance and Behavior of Juvenile Steelhead in a Coastal California Lagoon

Abstract: In California (USA), seasonal lagoons provide important oversummer rearing habitat for juvenile steelhead trout (anadromous Oncorhynchus mykiss). However, key water quality parameters such as temperature and dissolved oxygen concentration can periodically approach or exceed the physiological tolerances of steelhead during the protracted dry season. A field study employing distributed temperature sensing technology, water quality monitoring, habitat mapping, and mark-recapture sampling was conducted to examine how shifting environmental conditions affected the performance and behavior of juvenile steelhead in the Scott Creek estuary/lagoon (Santa Cruz County). Abiotic conditions were driven by episodic inputs of seawater to the typically freshwater lagoon. During midsummer, the water column was vertically stratified which reduced suitable lagoon rearing habitat by approximately 40%. Nevertheless, steelhead abundance, growth, and condition factor were high during the summer and decreased in autumn following lagoon destratification and cooling. Unlike previous work, this study identified limited emigration from the lagoon to riverine habitat during the summer. Instead, juvenile steelhead exhibited crepuscular movement patterns within the lagoon, with peaks in upstream (to upper lagoon habitat) and downstream (to lower lagoon habitat) movement occurring at dawn and dusk, respectively. This study underscores that habitat complexity and connectivity are critical for juvenile steelhead production and persistence and provides insight into steelhead habitat use and behavior in seasonal lagoons

Anthropogenic land-use signals propagate through stream food webs in a California, USA, watershed

a b s t r a c t Human development of watersheds can change aquatic ecosystems via multiple pathways. For instance, human rural development may add nutrients to ecosystems. We used naturally occurring stable isotopes in stream food webs to investigate how land use affects stream ecosystems across a gradient of land development in the San Lorenzo watershed, California. Road density was used as a proxy for land development. We found that streams in watersheds with higher road densities had elevated concentrations of phosphate and nitrate. Furthermore, algal ␦ 15 N values increased as a function of nitrate concentration, but saturated at approximately 6‰. This saturating pattern was consistent with a two-source mixing model with anthropogenic and watershed sources, fit using Bayesian model fitting. In sites that had >2.6 km roads km −2 , anthropogenic sources of N were estimated to represent >90% of the N pool. This anthropogenic N signal was propagated to stream consumers: rainbow trout (Oncorhynchus mykiss), signal crayfish (Pacifasticus leniusculus), and benthic invertebrate ␦ 15 N were positively correlated with algal ␦ 15 N. Even relatively low density rural human land use may have substantial impacts on nutrient cycling of stream ecosystems. © 2014 Elsevier GmbH. All rights reserved. Introduction Human land-use impacts freshwater ecosystems via multiple pathways, such as through nutrient loading and habitat alteration Stable isotopes are increasingly used to investigate how anthropogenic land-use alters aquatic ecosystems. For example, nitrogen stable isotope ratios (␦ 15 N) can identify potential sources of nitrogen as well as inform rates of nutrient transformations The ecological effects of human land use can be illuminated through the study of gradients that span urban to rural developments Materials and methods Study system We examined 12 sites within the San Lorenzo River watershed (Santa Cruz County, CA, USA) that spanned a gradient of human land-use intensity. Sites were located on the numerous relatively small (first and second order) streams within the watershed and were part of a larger study (D. B. Herbst, unpublished data). From this larger set of candidate sites, sites were chosen to stratify a gradient in human land-use intensity and minimize differences in gradient and stream size. With two exceptions, sites were located on different streams. Elevations in this coastal watershed range from 979 m to sea level where the San Lorenzo River enters the Monterey Bay. The climate is Mediterranean, with 76-153 cm rain yr −1 . Tributaries drain steep soils of weathering granite, schists, marble, and marine deposits consisting of sandstones, shales and mudstones The San Lorenzo watershed has a history of excess anthropogenic nutrient inputs Field study Each site consisted of a riffle-pool sequence ranging from 40 to 60 m in length. Sites encompassed an anthropogenic gradient of the watershed which ranged from locations with little anthropogenic influence to locations with higher levels of anthropogenic influence such as road crossing and rural development. We used catchment road density (km km −2 ) as an index of human land-use intensity for each site. At each site, we collected primary producers (periphyton), and consumers (benthic invertebrates; rainbow trout, Oncorhynchus mykiss; signal crayfish, Pacifasticus leniusculus) for stable isotope analyses and water samples to obtain nutrient concentrations. All sampling was conducted in June 20-26, 2009 at near base stream flow conditions. Primary producer biomass was characterized by algae (periphyton) scraped from cobbles collected from both a region of slowand fast-water within the sampling site. Previous work has found that water velocity can influence algal stable isotope signatures At each site benthic invertebrates were collected by a Surber sample (0.5 mm mesh; sampling to a depth of 10 cm) in both a region of fast and slow stream flow. Samples were preserved in 70% ethanol. We note that preservation in ethanol can slightly alter isotope signatures (shifting ␦ 13 C approximately 1‰ and ␦ 15 N approximately 0.4 ‰; Venturra and Jeppesen, 2009). We did not adjust for this shift because it is likely relatively consistent within invertebrates that have fairly constrained stoichiometry (as opposed to across taxonomic groups with vastly different stoichiometry). Prior to preparation for stable isotope analysis, invertebrates were sorted and identified to family and functional feeding group (filterer, detritivore, herbivore, or predator) according to Fish and crayfish were collected by three-pass depletion electrofishing. Block nets at the upper and lower extent of each site prevented movement in or out of the site during surveys. Signal crayfish (Pacifastacus leniusculus; n = 35, approximately three per site) and rainbow trout (O. mykiss; n = 65, approximately five per site) were selected as focal species as they were the most abundant top aquatic consumers present across the different sampling sites. Orbital carapace length (crayfish) or total fork length (trout) and wet weight (to the nearest 0.1 g) were measured on-site for each sampled organism. Crayfish muscle tissue and rainbow trout caudal fin clips Algae and benthic invertebrates were oven dried until they reached a constant weight (approximately 48 h at 60 • C), whereas crayfish and trout samples were freeze-dried. To remove 13 Cdepleted lipids, all consumer samples were flushed with three cycles of petroleum ether at 1200 psi in a Dionex ASE 200 Accelerated Solvent Extraction System. Algae and crayfish tissue samples were ground into a homogenous powder with an agate mortar and pestle. Larger benthic invertebrates were also ground into a fine powder, whereas multiple individuals of the same species were aggregated into one sample for smaller invertebrates. Trout fin clips were left intact. Samples were weighed into 5 mm × 9 mm (algae, mean ± standard deviation = 4800 ± 31 g) or 3 mm × 5 mm (benthic invertebrates, 589 ± 53 g; crayfish muscle, 698 ± 30 g; fish fins, 673 ± 78 g) tin capsules (Costech Analytical Technologies). Stable isotope analyses Sample ␦ 13 C and ␦ 15 N were measured on a Carlo Erba 1108 elemental analyzer coupled to a ThermoFinnigan Delta Plus XP isotope ratio mass spectrometer (University of California, Santa Cruz Stable Isotope Laboratory). Sample isotope ratios were corrected relative to working standards with C:N ratios similar to the samples. For internationally calibrated in-house standards, analytical precision of ␦ 13 C and ␦ 15 N was less than 0.2‰. Water nutrients We collected duplicate water samples to measure total nitrate and phosphate concentrations at each site. Unfiltered water samples were collected just under the water surface in acid-rinsed high-density polyethylene (HDPE) bottles and were kept frozen at −20 • C until analysis. Frozen water samples were thawed, filtered through 0.7 m GF/F filters (Whatman), and analyzed for nitrate and phosphate concentrations using a QuikChem 8000 Flow Injection analyzer (Lachat Instruments) at the University of California, Santa Cruz Marine Analytical Labs. Isotopes and mixing models To address our question of how human land-use alters stream N cycling, we used a Bayesian information theoretic approach to estimate models and parameters. Bayesian mixing models allow the use of prior information for parameters and the propagation of uncertainty into estimates of posterior probability distributions of contributions to isotopic mixtures (e.g., where the stable isotope signature of the mixture (␦ 15 N mix ) was a function of proportional contributions of the two sources, anthropogenic sources (f a ) and background watershed sources (f b ), and their respective isotope signatures (␦ 15 N a and ␦ 15 N b ) and where f a + f b = 1. While ␦ 15 N a and ␦ 15 N b were not measured in this study, previous studies have indicated that ␦ 15 N a are generally enriched (prior probability distribution was uniformly distributed between 5‰ and 18‰) and ␦ 15 N b are generally depleted (prior probability distribution uniformly distributed between −2‰ and 2‰) (e.g., where N t was the measured concentration of nitrate at the site and N b was the unknown amount of that nitrate that was from background watershed sources. The prior probability distribution for N b was uniformly distributed between 0 and 10 M NO 3 − ; we did not have direct or published measurements of this prior and thus used a wide and uninformative prior. Through assuming that background watershed nitrate was constant but unknown across sites and estimating this parameter, we estimated the contribution of anthropogenic nitrate as N t − N b . It is important to note that these analyses were focused on the N that was available for algae to uptake and transfer into their stable isotope signatures. Given that algae have relatively fast turnover times, we think that our single measurements of nitrate during low summer baseflow are likely good approximations of N available to algae during this summer baseflow period. Therefore, our period of inference was restricted to this time frame. Furthermore, it is possible that other processes are contributing to observed isotopic patterns which would suggest a potentially different model structure (see section "Discussion"). Bayesian parameter estimation We estimated the posterior probability distributions of parameters and fit models using a Bayesian approach. Specifically, we estimated posterior probability distributions for N b , ␦ 15 N a , and ␦ 15 N b based on the data and our prior specifications. Bayesian modeling is based on the premise that the probability of a given combination of parameter values is defined by the likelihood of the data given the model and the prior belief in the parameter set. Markov chain Monte Carlo (MCMC) sampling was performed in JAGS in R Results The streams sampled in this survey of the San Lorenzo watershed varied in their land-use intensity. Specifically, the road density of the stream basins ranged from 0.89 to 6.13 road km km −2 . Sites with higher human land-use generally had higher nutrient concentrations N isotopes of algae showed a strong positive saturating relationship with nitrate concentratio

Assessing the Relationship between Gulls <i>Larus</i> spp. and Pacific Salmon in Central California Using Radiotelemetry

<div><p></p><p>Predation by marine birds has resulted in substantial losses to runs of Pacific salmon <i>Oncorhynchus</i> spp., in some cases necessitating management action. Recovery of PIT tags on a seabird breeding colony (Año Nuevo Island) indicated that western gulls <i>Larus occidentalis</i> prey upon federally listed Coho Salmon <i>Oncorhynchus kisutch</i> and steelhead <i>O. mykiss</i> in central California. Whereas salmonid populations in central California have decreased in recent decades, the western gull population on Año Nuevo Island has increased. We observed gulls <i>Larus</i> spp. within estuaries to document predation and used radiotelemetry to examine gull movement in relation to the availability of salmonids. During 2008 and 2009, observed predation events of out-migrating salmonids by gulls were rare; 21 events occurred during 338 h of observations at two estuaries. During the prehatch and chick-rearing phases of breeding, which coincided with migration of salmonids from fresh to salt water, 74% of the detections of radio-tagged western gulls occurred within 25 km of Año Nuevo Island, suggesting that the relative susceptibility of predation by western gulls using Año Nuevo Island decreased with distance from the island. Western gull presence at creek mouths was greatest during daylight hours (91% of detections), while juvenile salmonids were present predominantly at night (65% of detections). The greatest overlap between western gulls and salmonids occurred at dusk, and predation of out-migrating salmonids was likely opportunistic. Deterring gulls from creek mouths when overlap between predator and prey might otherwise occur may buffer out-migrating salmonids from predation. Our results will inform management strategies to most effectively reduce the impacts of gull predation on central California salmonids. </p><p>Received May 8, 2014; accepted March 11, 2015</p></div