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Microplastics are emergent contaminants threatening aquatic organisms including aquacultured fish. This study investigated the effects of high-density polyethylene (HDPE, 100 to 125μm) on yellow perch (Perca flavescens) based on integrative evaluation including growth performance, nutritional status, nutrient metabolism, fish health, and gut microbial community. Five test diets (0, 1, 2, 4, or 8g HDPE/100g diet) containing41% protein and 10.5% lipid were fed to juvenile perch (average body weight, 25.9±0.2g; n=15) at a feeding rate of 1.5% to 2.0% body weight daily. The feeding trial was conducted in a flow-through water system for 9wk with 3 tanks per treatment and 15 yellow perch per tank. No mortality or HDPE accumulation in the fish was found in any treatments. Weight gain and condition factor of fish were not significantly impacted by HDPE (P>0.05). Compared to the control group, fish fed the 8% HDPE diet had significantly decreased levels of protein and ash (P&nbsp

Cyanobacteria harmful algal blooms (CHABs) became a concern in the upper San Francisco Estuary, California beginning in 1999, when yearly blooms of Microcystis began in the Delta region. Subsequent research identified that the increase in the magnitude, duration and toxicity of Microcystis blooms was associated with drought related conditions of elevated water temperature and low streamflow. However, the impact of extreme conditions on the resilience of the bloom was unknown. The 2014 and 2017 water years provided a unique opportunity to determine the effect of climatic “whiplash” produced by the occurrence of extreme wet conditions following extreme dry conditions on the Microcystis bloom. We hypothesized that the period of record wet conditions in 2017 (1906-2018) would eliminate the Microcystis bloom for that year and perhaps revert the estuary phytoplankton community back to pre-bloom conditions due to extreme flushing, despite the increase in magnitude and spatial and temporal distribution of the Microcystis bloom during the 2014 extreme dry year. Field sampling was conducted at 2-week or 4-week intervals between July and November at stations throughout the Delta for both years and included a suite of physical, chemical and biological factors. Using PRIMER-e DISTLM, we determined that retention time in the upper estuary and water temperature were key environmental correlates with the Microcystis bloom amplitude and in regression models described 58-78% of the variation of the bloom surface biovolume or subsurface abundance. The period of record high streamflow in 2017 was not enough to eliminate the Microcystis bloom. However, the bloom was small in 2017, with a low abundance, late initiation, short duration, narrow distribution and low toxin production. Warm water temperature enabled the bloom to flower in late summer despite streamflow many times those measured previously. In addition, although conditions early in the summer of 2017 favored diatoms, the summer was characterized by an abundance of other non-Microcystis cyanobacteria. We conclude that once established, Microcystis is likely to be resistant to extreme wet conditions, as long as water temperature and other key water quality conditions are favorable.

Temperature and salinity often define the distributions of aquatic organisms. This is at least partially true for Delta Smelt, an imperiled species endemic to the upper San Francisco Estuary. While much is known about the tolerances and distribution of Delta Smelt in relation to these parameters, little is known regarding the temperature and salinity preferences of the species. Therefore, the temperature and salinity preferences of sub-adult Delta Smelt were investigated across a wide range of thermal (8–28 °C) and salinity (0–23 ppt) conditions. Replicates of ten fish were allowed to swim between two circular chambers with different temperature or salinity, and the distribution of fish between the chambers was recorded. We found that Delta Smelt showed no temperature preference below 15 °C, a modest aversion to the warmer tank from 15 to 28 °C, and a strong aversion to the warmer tank with elevated mortality at temperatures above 28 °C. Delta Smelt also preferred lower salinities, and this preference became more pronounced as salinity increased toward 23 ppt. These results indicate that Delta Smelt can tolerate high temperatures and salinities for a short time, and that their preferences for lower temperature and salinity strengthens as these variables increase.

Developmental abnormalities in otoliths can impact growth and survival in teleost fishes. Here, we quantified the frequency and severity of developmental anomalies in otoliths of delta smelt (Hypomesus transpacificus), a critically endangered estuarine fish that is endemic to the San Francisco Estuary. Left–right asymmetry and anomalous crystalline polymorphs (i.e., vaterite) were quantified and compared between wild and cultured populations using digital image analysis. Visual estimates of vaterite were validated using X-ray diffraction, Raman spectroscopy, laser ablation ICPMS, and electron probe microanalysis. Results indicated that cultured delta smelt were 80 times more likely to contain a vateritic otolith and 18 times more likely to contain relatively large (≥ 15%) amounts of vaterite. Similarly, cultured fish exhibited 30% greater asymmetry than wild fish. These results indicate that cultured delta smelt exhibit a significantly higher frequency of vestibular abnormalities which are known to reduce fitness and survival. Such hatchery effects on otolith development could have important implications for captive culture practices and the supplementation of wild fish populations with cultured individuals.