Molecular diet analysis of the marine fish-eating bat, Myotis vivesi Menegaux, 1901, and potential mercury exposure

Mercury is a toxic element acquired by animals through feeding and that can accumulate within food chains through biomagnification. This possesses particular risks to higher trophic levels and may unduly impact marine foraging species or individuals. The fish-eating bat, Myotis vivesi Menegaux, 1901, inhabits islands in the Gulf of California and can act as a predator in the marine environment. A predominantly marine diet and a high trophic position increase the risk of mercury exposure as a consequence of increased bioaccumulation. Using molecular techniques to reconstruct diet, we show that M. vivesi regularly feeds on small fishes and crustaceans, particularly on the northern anchovy (Engraulis mordax Girard, 1854) and the krill species (Nyctiphanes simplex Hansen, 1911). Additionally, we identify significant inter-annual variation in diet composition within this population, but measured levels of total mercury in faecal samples were not related to dietary diversity or trophic level.

Use of the SmeltCam as an Efficient Fish Sampling Alternative Within the San Francisco Estuary

Resource managers often rely on long-term monitoring surveys to detect trends in biological data. However, no survey gear is 100% efficient, and many sources of bias can be responsible for detecting or not detecting biological trends. The SmeltCam is an imaging apparatus developed as a potential sampling alternative to long-term trawling gear surveys within the San Francisco Estuary, California, to reduce handling stress on sensitive species like the Delta Smelt (Hypomesus transpacificus). Although believed to be a reliable alternative to closed cod-end trawling surveys, no formal test of sampling efficiency has been implemented using the SmeltCam. We used a paired deployment of the SmeltCam and a conventional closed cod-end trawl within the Napa River and San Pablo Bay, a Bayesian binomial N-mixture model, and data simulations to determine the sampling efficiency of both deployed gear types to capture a Delta Smelt surrogate (Northern Anchovy, Engraulis mordax) and to test potential bias in our modeling framework. We found that retention efficiency—a component of detection efficiency that estimates the probability a fish is retained by the gear, conditional on gear contact—was slightly higher using the SmeltCam (mean = 0.58) than the conventional trawl (mean = 0.47, Probability SmeltCam retention efficiency > trawl retention efficiency = 94%). We also found turbidity did not affect the SmeltCam’s retention efficiency, although total fish density during an individual tow improved the trawl’s retention efficiency. Simulations also showed the binomial model was accurate when model assumptions were met. Collectively, our results suggest the SmeltCam to be a reliable alternative to sampling with conventional trawling gear, but future tests are needed to confirm whether the SmeltCam is as reliable when applied to taxa other than Northern Anchovy over a greater range of conditions.

Climate-driven aerobic habitat loss in the California Current System

Climate warming is expected to intensify hypoxia in the California Current System (CCS), threatening its diverse and productive marine ecosystem. We analyzed past regional variability and future changes in the Metabolic Index (Φ), a species-specific measure of the environment’s capacity to meet temperature-dependent organismal oxygen demand. Across the traits of diverse animals, Φ exhibits strong seasonal to interdecadal variations throughout the CCS, implying that resident species already experience large fluctuations in available aerobic habitat. For a key CCS species, northern anchovy, the long-term biogeographic distribution and decadal fluctuations in abundance are both highly coherent with aerobic habitat volume. Ocean warming and oxygen loss by 2100 are projected to decrease Φ below critical levels in 30 to 50% of anchovies’ present range, including complete loss of aerobic habitat—and thus likely extirpation—from the southern CCS. Aerobic habitat loss will vary widely across the traits of CCS taxa, disrupting ecological interactions throughout the region.

Sixty-five years of northern anchovy population studies in the southern California Current: a review and suggestion for sensible management

The central stock of northern anchovy (CSNA; Engraulis mordax), the most abundant small pelagic fish in the southern California Current, is key to ecosystem functions. We review drivers of its population dynamics in relation to management. Springtime upwelling intensity lagged by 2 years co-varied positively with CSNA biomass, as did the abundance of Pacific sardine (Sardinops sagax; weakly negative). CSNA population dynamics indicate the need for a multi-species stock assessment, but given serious challenges with modelling population collapse and recovery dynamics, and its moderate fisheries, we suggest that sensible management could be a simple 2-tier harvest control rule designed to emphasize the key trophic role of CSNA in the ecosystem while maintaining moderate socio-economic services. We recommend a monitoring fishery of no more than 5 KMT year−1 split between central and southern California when the stock falls below the long-term median abundance estimate of 380 KMT across the California portion of its range, and a catch limit of 25 KMT year−1 when the stock is above this reference point. This rule would be precautionary, serving to maintain the most important small pelagic forage in the ecosystem, various fisheries interests, and information streams when the population is in a collapsed state.

Influence of wind events on larval fish mortality rates in the southern California Current Ecosystem

Wind-induced mixing can affect the vertical distribution of plankton in the upper water column, influencing the prey available for larval fishes. The stable ocean hypothesis proposes that periods of calm winds facilitate the development of plankton layers at concentrations sufficient for successful larval foraging and increased survival. Conversely, storm events redistribute prey, leading to reduced foraging success. Here, we investigate this hypothesis by comparing larval fish mortality rates estimated from 37 years of ichthyoplankton data against metrics of wind events defined as storms and calm periods. Contrary to expectations, we found that mortality for Pacific hake (Merluccius productus) significantly decreased as storm events increased in the southern California Current Ecosystem. Mortality rates for northern anchovy (Engraulis mordax), Pacific sardine (Sardinops sagax), Pacific mackerel (Scomber japonicus), and jack mackerel (Trachurus symmetricus) had no relationship to storms, and no species’ mortality rates were related to the number of calm events. Our results highlight the differing sensitivities of larval survival among fishes in the region and indicate that responses to atmospheric processes are species-dependent.

Errors associated with compound specific δ15N analysis of amino acids in preserved fish samples purified by high pressure liquid chromatography

Compound specific isotopic analysis of amino acids (CSIA-AA) is increasingly used in ecological and biogeochemical studies tracking the origin and fate of nitrogen (N). Its advantages include the potential for resolving finer-scale trophic dynamics than possible with standard bulk SIA and for reconstructing historical changes in the food webs of consumers from analyses of specimens in preserved sample archives. For the latter, assessing the effects of chemical preservatives on δ15NAA has been inconclusive because the conventional CSIA approach for derivatized AAs by gas chromatography – combustion – isotope ratio mass spectrometry (GC-C-IRMS) has analytical errors (0.4 - 1.0 ‰) in the range expected from chemical preservation. Here, we show improved analytical precision (0.15 ± 0.08 ‰) for 11 underivatized AA standards analyzed by high pressure liquid chromatography followed by offline elemental analysis – IRMS (HPLC/EA-IRMS), an approach originally developed by Broek and McCarthy (2014). Using this method, we report the first high-precision tests of preservation effects on δ15NAA in Northern Anchovy (Engraulis mordax) kept 1½-year in ethanol and up to 27-years in formaldehyde. We found minimal methodological induced fractionation for 8 AAs, and preservation effects on δ15N were similar regardless of duration and preservative used. Although some of the AAs differed significantly from frozen control samples (average +1.0 ± 0.8 ‰), changes in δ15N in the source AA Phenylalanine and trophic position estimates were statistically insignificant. Our results are encouraging for resolving fine-scale natural variability using HPLC/EA-IRMS on chemically preserved specimens and for ultimately reconstructing biogeochemical records and trophic dynamics over long time scales.

Variation in opsin transcript expression explains intraretinal differences in spectral sensitivity of the northern anchovy

Vertebrate retinal photoreceptors house visual pigments that absorb light to begin the process of vision. The light absorbed by a visual pigment depends on its two molecular components: protein (opsin) and chromophore (a vitamin A derivative). Although an increasing number of studies show intraretinal variability in visual pigment content, it is only for two mammals (human and mouse) and two birds (chicken and pigeon) that such variability has been demonstrated to underlie differences in spectral sensitivity of the animal. Here, we show that the spectral sensitivity of the northern anchovy varies with retinal quadrant and that this variability can be explained by differences in the expression of opsin transcripts. Retinal (vitamin A1) was the only chromophore detected in the retina, ruling out this molecular component as a source of variation in spectral sensitivity. Chromatic adaptation experiments further showed that the dorsal retina had the capacity to mediate color vision. Together with published results for the ventral retina, this study is the first to demonstrate that intraretinal opsin variability in a fish drives corresponding variation in the animal’s spectral sensitivity.

A vertebrate retina with segregated colour and polarization sensitivity

Besides colour and intensity, some invertebrates are able to independently detect the polarization of light. Among vertebrates, such separation of visual modalities has only been hypothesized for some species of anchovies whose cone photoreceptors have unusual ultrastructure that varies with retinal location. Here, I tested this hypothesis by performing physiological experiments of colour and polarization discrimination using the northern anchovy, Engraulis mordax . Optic nerve recordings showed that the ventro-temporal (VT), but not the ventro-nasal (VN), retina was polarization sensitive, and this coincided with the exclusive presence of polarization-sensitive photoreceptors in the VT retina. Spectral (colour) sensitivity recordings from the VN retina indicated the contribution of two spectral cone mechanisms to the optic nerve response, whereas only one contributed to the VT retina. This was supported by the presence of only one visual pigment in the VT retina and two in the VN retina, suggesting that only the VN retina was associated with colour sensitivity. Behavioural tests further demonstrated that anchovies could discriminate colour and the polarization of light using the ventral retina. Thus, in analogy with the visual system of some invertebrates, the northern anchovy has a retina with segregated retinal pathways for colour and polarization vision.