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ObjectiveAnadromous Striped Bass Morone saxatilis are dominant predators in estuaries and coastal areas along the U.S. Atlantic coast, with the potential to exert top-down control on prey populations. Although Striped Bass diets have been studied previously, spatiotemporal patterns of diet across ontogeny remain poorly understood, especially for young fish in shallow nursery habitats.MethodsWe collected and examined gut contents from adult, juvenile, and young-of-year (age-0) Striped Bass from nine rivers across the Maryland and Virginia portions of Chesapeake Bay during summer and fall 2018. We compared the use of traditional morphological inspection and new amplicon-based next-generation sequencing methods for identifying gut contents.ResultStriped Bass in shallow tributary habitats of Chesapeake Bay had diverse diets that varied strongly with ontogeny and salinity zone. In particular, the diet of age-0 Striped Bass varied greatly from those of juveniles and adults when age-0 fish foraged in freshwater habitats. Although our results on prey consumed aligned with previous surveys, we identified additional taxa as important prey for these young fish, including dipteran insects, Banded Killifish Fundulus diaphanus, Inland Silverside Menidia beryllina, bay barnacle Amphibalanus improvisus, and grass shrimp Palaemon spp. Comparison of methodologies indicated that 40% of prey by weight could not be identified with morphological analysis, while 76% of mitochondrial cytochrome oxidase I sequences could be assigned binomial names, allowing for high-resolution taxonomic comparisons.ConclusionThis study adds to the growing body of evidence that amplicon-based next-generation sequencing methods are far superior to traditional morphological analyses of gut contents for fine-scale taxonomic resolution of prey.

Background Conserving marine predators is tantamount to maintaining functional marine ecosystems. Though intensively studied in other regions, little is known about at-sea movements and diving behaviors of a recovering population of grey seals ( Halichoerus grypus atlantica ) inhabiting northeastern United States continental shelf waters. Young-of-year grey seals may be particularly vulnerable to threats due to a lack of parental care postweaning. There is a need to establish baseline knowledge of at-sea behaviors in the face of large-scale ocean industrialization. We deployed 63 satellite relay data loggers on young-of-year grey seals to investigate postweaning at-sea movements and dive behaviors prior to the installation of offshore wind turbines. Results Young-of-year grey seals dispersed widely across the continental shelf waters. Collective utilization distributions of seals overlapped with offshore wind planning areas in the winter and spring months, and overlap was lowest in the summer when the seals dispersed northeastward. Maximum diving depth and duration increased in the first two months of nutritional independence and stabilized by April. Dives were classified as being either benthic or pelagic depending on dive depth relative to bathymetry. Seals conducted more benthic diving in the spring and this coincided spatiotemporally with habitat and phenology of an important prey species. Following a diel trend, benthic diving peaked during daylight hours, while pelagic diving occurred more frequently at night. Benthic dives occurred more frequently than pelagic dives in sandy shoals and banks. Furthermore, seals conducted more benthic than pelagic dives in wind energy planning areas. Conclusions Ours is the first comprehensive study on the horizontal movement and diving behaviors in the U.S. population of grey seals, contributing knowledge on the at-sea habits of a vulnerable demographic in relation to other anthropogenic uses of the marine environment. This information will serve as valuable input to conservation management and mitigation plans, and it contributes necessary regional context to the broader understanding of grey seal ontogeny across the North Atlantic. Furthermore, these results provide important baseline information for future comparative analyses of grey seal behavior as offshore wind development expands in scope in this region.

Population declines in shark species have been reported on local and global scales, with overfishing, habitat destruction and climate change posing severe threats. The lack of species-specific baseline data on ecology and distribution of many sharks, however, makes conservation measures challenging. Here, we present a fisheries-independent shark survey from the Fiji Islands, where scientific knowledge on locally occurring elasmobranchs is largely still lacking despite the location’s role as a shark hotspot in the Pacific. Juvenile shark abundance in the fishing grounds of the Ba Estuary (north-western Viti Levu) was assessed with a gillnet-and longline-based survey from December 2015 to April 2016. A total of 103 juvenile sharks identified as blacktip Carcharhinus limbatus (n = 57), scalloped hammerhead Sphyrna lewini (n = 35), and great hammerhead Sphyrna mokarran (n = 11) sharks were captured, tagged, and released. The condition of umbilical scars (68% open or semihealed), mean sizes of individuals (±SD) (C. limbatus: 66.5 ± 3.8 cm, S. lewini: 51.8 ± 4.8 cm, S. mokarran 77.4 ± 2.8 cm), and the presence of these species over recent years (based on fishermen interviews), suggest that the Ba Estuary area is a critical habitat for multiple species that are classified as “Near Threatened” or “Endangered.” Specifically, the area likely acts as a parturition ground over the studied period, and potentially as a subsequent nursery area. We identified subareas of high abundance and found that temperature, salinity and depth acted as small-scale environmental drivers of shark abundance. The data suggests a tendency for species-specific spatial use, both horizontally (i.e., between sampling areas) and vertically (i.e., across the water column). These results enhance the understanding of shark ecology in Fiji and provide a scientific basis for the implementation of local conservation strategies that contribute to the protection of these threatened species.

Seagrasses, oyster reefs, and salt marshes are critical coastal habitats that support high densities of juvenile fish and invertebrates. Yet which species are enhanced through these nursery habitats, and to what degree, remains largely unquantified. Densities of young-of-year fish and invertebrates in seagrasses, oyster reefs, and salt marsh edges as well as in paired adjacent unstructured habitats of the northern Gulf of Mexico were compiled. Species consistently found at higher densities in the structured habitats were identified, and species-specific growth and mortality models were applied to derive production enhancement estimates arising from this enhanced density. Enhancement levels for fish and invertebrate production were similar for seagrass (1370 [SD 317] g m⁻² y⁻¹ for 25 enhanced species) and salt marsh edge habitats (1222 [SD 190] g m⁻² y⁻¹, 25 spp.), whereas oyster reefs produced ~650 [SD 114] g m⁻² y⁻¹ (20 spp). This difference was partly due to lower densities of juvenile blue crab (Callinectes sapidus) on oyster reefs, although only oyster reefs enhanced commercially valuable stone crabs (Menippe spp.). The production estimates were applied to Galveston Bay, Texas, and Pensacola Bay, Florida, for species known to recruit consistently in those embayments. These case studies illustrated variability in production enhancement by coastal habitats within the northern Gulf of Mexico. Quantitative estimates of production enhancement within specific embayments can be used to quantify the role of essential fish habitat, inform management decisions, and communicate the value of habitat protection and restoration.

Animals play an important and sometimes overlooked role in nutrient cycling. The role of animals in nutrient cycling is spatially and temporally variable, but few studies have evaluated the long-term importance of animal-mediated nutrient cycling in meeting nutrient demand by primary producers. We quantified the proportion of phytoplankton nutrient (phosphorus, P) demand met by excretion by gizzard shad (Dorosoma cepedianum) in a eutrophic reservoir where this species dominates fish biomass. From 2000 to 2014, gizzard shad excretion supported a variable proportion of phytoplankton P demand, averaging 7–27% among years over the growing season (spring and summer). Temporal patterns emerged, as gizzard shad consistently supported a higher proportion of demand during summer (mean 31%) than spring (8%). In spring, the proportion of demand met from gizzard shad excretion was best predicted by gizzard shad population biomass, stream discharge, and temperature. In summer, this proportion was best predicted only by biomass of the young-of-year (YOY) gizzard shad. Thus, variation in YOY shad biomass significantly alters nutrient supply, and future studies should explore the long-term role of animal population dynamics in nutrient cycling. Our study shows that several years of data are needed to perform a critical evaluation of the importance of animals in meeting ecosystem nutrient demand.

Invasive species are a major threat to ecosystem structure and function. For example, Bythotrephes cederströmii (Bythotrephes hereafter) invasions have significantly reduced native zooplankton density and biomass, resulting in competitive interactions with zooplanktivorous fishes. Young of year (YOY) walleye (Sander vitreus) are initially zooplanktivorous and have recently been shown to display reduced growth in Bythotrephes-invaded lakes. Here, we combined a bioenergetics model for larval walleye with changes in the zooplankton community following Bythotrephes invasion and predicted reduced larval walleye growth in the presence of Bythotrephes, supporting field observations. The model predicted greater negative impacts on larval walleye growth in oligotrophic compared with mesotrophic lakes, though reduced growth was only significant under oligotrophic conditions. Under Bythotrephes invasion, net energy available to growth over the simulated period was often observed to be negative (indicating mass loss). These combined results from the model suggest that Bythotrephes invasion could potentially lead to walleye recruitment failure, especially in low nutrient environments. This result was insensitive to differences in annual mean water temperatures ranging from 18.5 to 23.5 °C. As YOY growth, survival, and recruitment are ultimately linked to adult abundance and sustainability of managed stocks, our results highlight the potential impacts of Bythotrephes on the sustainability of walleye populations in boreal lakes.

Decadal changes in the life history events of marine species are becoming increasingly important to identify under a warming climate, yet many long-term monitoring programs do not collect data at the spatiotemporal resolution needed to describe them. Such data are vital for understanding the southern New England/Mid-Atlantic winter flounder Pseudopleuronectes americanus stock, a species hypothesized to be adversely impacted by warming waters via increased temporal overlap between its early life stages and predators. To provide insight into winter flounder early life history dynamics and context for resiliency of the stock, we examined ichthyoplankton data collected from 2 monitoring programs within Narragansett Bay, Rhode Island, USA: one spatially comprehensive dataset across the bay proper (2001–2008, 2016–2017), and the other a longer time series centered in one of the sub-estuaries of the bay (Mount Hope Bay, 1972–2017). By leveraging these datasets together, we conducted a spatiotemporal synthesis of early life stage winter flounder by evaluating changes in larval phenology, decadal coherence in larval spatial patterns, and correspondence between larvae and their subsequent life stage. We identified changes in larval phenology via earlier seasonal peaks in density through time. Results also indicated stable larval spatial patterns during a period of larval decline, as well as spatial coherence between larval and young-of-the-year stages. Using winter flounder as a model species, our results highlight the importance of high-resolution spatiotemporal ichthyoplankton sampling to identify changes in phenology and site fidelity for marine fishes.

Young-of-the-year fish communities are widely used as bioindicators of various environmental disturbances. This study was conducted from 1997 to 2015 and aims to develop fish trait–based indices of changes in the temperature regime and eutrophication of water bodies in the Dnipro River basin. We identified fish traits that significantly correlate with both temperature and chlorophyll-a concentration optimum: reproduction habitat, oxygen tolerance, and toxicity tolerance. Compared to other ecological groups, lithophilic species exhibited the lowest degree of thermal and eutrophication optimum, indicating this species’ greater vulnerability to environmental alteration. Fish species that are intolerant to water quality and low oxygen concentration were the most sensitive to changes in temperature regime and eutrophication level. Salinity preferences and water quality tolerance emerged as reliable predictors of temperature optimum. Freshwater fish had an average temperature optimum that was 4.5% higher than that of freshwater-brackish and freshwater-brackish-marine fish. Species tolerance to the temperature factors and nutrient loads correlated only with rheophily, with rheophilic species having an average 13.8% higher temperature tolerance than other fish species and a 10.4% higher chlorophyll-a concentration tolerance. The fish temperature index increased over time during the study period in all the studied water bodies, consistent with ongoing warming affecting all sites. In contrast, the Fish Eutrophication Index showed greater temporal heterogeneity in studied water bodies, indicating various adaptative potentials of fish communities to eutrophication. These indices can be relevant for assessing disturbed situations caused by changes in climatic and anthropogenic impacts on water bodies.

Flooding regulates the amount and quality of habitat available for fish populations in river-floodplains. Although previous studies assessed fish population responses to river hydrology, the processes by which flooding affects fish abundance and catch remain unclear. Here, we investigated whether degree of flooding affects abundance and catch of Colossoma macropomum, a long-lived, overexploited fish population of the central Amazon Basin. We computed the degree of flooding corresponding to the feeding area of young-of-the-year C. macropomum as the annual magnitude of the moving littoral zone (ML). We estimated abundance of age classes one, two, and three of C. macropomum using a modeling program based on catch, fishing effort, and fish length frequency data from the principal commercial fishery. We found that flooding positively and non-linearly affected abundance of the age-one cohort but not of older age classes. ML data corresponding to a late rising water phase in which zooplankton, seeds, and fruits dominated the diet provided the strongest effect on age-one abundance. However, flooding effects on total catch were not found, likely due to catches comprising several age classes. These results provide support to existing evidence that the magnitude of the moving littoral zone regulates abundance of juvenile fish. Because the ML quantifies food and habitat availability for various other fish species, it may constitute an important control of fish abundance in these systems. Management of these fisheries may be improved by adjusting fishing effort based on hydrology. More generally, the information also serves to assess the impacts of hydrological alterations (e.g., dams) on fish recruitment.