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Marine fisheries management strives to maintain sustainable populations while allowing exploitation. However, well-intentioned management plans may not meet this balance as most do not include the effect of climate change. Ocean temperatures are expected to increase through the 21st century, which will have far-reaching and complex impacts on marine fisheries. To begin to quantify these impacts for one coastal fishery along the east coast of the United States, we develop a coupled climate–population model for Atlantic croaker (Micropogonias undulatus). The model is based on a mechanistic hypothesis: recruitment is determined by temperature-driven, overwinter mortality of juveniles in their estuarine habitats. Temperature forecasts were obtained from 14 general circulation models simulating three CO 2 emission scenarios. An ensemble-based approach was used in which a multimodel average was calculated for a given CO 2 emission scenario to forecast the response of the population. The coupled model indicates that both exploitation and climate change significantly affect abundance and distribution of Atlantic croaker. At current levels of fishing, the average (2010–2100) spawning biomass of the population is forecast to increase by 60–100%. Similarly, the center of the population is forecast to shift 50–100 km northward. A yield analysis, which is used to calculate benchmarks for fishery management, indicates that the maximum sustainable yield will increase by 30–100%. Our results demonstrate that climate effects on fisheries must be identified, understood, and incorporated into the scientific advice provided to managers if sustainable exploitation is to be achieved in a changing climate.

Hypoxia is thought to have negative effects on fish in coastal ecosystems, but quantifying those effects can be difficult. Direct exposure to hypoxia can reduce fish growth or survival, but fish can also rapidly detect and avoid low dissolved oxygen levels. However, avoidance behavior may result in indirect effects that reduce fish growth. For example, when hypoxic conditions expand, fish densities may increase in nearshore oxygenated refuges, potentially causing density-dependent reductions in growth. We evaluated this hypothesis for juvenile demersal fish species (primarily spot Leiostomus xanthurus and Atlantic croaker Micropogonias undulatus). By monitoring water quality and fish density across the Neuse River Estuary, North Carolina, USA, under varying water quality conditions during summer 2007, we showed that fish effectively avoided hypoxia despite rapidly changing conditions (minutes to hours), moving away from incursions of hypoxic water and then rapidly redistributing into affected areas after these events passed. Fish densities in nearshore oxygenated refuges increased nearly 2-fold when habitat was compressed by hypoxic waters. Spot in compressed refuges also had significantly less food in their stomachs during June. Based on published estimates of density-dependent spot growth, we estimated that average spot growth rate was reduced 17% during habitat compression events, which occurred 21.5% of the time, translating into an average reduction in growth rate of 4% over the summer. This likely is a conservative estimate of indirect hypoxia effects on growth, as hypoxia was relatively mild in 2007, and density dependence is only one indirect mechanism by which hypoxia may potentially reduce growth.

Seasonal hypoxia may alter trophic relationships between benthic prey and mobile predators if consumers with low hypoxia tolerance are vertically displaced from bottom waters and switch to pelagic prey. Alternatively, consumers with greater hypoxia tolerance may continue to forage on stressed benthic prey. Identifying the trophic effects of hypoxia on mobile fishes requires long-term records of individual exposure histories. Elemental profiles in otoliths of demersal Atlantic croaker Micropogonias undulatus from the northern Gulf of Mexico (nGoM) were used to quantify hypoxia exposure and estuarine occupancy histories over the 2 to 3 mo prior to capture. Elemental patterns in croaker otoliths clustered fish into 4 groups: early or late estuarine migrants and normoxic or hypoxic coastal residents. Stable isotope values of δ13C and δ15N in croaker muscle were compared between clusters to determine trophic shifts associated with environmental histories, while isotope niche areas of clusters indicated whether trophic shifts were uniform or variable among individuals within each cluster. Estuarine migrants displayed lower δ13C and δ15N values, indicating greater contribution of terrestrially derived diets, although the relatively larger isotope niche areas for estuarine clusters was consistent with individually variable emigration timings. Coastal normoxic and hypoxic fish both had similar δ13C and δ15N values, suggesting limited vertical displacement to pelagic food webs in hypoxic fish. These results indicate trophic resilience of demersal croaker to seasonal hypoxia in the nGoM, with no detectable change in trophic dynamics over monthly time scales. This paired natural tag approach further enhances our understanding of sublethal trophic responses to hypoxia and consequences for ecosystem functioning.

In estuarine and coastal systems, anthropogenic activities and directional changes in global air temperatures have led to increased water temperatures, as well as increased frequency and severity of episodic hypoxia. These alterations have had population-level effects on aquatic organisms, including changes in species-specific distributions. Because physiology is the transfer function connecting environmental conditions to individual behaviors and eventually to population-level effects, we used individual-based models (IBMs) that incorporate changes in metabolic scope as motivation for movement. Our objective was to investigate the effects of temperature and hypoxia on the distribution of Atlantic croaker and spot in Chesapeake Bay. We compared the predicted monthly fish distributions from the IBMs with apparent fish distributions recorded by the VIMS Juvenile Fish Trawl Survey during 1988–2014. IBMs failed to reproduce accurately the apparent spatial distributions of Atlantic croaker and spot. More specifically, compared with the trawl survey, IBMs predicted larger proportions of these two species would be captured in the lower regions of Chesapeake Bay and smaller proportions in the York and Rappahannock rivers. We postulate that, because similar thermal and oxygen conditions were occupied by fish in the IBMs and in the wild, temperature may not be the most important factor motivating the movement of Atlantic croaker and spot in Chesapeake Bay, and other spatial factors (e.g., prey availability) act in concert with temperature and hypoxia to determine the spatial distributions of Atlantic croaker and spot in Chesapeake Bay. Alternatively, surveys used to estimate species occurrences could be biased if gear vulnerability is affected by temperature or oxygen conditions (or both). Additional research is needed to clarify which condition is true or if a combination of these factors (and others) led to discrepancies between predicted and apparent fish distributions based on trawl survey data.

The condition of individuals in a year class may contribute to recruitment variability due to differential survival of poor- and well-conditioned fish, but the temporal dynamics of juvenile fish condition are poorly understood. We examined inter- and intra-annual dynamics of condition for juveniles of 3 species collected from estuarine nursery areas of Chesapeake Bay from November 2010 to June 2014. We describe temporal patterns in length-based indices, the hepatosomatic index (HSI), and relative subdermal lipid estimates for juvenile summer flounder Paralichthys dentatus (n = 1771), Atlantic croaker Micropogonias undulatus (n = 3911), and striped bass Morone saxatilis (n = 874). Multiple indices provided a more complete understanding of energy-storage strategies for juveniles because temporal patterns among condition indices were not congruent for a given species. Most juvenile summer flounder and Atlantic croaker migrate from Chesapeake Bay in the fall, and both species exhibited increases in subdermal lipids in the time period prior to migration. For all species, individuals that remained in the estuary during winter exhibited high HSI values, indicating a common energy-storage strategy during winter. Mean condition of juveniles varied among year classes, but differences were inconsistent among indices, suggesting that energy was differentially stored among tissues for these year classes. Density-dependent effects contributed to variation in mean condition for summer flounder and striped bass. Our understanding of recruitment variability may be improved by assessing annual differences in mean condition as revealed by multiple indices.

Hypoxia is a worldwide natural phenomenon that is becoming increasingly more severe in the northern Gulf of Mexico (nGoMex) where it is driven by anthropogenic nutrient loading in riverine runoff. In addition to outright mortality, hypoxia can have a range of ecophysiological effects on demersal and pelagic organisms, from altering species niche widths to stifling growth rates. While hypoxia exposure is known to have many sub-lethal effects for early life history stages of demersal fishes, quantifying exposure histories and consequences for the entire lifespan of exposed individuals is critical for understanding how hypoxia impacts ecosystem functions. Lifetime hypoxia exposure patterns and potential consequences to metabolism were reconstructed using otolith chemical analyses (Mn:Ca, Mg:Ca and Mn:Mg values) in Atlantic Croaker (Micropogonias undulatus) collected in the northern Gulf of Mexico. Age-0 croaker were found to have the highest Mn:Ca duration fractions, indicating that a large proportion of juveniles are exposed to hypoxia within the first year of life. However, otolith chemical proxies of metabolism (Mg:Ca) did not differ by hypoxia exposure groups until Age-1 and Age-2. These patterns were unexpected and contrary to those found for comparable assessments of putative metabolic response to hypoxia in other systems, potentially due to the relative hypoxia tolerance of Atlantic Croaker that may allow increased foraging rates in low oxygen waters. Understanding the complex interactions between hypoxia and the lifetime growth and metabolism in mobile fishes is necessary to make robust predictions about the ecosystem consequences of this pervasive and growing environmental stressor in the Gulf of Mexico.

High-quality nursery habitats support greater numbers of juveniles that survive to adulthood, but characteristics underlying high-quality habitats remain elusive because their productivity varies spatially and temporally and may be species-specific. Fish condition is an energy-integrative measure of ecological interactions, stress, and activity within a habitat, such that juvenile fish condition is representative of the quality of nursery habitats. We apply spatially explicit models to examine patterns in nursery habitat quality of Virginia estuaries based on nutritional condition for Atlantic croaker Micropogonias undulatus, summer flounder Paralichthys dentatus, and striped bass Morone saxatilis. Environmental factors (water temperature, dissolved oxygen, salinity, and depth) positively influenced the condition of Atlantic croaker but had mixed effects on striped bass and summer flounder condition, despite co-location of these species. Over the observed range of environmental conditions, the potential impact of salinity was 1.7 to 6 times that of other environmental factors. The condition of Atlantic croaker was negatively influenced by conspecific density, indicating local abundance mediates habitat quality. Regional habitat effects on fish condition were apparent after adjusting for broad-scale environmental effects: on average, striped bass condition was 22% greater in habitats near tributary mouths, Atlantic croaker condition was 28% greater in upestuary habitats, and the adjusted mean condition of summer flounder associated with coastal lagoons and the eastern side of Chesapeake Bay was 100% greater than conspecifics from other regions. Not all habitats are equally suitable for juvenile stages of estuarine species, but locations that produce well-conditioned individuals reveal the environmental characteristics associated with high-quality nursery habitats that contribute to species-specific productivity.

The long-term effects on marine fish populations of the recent increase worldwide in the incidence of coastal hypoxia are unknown. Here we show that chronic environmental exposure of Atlantic croaker (Micropogonias undulatus) to hypoxia in a Florida estuary caused marked suppression of ovarian and testicular growth which was accompanied by endocrine disruption. Laboratory hypoxia studies showed that the endocrine disruption was associated with impairment of reproductive neuroendocrine function and decreases in hypothalamic serotonin (5-HT) content and the activity of the 5-HT biosynthetic enzyme, tryptophan hydroxylase. Pharmacological restoration of hypothalamic 5-HT levels also restored neuroendocrine function, indicating that the stimulatory serotonergic neuroendocrine pathway is a major site of hypoxia-induced inhibition. Inhibition of tryptophan hydroxylase activity to downregulate reproductive activity could have evolved as an adaptive mechanism to survive periodic hypoxia, but in view of the recent increased incidence of coastal hypoxia could become maladaptive and potentially affect fish population abundance and threaten valuable fishery resources.

Throughout the Gulf of Mexico (GoM), coastal inlets provide vital pathways for marine fish larvae spawned offshore to reach estuarine nursery habitat. While survivorship of fish larvae is dependent on many factors, feeding success has been highlighted as the most important. Currently, little is known about the feeding ecology of larval fish during their passage through coastal inlets. The purpose of this study was to investigate the trophodynamics of fish larvae passing through a GoM estuarine tidal inlet during a fall spawning season. Several trophic niches existed within the δ13C’ and δ15N’ (lipid and baseline corrected values) stable isotope-derived food web of larval fish, with multiple taxa present within most trophic niches. These trophic niches changed throughout the fall season, however. Micropogonias undulatus, an estuarine-dependent species common to the region, was selected for a closer look at dietary shifts through ontogeny and showed an overall shift to higher trophic level prey with size, although there was not yet evidence of a shift towards estuarine prey indicative of settlement. Their diet mainly consisted of Calanoida copepods across all sizes of larvae with significant electivity for only Acartia (Calanoida), and this selection was likely due to a difference in behavior rather than taxonomy or size. This study provides information about larval fish trophodynamics at coastal inlets, advancing our understanding of larval fish survivorship and recruitment to adult fish stocks.

The long-term impacts on marine ecosystems of the recent dramatic worldwide increase in the incidence of coastal hypoxia are unknown. Here, we show widespread reproductive disruption in Atlantic croakers collected from hypoxic sites approximately 120 km apart in the extensive northern Gulf of Mexico continental shelf hypoxic zone. Gonadal growth and gamete production were impaired in croakers from hypoxic sites compared with fish from reference normoxic sites east of the Mississippi River Delta. Male germ cells were detected in approximately 19 per cent of croaker ovaries collected in the hypoxic region, but were absent in ovaries from normoxic sites. In addition, the sex ratio was skewed towards males at the hypoxic sites. The masculinization and other reproductive disruptions were associated with declines in neuroendocrine function, as well as ovarian and brain expression of aromatase (the enzyme that converts androgens to oestrogens). A similar incidence of ovarian masculinization and decline in ovarian aromatase expression were observed in croaker after chronic laboratory hypoxia exposure, indicating that ovarian masculinization is a specific hypoxia response and is due to decreased aromatase activity. The results suggest severe reproductive impairment can occur over large coastal regions in marine fish populations exposed to seasonal hypoxia, with potential long-term impacts on population abundance.