Abstract. Genetically distinct anadromous (sockeye) and nonanadromous (kokanee) morphs of the Pacific salmon, Oncorhynchus nerka, develop identical, brilliant red color at maturity during sympatric breeding in freshwater streams. The marine and lacustrine environments they occupy prior to maturity, however, appear to differ in the availability of dietary carotenoid pigments necessary to produce red coloration. We tested the hypothesis that kokanee, which occupy carotenoid-poor lakes, are more efficient at using the dietary pigments than are sockeye, which occupy the more productive North Pacific Ocean. In a 2-year controlled breeding study, flesh and skin color of mature and immature crosses fed a low-carotenoid diet were quantified with both a chromameter and by chemical extraction of carotenoid pigments. Results revealed striking countergradient variation in carotenoid use, with kokanee approximately three times more efficient at sequestering the pigments to the flesh musculature than similar age sockeye. This difference translated into virtually nonoverlapping differences between pure crosses in secondary sexual color at maturity, when the pigments are mobilized and transported to the skin. Kokanee crosses turned pinkish red over most of their body, whereas sockeye turned olive green. The olive green was similar to the breeding color of residuals in the wild, the progeny of anadromous sockeye that remain in fresh water and are believed to have given rise to kokanee on numerous independent occasions. Reciprocal hybrids were similar to each other and intermediate to the pure crosses, indicating additive genetic inheritance. Mate choice trials with sockeye males in the wild showed the ancestral morph strongly preferred red over green models. These results suggest a preference for red mates maintained in nonanadromous breeding populations drove the reevolution of the red phenotype in kokanee via more efficient use of dietary carotenoid pigments. This is a novel, yet hidden, mechanism by which sexual selection promotes the genetic differentiation of these sympatric populations.

The northwestern Gulf of Mexico shelf experiences the largest seasonal hypoxic (dissolved oxygen, DO ≤ 2.0 mg l −1 ) zone in the western hemisphere. This study uses bottom trawl and hydrographic surveys over 3 yr to quantify low DO avoidance thresholds, patterns of aggregation in nearby oxygenated refuge habitats, and spatial overlap of brown shrimp Farfantepenaeus aztecus and several finfishes on the nearshore Louisiana shelf. On average, DO avoidance thresholds were low (1 to 3 mg l −1 ) and near incipient lethal levels for similar species, suggesting organisms avoid the lowest, lethal DO levels on the shelf. Avoidance thresholds varied both within and among years, indicating that behavioral responses to low DO are context-dependent and vary in relation to the severity of hypoxia and possibly other factors. Despite the absence of physical barriers to movement, evading organisms aggregated at short distances (1 to 3 km) just beyond the margins of the hypoxic zone, indicating that sublethal and indirect effects of hypoxia are probably most intense within a relatively narrow region along the hypoxic edge. DO avoidance thresholds and patterns of aggregation were similar between brown shrimp, the primary target of the commercial shrimp trawl fishery, and several juvenile and small adult finfishes that comprise most of the bycatch. In addition, spatial overlap between brown shrimp and finfishes was highest in the years when hypoxia was most severe, and this effect was stronger for benthic fishes than for pelagic fishes. These results suggest the potential for enhanced harvest and bycatch interactions along the margins of the hypoxic zone as an indirect effect of hypoxia-induced shifts in spatial patterns. Such spatially mediated indirect effects are an important means by which hypoxia influences mobile species in the Gulf.

Seasonal, summertime hypoxia (dissolved oxygen ≤ 2 mg l -1 ) has occurred over large areas (~1000 to 20 000 km 2 ) of the northwestern Gulf of Mexico shelf during several years since at least the mid-1980s, resulting in habitat loss for demersal species. To evaluate the effects of hypoxiainduced habitat loss on Atlantic croaker Micropogonias undulatus and brown shrimp Farfantepenaeus aztecus, we compared species' spatial distributions and relationships to abiotic factors (temperature, dissolved oxygen, salinity) across years differing in the spatial extent of hypoxia. Analysis of 14 yr of fishery-independent research trawl and environmental data (July) indicated that hypoxiainduced shifts in spatial distribution result in considerable shifts in the temperature and oxygen conditions that croaker and brown shrimp experience. Croaker, which typically occupy relatively warm, inshore waters, remain in the warmest waters inshore of the hypoxic region but also are displaced to cooler offshore waters. Brown shrimp, which are typically distributed more broadly and further offshore, shift to relatively warm inshore waters as well as cooler waters near the offshore hypoxic edge. These shifts in the species' spatial distribution are reflected in long-term decreases and increases in the mean temperatures occupied by croaker and brown shrimp, respectively, as well as increases in the variance in occupied temperatures for both species. Despite avoidance of the lowest oxygen waters, high densities of croaker and brown shrimp occur in areas of moderately low oxygen concentration (35 to 60% air saturation, 1.6 to 3.7 mg l -1 ) near the offshore hypoxic edge. Because temperature and dissolved oxygen are important abiotic factors that impact metabolic scope, these shifts in spatial distribution during severe hypoxia may impact organism energy budgets. High croaker and shrimp densities near the hypoxic edge likely have implications for trophic interactions as well as the harvest of both target (brown shrimp) and nontarget (croaker) species by the commercial shrimp fishery.

Coastal acidification in southeastern U.S. estuaries and coastal waters is influenced by biological activity, runoff from the land, and increasing carbon dioxide in the atmosphere. Acidification can negatively impact coastal resources such as shellfish, finfish, and coral reefs, and the communities that rely on them. Organismal responses for species located in the U.S. Southeast document large negative impacts of acidification, especially in larval stages. For example, the toxicity of pesticides increases under acidified conditions and the combination of acidification and low oxygen has profoundly negative influences on genes regulating oxygen consumption. In corals, the rate of calcification decreases with acidification and processes such as wound recovery, reproduction, and recruitment are negatively impacted. Minimizing the changes in global ocean chemistry will ultimately depend on the reduction of carbon dioxide emissions, but adaptation to these changes and mitigation of the local stressors that exacerbate global acidification can be addressed locally. The evolution of our knowledge of acidification, from basic understanding of the problem to the emergence of applied research and monitoring, has been facilitated by the development of regional Coastal Acidification Networks (CANs) across the United States. This synthesis is a product of the Southeast Coastal and Ocean Acidification Network (SOCAN). SOCAN was established to better understand acidification in the coastal waters of the U.S. Southeast and to foster communication among scientists, resource managers, businesses, and governments

Temperature-dependent sex determination has been demonstrated in several animals, but in salmonids sex is generally believed to be under strict genetic control. We observed distorted sex ratios, attributable to a temperature manipulation during embryonic development, in experiments conducted during 1991 with the anadromous and nonanadromous (kokanee) forms of sockeye salmon (Oncorhynchus nerka) from Takla Lake, British Columbia. Sex ratios ranged from 62 to 84% female, and the biased ratio could not be accounted for by differential mortality by sex. The effect was observed independently in sockeye and kokanee crosses, as well as in the reciprocal hybrids. Similar crosses from Takla Lake in a previous year (1989) in which incubation temperature was not manipulated resulted in normal (1:1) sex ratios. Other populations of sockeye and kokanee from the same year (1991) but undergoing no temperature manipulation maintained normal sex ratios, as did populations from several disparate locations and years (1986-1994). The parsimonious conclusion is that the temperature manipulation during development was responsible for the biased sex ratio through a direct influence on sex differentiation. Hence, the possibility that temperature-dependent sex determination occurs in O. nerka, and perhaps other salmonids, deserves rigorous testing. Résumé : Chez plusieurs animaux, on a montré que la détermination du sexe peut être dépendante de la température, mais on considérait généralement que, chez les salmonidés, elle était régie par un strict contrôle génétique. Nous avons toutefois observé des distorsions du rapport des sexes, attribuables à une manipulation de la température pendant le développement embryonnaire, dans des expériences menées en 1991 sur les formes anadromes et non anadromes (kokani) de saumon rouge (Oncorhynchus nerka) du lac Takla, en Colombie-Britannique. Le rapport des sexes était de 62 à 84% en faveur des femelles, et cette distorsion ne pouvait pas s'expliquer par des différences dans la mortalité selon le sexe. L'effet a été observé de façon indépendante chez des croisements de saumon rouge et de kokani, ainsi que chez des hybrides réciproques. Des croisements semblables du lac Takla d'une année antérieure (1989) pendant laquelle la température d'incubation n'avait pas été manipulée a donné des rapports des sexes normaux (1:1). Chez d'autres populations de saumon rouge et de kokani de la même année (1991), mais n'ayant pas subi de manipulation thermique, les rapports des sexes étaient normaux, tout comme chez des populations provenant d'endroits divers et d'années différentes (1986-1994). Nous pouvons conclure avec circonspection que la manipulation de la température pendant le développement est responsable de la distorsion du rapport des sexes par influence directe sur la différenciation sexuelle. La possibilité que la détermination du sexe soit dépendante de la température chez O. nerka, et peut-être chez d'autres salmonidés, mérite donc une vérification rigoureuse. [Traduit par la Rédaction]

Nearly every summer, a large hypoxic zone forms in the northern Gulf of Mexico. Research on the causes and consequences of hypoxia requires reliable estimates of hypoxic extent, which can vary at submonthly time scales due to hydro-meteorological variability. Here, we use an innovative space-time geostatistical model and data collected by multiple research organizations to estimate bottom-water dissolved oxygen (BWDO) concentrations and hypoxic area across summers from 1985 to 2016. We find that 27% of variability in BWDO is explained by deterministic trends with location, depth, and date, while correlated stochasticity accounts for 62% of observational variance within a range of 185 km and 28 days. Space-time modeling reduces uncertainty in estimated hypoxic area by 30% when compared to a spatial-only model, and results provide new insights into the temporal variability of hypoxia. For years with shelf-wide cruises in multiple months, hypoxia is most severe in July in 59% of years, 29% in August, and 12% in June. Also, midsummer cruise estimates of hypoxic area are only modestly correlated with summer-wide (June-August) average estimates (r 2 = 0.5), suggesting midsummer cruises are not necessarily reflective of seasonal hypoxic severity. Furthermore, summer-wide estimates are more strongly correlated with nutrient loading than midsummer estimates.

Both fisheries exploitation and increased nutrient loadings strongly affect fish and shellfish abundance and production in estuaries. These stressors do not act independently; instead, they jointly influence food webs, and each affects the sensitivity of species and ecosystems to the other. Nutrient enrichment and the habitat degradation it sometimes causes can affect sustainable yields of fisheries, and fisheries exploitation can affect the ability of estuarine systems to process nutrients. The total biomass of fisheries landings in estuaries and semi-enclosed seas tends to increase with nitrogen loadings in spite of hypoxia, but hypoxia and other negative effects of nutrient over-enrichment cause declines in individual species and in parts of systems most severely affected. More thoroughly integrated management of nutrients and fisheries will permit more effective management responses to systems affected by both stressors, including the application of fisheries regulations to rebuild stocks negatively affected by eutrophication. Reducing fishing mortality may lead to the recovery of depressed populations even when eutrophication contributes to population declines if actions are taken while the population retains sufficient reproductive potential. New advances in modeling, statistics, and technology promise to provide the information needed to improve the understanding and management of systems subject to both nutrient enrichment and fisheries exploitation.Electronic supplementary material The online version of this article (

We used fishery-independent hydrographic and bottom trawl surveys on the northwestern Gulf of Mexico shelf from 1983–2000 to test for density dependence and effects of hypoxia (dissolved oxygen ≤ 2.0 mg·L–1) on the spatial distribution of brown shrimp (Farfantepenaeus aztecus). Spatial distribution of shrimp was positively related to abundance on the Texas shelf but negatively related to abundance on the Louisiana shelf. Density dependence was weak, however, and may have been due to factors other than density-dependent habitat selection. Males were distributed over a broader area and further offshore than were females, though differences in spatial distribution between sexes were not large (~10%–15%). Large-scale hypoxia (up to ~20 000 km2) on the Louisiana shelf occurs in regions of typically high shrimp density and results in substantial habitat loss (up to ~25% of the Louisiana shelf), with shifts in distribution and associated high densities both inshore and offshore of the hypoxic region. We discuss these results in terms of the generality of density-dependent spatial distributions in marine populations and potential consequences of habitat loss and associated shifts in distribution due to low dissolved oxygen.