Occurrence of demersal fishes in relation to near‐bottom oxygen levels within the California Current large marine ecosystem

Various ocean‐climate models driven by increased greenhouse gases and higher temperatures predict a decline in oceanic dissolved oxygen (DO) as a result of greater stratification, reduced ventilation below the thermocline, and decreased solubility at higher temperatures. Since spreading of low oxygen waters is underway and predicted to increase, understanding impacts on higher trophic levels is essential. Within the California Current System, shoaling of the oxygen minimum zone (OMZ) is expected to produce complex changes. Onshore movement of the OMZ could lead to habitat compression for species with higher oxygen requirements while allowing expansion of species tolerant of low bottom DO. As part of annual groundfish surveys, we sampled catch across a range of conditions from the upper to the lower limit of the OMZ and shoreward across the continental shelf of the US west coast. DO ranged from 0.02 to 4.25 mL L^?1 with 642 stations (of 1020 sampled) experiencing hypoxic conditions in 2008–2010. Catch and species richness exhibited significant and positive relationships with near‐bottom oxygen concentration. The probability of occurrence was estimated for four species (spotted ratfish, petrale sole, greenstriped rockfish and Dover sole) using a binomial Generalized Additive Model. The models for each species included terms for position, day of the year, salinity, near‐bottom temperature and the interaction term between depth and near‐bottom DO. Spotted ratfish and petrale sole were sensitive to changes in near‐bottom oxygen, while greenstriped rockfish and Dover sole show no changes in probability of occurrence in relation to changes in oxygen concentration.     

Ichthyoplankton distribution and abundance in relation to nearshore dissolved oxygen levels and other environmental variables within the Northern California Current System

Nearshore hypoxia along the coast of Oregon and Washington is a seasonal phenomenon that has generated concern among scientists studying this temperate upwelling ecosystem. These waters are affected by coastal upwelling‐induced hypoxia mainly during late summer and fall, but the effects of low oxygen levels on fish and invertebrate communities, particularly during the early‐life history stages, are poorly known. We investigated the effects of hypoxia and other variables on the species composition, density, vertical and horizontal distribution of fish larvae along the Oregon and Washington coasts during the summers of 2008, 2009 and 2010. Bottom‐dissolved oxygen (DO) values ranged from 0.49 to 4.79 mL L^?1, but the overall water column DO values were only moderately hypoxic during the 3 yr of sampling compared with recent extreme years. In this study, DO was found to be an environmental parameter affecting the species composition, but other variables such as season, year and depth of capture were also important. Although the overall density of fish larvae increased with increasing bottom‐DO values, the effect on individual species density was limited. Slender sole (Lyopsetta exilis) and sand sole (Psettichthys melanostictus) were the only species to have a weak trend of density with DO, but both showed negative relationships and neither relationship was significant. Our results indicate that larval fish spatial distribution was only moderately affected within the range of observed oxygen values, but low DO may be an important factor under intense hypoxic conditions.     

Effect of Sublethal Hypoxia on the Immune Response and Susceptibility of Channel Catfish, Ictalurus punctatus, to Enteric Septicemia

The effect of sublethal hypoxia exposure on stress and immune responses and susceptibility to Edwardsiella ictaluri infection in juvenile channel catfish, Ictalurus punctatus, was investigated. Fish were monitored for temporal changes in glucose and cortisol concentrations before, during, and after 2 h exposure to sublethal hypoxia (<2 mg/L dissolved oxygen [DO]) and when maintained under normoxic conditions (6.0 ± 0.3 mg/L DO). Both blood glucose and plasma cortisol increased significantly in response to hypoxic conditions. Fish exposed to hypoxic or normoxic conditions were challenged with a high dose (1.3 × 10⁷ colony‐forming units [CFU]/mL) or a low dose (1.3 × 10⁵ CFU/mL) of E. ictaluri or sterile culture broth by 30‐min immersion bath. Approximately 1% of fish in both the normoxic and the hypoxic groups died when challenged with the low dose of E. ictaluri. However, when challenged with the high dose of E. ictaluri, catfish exposed to hypoxic conditions had significantly higher cumulative mortality (36 ± 12.1%) than those maintained under normoxic conditions (12 ± 1.1%). Total hemolytic complement and bactericidal activities and antibody response were lower in hypoxia‐exposed channel catfish, indicating that increased susceptibility of channel catfish to E. ictaluri may be the result of the immunosuppressive effects of the stress response to hypoxia.     

Distribution patterns of Pacific halibut (Hippoglossus stenolepis) in relation to environmental variables along the continental shelf waters of the US West Coast and southern British Columbia

Knowing how Pacific halibut (Hippoglossus stenolepis) distribute in relation to ocean conditions is of primary importance to halibut managers, as they are tasked with estimating stock size and designing effective monitoring programs amidst a changing climate. This research examined near‐bottom environmental data alongside halibut survey catch data for the years 2006–2009 on the continental shelf of Oregon, Washington, and southern British Columbia. The objectives of the research were to: (1) characterize summer environmental conditions and halibut distribution; (2) explore ranges and possible tolerance thresholds for halibut in relation to temperature, dissolved oxygen (DO), salinity, and pH; and (3) identify the primary environmental factors affecting distribution of halibut and model the observed relationships. Seasonal hypoxia is an annual feature of the study area and results suggest halibut exhibited an apparent DO minimum threshold of 0.9 mL L^?1. Ordinary least squares multiple regression analysis indicated that depth, temperature, and DO were significant variables in predicting halibut distribution, whereas salinity and bottom type were not. Ambiguity in model results led to the use of two additional analytical methods, geographically weighted regression (GWR) and tree regression, to examine regional variation and the overarching structure of halibut distribution. The three models yielded similar results indicating the importance of DO and temperature as variables describing structure. The GWR model yielded the best fit of the three when using DO as a predictor variable, indicating that regional variation is a factor. These results suggest that low, but above‐threshold, DO may be contributing to catchability differences in the survey.     

渤海中部海域低氧区的发生记录

2014年8月对渤海中部海域的水文(温度、盐度)、化学(溶解氧DO、营养盐和化学耗氧量COD)和生物要素(叶绿素a Chl-a)的空间分布进行了调查。研究了该海域底层水体低氧(DO<3.0 mg/L)的分布特征,深入分析了低氧区发生的关键因素。结果显示,底层水体DO浓度最小值为2.30 mg/L,低氧面积达1200 km2,呈西北–东南走向。调查海域的西部和西南海域呈现出明显的温度层化,尤其在低氧区附近形成了一个表层与底层水体温度差(δT)>5℃的区域,δT 最高值达到7.3℃。水体密度层化与温度层化特征相似,在低氧区附近形成了一个底表层密度差(δρ)>2 g/L的等值线闭合圈。温度层化是低氧产生的主要物理因素。表层水体 COD 高值区主要分布在调查海域的西部,覆盖大部分的低氧海域。表层水体中的Chl-a(>4μg/L)和PO43--P (>6μg/L)浓度高值区主要分布在调查海域的西南部,部分与低氧区重合。本研究可为探索渤海海域富营养化演变过程提供借鉴。     

Amazon River water in the northeastern Caribbean Sea and its effect on larval reef fish assemblages during April 2009

During April to June 2009, a large bolus of Amazon River water impacted the northeastern Caribbean Sea. Shipboard observations collected near Saba Bank, the U.S. and British Virgin Islands, and the Anegada Passage showed low surface salinity (35.76 ± 0.05 Practical Salinity Unit (PSU)), elevated surface temperature (26.77 ± 0.14°C), high chlorophyll‐a (1.26 ± 0.21 mg m^?3) and high dissolved oxygen (4.90 ± 0.06 mL L^?1) in a 20‐ to 30‐m thick surface layer in the riverine plume. The water was ~1°C warmer, 1 PSU fresher, 0.3 mL L^?1 higher in oxygen and 1.2 mg m^?3 higher in chlorophyll‐a than Atlantic Ocean waters to the north, with Caribbean surface waters showing intermediate values. Plankton net tows obtained in the upper 100 m of the water column revealed larval fish assemblages within the plume that were significantly different from those of the surrounding waters and from those encountered in the area in previous years. The plume waters contained higher concentrations of mesopelagic fish larvae from the families Myctophidae and Nomeidae, which as adults typically inhabit offshore, deep water habitats. Concentrations of larvae from inshore and reef‐associated families such as Scaridae, Serranidae, Labridae and Clupeidae were lower than those found outside the plume in similar shallow areas, particularly in near‐surface waters. An event like the one observed in 2009 had not been documented in at least the past 30 yr, and yet it was followed by another similarly extreme event in 2010. The ecological implications, including any long‐term consequences of such recent extreme events, are important and merit further study.     

Effects of hypoxia on lysozyme activity and antioxidant defences in the kidney and spleen of Carassius auratus

Assessing the responses of lysozyme activity and antioxidant defences to hypoxia is important to understand the adaptation and tolerance strategies of fish under hypoxia. This study investigated the effects of different dissolved oxygen (DO) levels on Carassius auratus, a natural triploid fish, from Qihe River. The content of malondialdehyde (MDA) and the activities of lysozyme and antioxidant enzymes were measured in the kidney and spleen after hypoxic exposure. At the DO concentrations of 1 and 2 mg L^?1, the activities of antioxidant enzymes and lysozyme significantly decreased and the content of MDA significantly increased (P < 0.01). This result suggests that hypoxia decreased antioxidant enzyme and lysozyme activities and caused MDA accumulation in a concentration‐dependent manner. The DO level of 4 mg L^?1 increased the activities of superoxide dismutase and catalase in the kidney and the activities of glutathione peroxidase and catalase in the spleen (P < 0.05). This result implies that slight hypoxic stress can enhance antioxidant defences to alleviate the damage of oxidative stress. The reduced activities of antioxidant enzymes and the accumulation of MDA at the DO level of ≤2 mg L^?1 implied the decrease in antioxidative ability and the occurrence of oxidative stress. The decrement in lysozyme activity indicated that the antibacterial ability was weakened to some degree. Therefore, hypoxic stress at DO levels ≤2 mg L^?1 should be removed by aeration to avoid the oxidative damage resulting from the reduced antioxidative ability and prevent the outbreak of diseases caused by weakened antibacterial effects.     

Spatial distribution of ocean habitat of yearling Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon off Washington and Oregon,USA

We determined the habitat usage and habitat connectivity of juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon in continental shelf waters off Washington and Oregon, based on samples collected every June for 9 yr (1998–2006). Habitat usage and connectivity were evaluated using SeaWiFS satellite‐derived chlorophyll a data and water depth. Logistic regression models were developed for both species, and habitats were first classified using a threshold value estimated from a receiver operating characteristic curve. A Bernoulli random process using catch probabilities from observed data, i.e. the frequency of occurrence of a fish divided by the number of times a station was surveyed, was applied to reclassify stations. Zero‐catch probabilities of yearling Chinook and yearling coho salmon decreased with increases in chlorophyll a concentration, and with decreases in water depth. From 1998 to 2006, ～ 47% of stations surveyed were classified as unfavorable habitat for yearling Chinook salmon and ～ 53% for yearling coho salmon. Potentially favorable habitat varied among years and ranged from 9 856 to 15 120 km² (Chinook) and from 14 800 to 16 736 km² (coho). For both species, the smallest habitat area occurred in 1998, an El Niño year. Favorable habitats for yearling Chinook salmon were more isolated in 1998 and 2005 than in other years. Both species had larger and more continuous favorable habitat areas along the Washington coast than along the Oregon coast. The favorable habitats were also larger and more continuous nearshore than offshore for both species. Further investigations on large‐scale transport, mesoscale physical features, and prey and predator availability in the study area are necessary to explain the spatial arrangement of juvenile salmon habitats in continental shelf waters.     

Ocean scale hypoxia‐based habitat compression of Atlantic istiophorid billfishes

Oxygen minimum zones (OMZs) below near‐surface optimums in the eastern tropical seas are among the largest contiguous areas of naturally occurring hypoxia in the world oceans, and are predicted to expand and shoal with global warming. In the eastern tropical Pacific (ETP), the surface mixed layer is defined by a shallow thermocline above a barrier of cold hypoxic water, where dissolved oxygen levels are ≤3.5 mL L^?1. This thermocline (～25–50 m) constitutes a lower hypoxic habitat boundary for high oxygen demand tropical pelagic billfish and tunas (i.e., habitat compression). To evaluate similar oceanographic conditions found in the eastern tropical Atlantic (ETA), we compared vertical habitat use of 32 sailfish (Istiophorus platypterus) and 47 blue marlin (Makaira nigricans) monitored with pop‐up satellite archival tags in the ETA and western North Atlantic (WNA). Both species spent significantly greater proportions of their time in near‐surface waters when inside the ETA than when in the WNA. We contend that the near‐surface density of billfish and tunas increases as a consequence of the ETA OMZ, therefore increasing their vulnerability to overexploitation by surface gears. Because the ETA OMZ encompasses nearly all Atlantic equatorial waters, the potential impacts of overexploitation are a concern. Considering the obvious differences in catchability inside and outside the compression zones, it seems essential to standardize these catch rates separately to minimize inaccuracies in stock assessments for these species. This is especially true in light of global warming, which will likely exacerbate future compression impacts.     

Effect of dissolved oxygen levels on growth performance,energy budget and antioxidant responses of yellow catfish,Pelteobagrus fulvidraco (Richardson)

This study was conducted to determine the effect of dissolved oxygen (DO) levels on growth performance, energy budget and antioxidant responses of yellow catfish Pelteobagrus fulvidraco. Yellow catfish was exposed to four levels of DO, consisting of hypoxia (2.28 mg L^?1), moderate hypoxia (4.04 mg L^?1), saturation (6.51 mg L^?1) and super‐saturation groups (9.11 mg L^?1), respectively, for 8 weeks. Specific growth rate and feed efficiency in dry matter were lowest in hypoxia and highest in the saturation and super‐saturation groups. Apparent digestibility coefficients of dry matter and energy increased with increasing DO levels. Gross energy and growth energy were lowest for hypoxia, followed by moderate hypoxia and the highest for other two groups. Faecal energy was highest in hypoxia and lowest in saturation and super‐saturation group. DO levels also significantly influenced activities of antioxidant enzymes and malondialdehyde level in liver and serum. Based on the observation described previously, saturated DO level helps to improve growth performance, feed utilization and antioxidant responses in yellow catfish. Super‐saturation did not increase fish performance. To our knowledge, this is the first study involved in the effect of DO levels on energy budget of fish and provides new insight into aeration regime for yellow catfish culture.     

Broad‐scale movements and pelagic habitat of the dusky shark Carcharhinus obscurus off Southern Australia determined using pop‐up satellite archival tags

We investigated the broad‐scale movements and pelagic habitats of large juvenile dusky sharks (Carcharhinus obscurus) off Southern Australia. Pop‐up satellite archival tags (PSATs) were deployed on three large juvenile dusky sharks (～2.2–2.6 m total length) for 6 months in Spencer Gulf during 2010. Tagged dusky sharks all migrated westward and across the Great Australian Bight (GAB) during autumn to offshore shelf waters off Western Australia. Estimated minimum distances travelled ranged from 1760 to 2736 km. Depths occupied by tagged dusky sharks ranged from the surface to 355 m. The most common thermal ranges occupied were 19–22°C. Broad‐scale movements of large juvenile dusky sharks across the continental shelves combined with periods of residency in semi‐protected gulf waters indicated that a multi‐jurisdictional management approach may be appropriate for this species.     

Living on the continental shelf edge: habitat use of juvenile shortfin makos Isurus oxyrinchus in the Great Australian Bight,southern Australia

We used satellite telemetry data to investigate the movement patterns and habitat use of juvenile shortfin makos Isurus oxyrinchus (Lamnidae) tagged in the Great Australian Bight, southern Australia. Tracking durations ranged from 49–672 days and six deployments were > 1 year. During winter and spring, some shortfin makos migrated to the tropical NE Indian Ocean and Coral Sea, and the Subtropical Front region. One shortfin mako undertook an extended migration of 25 550 km across the Indian Ocean. Areas characterized by sea‐mounts in the NE Indian Ocean, the oceanic Subtropical Front region, and the continental shelf edge (200‐m depth) and slope canyons were visited by several sharks. Juvenile shortfin makos used the outer continental shelf, the shelf edge, the slope and oceanic waters during migrations and mostly exhibited fidelity in the mid‐outer shelf, the shelf edge and slope habitats characterized by high bathymetric relief and oceanographic frontal gradients. Our findings highlighted that the continental shelf and slope and associated submarine canyons of the Great Australian Bight represent ecologically important habitats for juvenile shortfin makos. The findings of this study will be pertinent during future management processes for this highly migratory species in this Southern Hemisphere region.     

Spatial distribution of Greenland halibut Reinhardtius hippoglossoides in relation to abundance and hypoxia in the estuary and Gulf of St. Lawrence

Annual bottom‐trawl surveys (1990–2010) were used to examine associations between environmental conditions, spatial distribution, and size‐specific abundance of Greenland halibut Reinhardtius hippoglossoides in the estuary and Gulf of St. Lawrence (EGSL), and to test the influence of hypoxic conditions on habitat selection. Size classes representing juvenile, immature and adult fish were used for the analyses. The highest concentrations of fish were found in the St. Lawrence estuary at both high and low levels of stock abundance. The areas occupied by 50, 75, and 95% of juvenile fish expanded with higher population abundance. However, contrary to our predictions, densities in marginal habitats did not increase at a higher rate than in optimal habitats. Fish longer than 32 cm were distributed over a broader area than juvenile fish. Their abundance explained a limited proportion of the variability in spatial distribution. The spatial dynamics of Greenland halibut in the EGSL is best described by a proportional density model where the rate of increase in local density is associated with population abundance. Habitats selected by Greenland halibut were characterized by low dissolved oxygen (DO) levels. The strong association between high fish densities and low DO concentrations indicates a high tolerance of Greenland halibut to hypoxia. It also indicates that negative effects, if present, could be compensated by other factors such as food availability and/or refuge from predation. The results of this study also clearly define the St. Lawrence estuary as the major nursery area for the EGSL population.     

Advection of anchovy (Engraulis encrasicolus) larvae along the Catalan continental slope (NW Mediterranean)

The Gulf of Lions is one of the main anchovy (Engraulis encrasicolus) spawning areas in the NW Mediterranean. During the spring, low‐salinity surface water from the outflow of the Rhône is advected by the shelf‐slope current along the continental slope off the Catalan coast. In June 2000, a Lagrangian experiment tracking these low‐salinity surface waters was conducted to assess the importance of this transport mechanism for anchovy larvae and to determine the suitability of the tracked surface waters for survival of anchovy larvae. The experiment consisted of sampling the tracked water parcel for 10 days with three drifters launched at the core of the shelf‐slope current where low‐salinity surface waters were detected. The survey was completed by sampling the surrounding waters. Anchovy larvae from the spawning area in the Gulf of Lions were advected towards the south in the low‐salinity waters. The size increase of anchovy larvae throughout the Lagrangian tracking closely followed the general growth rate calculated by otolith analysis (0.65 mm day⁻¹). However, advection by the current was not the only mechanism of anchovy larval transport. A series of anticyclonic eddies, originated in the Gulf of Lions and advected southwards, seemed to play a complementary role in the transport of larvae from the spawning ground towards the nursery areas. These eddies not only contributed to larval transport but also prevented their dispersion. These transport and aggregation mechanisms may be important for anchovy populations along the Catalan coast and require further study.     

Ocean distribution and habitat associations of yearling coho (Oncorhynchus kisutch) and Chinook (O. tshawytscha) salmon in the northern California Current

Yearling juvenile coho and Chinook salmon were sampled on 28 cruises in June and September 1981–85 and 1998–07 in continental shelf and oceanic waters off the Pacific Northwest. Oceanographic variables measured included temperature, salinity, water depth, and chlorophyll concentration (all cruises) and copepod biomass during the cruises from 1998–07. Juvenile salmonids were found almost exclusively in continental shelf waters, and showed a patchy distribution: half were collected in ～5% of the collections and none were collected in ～40% of the collections. Variance‐to‐mean ratios of the catches were high, also indicating patchy spatial distributions for both species. The salmon were most abundant in the vicinity of the Columbia River and the Washington coast in June; by September, both were less abundant, although still found mainly off Washington. In June, the geographic center‐of‐mass of the distribution for each species was located off Grays Harbor, WA, near the northern end of our sampling grid, but in September, it shifted southward and inshore. Coho salmon ranged further offshore than Chinook salmon: in June, the average median depth where they were caught was 85.6 and 55.0 m, respectively, and in September it was 65.5 and 43.7 m, respectively. Abundances of both species were significantly correlated with water depth (negatively), chlorophyll (positively) and copepod biomass (positively). Abundances of yearling Chinook salmon, but not of yearling coho salmon, were correlated with temperature (negatively). We discuss the potential role of coastal upwelling, submarine canyons and krill in determining the spatial distributions of the salmon.     

Movement,depth distribution and survival of spinetail devilrays (Mobula japanica) tagged and released from purse‐seine catches in New Zealand

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Physiological and behavioural responses to hypoxia in an invasive freshwater fish species and a native competitor

The spread of invasive species is one of the major environmental concerns which can have negative effects on biodiversity. While several life history traits have been identified as being important for increasing the invasiveness of introduced species, the physiological factors that allow certain species to become successful invaders remain poorly understood. It has been speculated that good invaders are thriving in disturbed environments. In unfavourable conditions, as during hypoxic events, invasive species might be better adapted in their physiological and behavioural responses towards this stressor. We compared physiological and behavioural traits between two freshwater fish species: the European bullhead (Cottus gobio), an invasive fish species in Scotland, and its native competitor the stone loach (Barbatula barbatula) over different dissolved oxygen concentrations (DO). Contrary to expectations, bullhead displayed a reduced hypoxia tolerance as compared to stone loach, indicated by a higher threshold (P_crit) for the maintenance of standard metabolism. Avoidance behaviour during progressive hypoxia was similar between bullhead and stone loach. When given a choice between an open normoxic zone and a shelter located in hypoxia, both species spent most of their time hiding under the shelter in hypoxic conditions (bullhead: 100%; stone loach: 93.93%–99.73%), although stone loach showed brief excursions into normoxic conditions under 25% DO level. These results suggest that stone loach might be more resistant to hypoxia as compared to bullhead, and thus that increased hypoxia tolerance is likely not a trait by which bullhead have been able to expand their range within the United Kingdom.     

大鹏澳养殖水域溶解氧的变化及其与生态结构的关系

根据2001年6月至2002年6月大鹏澳网箱养殖水域周年的溶解氧(DO)水平监测数据,阐述网箱养殖区水体DO的水平、季节的变化、垂直变化和网箱养殖水体DO质量评价以及DO与水环境因子的相关性。结果表明,大鹏湾网箱养殖水域DO周年水平为3 86～8 01mg/L,氧饱和度为51 6%～103 6%,养殖水域DO水平低于非养殖水域的;冬、春季DO水平较高,均符合我国渔业水质标准要求。夏季DO水平普遍较低,超标率为95%;表层水DO水平略高于底层水,DO水平超标率分别为46%和66 7%;DO水平(全年)与pH值、盐度、叶绿素水平均呈显著性正相关(P<0 02,n=32),与水温、无机磷和无机氮水平呈显著性负相关(P<0 02,n=32)。其中,在春季相关最为明显。