Increase In Swimming Speed Research Articles

The low O2 avoidance strategy of the Cape silverside Atherina breviceps (Teleostei)

Aquatic low O2 (hypoxia) is not uncommon but mobile organisms do not have to suffer low O2 stress if they engage in a safe avoidance strategy by moving away early and quickly. Within a behavioural choice chamber system at 20°C, the Cape silverside (Atherina breviceps), a common species from South Africa, was found to seek out and select high O2 saturated water when water O2 saturations declined to 23% saturation. Atherina breviceps also avoided low O2 with a modest increase in swimming speed but no correlation between hypoxic swimming speed and avoidance could be found implying that heightened swimming speed does not necessarily lead to faster avoidance on an individual basis. The overall low O2 response of A. breviceps was seen to reflect an effective avoidance strategy that is compatible with the survival of this species facing low O2 challenge in the wild.

The hydrodynamic function of shark skin and two biomimetic applications

It has long been suspected that the denticles on shark skin reduce hydrodynamic drag during locomotion, and a number of man-made materials have been produced that purport to use shark-skin-like surface roughness to reduce drag during swimming. But no studies to date have tested these claims of drag reduction under dynamic and controlled conditions in which the swimming speed and hydrodynamics of shark skin and skin-like materials can be quantitatively compared with those of controls lacking surface ornamentation or with surfaces in different orientations. We use a flapping foil robotic device that allows accurate determination of the self-propelled swimming (SPS) speed of both rigid and flexible membrane-like foils made of shark skin and two biomimetic models of shark skin to measure locomotor performance. We studied the SPS speed of real shark skin, a silicone riblet material with evenly spaced ridges and a Speedo® 'shark skin-like' swimsuit fabric attached to rigid flat-plate foils and when made into flexible membrane-like foils. We found no consistent increase in swimming speed with Speedo® fabric, a 7.2% increase with riblet material, whereas shark skin membranes (but not rigid shark skin plates) showed a mean 12.3% increase in swimming speed compared with the same skin foils after removing the denticles. Deformation of the shark skin membrane is thus crucial to the drag-reducing effect of surface denticles. Digital particle image velocimetry (DPIV) of the flow field surrounding moving shark skin foils shows that skin denticles promote enhanced leading-edge suction, which might have contributed to the observed increase in swimming speed. Shark skin denticles might thus enhance thrust, as well as reduce drag.

Rajiform locomotion: three-dimensional kinematics of the pectoral fin surface during swimming by freshwater stingray Potamotrygon orbignyi

Rajiform locomotion in fishes is dominated by distinctive undulations of expanded pectoral fins. Unlike other fishes, which typically interact with the fluid environment via multiple fins, undulating rays modulate a single control surface, the pectoral disc, to perform pelagic locomotion, maneuvering and other behaviors. Complex deformations of the broad, flexible pectoral fins occur as the undulating wave varies in three dimensions; pectoral fin kinematics and changes in waveform with swimming speed cannot be fully quantified by two-dimensional analyses of the fin margin. We present the first three-dimensional analysis of undulatory rajiform locomotion in a batoid, the freshwater stingray Potamotrygon orbignyi. Using three cameras (250 frames s(-1)), we gathered three-dimensional excursion data from 31 points on the pectoral fin during swimming at 1.5 and 2.5 disc lengths s(-1), describing the propulsive wave and contrasting waveforms between swimming speeds. Only a relatively small region of the pectoral fin (~25%) undulates with significant amplitude (>0.5 cm). Stingrays can maintain extreme lateral curvature of the distal fin margin in opposition to induced hydrodynamic loads, 'cupping' the edge of the pectoral fin into the flow, with potential implications for drag reduction. Wave amplitude increases across both anteroposterior and mediolateral fin axes. Along the anteroposterior axis, amplitude increases until the wave reaches mid-disc and then remains constant, in contrast to angulliform patterns of continuous amplitude increase. Increases in swimming speed are driven by both wave frequency and wavespeed, though multivariate analyses reveal a secondary role for amplitude.

Levels of Predation Modulate Antipredator Defense Behavior and Metamorphic Traits in the Toad Bufo melanostictus

The influence of the presence of nonlethal predators on antipredator defense behavior and metamorphic traits was studied in the toad Bufo melanostictus. Tadpoles reared with caged predaceous tadpoles of Hoplobatrachus tigerinus delayed metamorphosis and also emerged at a smaller size compared to those in a predator-free environment. Different predaceous cues (kairomones, alarm cues and dietary cues, derived after feeding the predator with conspecific prey) induced increase in stationary phase and swimming speed and decrease in swimming spurts and time used for feeding. The intensity of defense behaviors varied with the cue. They were more intense in response to cues of predators fed on conspecific prey item. The response to alarm cues or kairomones or predators fed on heterogeneric tadpole species was similar but lower in magnitude compared to that elicited in response to predators fed on conspecific prey item. Most intense defense behavior in response to the odors of the predator that consumed conspecific members is possibly caused by a combined effect of kairomones, alarm, and dietary cues. The findings suggest that B. melanostictus tadpoles are capable of assessing levels of predation risk and modulating the intensity of their defense behavior in accordance with the perceived threat. Interestingly, despite longer larval period, size at metamorphosis of the toadlets was smaller in the presence of caged predator.

The effect of statins on performance in the Morris water maze in guinea pig

Statins play a crucial role in reducing the risk of death from cardiovascular disease in millions of people worldwide. Recently, pharmacovigilance data has suggested that statin drugs may have rare but significant adverse psychiatric effects, such as amnesia, anxiety and even aggression. In order to investigate the effects of statins on cognitive function in an animal model, we studied the effect of 6weeks of daily administration of oral simvastatin (1mg/kg) or atorvastatin (0.5mg/kg) in guinea pig on performance in the Morris water maze (MWM). Animals were also re-tested in the MWM, 2weeks after drug cessation, to test for any changes in performance as a result of drug de-challenge. Guinea pigs treated with either statin showed a significant (P<0.001) decrease in total cholesterol and low density lipoprotein-cholesterol (LDL-C), which remained partially reduced after the 2week drug washout period. Guinea pigs receiving either statin did not show any difference in latency to reach the platform, nor any difference in total distance travelled during testing. Also, analysis of probe trials revealed no significant differences between drug and vehicle groups. However, both groups spent a significantly (P<0.01) greater proportion of time in the outer zone of the maze (indication of increased anxiety) and showed an increase in swimming speed (P<0.05) compared with the vehicle group. Differences between groups for swimming speed, and time spent in the outer zone, were not retained in the drug de-challenge phase. Our results show that low dose treatment with statins can induce mild but significant anxiety in guinea pigs.

Changes in wingstroke kinematics associated with a change in swimming speed in a pteropod mollusk,Clione limacina

In pteropod mollusks, the gastropod foot has evolved into two broad, wing-like structures that are rhythmically waved through the water for propulsion. The flexibility of the wings lends a tremendous range of motion, an advantage that could be exploited when changing locomotory speed. Here, we investigated the kinematic changes that take place during an increase in swimming speed in the pteropod mollusk Clione limacina. Clione demonstrates two distinct swim speeds: a nearly constant slow swimming behavior and a fast swimming behavior used for escape and hunting. The neural control of Clione's swimming is well documented, as are the neuromuscular changes that bring about Clione's fast swimming. This study examined the kinematics of this swimming behavior at the two speeds. High speed filming was used to obtain 3D data from individuals during both slow and fast swimming. Clione's swimming operates at a low Reynolds number, typically under 200. Within a given swimming speed, we found that wing kinematics are highly consistent from wingbeat to wingbeat, but differ between speeds. The transition to fast swimming sees a significant increase in wing velocity and angle of attack, and range of motion increases as the wings bend more during fast swimming. Clione likely uses a combination of drag-based and unsteady mechanisms for force production at both speeds. The neuromuscular control of Clione's speed change points to a two-gaited swimming behavior, and we consider the kinematic evidence for Clione's swim speeds being discrete gaits.

Sperm competition differentially affects swimming velocity and size of spermatozoa from closely related muroid rodents: head first

Sperm competition favours an increase in sperm swimming velocity that maximises the chances that sperm will reach the ova before rival sperm and fertilise. Comparative studies have shown that the increase in sperm swimming speed is associated with an increase in total sperm size. However, it is not known which are the first evolutionary steps that lead to increases in sperm swimming velocity. Using a group of closely related muroid rodents that differ in levels of sperm competition, we here test the hypothesis that subtle changes in sperm design may represent early evolutionary changes that could make sperm swim faster. Our findings show that as sperm competition increases so does sperm swimming speed. Sperm swimming velocity is associated with the size of all sperm components. However, levels of sperm competition are only related to an increase in sperm head area. Such increase is a consequence of an increase in the length of the sperm head, and also of the presence of an apical hook in some of the species studied. These findings suggest that the presence of a hook may modify the sperm head in such a way that would help sperm swim faster and may also be advantageous if sperm with larger heads are better able to attach to the epithelial cells lining the lower isthmus of the oviduct where sperm remain quiescent before the final race to reach the site of fertilisation.

Sucralose – An ecotoxicological challenger?

The non-calorie sweetener sucralose – sucrose containing three chlorine atoms – is intensively sweet and has become a popular substitute for sugar. Its widespread use, exceptional stability in combination with high water solubility have thus resulted in contamination of recipient waters. Earlier studies on sucralose in aquatic organisms indicate low bioaccumulation potential and negligible acute/chronic toxicity, but the close structural resemblance with sucrose in combination with the importance of sugar in nature, warrant a more detailed ecotoxicological assessment. The aim of this investigation was therefore to study behavioural and physiological effects of sucralose in crustaceans. Our results show that both physiology and locomotion behaviour were affected by exposure to sucralose. In Daphnia magna, the behavioural response was manifested as altered swimming height and increased swimming speed, whereas in gammarids the time to reach food and shelter was prolonged. Regardless if these behavioural responses were initiated via traditional toxic mechanisms or stimulatory effects, they should be considered as a warning, since exposed organisms may diverge from normal behaviour, which ultimately can have ecological consequences.

Abnormal swimming behavior and increased deformities in rainbow trout Oncorhynchus mykiss cultured in low exchange water recirculating aquaculture systems

Two studies were conducted to evaluate rainbow trout Oncorhynchus mykiss health and welfare within replicated water recirculating aquaculture systems (WRAS) that were operated at low and near-zero water exchange, with and without ozonation, and with relatively high feed loading rates. During the first study, rainbow trout cultured within WRAS operated with low water exchange (system hydraulic retention time (HRT)=6.7 days; feed loading rate=4.1kg feed/m3 daily makeup flow) exhibited increased swimming speeds as well as a greater incidence of “side swimming” behavior as compared to trout cultured in high exchange WRAS (HRT=0.67 days; feed loading rate=0.41kg feed/m3 daily makeup flow). During the second study, when the WRAS were operated at near-zero water exchange, an increased percentage of rainbow trout deformities, as well as increased mortality and a variety of unusual swimming behaviors were observed within WRAS with the highest feed loading rates and least water exchange (HRT≥103 days; feed loading rate≥71kg feed/m3 daily makeup flow). A wide range of water quality variables were measured. Although the causative agent could not be conclusively identified, several water quality parameters, including nitrate nitrogen and dissolved potassium, were identified as being potentially associated with the observed fish health problems.

Scopolamine impairs behavioural function and arginine metabolism in the rat dentate gyrus

Alzheimer’s disease (AD) is a neurodegenerative disorder with progressive memory loss. It has been shown that the cholinergic neurotransmission deficit is one of the neurochemical characteristics of AD, and that l-arginine and its metabolites also play a prominent role in AD pathogenesis. Scopolamine, a non-selective muscarinic receptor antagonist, blocks cholinergic neurotransmission and impairs behavioural function, including learning and memory. This study investigated the effects of scopolamine on animals’ behavioural performance and l-arginine metabolism in the hippocampus and prefrontal cortex. Rats were given intraperitoneal injections of scopolamine (0.8 mg/kg) or saline (1 ml/kg) and tested in the Y-maze, open field, water maze and elevated plus maze 30 min post-treatment. After completion of the behavioural testing, the CA1, CA2/3 and dentate gyrus (DG) sub-regions of the hippocampus and the prefrontal cortex were harvested to measure the activity and protein expression of nitric oxide synthase (NOS) and arginase, and the levels of l-arginine, l-citrulline, l-ornithine, agmatine, putrescine, spermidine, spermine, glutamate and GABA. Scopolamine treated rats displayed reduced alternation and exploratory behaviour, increased swimming speed and impaired spatial learning and memory. There were significantly decreased NOS activity, increased arginase activity, and increased l-ornithine and putrescine levels in the DG, but not other regions examined, in the scopolamine treated rats as compared to the controls. These findings suggest that scopolamine impairs behavioural function and alters l-arginine metabolism in the DG sub-region of the hippocampus specifically. The underlying mechanisms of it remain to be explored further.

Metachronal swimming in Antarctic krill: gait kinematics and system design

Metachronal swimming, in which adjacent appendages stroke in sequence, is widespread among crustaceans inhabiting the transitional flow realm in which both viscosity and inertia effects are important. However, the design and operation of this propulsion system in response to various hydrodynamic, energetic, and behavioral needs have not been well investigated. We examine free-swimming Antarctic krill (Euphausia superba) as a model species and identify three distinct behavioral swimming gaits. The pleopod kinematics of these gaits, hovering, fast-forward swimming, and upside-down swimming, are quantified via image analysis of high-speed video. Pleopod stroke amplitude and frequency were found to vary significantly among these swimming modes. In order to increase swimming speed, krill were found first to increase stroke amplitude and secondarily to increase beat frequency. The kinematics of these distinct swimming modes provide insight as we consider multi-appendage metachronal swimming from a design standpoint. The ratio of the distance between adjacent appendage bases and appendage length is identified as a key parameter in metachrony, the value of which is constrained to a narrow range for a wide variety of species.

Investigating Behaviour and Population Dynamics of Striped Marlin (Kajikia audax) from the Southwest Pacific Ocean with Satellite Tags

Behaviour and distribution of striped marlin within the southwest Pacific Ocean were investigated using electronic tagging data collected from 2005–2008. A continuous-time correlated random-walk Kalman filter was used to integrate double-tagging data exhibiting variable error structures into movement trajectories composed of regular time-steps. This state-space trajectory integration approach improved longitude and latitude error distributions by 38.5 km and 22.2 km respectively. Using these trajectories as inputs, a behavioural classification model was developed to infer when, and where, ‘transiting’ and ‘area-restricted’ (ARB) pseudo-behavioural states occurred. ARB tended to occur at shallower depths (108±49 m) than did transiting behaviours (127±57 m). A 16 day post-release period of diminished ARB activity suggests that patterns of behaviour were affected by the capture and/or tagging events, implying that tagged animals may exhibit atypical behaviour upon release. The striped marlin in this study dove deeper and spent greater time at ≥200 m depth than those in the central and eastern Pacific Ocean. As marlin reached tropical latitudes (20–21°S) they consistently reversed directions, increased swimming speed and shifted to transiting behaviour. Reversals in the tropics also coincided with increases in swimming depth, including increased time ≥250 m. Our research provides enhanced understanding of the behavioural ecology of striped marlin. This has implications for the effectiveness of spatially explicit population models and we demonstrate the need to consider geographic variation when standardizing CPUE by depth, and provide data to inform natural and recreational fishing mortality parameters.

Redefining membership in animal groups

Groups of animals-flocks, herds, shoals, and swarms-are often dynamical entities. Relative positions of group members change, and most groups divide and re-form on multiple timescales. Few studies, however, have attempted to define when an animal is or is not within a group. Most authors adopt arbitrary distance thresholds, such as the elective group size (EGS), which assume that animals closer than some threshold distance are in the same group. In the present article, we define a group-membership criterion derived from dynamical statistical considerations and based on detailed trajectories of all members of a moving group. We demonstrate the use of our criterion to track the comings and goings in shoals of zebrafish (Danio rerio) and the gradual dissolution of the shoal across multiple exposures to a testing tank. We present a novel measure of group cohesion based on our group membership criterion and demonstrate that excursions away from the shoal explain some previous observations of the dynamics of shoaling. Finally, we show that excursions away from a shoal are accompanied by an increase in swimming speed. Applying similar criteria to data from other species may clarify some of the common features of animal collective motion.

Chemoattraction to lysophosphatidic acid does not require a change in membrane potential inTetrahymena thermophila

LPA (lysophosphatidic acid), a known chemoattractant for many types of eukaryotic cells, is also a reliable chemoattractant for Tetrahymena. Since LPA receptors are GPCRs (G-protein coupled receptors) in many cell types and several putative GPCR sequences can be found in the Tetrahymena Genome Database, we are interested to determine whether similar GPCR pathways can be used for chemosensory transduction in Tetrahymena. To confirm our procedures, we tested the known chemoattractant proteose peptone (at 1.0 mg/ml), which caused hyperpolarization and increased forward swimming speed in Tetrahymena, consistent with the current model for ciliate chemoattraction. Although 10 µM LPA did not produce these same responses, it was still an effective chemoattractant. PTX (pertussis toxin) blocked attraction to both of these compounds, suggesting a possible G-protein involvement in chemoattraction. Both of these chemoattractants also decreased the basal percent of cells showing direction changes [PDC (percent directional change)] and the duration of backward swimming in 0.5 mM Ba2+ (a general excitability assay). LPA probably causes chemoattraction in Tetrahymena by decreasing the basal PDC without changing either membrane potential or swim speed. Since a pertussis-sensitive G-protein might modulate the ciliate voltage-dependent Ca2+ channels, we propose that LPA acts through an uncharacterized GPCR to lower the PDC by decreasing cellular excitability. These combined behavioural and electrophysiological analyses support the novel hypothesis that chemoattraction to some attractants, like LPA, can occur without hyperpolarization and increased swim speed in Tetrahymena.

Changes in the scotopic vision of juvenile Pacific bluefin tuna (Thunnus orientalis) with growth

In cultured juvenile Pacific bluefin tuna (Thunnus orientalis), reducing the mass deaths caused by collision or contact with tank or net walls at night is a priority for seedling production. Pacific bluefin tuna is a visually dependant species, although its scotopic vision is poor. We recorded electroretinograms to investigate the visual function with growth in the dark-adapted eyes of juvenile Pacific bluefin tuna. Peak wavelengths of spectral sensitivity [38-62 days posthatch (dph), 77-167 mm standard length (SL)] were observed between 474 and 494 nm. Visual light sensitivity has a tendency to increase slightly with growth at 28-64 dph in individuals that measured 29-175 mm SL. However, visual temporal resolution did not significantly increase with growth at 38-62 days dph in individuals that measured 77-167 mm SL. These results suggest that the mass death continues between 28 and 64 dph because of low visual function and increasing swimming speed with growth.

Time for Zebrafish

Time for Zebrafish

Survival selection imposed by predation on a physiological trait underlying escape speed

Summary 1. In contrast to other phenotypic traits, selection on physiological traits remains largely undocumented. We have evaluated survival selection imposed by predation by dragonflies on the activity of arginine kinase (Ak), a key enzyme delivering energy for escape performance in invertebrates. 2. To accomplish this, we conducted a semi‐natural field enclosure experiment in which we manipulated predation by large dragonfly predators, and quantified escape swimming speed and Ak in the prey, the damselfly Enallagma vesperum. To avoid confounding selection on Ak with selection on other swimming speed‐related variables, we also scored all morphological and behavioural traits thought to underlie swimming speed in these damselflies. 3. Dragonfly predators imposed considerable mortality and selected for faster swimming speed and higher activity levels of Ak. Furthermore, higher Ak levels contributed to higher swimming speeds, confirming the mechanistic role of Ak for escape performance. Although morphological (size of the caudal lamellae which generate trust) and behavioural (number of beats made by the abdomen during swimming and the start angle of the C‐start) variables contribute to increasing swimming speed, we detected no selection on these variables. This may be due to functional redundancy. 4. Taken together, our results indicated selection on Ak and suggested that selection on physiological traits may be as strong as selection on morphology and life history traits.

Displacement, velocity preference, and substrate use of three native California stream fishes in simulated pulsed flows

We studied whether juvenile fishes were able to maintain swimming speed and position during simulated river pulsed flows in a laboratory flume. We used a glass flume (15.24×0.6 m) with river- rock substrate to determine the longitudinal displace- ment, movement distances and frequencies, velocity selection, and substrate use of juvenile (SL range: 6.1± 0.2 cm) hardhead Mylopharodon conocephalus (n=13), rainbow trout Oncorhynchus mykiss (n=11), and Sacra- mento sucker Catostomus occidentalis (n=12) during a 100-min flow pulse, as velocity changed from slow to medium, fast, medium, and slow. Fish were capable of maintaining swimming speed and position up to the maximum flume velocity of 0.46 m·s −1 , except for one hardhead that impinged on the rear fish screen. Fish swam faster in the flume during the medium and fast intervals than the slow intervals, but fish speeds were similar among the medium and faster intervals, when some fish took cover behind the rock substrate. In comparison with a Brett-type swim-tunnel, fish showed less increase in mean swimming speed as the flume velocity increased. Fish in the flume were able to use the rock substrate as hydraulic cover, decreasing the encoun- tered water velocity, and, presumably, conserving energy.

Behavioural responses to pressure changes in cultured Atlantic cod ( Gadus morhua): Defining practical limits for submerging and lifting sea-cages

Farmed Atlantic cod ( Gadus morhua) are occasionally exposed to buoyancy changes in sea-cages, through lifting or lowering of cage nets. Physiological processes regulate the level of gas in the closed swim bladders of cod and thus the ability of cod to control their buoyancy. Rapid net lifting may cause positive buoyancy, leading to barotrauma, while net lowering may lead to negative buoyancy and alter cod behaviours. We tested how groups of farmed cod responded immediately after lifting events from 5 different start depths equivalent to 40% pressure reductions, and how long they took to return to pre-lifting pressure levels. In addition, we tested immediate responses and recovery times to cage lowering events equivalent to 100–300% pressure increases. Trials were conducted with 100 cod of 1.1–1.7 kg in a 63 m 3 sea-cage at the lower (5 °C) and upper (16 °C) water temperature limits experienced during culture. Swimming behaviours were measured at fixed intervals before and after cage lifting or lowering, and a feeding test was used to assess appetite. In general, lifting events increased swimming speeds 1.5–4 times and tail beats 2–3 times and fish swam with an average −14° head-down angle, indicating positive buoyancy. The depth before lifting affected the immediate response as the fish became more active after lifting events from shallow compared to deeper depths. Appetite levels decreased for about 2 h after cage lifting, independent of temperature or start depth. The overall recovery time of 8 h after lifting did not depend upon start depth or temperature. Lowering events appeared to cause negative buoyancy. Swimming speeds (1.3–2.3 times) and tail beat frequencies (1.4–2.3 times) increased immediately after cage lowering, and cod swam with an average 30° head-up swimming angle. Neither pressure level nor temperature affected this immediate response. Time to recover to neutral buoyancy for 300% pressure increases took 42–90 h, but only 18–34 h for 100% pressure increases. We conclude that a 40% pressure reduction is an upper limit for lifts of healthy farmed cod. Secondary lifts should not be done until at least 10 h after the first lift. Cage lowering should be done slowly to avoid potentially stressful crowding of negatively buoyant fish on the cage bottom, especially at low temperatures.

Characterizing the escape response of juvenile summer flounder Paralichthys dentatus to diel‐cycling hypoxia

Swimming speed, angular correlation and expected displacement were measured in juvenile summer flounder Paralichthys dentatus acclimated to either oxygen saturation (c. 7.8 mg O(2) l(-1); saturation-acclimated fish) or diel-cycling hypoxia (cycling between 11.0 and 2.0 mg O(2) l(-1)) for 10 days and subsequently exposed to more severe diel-cycling hypoxia (cycling between 7.0 and 0.4 mg O(2) l(-1)). Saturation-acclimated P. dentatus exhibited an active response to declining dissolved oxygen (DO) by increasing swimming speed, angular correlation and expected displacement to peak levels at 1.4 mg O(2) l(-1) that were 3.5, 5.5 and 4.2 fold, respectively, greater than those at DO saturation. Diel-cycling hypoxia-acclimated P. dentatus also exhibited an active response to declining DO, although it was relatively less pronounced. Diel-cycling hypoxia-acclimated P. dentatus swimming speed, however, still doubled as DO decreased from 7.0 to 2.8 mg O(2) l(-1). Diel-cycling hypoxia-acclimated P. dentatus did not recover as well from low DO exposure as did saturation-acclimated fish. This was reflected in their relatively more random swimming (low angular correlation between successive moves) and poor maintenance of rank order between individuals during the recovery phase. Even saturation-acclimated P. dentatus did not resume swimming at speeds observed at saturation until DO was 4.2 mg O(2) l(-1). Paralichthys dentatus were very sensitive to decreasing DO, even at DO levels that were not lethal or growth limiting. This sensitivity and their poor recovery may preclude juvenile P. dentatus from using highly productive nursery habitats affected by diel-cycling hypoxia.