Effects of physical factors on the vertical distribution of larval walleye pollock Theragra chalcogramma under controlled laboratory conditions

This study examined, under controlled laboratory conditions, behavioral responses of larval walleye pollock (4 to 8 mm) to various physical factors that may potentially play a role in vertical movements and distribution. Diel periodicity in vertical distribution was evident and appeared to be under exogenous control. During the day, with light intensity at 70 μE/m⁻²s⁻¹, larvae moved downward from the surface. As light intensity decreased during simulated evening twilight, larvae moved upward. At night, with no available light, larvae continued to swim upward, apparently the result of negative geotaxis. They remained near the surface until morning when they then began moving downward. Changes in vertical distribution occurred concomitantly with changes in activity, orientation and behavior. Formation of a vertical thermal gradient caused larvae to move upward and away from cold water. Turbulence at the surface, induced by an airstream, elicited an avoidance response and resulted in larvae moving downward.Keywords: diel periodicity, vertical migrations, light intensity, Theragra chalcogramm

Light-induced changes in the prey consumption and behavior of two juvenile planktivorous fish

Walleye pollock and sablefish, as 0+ yr juveniles, are pelagic particulate feeding planktivores. We conducted a series of laboratory experiments to determine how illumination influences prey consumption in these species, utilizing live Artemia sp. as prey. Both juvenile walleye pollock and sablefish were characterized by a sigmoidal relationship between the log of illumination and the number of prey consumed, with greater prey consumption at higher illuminations. The threshold illumination below which fish were no longer able to visually forage was approximately 5 x 10⁻⁷ μEs⁻¹m⁻² for walleye pollock and 5 x 10⁻⁵ μEs⁻¹m⁻² for sablefish, indicating that walleye pollock are better adapted for visual feeding at depth or at night than are sablefish. This is consistent with what is known about their vertical distributions at this Life stage; walleye pollock make daily vertical migrations which keep them at lower illuminations than sablefish, which remain at or near the water surface throughout the diel cycle. Although feeding more effectively in the light, both species were capable of detecting and capturing prey in darkness.The published version of this article is copyrighted by Inter-Research and can be found here: http://www.int-res.com/journals/meps/meps-home/Keywords: Visual threshold, Nonvisual feeding behavior, Illumination, Nocturna

Food deprivation affects vertical distribution and activity of a marine fish in a thermal gradient: potential energy-conserving mechanisms

The effects of reduced food availability on the behavior of juvenile walleye pollock Theragra chalcogramma were examined in laboratory experiments designed to test for potential energy-conserving responses. Groups of juvenile fish were held on 1 of 6 ration treatments ranging from ad libitum to near starvation, and then vertical distribution and activity levels were quantified in a 2.5 m deep water column under isothermal and thermally stratified conditions. Stratification resulted in a general shift to the upper, warmer layer in the 2 experiments employing a sharp thermocline at mid-depth, but the occurrence of fish in the colder bottom layer varied with different ration treatments. Movement into cold water increased in intermediate ration groups compared to high ration groups. Since reduced temperatures should reduce metabolic costs, this behavior is consistent with our hypothesis that food deprivation should invoke energy-saving behaviors. However, activity levels increased for fish held on intermediate rations, suggesting that the greater movement into cold water was a corollary result of increased searching for food. Fish in the lowest ration treatments had decreased activity levels, but also decreased their movement into cold water when a sharp thermocline was present, negating potential bioenergetic benefits. In the third experiment, there was a gradual thermal gradient from surface to bottom rather than a sharp thermocline. Temperatures associated with vertical positions of the fish were determined. In this experiment, clear energy-conserving responses to temperature were displayed by food-deprived fish; the average temperatures occupied by fish on starvation rations were 3 to 4 degree C colder than those of the higher ration groups. Based on the high Q sub(10) for metabolic rates of juvenile pollock, these reduced temperatures potentially conferred energy savings of up to 34%, relative to the metabolic expenditures of fish on high rations. The contrast in behavior for the lowest ration groups between sharply stratified and gradually stratified conditions suggested that the severity of the temperature gradient influenced the fishes' ability to take advantage of cold water as an energetic refuge. The behavior of fish in the laboratory was consistent with prior observations in the Bering Sea, where juvenile walleye pollock remained in surface waters if food availability was high, but initiated vertical migration into deeper, colder water with reduced prey densities. Results of this study demonstrated a broad flexibility in the behavioral mechanisms used by walleye pollock to deal with declining food levels. The initial response to food limitation was increased activity, indicative of greater searching behavior. With extended food deprivation, a switch to energy-conserving behavior was evident. The temperature responses of fish experiencing severe food limitation provided support for a bioenergetic hypothesis of diel vertical migration.Keywords: Starvation, Bioenergetics, Walleye pollock, Temperature, BehaviorKeywords: Starvation, Bioenergetics, Walleye pollock, Temperature, Behavio

Altered search speed and growth: social versus independent foraging in two pelagic juvenile fishes

Prior studies have demonstrated that juvenile walleye pollock Theragra chalcogramma forage socially in schools for spatially and temporally clumped food, but forage more independently for spatially and temporally dispersed food. One advantage of social foraging is that fish in schools may be able to locate more food clumps than fish foraging individually. However, data also indicate that walleye pollock swim faster when foraging socially. We conducted laboratory experiments to evaluate the effect of food distribution upon the energetic foraging costs incurred by juvenile walleye pollock and sablefish Anaplopoma fimbria. We predicted that when given identical rations, fish receiving clumped food would swim faster, expending more energy, and therefore grow more slowly than fish receiving dispersed food. After 2 wk under these 2 foraging regimes, juvenile walleye pollock receiving clumped food swam 50 % faster, but experienced 19 % lower growth, than walleye pollock receiving dispersed food. Sablefish demonstrated only a weak swim speed response, with no difference in growth between food distributions. Our results demonstrate that although social foraging may increase encounter rates with food, in some species there may also be an energetic cost for this behavior, which will have an influence upon energetic efficiency, potentially affecting growth and survival.Keywords: Swimming, Growth, Foraging strategy, Bioenergetics, Milkshake effect, Shoalin

Stress and delayed mortality induced in Pacific halibut by exposure to hooking, net towing, elevated seawater temperature and air: implications for management of bycatch

The extent of stress and eventual mortality in Pacific halibut Hippoglossus stenolepis that resulted from simulated capture by hooking or towing in a net, followed by abrupt exposure to warmer seawater temperature and air, were determined under laboratory conditions. Abrupt exposure to 16degreesC seawater and air after either method of capture increased capture-induced stress, with accompanying mortality of 33% for hooked fish and 78% for fish towed in a net. Moreover, these deaths occurred as long as 30 d after experimental treatment, Suggesting that delayed mortality should be considered in any study of Pacific halibut bycatch mortality. Stress induced by hooking or towing in a net followed by air exposure was reflected in cessation of negative phototaxis and feeding, both of which were resumed after 5 d with no mortality occurring. The results of this study clearly show that seasonal increases in temperature associated with thermoclines and deck conditions have the potential for markedly increasing the mortality of Pacific halibut that might otherwise survive capture and release in colder seasons. Strategies for effective management of Pacific halibut bycatch need to include consideration of seasonal temperature increases and how this factor might increase mortality.Keywords: Hippoglossus stenolepis, bycatch, mortalit

Effect of light on juvenile walleye pollock shoaling and their interaction with predators

Research was undertaken to examine the influence of light intensity on the shoaling behavior, activity and anti-predator behavior of juvenile walleye pollock Theragra chalcogramma. Under a 12 h light/12 h dark photoperiod, juveniles displayed a diurnal shoaling and activity pattern, characterized by fish swimming in cohesive groups during the day, with a cessation of shoaling and decreased swimming speeds at night. Prior studies of schooling fishes have demonstrated distinct light thresholds below which schooling abruptly ceases. To see if this threshold effect occurs in a predominantly shoaling species, Like juvenile walleye pollock, another experiment was undertaken in which illumination was lowered by orders of magnitude, giving fish 20 min to adapt to each light intensity. Juvenile walleye pollock were not characterized by a distinct light threshold for shoaling; groups gradually dispersed as light levels decreased and gradually recoalesced as light levels increased. At light levels below 2.8 x 10(-6) mu E s(-1) m(-2), juvenile walleye pollock were so dispersed as to no longer constitute a shoal. Exposure to simulated predation risk had differing effects upon fish behavior under light and dark conditions. Brief exposure to a model predator in the dark caused fish to swim faster, for 5 or 6 min, than fish which had been similarly startled in the Light. Chronic exposure to a living predator produced similar results: fish tended to swim slower when a predator was present in the light, but faster when a predator was present in the dark. In the Light, shoaling and/or schooling provide protection against predators. But in the dark, with fish unable to see one another, increased prey activity resulting from predator disturbance may lead to accelerated dispersal of prey shoals. Thus, perceived predation risk may have different effects upon the spatial distribution of juvenile walleye pollock under light and dark conditions. This has implications for survival, as fish which have become widely scattered during the darkness may take longer to reform shoals at dawn, resulting in greater predation risk.Keywords: Illumination, Twilight hypothesis, Schooling, Swimming speed, Spatial distribution, Predator-preyKeywords: Illumination, Twilight hypothesis, Schooling, Swimming speed, Spatial distribution, Predator-pre

Comparison of Predator Avoidance Capabilities with Corticosteroid Levels Induced by Stress in Juvenile Coho Salmon

The relationship between predator avoidance deficits induced by a simple handling stress and the level of plasma corticosteroids was determined for juvenile coho salmon Oncorhynchus kisutch. Groups of fish were held out of water for 1 min, then allowed to recover from this stress for 1, 90, or 240 min. After each recovery period, some unstressed and stressed fish were sampled for plasma cortisol, and others were subjected to predation by lingcod Ophiodon elongatus. Levels of corticosteroids in stressed fish remained high throughout the 240-min period of recovery, but predator avoidance returned to control levels in less than 90 min. Results suggest that juvenile coho salmon quickly recover basic survival skills of predator avoidance after mild stress, even though cortisol levels continue to indicate a stressed condition.Keywords: Oncorhynchus kisutch smolts, Corticosteroids, stress reactionKeywords: Oncorhynchus kisutch smolts, Corticosteroids, stress reactio

Capture-Related Stressors Impair Immune System Function in Sablefish

The sablefish Anoplopoma fimbria is a valuable North Pacific Ocean species that, when not targeted in various commercial fisheries, is often a part of discarded bycatch. Predictions of the survival of discarded fish are dependent on understanding how a fish responds to stressful conditions. Our objective was to describe the immunological health of sablefish exposed to capture stressors. In laboratory experiments designed to simulate the capture process, we subjected sablefish to various stressors that might influence survival: Towing in a net, hooking, elevated seawater and air temperatures, and air exposure time. After stress was imposed, the in vitro mitogen-stimulated proliferation of sablefish leukocytes was used to evaluate the function of the immune system in an assay we validated for this species. The results demonstrated that regardless of fishing gear type, exposure to elevated seawater temperature, or time in air, the leukocytes from stressed sablefish exhibited significantly diminished proliferative responses to the T-cell mitogen, concanavalin A, or the B-cell mitogen, lipopolysaccharide. There was no difference in the immunological responses associated with seawater or air temperature. The duration and severity of the capture stressors applied in our study were harsh enough to induce significantly elevated levels of plasma cortisol and glucose, but there was no difference in the magnitude of levels among stressor treatments. These data suggest that immunological suppression occurs in sablefish subjected to capture-related stressors. The functional impairment of the immune system after capture presents a potential reason why delayed mortality is possible in discarded sablefish. Further studies are needed to determine whether delayed mortality in discarded sablefish can be caused by increased susceptibility to infectious agents resulting from stressor-mediated immunosuppression.Keywords: Anoplopoma fimbria, immune system, stress, bycatc

Characterization of the physiological stress response in lingcod

The goal of this study was to describe the duration and magnitude of the physiological stress response in lingcod Ophiodon elongatus after exposure to brief handling and sublethal air stressors. The response to these stressors was determined during a 24‐h recovery period by measuring concentrations of plasma cortisol, lactate, glucose, sodium, and potassium. Lingcod were subjected to brief handling followed by either a 15‐min or a 45‐min air stressor in the laboratory. After the 15‐min stressor, an increase in cortisol or glucose could not be detected until after 5 min of recovery. Peak concentrations were measured after 30 min for cortisol and after 60 min for glucose and lactate. Glucose and lactate had returned to basal levels after 12 h, whereas cortisol did not return to basal levels until after 24 h of recovery. Immediately following a 45‐min air stressor, all measured parameters were significantly elevated over levels in prestressor control fish. Cortisol concentrations tended to increase and reached a measured peak after 8 h of recovery, whereas glucose and lactate reached a measured peak after 1 h of recovery. Cortisol and lactate returned to basal levels within 24 h. Glucose, however, remained elevated even after 24 h of recovery. Plasma ions initially increased during the first hour of recovery, and the concentrations then declined to a level below that measured in control fish for the remainder of the 24‐h recovery period. In addition, we evaluated the effect of fish size on the stress response. There was no significant difference between the stress response of smaller (41–49‐cm [total length] and larger (50–67‐cm) lingcod after 45 min air exposure. In general, both the magnitude and duration of the primary and secondary stress responses in lingcod are comparable to those of salmonids

THE EFFECT OF TEMPERATURE ON THE ACTIVITY OF BLUEFISH, POMATOMUS SALTATRIX L

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