Oriented swimming at an angle to light in a schooling characid, Moenkhausia dichrowa Kner

Two groups each of approximately 100 Moenkhausia dichroua , a schooling characid, showed a long–lasting, constant–oriented swimming when placed in a light–centred circular channel. This apparatus consists of a 1–m diameter circular channel illuminated by either a central or a peripheral light system, so that the light angle is constant all around the channel. With the central light at a fixed angle, fish swam for several months in one direction and reversed direction at a certain date. When the light angle was increased by 10° every other day between 0° (horizontal) and 90° (vertical), swimming direction was reversed at a particular angle in each experiment. This response to artificial light suggests that this small schooling fish uses the sun as an orientation clue in its seasonal migrations.     

Reactions in individual fish to strobe light. Field and aquarium experiments performed on whitefish (Coregonus lavaretus)

This study describes how individual whitefish Coregonus lavaretusreact to strobe light. Field experiments were performed in a net enclosure on fish tagged with ultrasonic transmitters. A strobe light array was switched on near the tagged fish. The fish moved away from the light and increased their swimming speed. Aquarium experiments under controlled conditions were carried out in rearing tanks at Saimaa Fisheries and Aquaculture Station in Finland. A strobe light was directed from the side of the basin just ahead of, directly at, and behind the fish at a close range. In the first two cases fish responded by a distinct turn and a change in swimming direction away from the light. The fish did not change its swimming direction when light was aimed from behind. It is concluded that strobe light may be used to prevent fish from swimming into a specific area. Implications for development of new fishing equipment and research concerning fishes in areas with water power stations is briefly discussed.     

Day-night variations in plasma melatonin and arginine vasotocin concentrations in chronically cannulated flounder (Platichthys flesus).

Chronically catheterised, free swimming flounder (Platichthys flesus) have been used in experiments examining the day-night variations in circulating levels of melatonin (Mel) and arginine vasotocin (AVT). Under normal photoperiod (16 h light/8 h dark) serial blood samples taken from individual fish demonstrated a Mel rhythm with daytime levels at 09.00 and 15.00 h (238+/-14 and 179+/-12 fmol x ml(-1), respectively) lower than those at 23.00 h (1920+/-128 fmol x ml(-1)). Maintenance of fish in 24-h light abolished the light/dark Mel rhythm and circulating levels were comparable to those measured during the day in fish under normal photoperiod illumination. In fish maintained under 24 h dark, although a daily rhythm was still apparent, at the time when it would be normally dark, plasma Mel concentration was reduced and at times when it would be normally light, levels were higher than in fish maintained under normal light/dark illumination. Plasma AVT concentrations were higher in fish during the day (4.4+/-0.8 fmol x ml(-1)) than those at night (1.5+/-0.4 fmol x ml(-1)), the opposite to that seen with Mel. During acute study infusion of AVT resulted in reduced levels of plasma Mel, although this did not achieve statistical significance. Infusion of Mel did not alter circulating AVT concentration.     

Action spectra for phototaxis in zoospores of the brown algaPseudochorda gracilis

Summary Action spectra for phototaxis in zoospores of brown alga,Pseudochorda gracilis (Laminariales), were examined in the wavelength range between 300 and 600 nm using the Okazaki Large Spectrograph and a video tracking system. The direction of swimming (both in percent cells swimming in parallel with the stimulating light, and in mean angle of cell movement) was dependent on the wavelength. The action spectra had two peaks at 420 and 460 nm, while light above 500 nm was not effective in changing the swimming direction of the cells.Abbreviations TCMA tracker-cell movement analyzer system - CMA cell movement analyzer program     

Water temperature, light intensity and zooplankton density and the feeding activity of juvenile brook charr (Salvelinus fontinalis)

SUMMARY 1. The objective of this study was to evaluate the effects of zooplankton biomass (as a measure of density), fish biomass, light intensity and water temperature on the attack rate and swimming characteristics (i.e. swimming speed and angle of turn) of juvenile (1+) brook charr (Salvelinus fontinalis) in field enclosures. We used a portable underwater camera system in a series of pelagic enclosures to quantify the feeding behaviour of brook charr over a gradient of natural conditions. 2. In simple linear or non‐linear regression models we found (i) that attack rate and angle of turn were positively related to water temperature, (ii) that attack rate and swimming speed were positively related to zooplankton biomass and light intensity and (iii) that attack rate was positively related to swimming speed. In multiple regression models, fish biomass, light intensity and variance of the angle of turn accounted for 87% of the variation in attack rate. Light intensity and water temperature accounted for 86% of the variation in swimming speed. Fish gut fullness and attack rate accounted for 83% of the variation in the variance of the angle of turn executed by fish. 3. The increase in the number of attacks as zooplankton biomass increases conforms to the general positive functional response observed in other fish species. Our results also support the hypothesis that swimming speed increases with prey biomass. We did not observe a plateau in attack rate as zooplankton biomass increased. As our experiments were performed under natural biotic and abiotic conditions, factors other than zooplankton biomass might affect or limit this response, such as water temperature and light intensity. 4. Because zooplankton biomass was correlated with water temperature and light intensity, it was not possible to evaluate the independent contribution of these factors on the attack rate and swimming characteristics (swimming speeds and angle of turn) of brook charr. However, this study highlighted the impact of these factors on the feeding behaviour of juvenile brook charr when feeding in the pelagic habitat under natural conditions, and their importance in future models of optimal foraging and fish habitat quality.     

Diurnal Variations in Swimming Activity of Rutilus rutilus (Cyprinidae) in a Group under Tank Conditions

Measurements of the swimming activity of a group of roach (12–19 cm TL, average) in a circular swimming chamber revealed two distinct activity patterns: a diurnal and a nocturnal one. The experiments showed that, having the choice, two factors stimulated the rhythmicity of the swimming behaviour of the fish, i.e. light intensity and the presence of a current field in the proximity of the fish. During daytime (bright light conditions) the fish moved into the current field and swam on average at 0.4 BL/s (resting swimming). The roach remained swimming at this speed even if no current field was established, however, then distributed evenly in the basin. By contrast, during night (dim light conditions) the fish predominantly chose the still water section but swam on average with a cruising speed of 1.6 BL/s (night swimming). Accordingly, they did not seek the still water section for night swimming if the light was kept on. Then again, the fish distributed more or less evenly in the basin. The results support the hypothesis that the fish migrate during night-time and do this preferably in still water.     

Evaluation of Adult White Sturgeon Swimming Capabilities and Applications to Fishway Design

Concern over passage of sturgeon barriers, has focused attention on fishway design that accommodates its swimming performance. In order to evaluate swimming performance, regarding fish ladder type partial barriers, wild adult sturgeons, Acipenser transmontanus; 121–76m fork length, were captured in the San Francisco Bay Estuary and Yolo Bypass toe drain. Hydrodynamic forces and kinematic parameters for swimming performance data were collected in a laboratory flume under three flow conditions through barriers and ramp. The experiments were conducted in a 24.4 m long, 2.1 m wide, and 1.62 m deep aluminum channel. Two geometric configurations of the laboratory model were designed based on channel characteristics that have been identified in natural river systems. At a given swimming speed and fish size, the highest guidance efficiencies of successful white sturgeon passage as a function of flow depth, flow velocity, turbulence intensity, Reynolds number, Froude number and shear velocity observed in the steady flow condition, tested with the horizontal ramp structure, occurred at an approach velocity of 0.33 ms^-1. The guidance efficiency of successful sturgeon passage increased both with increasing flow velocity and Froude number, and decreased both with the flow depth and the turbulence intensity. This study also provides evidence that tail beat frequency increases significantly with swimming speed, but tail beat frequency decreases with fish total length. Stride length increases both with swimming speed and fish total length. The importance of unsteady forces is expressed by the reduced frequency both with swimming speed and fish total length. Regression analysis indicates that swimming kinematic variables are explained by the swimming speed, the reduced frequency and the fish total length. The results emphasize the importance of fish ladder type patchiness when a fishway is designed for the passage of sturgeon.     

水流对草鱼幼鱼趋光行为的影响

为了探讨水流对鱼类趋光性的影响, 利用自制的循环水槽装置, 以草鱼(Ctenpharyngodon idellus)幼鱼为研究对象, 研究其在光照度为300 lx, 不同流速工况(0、0.1和0.2 m/s)下的趋光性行为, 同时设黑暗静水工况为对照组。结果表明: (1) 0.2 m/s的流速可完全激发草鱼幼鱼的趋流性, 使其游泳方向多数与顶流方向呈± 20°。(2)根据草鱼幼鱼在不同流速工况下随光照度衰减在水槽内的分布情况, 计算得其在3种流速工况下的光强期望值分别为: 52.45, 34.62和37.86 lx。(3)当照度为300 lx时, 静水工况下的实验鱼在水槽中呈现“两头高, 中间低”的分布情况, 并未表现出对某一光强范围的偏好行为; 在小于感应值的低流速下, 草鱼幼鱼的分布情况总体趋势与静水工况类似, 但在远离光源处的分布较多, 多呈“逆流后退”行为; 当流速值超过感应流速时, 在趋流性的作用下, 鱼类在尾部的聚集情况明显下降, 同时在水槽中的分布更加均匀, 其原有光环境的作用减弱。研究初步证明了略大于感应值的小流速所引发的草鱼趋流性即可对其光环境响应行为产生影响。     

Artificial light and season affects vertical distribution and swimming behaviour of post-smolt Atlantic salmon in sea cages

In winter, post-smolts of Atlantic salmon Salmo salar exposed to continuous additional light of different intensities (LL) in 14 m deep sea cages maintained a constant swimming speed in circular polarized schools with maximum fish density in the warmest water layers at 11 m depth. By contrast, fish exposed to natural light only (NL) ceased swimming at dusk, ascended from the warm layer to the thermocline c. 3 m depth and were more dispersed in the whole water column during the dark phase. In early spring, fish exposed to LL of medium (LL-MED) or high (LL-HIGH) light intensity ascended and maximum fish density was at 5 m depth. A similar ascent was delayed to late spring in both the low intensity group (LL-LOW) and the NL group, coinciding with a shift in maximum temperature to this depth. The advanced ascent in the LL-HIGH and LL-MED groups is interpreted as a light-induced shift in a seasonal rhythm of feeding motivation. In summer, most of the fish in all four treatment groups were observed in the warm and less saline 4 m surface layer. It is suggested that the seasonal and diel changes in vertical distribution of Atlantic salmon are related inversely to feeding motivation, with preferences for maximum temperature and darkness acting as modifying factors.     

Eyeless Mexican Cavefish Save Energy by Eliminating the Circadian Rhythm in Metabolism

The eyed surface form and eyeless cave form of the Mexican tetra Astyanax mexicanus experience stark differences in the daily periodicities of light, food and predation, factors which are likely to have a profound influence on metabolism. We measured the metabolic rate of Pachón cave and surface fish at a fixed swimming speed under light/dark and constant dark photoperiods. In constant darkness surface forms exhibited a circadian rhythm in metabolism with an increase in oxygen demand during the subjective daytime, whereas cave forms did not. The lack of circadian rhythm in metabolism leads to a 27% energy savings for Pachón cave fish compared to surface fish when comparing both forms in their natural photoperiods. When surface forms were tested under constant dark conditions they expended 38% more energy than cave forms under equivalent conditions. Elimination of the circadian rhythm in metabolism may be a general feature of animals that live in perpetually dark food-limited environments such as caves or the deep sea.     

Near-infrared orientation of Mozambique tilapia Oreochromis mossambicus

Light plays a pivotal role in animal orientation. Aquatic animals face the problem that penetration of light in water is restricted through high attenuation which limits the use of visual cues. In pure water, blue and green light penetrates considerably deeper than red and infrared spectral components. Submicroscopic particles and coloured dissolved organic matter, however, may cause increased scattering and absorption of short-wave components of the solar spectrum, resulting in a relative increase of red and infrared illumination. Here we investigated the potential of near-infrared (NIR) light as a cue for swimming orientation of the African cichlid fish (Cichlidae) Oreochromis mossambicus. A high-throughput semi-automated video tracking assay was used to analyse innate behavioural NIR-sensitivity. Fish revealed a strong preference to swim in the direction of NIR light of a spectral range of 850-950nm at an irradiance similar to values typical of natural surface waters. Our study demonstrates the ability of teleost fish to sense NIR and use it for phototactic swimming orientation.     

Characteristics of reaction field and the reactive distance of a planktivore, Pseudorasbora parva (Cyprinidae), in various environmental conditions

The characteristics of the reaction field and the reactive distance of the Stone moroko (Pseudorasbora parva) were studied under three environmental conditions (structural complexity, light intensity and turbidity) and three prey sizes. In optimal experimental conditions, under no structural complexity, light intensity of 200 lux and turbidity less than 1 NTU (Nephelometric Turbidity Units), the cross-section of the reaction field was found to be elliptic with a bearing angle larger than the elevation angle, but both angles changed slightly depending on environmental conditions. The reactive distance was large, and the fish frequently attacked prey that was located within 15–60 degrees to each side from the frontal direction of a fish (i.e., ± 15 degrees from the axis of the fish body) horizontally. In the light intensity below 50 lux or turbidity above 10 NTU, however, the attack frequency and the reactive distance in the frontal direction of a fish did not differ from other horizontal directions in the reaction field. The average reactive distance increased proportionally with increasing strand distance, but it gradually reached a constant value for strand distances greater than about 3.6 times the fish body length. The average reactive distance increased in the light intensity range of 10–200 lux and decreased negatively with turbidity increasing. The average reactive distance increased with larger prey size, but the rate of increase of the reactive distance gradually decreased.     

The photobehaviour of Daphnia spp. as a model to explain diel vertical migration in zooplankton

Many pelagic animal species in the marine environment and in lakes migrate to deeper water layers before sunrise and return around sunset. The amplitude of these diel vertical migrations (DVM) varies from several hundreds of metres in the oceans to approx. 5–20 m in lakes. DVM can be studied from a proximate and an ultimate point of view. A proximate analysis is intended to reveal the underlying behavioural mechanism and the factors that cause the daily displacements. The ultimate analysis deals with the adaptive significance of DVM and the driving forces that were responsible for the selection of the traits essential to the behavioural mechanism. The freshwater cladoceran Daphnia is the best studied species and results can be used to model migration behaviour in general. Phototaxis in Daphnia spp., which is defined as a light-oriented swimming towards (positive phototaxis) or away (negative phototaxis) from a light source, is considered the most important mechanism basic to DVM. A distinction has been made between primary phototaxis which occurs when light intensity is constant, and secondary phototaxis which is caused by changes in light intensity. Both types of reaction are superimposed on normal swimming. This swimming of Daphnia spp. consists of alternating upwards and downwards displacements over small distances. An internal oscillator seems to be at the base of these alternations. Primary phototaxis is the result of a dominance of either the upwards or the downwards oscillator phase, and the direction depends on internal and external factors: for example, fish-mediated chemicals or kairomones induce a downwards drift. Adverse environmental factors may produce a persistent primary phototaxis. Rare clones of D. magna have been found that show also persistent positive or negative primary phototaxis and interbreeding of the two types produces intermediate progeny: thus a genetic component seems to be involved. Also secondary phototaxis is superimposed on normal swimming: a continuous increase in light intensity amplifies the downwards oscillator phase and decreases the upwards phase. A threshold must be succeeded which depends on the rate and the duration of the relative change in light intensity. The relation between both is given by the stimulus strength versus stimulus duration curve. An absolute threshold or rheobase exists, defined as the minimum rate of change causing a response if continued for an infinitely long time. DVM in a lake takes place during a period of 1-5-2 h when light changes are higher than the rheobase threshold. Accelerations in the rate of relative increase in light intensity strongly enhance downwards swimming in Daphnia spp. and this enhancement increases with increasing fish kairomone and food concentration. This phenomenon may represent a ‘decision-making mechanism’ to realize the adaptive goal of DVM: at high fish predator densities, thus high kairomone concentrations, and sufficiently high food concentrations, DVM is profitable but not so at low concentrations. Body axis orientation in Daphnia spp. is controlled with regard to light-dark boundaries or contrasts. Under water, contrasts are present at the boundaries of the illuminated circular window which results from the maximum angle of refraction at 48–9° with the normal (Snell's window). Contrasts are fixed by the compound eye and appropriate turning of the body axis orients the daphnid in an upwards or an obliquely downwards direction. A predisposition for a positively or negatively phototactic orientation seems to be the result of a disturbed balance of the two oscillators governing normal swimming. Some investigators have tried to study DVM at a laboratory scale during a 24 h cycle. To imitate nature, properties of a natural water column, such as a large temperature gradient, were compressed into a few cm. With appropriate light intensity changes, vertical distributions looking like DVM were obtained. The results can be explained by phototactic reactions and the artificial nature of the compressed environmental factors but do not compare with DVM in the field. A mechanistic model of DVM based on phototaxis is presented. Both, primary and secondary phototaxis is considered an extension of normal swimming. Using the light intensity changes of dawn and the differential enhancement of kairomones and food concentrations, amplitudes of DVM could be simulated comparable to those in a lake. The most important adaptive significance of DVM is avoidance of visual predators such as juvenile fish. However, in the absence of fish kairomones, small-scale DVMs are often present, which were probably evolved for UV-protection, and are realized by not enhanced phototaxis. In addition, the ‘decision-making mechanism’ was probably evolved as based on the enhanced phototactic reaction to accelerations in the rate of relative changes in light intensity and the presence of fish kairomones.     

Nitrogenase and photosynthetic activities of chemostat cultures of Rhodobacter capsulatus 37b4 grown under different illuminations

Nitrogen fixation as well as structural and functional properties of the photosynthetic apparatus were studied with phototrophically grown chemostat cultures of Rhodobacter capsulatus strain 37b4. Illumination was varied between 3,000 and 30,000 lx at a constant dilution rate of D=0.075 h^-1. Steady state parameters of growth revealed two forms of limitation, i.e. energy limitation in the range of 3,000 to about 10,000 lx and nitrogen limitation at higher illuminations. Over the entire range of illumination, the specific bacteriochlorophyll content and the amount of total bacteriochlorophyll per photochemical reaction center remained essentially constant. Photophosphorylation activity remained constant up to 20,000 lx but was slightly increased at 30,000 lx. Hydrogen evolution and acetylene reduction activities of cellular nitrogenase were assayed under saturating light conditions with samples taken from cultures growing under steady state conditions. In spite of the apparent constancy of the composition and activity of the photosynthetic apparatus under energy limitation, maximal specific acetylene reduction and hydrogen evolution activities increased by factors of 3 and 8, respectively, when illumination of the culture was raised from 3,000 to about 15,000 lx. Above 15,000 lx, both activities of nitrogenase approached constancy.We, therefore, conclude that neither under energy limitation nor under nitrogen limitation the function of nitrogenase depended on the photosynthetic activities. Moreover, it is suggested that light did not influence nitrogenase activity under conditions of nitrogen limitation, while under conditions of energy limitation light seemed to influence nitrogenase activities indirectly via glutamate consumption of the cells.     

Amphidromous fish school migration revealed by combining fixed sonar monitoring (horizontal beaming) with fishing data

Coastal and estuarine environments are particularly productive ecological systems and can provide protein and nutrient exports to adjacent marine ecosystems. In spite of this, studies of fish school migration patterns between lagoons and the sea are lacking. In 1999, fish samplings combining fishing and acoustic sonar field data collection were performed in two shallow water channel lagoons to monitor fish school diel migration (lagoon-sea). Lagoon fish assemblages included 15 species and 10 families. The estimated abundances and the fishes' swimming characteristics permit those detected by sonar to be selected according to 3 criteria. Direct sampling by cast net confirmed that Dicentrarchus labrax schools were present during the autumn migration period. In situ horizontal sonar observation in shallow water bodies constitutes a powerful tool for the study of fish behaviour. Fish school migration within the range of values recorded was not affected by current velocity (maximum 0.83 m/s). No relationships among school shape, surface area or migratory direction were found. The amphidromous schools were small (0.3 to 15 m²) and observed mainly at night. Migratory behaviour appeared to be determined to some extent by fluctuations in lagoon salinity and temperature. Consistent with the ‘multi-transit’ hypothesis, which states that schools pass several times in front of the sonar transducer before moving toward or away from the lagoon, the net flow of fish was less than that predicted by the sonar methodology. Thus the multi-transit behaviour hypothesis should be considered when interpreting fish population transfer data gathered with acoustic methods using a single transducer in rivers, estuaries, or channels. In addition, the exclusive use of echosounder could generate major biomass underestimation when the fish are grouped in schools.     

Metabolism and photoperiod in juvenile lake charr and walleye

Synopsis Basal and swimming oxygen consumption of juvenile lake charr (8 and 12°C and walleye (8°C) were measured for fish exposed either to a natural or constant photoperiod. Seasonal changes in oxygen consumption were not demonstrable at the basal level or for swimming at comparable speeds between 20 and 45 cm s^–1. The absence of a seasonal change in oxygen consumption among juvenile fish is in marked contrast to the seasonal pattern described in other studies at the basal and active level in mature fish.     

Pectoral fin beat frequency predicts oxygen consumption during spontaneous activity in a labriform swimming fish (Embiotoca lateralis)

The objective of this study was to identify kinematic variables correlated with oxygen consumption during spontaneous labriform swimming. Kinematic variables (swimming speed, change of speed, turning angle, turning rate, turning radius and pectoral fin beat frequency) and oxygen consumption (MO₂) of spontaneous swimming in Embiotoca lateralis were measured in a circular arena using video tracking and respirometry, respectively. The main variable influencing MO₂ was pectoral fin beat frequency (r ² = 0.71). No significant relationship was found between swimming speed and pectoral fin beat frequency. Complementary to other methods within biotelemetry such as EMG it is suggested that such correlations of pectoral fin beat frequency may be used to measure the energy requirements of labriform swimming fish such as E. lateralis in the field, but need to be taken with great caution since movement and oxygen consumption patterns are likely to be quite different in field situation compared to a small lab tank. In addition, our methods could be useful to measure metabolic costs of growth and development, or bioassays for possible toxicological effects on fish.     

Empirical analysis of the influence of swimming pattern on the net energetic cost of swimming in fishes

We tested the hypothesis that the energetics of swimming in a flume accurately represent the costs of various spontaneous movements using empirical relationships between fish swimming costs, weight, and speed for three swimming patterns: (1) 'forced swimming' corresponded to movements adopted by fish forced to swim against a unidirectional current of constant velocity; (2) 'directed swimming' was defined as quasi-rectilinear movements executed at relatively constant speeds in a stationary body of water and (3) 'routine swimming' was characterized by marked changes in swimming direction and speed. Weight and speed explained between 76% (routine swimming) and 80% (forced swimming) of net swimming cost variability. Net costs associated with different swimming patterns were compared using ratios of model predictions (swimming cost ratio; SCR) for various weight and speed combinations. Routine swimming was the most expensive swimming pattern (SCR for routine and forced swimming =6.4 to 14.0) followed by directed (SCR for directed and forced swimming =0.9 to 2.8), and forced swimming. The magnitude of the difference between the net costs of forced and spontaneous swimming increases with movement complexity and decreases as fish weight increases.     

Social spacing in the mormyrid fish Gnathonemus petersii (Pisces): A multisensory approach

The sensory basis of group cohesion in the weak-electric fish Gnathonemus petersii was investigated in a circular tank with groups of four fish each, interacting through a wide-meshed plastic screen with intact or operated conspecifics, or with other stimulus objects. We confined these stimuli to one or two peripheral holding compartments. The response measures were obtained from the free swimming fish and included (1) the time the fish spent together as a group, (2) the time they spent in front of the holding compartments, (3) the circular distribution of the fish's positions, and (4) the mean nearest neighbour distances. Under empty compartment conditions, four different groups were tested, consisting of either (1) intact, electrically active fish, or (2) electrically ‘silent’ fish (with their electric organ surgically rendered inoperative), or (3) blind, or (4) ‘silent’ and blind animals. The loss of either sensory modality, vision or feedback from electric organ discharge, led to changes of comparable size, decreasing the time spent as a group and increasing the mean nearest neighbour distance. In fish lacking both modalities, group cohesion was further impaired. With stimuli present in one or both holding compartments, the strength of social attraction depended on the nature of the stimulus: the more intact stimulus conspecifics were present, the more densely did the fish group in front of the stimulus compartment. ‘Wired-in’ electric organ discharges (simulating waveform and intensity) and electrically ‘silent’ fish were equally attractive, but only half as attractive as intact fish. Blind free swimming fish aggregated with intact and also with ‘silent’ conspecifics. Under dim light conditions, group cohesion was predominantly, though not exclusively, affected by electrosensory feedback from the electric organ discharge and visual input. Mechanical and olfactory cues may also be involved.     

Factors affecting feeding selectivity of visual predators on the copepod Acartia tonsa: locomotion,visibility and escape responses

Visual predation by fish on copepods involves prey encounter, attack and capture; during any of these processes prey selection can occur. Developmental changes in copepods, including increases in swimming speed, size and image contrast increase the encounter rate and distance at which they can be detected by predators. Copepods compensate for this increase vulnerability with age through diel vertical migration and improved escape capabilities. This study quantifies the changes in swimming speed and movement pattern with developmental stage of the copepod Acartia tonsa, using a video-computer system for motion analysis. Changes in visible size and image contrast with developmental stage were quantified under simulated natural illumination conditions using a video based image analysis system. The escape responses of the naupliar stages of the copepod Acartia tonsa were quantified in response to a stationary pipette sucking in water at a constant speed. Accurate quantification of the parameters that affect feeding selectivity of planktivorous fish will provide the basis for evaluation of their relative importance in future studies.