Swimming of Daphnia galeata x hyalina in response to changing light intensities: Influence of food availability and predator kairomone

Experiments showed that phototactic downward swimming in Daphnia galeata x hyalina as caused by a relative increase in light intensity (stimulus) is influenced by predator kairomone and food availability. The swimming responses at four different combinations of food availability and fish kairomone were analysed. Addition of both food and kairomone led to a significant increase in percentage of animals that responded to the light stimulus, but there was no significant interaction effect.We also found that kairomone and food had significant impact on displacement velocity and on the time between start of the stimulus and onset of the response.     

Temperature selection in Brook Charr: lab experiments,field studies,and matching the Fry curve

Zooplankton diel vertical migration (DVM) is an ecologically important process, affecting nutrient transport and trophic interactions. Available measurements of zooplankton displacement velocity during the DVM in the field are rare; therefore, it is not known which factors are key in driving this velocity. We measured the velocity of the migrating layer at sunset (upward bulk velocity) and sunrise (downwards velocity) in summer 2015 and 2016 in a lake using the backscatter strength (VBS) from an acoustic Doppler current profiler. We collected time series of temperature, relative change in light intensity chlorophyll-a concentration and zooplankton concentration. Our data show that upward velocities increased during the summer and were not enhanced by food, light intensity or by VBS, which is a proxy for zooplankton concentration and size. Upward velocities were strongly correlated with the water temperature in the migrating layer, suggesting that temperature could be a key factor controlling swimming activity. Downward velocities were constant, likely because Daphnia passively sink at sunrise, as suggested by our model of Daphnia sinking rate. Zooplankton migrations mediate trophic interactions and web food structure in pelagic ecosystems. An understanding of the potential environmental determinants of this behaviour is therefore essential to our knowledge of ecosystem functioning.

     

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.     

Impact of food concentration on diel vertical migration behaviour of <Emphasis Type="Italic">Daphnia pulex</Emphasis> under fish predation risk

Vertical migration of Daphnia represents the best-studied predator-avoidance behaviour known; yet the mechanisms underlying the choice to migrate require further investigation to understand the role of environmental context. To investigate the optimal habitat choice of Daphnia under fish predation pressure, first, we selected the individuals exhibiting strong migration behaviour. The animals collected from the hypolimnion during the daytime were significantly larger, being more conspicuous, and in turn performed stronger diel vertical migration (DVM) when exposed to fish cue. We called them strong migrants. Second, we provided the strong migrant D. pulex with food at high and intermediate (1 and 0.4 mg C l⁻¹, respectively) levels, which were well above the incipient limiting level and of high quality. They traded the benefits of staying in the warm water layer and moved down to the cold water in response to fish cue indicating fish predation. The availability of food allowed the animals to stay in the cold hypolimnion. However, at the low food level (0.1 mg C l⁻¹), which is an additional constraint on fitness, Daphnia moved away from the cold hypolimnion. Poor food condition resulted in strong migrant Daphnia to cease migration and remain in the upper warmer water layer. Although temperature is known to be a more important cost factor of DVM than food, our results clearly show that this is only true as long as food is available. It becomes clear that food availability is controlling the direction of vertical positioning when daphnids experience a dilemma between optimising temperature and food condition while being exposed to fish cue. Then they overlook the predation risk. Thus, the optimal habitat choice of Daphnia appears to be a function of several variables including temperature, food levels and fish predation. Handling editor: S. I. Dodson     

Ultrastructure of the epidermis of Meara stichopi (Platyhelminthes,Nemertodermatida) and associated extra-epidermal bacteria

This preliminary mechanistic model of normal swimming and phototactic behaviour in individual Daphnia was constructed using data and assumptions based on experiments and observations. Swimming under constant light intensity is characterized by short periods of upward movements alternating with equal periods of downward movements. Two oscillators are proposed that generate these phases in swimming. Unexpected shifts in depth, as observed in D. magna and D. longispina, are also present in the swimming of the computer daphnid and thus seem to be inherent to the underlying mechanism. As in real daphnids, during relative decreases in light intensity of low velocity, positive phototactic upward swimming is stepwise. With increasing velocity in the change in light, these steps disappear. When the model is triggered by a natural increase in light at dawn, a small downward movement results. Migration distance can be increased to commonly found depths of migrating Daphnia by the introduction of a fish exudate factor into the model, which enhances the phototactic response. Since attenuation of light in the water affects the phototactic swimming response, it also influences migration distance. The results of model calculations agree quite well with an empirical relationship between Secchi disc depth and amplitude of diel vertical migration in a number of lakes.     

Role of melatonin in the control of depth distribution of <Emphasis Type="Italic">Daphnia magna</Emphasis>

Previous studies confirmed the presence of melatonin in Daphnia magna and demonstrated diurnal fluctuations in its concentration. It is also known that in several invertebrate species, melatonin affects locomotor activity. We tested the hypothesis that this hormone is involved in the regulation of Daphnia diel vertical migration (DVM) behaviour that is well recognized as the adaptive response to predation threat. Using ‘plankton organs’, we studied the effect of three concentrations of exogenous melatonin (10⁻⁵, 10⁻⁷, 10⁻⁹ M) on DVM of both female and male D. magna in the presence or absence of chemical cue (kairomone) of planktivorous fish. Depth distribution was measured six times a day, using infrared-sensitive closed circuit television cameras. Our results showed a significant effect of melatonin on the mean depth of experimental populations, both males and females, but only when melatonin was combined with fish kairomone. Females stayed, on average, closer to the surface than males, both responding to the presence of kairomone by descending to deeper strata. In the presence of exogenous melatonin and with the threat of predation, Daphnia stayed closer to the surface and their distribution was more variable than that of individuals, which were exposed to the kairomone alone. Approaching the surface in the presence of predation threat seems to be maladaptive. We postulate the role of melatonin as a stress signal inhibitor in molecular pathways of response to predation threat in Cladocera.     

Coexistence of two similar copepod species, <Emphasis Type="Italic">Eudiaptomus gracilis</Emphasis> and <Emphasis Type="Italic">E. graciloides</Emphasis>: the role of differential predator avoidance

Habitat choice in relation to environmental factors of two coexisting calanoid copepod species, Eudiaptomus gracilis and E. graciloides, was studied in a mesotrophic lake and in large indoor mesocosms. Both species and sexes showed pronounced diel vertical migration (DVM) in the field. In 12 m deep mesocosms with free ranging fish DVM was observed and species increased day depth over time. No changes were observed in copepod day depth over time in experiments with fish kairomone. It is hypothesized that fish kairomone acts as an early warning system to copepods which respond by moving deeper, but only as far as the thermocline. For full DVM, a nearby mechanical stimulus is necessary. Thus, as fish go deeper to feed, copepods retreat. The response of copepods to fish predation, in the presence of low and high numbers of Daphnia,shows that copepods effectively use Daphniaas living shields to avoid predation. The two species adopt different vertical migration strategies depending on whether there are high or low numbers of Daphniapresent. A dominant feature of mesocosm experiments was the night time aggregating (lekking) of E. gracilis males at the surface. When the spring and autumn percentages of risk takers in the epilimnion were compared, E. gracilis, particularly males, suffered the greatest cost.     

Horizontal distribution of pelagic zooplankton in relation to predation gradients

An investigation was conducted examining the horizontal and vertical distribution of zooplankton in Lake Miramar, a southern California reservoir. Daphnia and Mesocyclops populations were most abundant offshore and in deeper water during the day but appeared to move toward shore and upward at night. The results of inshore zooplankton sampling provided no evidence chat the diel horizontal migration pattern was a result of sampler avoidance by zooplankton. Inshore-offshore differences in Daphnia and Mesocyclops abundance and diel migrations were reduced during winter and early spring. Rotifer zooplankters exhibited less seasonal variation in their horizontal distributions than did the large crustacean zooplankters at all times of the year. It is hypothesized that the spatial distribution of zooplankton is related to predation gradients in Lake Miramar. The dominant planktivore in the reservoir, young-of-the-year Micropterus salmoides. was abundant from late May through December and much less so from January to early May. They were largely restricted to the littoral zone and this produced horizontal gradients of planktivory which varied in strength seasonally and from day lo night. It appears that crustacean zooplankton in Lake Miramar avoid areas with abundant planktivores during the day but migrate into these areas at night when the intensity of planktivory is reduced. Rotifers exhibit less horizontal heterogeneity and no significant diel migrations, which is attributed to the reduced risk of predation that rotifers experience relative to crustacean zooplankters. A graphical model is proposed to integrate our understanding of diel vertical and horizontal migrations of zooplankton. In this model, gradients of predation are completely vertical in offshore areas and strongly horizontal in near shore areas. Gradients of food availability are roughly similar to those of predation intensity. Plankiers respond to these gradients by migrating in a path parallel to gradients of predation at dawn and parallel to gradients of food availability after dark.     

Thermal variation and factors influencing vertical migration behavior in Daphnia populations

The antipredator behavior diel vertical migration (DVM), common in aquatic keystone species Daphnia, involves daily migration from warmer surface waters before dawn to cooler deeper waters after dusk. Plasticity in Daphnia DVM behavior optimizes fitness via trade-offs between growth, reproduction, and predator avoidance. Migration behavior is affected by co-varying biotic and abiotic factors, including light, predator cues, and anthropogenic stressors making it difficult to determine each factor's individual contribution to the variation in this behavior. This study aims to better understand this ecologically significant behavior in Daphnia by: (1) determining how Daphnia pulicaria thermal preferences vary within and among natural populations; (2) distinguishing the role of temperature verses depth in Daphnia vertical migration; and (3) defining how two anthropogenic stressors (copper and nickel) impact Daphnia migratory behavior.Simulated natural lake stratification were constructed in 8 L (0.5 m tall, 14.5 cm wide) water columns to monitor under controlled laboratory conditions the individual effects of temperature gradients, depth, and metal stressors on Daphnia vertical migration. Three major findings are reported. First, while no difference in thermal preference was found among the four populations studied, within lake populations variability among isolates was high. Second, decoupling temperature and depth revealed that depth was a better predictor of Daphnia migratory patterns over temperature. Third, exposure to environmentally relevant concentrations of copper or nickel inhibited classic DVM behavior. These findings revealed the high variability in thermal preference found within Daphnia populations, elucidated the individual roles that depth and temperature have on migratory behavior, and showed how copper and nickel can interfere with the natural response of Daphnia to fish predator cues. Thus contributing to the body of knowledge necessary to predict how natural populations of Daphnia will be affected by climate related changes in lake temperatures and increased presence of anthropogenic stressors.     

Fish and mucus-dwelling bacteria interact to produce a kairomone that induces diel vertical migration in Daphnia

1. Bacterial populations associated with fish have previously been documented to be crucial for the production of chemical signals governing the interactions between predator fish and zooplankton prey. 2. In this study, we investigated the roles of fish and mucus‐dwelling bacteria in kairomone production by conducting two sets of experiments related to elimination of bacteria with antibiotics and using fish mucus in bioassays of Daphnia pulex’s diel vertical migration. 3. Daphnia’s migratory response to the antibiotic‐treated fish was about half the strength of the response to the fish cue treatment. Furthermore, when the same antibiotic‐treated fish were removed from the antibiotic‐containing water and transferred into control water for 24 and 48 h, the extent of D. pulex’s migration depended on the length of the incubation period, apparently corresponding to the regeneration of bacterial colonies associated with mucus. The migration pattern observed in the 24 h treatment was similar to that of antibiotic‐treated fish. On the other hand, a pronounced migration occurred in the 48 h following antibiotic treatment; here, we found a higher density of fish surface dwelling bacteria than at the start of the experiment. 4. In the experiment involving fish mucus, the mucus‐enriched control water induced a weak response similar to antibiotic‐treated fish. 5. On the basis of the results from the two experiments, we suggest that both fish and fish mucus‐dwelling bacteria interact in the release of kairomone in ecologically relevant quantities.     

Two examples of the interplay between field observations and laboratory experiments from 35 years of research with planktonic organisms

Two studies of complicated ecological phenomena in Lake Maarsseveen (The Netherlands) are presented to illustrate that a combination of field and laboratory analysis might be a successful approach. In the first one, the yearly varying ratio of population abundance of two diatoms, Asterionella formosa and Fragilaria crotonensis during early spring is explained by a combination of abiotic and biotic factors. A fungal parasite blocks population development of the first species at temperatures higher than 3–5 °C . The second species then competes successfully for phosphate and develops a large population. The interaction between Asterionella and the fungus proved to be intricate. Several physiological mechanisms operate together and combined with variable climatic factors makes the prediction of what species will be dominant, impossible. In the second example diel vertical migration of the hybrid Daphnia galeata × hyalina is analysed in the field and the laboratory. Migration is confined to six to seven weeks in June–July when large shoals of juvenile perch are present in the pelagic zone of the lake. These fish exudate a kairomone that enhances the reaction of Daphnia to relative increases in light intensity of dawn. Especially accelerations in rate of relative increases enhance swimming velocity. The extent of enhancement depends, however, on the concentration of the kairomone and on food concentration. A Decision Making Mechanism is introduced and the relation between the response mechanism and the adaptive relevance of diel vertical migration is made.Finally considerations about research strategies and the importance of fundamental research are made. It is concluded that the development of limnology and aquatic ecology might be in danger when the present tendency to give prevalence to applied research continues.     

<Emphasis Type="Italic">Chirocephalus sarpedonis</Emphasis> sp. nov. (Branchiopoda,Anostraca, Chirocephalidae) from Turkey questions the monophyly of the traditional <Emphasis Type="Italic">Chirocephalus</Emphasis> species-groups

Here we test for the possible coupling of two kairomone-induced, anti-fish defences in Daphnia, life-history changes (LHC) and diel vertical migration (DVM) mediated by the environmental factor light. A gradient of five different light intensities that represents naturally occurring intensities in the lake water column was used in life-history experiments, and we show that LHC of a single Daphnia clone are inversely coupled to the ambient light intensity. Furthermore, we could show that the light intensity has to exceed a threshold to induce the LHC. We also observed an effect of the light intensity on the fish kairomone-mediated expression of a candidate gene (actin 3) in a way that the gene response differs between groups of low and high light intensities. We argue that the ambient light intensity an individual is exposed to and that is dependent on the position in the water column of a lake inversely determines the degree of LHC. These findings suggest a plastic coupling of these two anti-fish defences in Daphnia (LHC and DVM), which allows for an adjustment to fluctuating environments of standing freshwater ecosystems.     

Interactive effects of fish kairomone and light on Daphnia escape behavior

We investigated the effects of fish kairomone and light intensity on the inducibility and effectiveness of escape behavior in four clones of the water flea Daphnia from different habitats. To characterize and determine the effectiveness of their escape responses, individuals were observed: (i) escaping from the hydrodynamic disturbances of a simulated predator (a small sphere dropped from above); (ii) being preyed upon by small fish (Poecelia reticulata); (iii) responding to encounters with conspecifics in crowded conditions. The simulated predation experiments revealed that when exposed to fish kairomone for 48 h, two Daphnia pulicaria clones, but not two hybrid clones, became about twice as sensitive to fluid disturbances when tested in the light, relative to no-kairomone and dark treatments. When tested in the dark, kairomone had no effect on sensitivity in any clone. All four clones had an all-or-none escape response, in which the strength of the response, as measured by escape distance and speed, was constant regardless of treatment. In the guppy predation trials, kairomone-treated D.pulicaria escaped significantly more often from guppies, in both bright- and dim-light conditions. In dim light, similar to natural lighting conditions, regardless of kairomone, all but the most weakly escaping clone were able to elude attacking guppies in a significant proportion of attacks. Finally, kairomone had no effect on the number of escapes performed by crowded individuals in response to the comparatively weak, non-threatening signals created by other Daphnia, indicating that the kairomone-induced alertness in the D.pulicaria clones did not make them 'excessively' sensitive. The results suggest that Daphnia escape behavior is under complex and efficient environmental regulation, and may play a significant role in aquatic trophic relationships.      

Diel horizontal migration of zooplankton: costs and benefits of inhabiting the littoral

1. In some shallow lakes, Daphnia and other important pelagic consumers of phytoplankton undergo diel horizontal migration (DHM) into macrophytes or other structures in the littoral zone. Some authors have suggested that DHM reduces predation by fishes on Daphnia and other cladocerans, resulting in a lower phytoplankton biomass in shallow lakes than would occur without DHM. The costs and benefits of DHM, and its potential implications in biomanipulation, are relatively unknown, however. 2. In this review, we compare studies on diel vertical migration (DVM) to assess factors potentially influencing DHM (e.g. predators, food, light, temperature, dissolved oxygen, pH). We first provide examples of DHM and examine avoidance by Daphnia of both planktivorous (PL) fishes and predacious invertebrates. 3. We argue that DHM should be favoured when the abundance of macrophytes is high (which reduces planktivory) and the abundance of piscivores in the littoral is sufficient to reduce planktivores. Food in the littoral zone may favour DHM by daphnids, but the quality of these resources relative to pelagic phytoplankton is largely unknown. 4. We suggest that abiotic conditions, such as light, temperature, dissolved oxygen and pH, are less likely to influence DHM than DVM because weaker gradients of these conditions occur horizontally in shallow lakes relative to vertical gradients in deep lakes. 5. Because our understanding of DHM is rudimentary, we highlight potentially important research areas: studying a variety of systems, comparing temporal and spatial scales of DHM in relation to DVM, quantifying positive and negative influences of macrophytes, focusing on the role of invertebrate predation, testing the performance of cladocerans on littoral versus pelagic foods (quantity and quality), investigating the potential influence of temperature, and constructing comprehensive models that can predict the likelihood of DHM. Our ability to biomanipulate shallow lakes to create or maintain the desired clear water state will increase as we learn more about the factors initiating and influencing DHM.     

Depth distribution of Daphnia in response to a deep-water algal maximum: the effect of body size and temperature gradient

1. In the absence of fish predation, Daphnia exploiting a deep‐water algal maximum are faced with a trade‐off. They can either dwell in the epilimnion where development in the warm water is fast, but food shortage causes low egg production, or in the hypolimnion, where food availability is high but development is slow because of low temperatures. 2. We tested the hypotheses that (i) depth distributions of various ontogenetic stages (size classes and egg‐bearing females) differ because daphnids react to light with size‐specific diel vertical migration (DVM) even in the absence of fish (residual predator avoidance hypothesis) and (ii) differently sized daphnids select different depths because the relative importance of temperature and food varies for ontogenetic stages (physiological hypothesis). We used large indoor mesocosms (Plankton Towers) to test these hypotheses experimentally. 3. Temperature was the strongest factor governing the distribution, with larger proportions of the population dwelling in the food‐rich hypolimnion if the temperature gradient was shallow. There were small but significant differences between ontogenetic stages during the day, but not at night. This suggested the existence of a ‘residual’ effect of light on depth distribution in the absence of a fish cue. 4. Although large individuals exhibited greater amplitude of DVM, the physiological hypothesis had to be rejected. A stage‐specific physiological effect is unlikely to be directly triggered by light, hence vertical movement of the individuals should not be synchronised. Rather, being forced into deeper layers by the residual light response during the day, large and egg‐bearing females experience a lower average temperature during day than juveniles. They probably compensate for this by spending longer time periods in warm waters at night.     

Finding the optimal vertical distribution: behavioural responses of Daphnia pulicaria to gradients of environmental factors and the presence of fish

1. The vertical distribution of zooplankton results from active habitat choice aiming to optimise fitness gain in a system of trade‐offs. 2. Using large, controlled indoor mesocosms (Plön Plankton Towers), we monitored the behavioural response of Daphnia pulicaria to vertical gradients of temperature, food, oxygen and light, in the presence and absence of fish predation. 3. In the absence of fish, Daphnia distributed as predicted by an ideal ‘free distribution with costs’. If the food was distributed homogeneously, they stayed in the warm epilimnion, while they balanced their time dwelling in epi‐ and hypolimnion if the food was concentrated in a deep‐water maximum. 4. However, oxygen depletion in the hypolimnion, representing an additional cost, prevented Daphnia from completely exploiting the hypolimnetic food maximum. Consequently, the proportion dwelling in the hypolimnion was larger if oxygen was not limiting. 5. Fish predation had an overwhelming effect, driving Daphnia into the hypolimnion under all experimental conditions. If permitted by oxygen availability, Daphnia used the whole hypolimnion, but oxygen depletion reduced their possible habitat to the upper hypolimnion with oxygen concentrations above c. 0.7 mg L^?1. As fish were less tolerant of low oxygen, the layer below the thermocline formed a predation refuge for Daphnia.     

Daphnia size structure,vertical migration,and phosphorus redistribution

The timing and magnitude of diel migration in two daphnid assemblages were determined from a series of vertical profiles of daphnid size distribution. Animals were collected concurrently for gut fullness determination. Only large daphnids (> 1.4 mm) migrated, but these animals could account for substantial vertical and diel differences in phosphorus excretion rate. Gut fullness measurements and time courses of diel vertical migration suggested that large Daphnia can cause a net downward flux of phosphorus during summer in thermally stratified lakes.     

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.     

Diel pattern of pelagic distribution and feeding in planktivorous fish

Summary The spatial distribution of juvenile roach (Rutilus rutilus), rudd (Scardinius erythrophtalamus), bream (Abramis brama) and bleak (Alburnus alburnus) was registered by echosounding during two years in small Bavarian lakes. The gut contents of the fish were analysed in order to reconstruct their rhythmicity of food intake. Fish were found in the pelagic zone only during the night, feeding before midnight with maximal rate. The vertical position of the fish was dependent on the water transparency following a light intensity in the order of 10^-3 Lux. The diel migration between littoral and pelagic zones was a mainly horizontal one with a vertical component. Both the speed of increase of fish density in the open water and of the vertical movement was correlated to the speed of change of light intensity. Shoals broke up into single fish during the offshore migration. The span of presence in the pelagic zone was identical with the duration of the dark period. During winter no diel migration nor periodical feeding took place.