Otolith microstructure during the pelagic, settlement and benthic phases in burbot

During the pelagic larval phase of burbot Lota lota L., the pattern of otolith increments changes, showing three, clearly distinguishable growth sectors: a first sector with faint increments, difficult to enumerate, comprising an average (±S.D.) of 17˙5±6˙7 increments, a second sector with distinct increments comprising an average of 33˙1±7˙6 increments and a third sector where increments again become faint and difficult to enumerate. Laboratory experiments conducted in parallel to the field investigation showed that settlement occurs after the formation of this third, faint sector and is marked by the formation of numerous accessory growth centres within the range of three to five daily increments. There was a strong linear relationship between sagittal width and total length of the burbot (r²=0˙928) over the range examined. Significantly different growth rates were calculated for the three otolith sectors (faint, distinct, faint) in burbot larvae, indicating large environmental changes during their pelagic larval phase in Lake Constance. These results suggest that inshore migration of burbot larvae does not take place in the warm epilimnetic surface waters but via an alternative pathway, the cold hypolimnion or profundal zone.     

Impact of temperature on food intake and growth in juvenile burbot

The effect of temperature on food consumption, food conversion and somatic growth was investigated with juvenile burbot Lota lota (age 0 years). Juvenile burbot showed a significant dome shaped relationship between relative daily food consumption ( C _R) and temperature ( T ) with C _R = − 0·00044 T ² + 0·01583 T  − 0·06010; ( n  = 90, r ² = 0·61). Maximum C _R was at 17·9° C (95% CL 17·2–18·6° C). The temperature related instantaneous growth rate ( G ) also followed a dome shaped function with G  = − 0·000063 T ² + 0·002010 T  − 0·007462; ( n  = 95, r ² = 0·57), with maximum growth rate at 16·0° C (95% CL 15·3–16·6° C). A significant linear relationship was found between the water temperature and the conversion coefficient ( C _C) with C _C = − 1·63 T  + 59·04; ( n  = 80, r ² = 0·74). The results indicate that juvenile burbot in large lakes benefit from higher water temperatures in the littoral zone, by increased food uptake and growth, especially during the warm summer months. Because profundal water temperatures do not reflect the optimal temperature for food consumption in large burbot, temperature is unlikely to be the main proximate factor for the obligate littoral‐profundal migration of juvenile burbot observed in many lake populations.     

Effects of regulated discharge on burbot migration

Burbot Lota lota movement and river discharge were studied in the Kootenai River, Idaho, U.S.A. and British Columbia, Canada, downstream of Libby Dam, Montana, U.S.A. A total of 24 adult burbot with transmitters were tracked from 1994 to 2000, for analysis of a travel distance of ≥5 km in ≤10 days termed 'stepwise movement'. Of 44 'stepwise movements', significantly greater movements during pre‐spawning and spawning were observed when average daily discharges from Libby Dam were <300 m³ s⁻¹, with a mean of 176 m³ s⁻¹, similar to pre‐dam conditions. Burbot travelled at a greater rate during all seasons (3·36 km day⁻¹) at discharges >300 m³ s⁻¹(mean = 1·84 km day⁻¹) than at discharges >300 m³ s⁻¹ but no difference was found for the pre‐spawning and spawning period. Burbot that started 'stepwise movements' in low discharge conditions frequently stopped during low discharges.     

Coupling of strontium and calcium cycles in Lake Constance

The contents of Sr and Ca were measured weekly in Lake Constance in 1986. Epilimnetic concentrations of Ca changed between 1.30 × 10^-3 mol l^-1 (during homothermy) and 0.9 × 10 -3 mol l^-1 (during thermal stratification). The seasonal fluctuations of Ca were correlated with those of Sr (between 4.61 and 5.36 mol l^-1). The epilimnion was permanently oversaturated with respect to calcite but not with respect to SrCO₃. Analysis of the settling process by use of sedimentation traps revealed two short episodes of very high authigenic settling fluxes of CaCO₃, triggered by phytoplankton diatoms. Seasonal changes of the Ca contents (between 4.1 and 30.7 percent of the dry weight) and of the Sr concentrations (from 12 to 75 × 10^-3 percent) in the settling material were closely correlated. This suggests a coprecipitation mechanism with a nearly constant stoichiometry of (atoms Sr/atoms Ca) × 1000 of 0.84. Coprecipitation of Sr or Ca with organic matter was insignificant. In the hypolimnion some Sr and Ca were released from the settling material. These results strongly suggest that the cycle of Sr in Lake Constance is driven predominantly by coprecipitation with calcite. The principal chemical mechanisms leading to coprecipitation are discussed.     

Human impact as registered in the pollen record: some results from the western Lake Constance region,Southern Germany

Pollen analytical results from a littoral profile taken in Lake Constance compared with pollen profiles from small kettle holes nearby form the basis for conclusions concerning human population density, the economy and environment from the Neolithic period to the Middle Ages. Early Neolithic human impact is implicated in a lime decline and also the expansion of beech. The late Neolithic lakeshore settlements caused a decline of elm, beech and lime and, by shifting cultivation, considerably changed the forest cover. The settlements were abandoned after less than 100 years. There were long periods without distinct human impact in the middle and towards the end of the late Neolithic period. Since at least the Late Bronze Age there has been permanent habitation in the region. Human impact was greatest in the High Medieval period and later, and was also substantial in the late La Tène and Roman periods. Distinct declines in human impact can be observed between the La Tène and Roman periods and in the Migration and Merovingian periods. In these intervals, open land and grazed oak forest were replaced by birch and later on by beech forests. The decreases in human impact are not of the same intensity in all diagrams.     

Trophic interactions among heterotrophic microplankton,nanoplankton, and bacteria in Lake Constance

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 µm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics.Clearance rates varied between 0.2 and 122.8 nl HNF^–1 h^–1 and between 0.2 and 53.6 µl ciliate^–1 h^–1, respectively.Ingestion and clearance rates measured for HNF and ciliates are in good agreement with results obtained by other investigators from different aquatic environments and from laboratory cultures. Both the abundance of all three major microheterotrophic categories — bacteria, HNF, and ciliates — and the grazing pressure within the microbial loop show pronounced seasonal variations.     

The influence of light on the diel vertical migration of young-of-the-year burbot Lota lota in Lake Constance

The diel vertical distribution of young-of-the-year (YOY) burbot Lota lota in the pelagic zone of Lake Constance was compared to light intensity at the surface and to the light intensity at their mean depth. Lota lota larvae inhabited the pelagic zone of Lake Constance from the beginning of May until the end of August. From early June, after the stratification of the water column, fish performed diel vertical migrations (DVM) between the hypolimnion and epilimnion. The amplitude of DVM increased constantly during the summer and reached 70 m by the end of August. Lota lota started their ascent to the surface after sunset and descended into the hypolimnion after sunrise. As the YOY fish grew from May to August, they experienced decreasing diel maximum light intensities: in May and early June L. lota spent the day at light intensities >40 W m⁻², but they never experienced light intensities >0·1 W m⁻² after the end of June. From this time, L. lota experienced the brightest light intensities during dusk and dawn, suggesting feeding opportunities at crepuscular hours. The present study implies, that YOY L. lota in the pelagic zone of Lake Constance increased their DVM amplitude during the summer to counteract a perceived predation risk related to body size and pigmentation.     

The role of phytoplankton in determining the underwater light climate in Lake Constance

At all seasons, the underwater light field of meso-eutrophic large (480 km²) deep (mean: 100 m) Lake Constance was studied in conjunction with the assessments of vertical distributions of phytoplankton chlorophyll concentrations. Vertical profiles of scalar, downwelling and upwelling fluxes of photosynthetically available radiation, as well as fluxes of spectral irradiance between 400 and 700 nm wavelength were measured.The overall transparency of the water for PAR is highly dependent on chlorophyll concentration. However, the spectral composition of underwater light is narrowing with water depth regardless of phytoplankton biomass.Green light is transmitted best, even at extremely low chlorophyll concentrations. This is explained by the selective absorption of blue light by dissolved organic substances and red light by the water molecules. Nevertheless, significant correlations were found between vertical attenuation coefficients of downwelling spectral irradiance and chlorophyll concentrations at all wavelengths. The slopes of the regression lines were used as estimates of chlorophyll-specific spectral vertical light attenuation coefficients (K _c()).The proportions of total upwelling relative to total downwelling irradiance (reflectance) increased with water depth, even when phytoplankton were homogeneously distributed over the water column. Under such conditions, reflectance of monochromatic light remained constant. Lower reflectance of PAR in shallow water is explained by smaller bandwidths of upwelling relative to downwelling light near the water surface. In deeper water, by contrast, the spectra of both upwelling and downwelling irradiance are narrowed to the most penetrating components in the green spectral range. Reflectance of PAR was significantly correlated with chlorophyll concentration and varied from 1% and 1-% at low and high phytoplankton biomass, respectively. Over the spectrum, reflectance exhibited a maximum in the green range. Moreover, in deeper layers, a red maximum was observed which is attributed to natural fluorescence by phytoplankton chlorophyll.     

Diel and seasonal distribution of perch in Lake Constance: a hydroacoustic study and in situ observations

A near-shore belt 50 km in length was surveyed parallel to the shoreline of Lake Constance, central Europe, with a single-beam echosounder five times between July 1993 and February 1994. The species and age composition of fish in the survey area was investigated by gillnet fishing and SCUBA-diving. In summer, the horizontal distribution of perch was patchy. Population density declined from east to west, and highest densities occurred in one shallow bay and close to ports and jetties at steeper shores. During daytime, perch stayed in the sublittoral zone between 3 and 15 m depth and between 2 and 6 m above the thermocline. Within this layer age classes were separated spatially: the relative number of young-of-the-year perch declined with depth whereas the relative number of adult perch (2+ and older) increased with depth. At dusk the fish migrated to the littoral zone, where they spent the night resting on the bottom. In winter, under almost homothermal conditions, perch of all ages were located between the 35 and 70 m depth contours, where they performed pronounced diel vertical migrations. They rested on, or close to, the bottom during daytime and ascended up to 20 m below the surface at night. During this season, horizontal distribution of perch was much more homogeneous than in summer.     

Acoustic observation of diel vertical migration and shoaling behaviour in Atlantic redfishes

Acoustic methods were used to study Atlantic redfishes Sebastes spp. vertical migration and shoaling behaviour in Newfoundland waters. Redfishes exhibited consistent patterns of vertical migration in winter, spring and summer, but pelagic shoals were not observed in winter. Pelagic daytime aggregations were generally in close proximity to dense patches of redfishes along the sea floor. Pelagic shoals exhibited high degrees of variability in size, shape and density. Attempts to explain variations in shoal density and area with features of shoal position and structure were unsuccessful. Nearest neighbour distance between fish in shoals had a lower limit near one body length. During the night, fishes were dispersed in the water column and distributions were more homogenous. Diel vertical migration appeared to be a foraging strategy, in which redfishes followed the migration of their euphausiid prey.     

Location-dependent infection of fish parasites in Lake Constance

Perch Perca fluviatilis and roach Rutilus rutilus were caught at various locations around Lake Constance (Bodensee). Infection of perch and roach with the eyeflukes Diplostomum spathaceum and Tylodelphis clavata was greatest in the western location (Bottighofen). The varied occurrence of parasite species at different locations indicated that maximum interchange between fish populations was slight. Whitefish Coregonus lavaretus migrating up the River Rhine were less infected with both the tapeworm Proteocephalus exiguus and the copepod Ergasilus sieboldi than whitefish from the middle of the lake. Some of the parasites had already left their hosts. Newly established infections were not found in migrating whitefish.     

Caridina nilotica in Lake Victoria: abundance, biomass, and diel vertical migration

Caridina nilotica (Decapoda: Atyidae) in offshore waters of Lake Victoria were investigated with both day and night sampling over a period of two years. Offshore populations are mainly planktonic rather than benthic, and the animals exhibit diel vertical migrations into near-surface waters at night. These changes in diel abundance as well as the size-frequency distribution of the migrating shrimp suggest that the migratory behavior is in response to visual planktivory, because only the very smallest individuals (2–4 mm) remain in surface waters during the day. During October 1992, abundances were estimated both by vertical net sampling and by underwater video transect methods. Concordance was established between abundances estimated by the two methods. Only about 9% (night) to 14% (day) of the Caridina population appeared to be epibenthic. We suggest that the behavior of the animal is consistent with the hypothesis that it is not a strict detritivore as previously reported; rather it may engage in facultative planktivory, especially at night.     

The role of plankton biomass in controlling fluctuations of suspended matter in Lake Constance

The concentrations of particulate matter, expressed as dry weight (DW), particulate organic (POM), and inorganic material were measured at regular intervals in Lake Constance between February 1980 and December 1982. Maximum particle concentrations were recorded for the euphotic zone in summer (7 mg l⁻¹), while minima occurred during the early summer and in winter. Annual mean concentrations of DW within the entire water column varied between 0.6 and 0.7 mg l⁻¹. In the euphotic zone nearly 70% of DW is organic material. The inorganic particles originate either from phytoplankton (diatomaceous silicon, biogenic decalcification) or from the tributaries. Although phytoplankton biomass only comprises a relatively small proportion (i.e. 30% at maximum) of organic matter, it is the primary source of POM. Therefore, seasonal variations in phytoplankton control epilimnetic concentrations of POM in Lake Constance. Inorganic material comprises smaller proportions of suspended matter. Seasonal variations are related predominantly to fluctuations in biomass and therefore particulate inorganic matter is suggested to originate mainly from autochthonous sources. At the sampling station concentrations of inorganic particles supported by the main tributary, the Alpenrhein, only occasionally vary concomitantly with runoff.     

What goes down must come up: symmetry in light-induced migration behaviour of Daphnia

During a short period of the year, Daphnia may perform a phenotypically induced diel vertical migration. For this to happen, light-induced swimming reactions must be enhanced both at dawn and at dusk. Enhanced swimming in response to light intensity increase can be elicited by fish-associated kairomone in the laboratory, if food is sufficiently available. However, during the light change at dusk the Daphnia are still in the hypolimnion, where no fish kairomone is present and both temperature and food availability is low. Still, what goes down must come up. This raises questions about how Daphnia tunes its light-induced swimming behaviour to prevailing conditions such that a normal diel vertical migration can be performed. We investigated the symmetry in behavioural mechanism underlying these diel vertical migrations in the hybrid Daphnia galeata × hyalina (Cladocera; Crustacea), with special interest for the environmental cues that are known to affect swimming in response to light increase. That is, we tested whether fish- associated kairomone, food availability, and temperature affected both swimming in response to light intensity increase and decrease similarly. We quantified swimming behaviour during a sequentially increased rate of light change. Vertical displacement velocity was measured and proved to be linearly related to the rate of the light change. The slope (PC) of the function depends on the value of the factors kairomone concentration, food availability, and temperature. The changes of the PC with kairomone concentration and with temperature were similar both at light intensity increases and decreases. The PC also increased with food concentration, although during light increases in a different way from during light intensity decreases. Low food availability inhibited swimming in response to light intensity increase, but enhanced swimming in response to light intensity decrease. Hence, ascent from the deep water layers with low food concentration at dusk is facilitated. These causal relations are part of a proximate decision-making mechanism which may help the individual Daphnia to tune migration to predation intensity and food availability.     

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0⁺ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.     

Diel patterns of feeding and vertical migration in daphnids and diaptomids during the clear water phase in Lake Geneva (France)

In lacustrine environments, little attention has been paid to small-scale interactions between zooplankton diel vertical migration (DVM) and feeding rhythms. Moreover, most of the information on in situ diel feeding and migratory rhythms is based on low sampling frequencies. The kinetics and the degree of coupling of these processes are thus only roughly known. Here, we present a study conducted on a diel cycle in Lake Geneva to establish the temporal and spatial relationships between DVM and grazing activity of the dominant planktonic crustaceans. Our methodological approach is based on reliable and frequent (every 30 minutes) sampling, and on gut fullness analysis. We test the hypothesis of temporal and spatial segregation in DVM and feeding activity of sympatric taxa to counteract resource competition. We also evaluate the variation in DVM and feeding activity between taxa, size and sexes. In Lake Geneva, the Daphnia complex of different species and size (D. hyalina × galeata) and the diaptomid (Eudiaptomus gracilis) have distinct DVM and diel feeding patterns which lead to temporal and spatial segregation. Differences arise from the amplitude and kinetics of DVM and diel feeding rhythms. A strong day/night contrast in depth distribution and feeding activity was observed for the large daphnids while the small daphnids and the diaptomids had lower amplitudes of DVM and weaker diel changes in feeding activity. Large Daphnia exhibited a bimodal feeding pattern coupled with dynamic interchange of individuals between the epi- and hypolimnetic layers at dusk and dawn. In contrast, little coupling between DVM and feeding patterns was found for the diaptomid. These distinct behaviours can be viewed as specific adaptive strategies developed by calanids and daphnids to limit interspecific competition and to compromise between avoidance of starvation in deep waters and avoidance of visual predators in surface layers. Our study supports the hypothesis of exogenous control of Daphnia DVM by the relative change in light intensity at dusk and dawn, but also suggests that small Daphnia (not large ones), are controlled by absolute light variations when this major stimulus is lacking. Our results also support the hypothesis that selective predation by fish is responsible for the observed differences in DVM and diel feeding patterns of sized-daphnids and diaptomids. Other factors explaining the coupling of DVM and feeding patterns are hunger, vertical temperature gradient and for daphnids, size. Thus, ecological plasticity in crustacean DVM and feeding patterns results from the interactive effect of multiple abiotic and biotic driving forces. Finally, our study also shows that large Daphnia have a marked contribution to the acceleration of downward nutrient fluxes in Lake Geneva, via their diurnal rhythm in feeding and vertical migration. Ecological implications of the study for lake management and sampling design of zooplankton grazing studies are also presented.

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Résumé Dans les écosystèmes lacustres, les interactions à fine échelle temporelle entre les patrons diurnes de migration verticale et de broutage du zooplancton sont peu étudiées. En outre, jusq'à présent, les études ont généralement été réalisées selon des chroniques temporelles assez lâches. La cinétique et le degré d'interaction entre les patrons journaliers de migration verticale et de broutage sont donc encore mal connus. La présente étude, conduite au Lac Léman (Lac de Genéve) au cours d'un cycle nycthéméral, tente de préciser les liaisons spatiales et temporelles existant entre les migrations journalières et la consommation de phytoplancton chez les taxons de Crustacés les mieux représentés. Notre approche méthodologique repose sur une maille temporelle d'échantillonnage fine et sur l'analyse de la fluorescence du contenu stomacal. Nous testons l'hypothèse d'une ségrégation spatio-temporelle visant à réduire la compétition entre les taxons sympatriques et reposant sur des différences entre les patrons journaliers respectifs de migration et d'alimentation. Nous évaluons pour ces rhythmes d'activité les différences entre les espèces, les classes de tailles et les sexes. Au lac Léman, le complexe de différentes espèces et tailles de daphnies (Daphnia hyalina × galeata) et le diaptomide (Eudiaptomus gracilis) présentent des patrons journaliers de migration et d'alimentation distincts, assurant une ségrégation spatio-temporelle. Les différences proviennent de variations dans la cinétique et l'amplitude des migrations et dans les niveaux d'alimentation. Les grandes daphnies affichent un fort contraste jour/nuit dans leur répartition verticale et leur état de réplétion, tandis que les petites daphnies et les diaptomides présentent une faible amplitude de migration et de variations circadiennes de réplétion. Les grandes daphnies ont un rythme alimentaire bimodal couplé avec un relais dynamique des organismes entre l'épilimnion et l'hypolimnion au crépuscule et à l'aube. Chez le diaptomide, les interactions sont au contraire faibles entre les patrons de migration verticale et de réplétion. Ces différents comportements peuvent être perçus comme des stratégies adaptives spécifiques développées par les daphnies et les diaptomides pour limiter la compétition interspécifique et aboutir à un compromis satisfaisant entre l'évitement de la famine en eaux profondes et de la prédation par les poissons dans les eaux superficielles. Notre étude conforte l'hypothèse d'un contrôle exogène de la migration verticale de Daphnia par les changements relatifs de la lumière au crépuscule et à l'aube. En l'absence de ce stimulus, la répartition verticale des petites daphnies semble par contre contrôlée par les variations absolues de lumière. Nos observations confortent également l'hypothèse que la prédation sélective par les poissons est responsable des différences observées dans les patrons de migration des grandes daphnies et ceux des petites daphnies et des diaptomides. Les autres facteurs pouvant influencer les patrons de migration et d'alimentation des crustacés du Lac Léman sont la famine, le gradient thermique vertical et, chez les daphnies, la taille. En définitive, la plasticité écologique des patrons journaliers de migration et d'alimentation résulte des effets interactifs de daphnies ont un rôle très important dans le transfert des nutriments dans les couches profondes durant l'été, via leurs migrations verticales et les variations circadiennes d'activité alimentaire. Les implications écologiques pour l'aménagement lacustre et la planification des études portant sur le broutage du zooplancton sont aussi présentées.

     

巢湖微囊藻和浮游甲壳动物昼夜垂直迁移的初步研究

2002年10月进行了巢湖微囊藻和几种优势浮游甲壳动物的昼夜垂直变化的研究,结果表明:微囊藻具有明显的昼夜垂直变化现象。白天上层水中的微囊藻密度显著高于下层水中,夜晚逐渐下沉使得下层水中的密度相对高于上层水。微囊藻与叶绿素a、水温、溶解氧和pH等均呈显著的正相关(p<0.01)。几种优势浮游甲壳动物的昼夜垂直迁移存在较大的差异。短尾秀体溞和角突网纹溞白天在下层水(1.5m和2.5m)中的密度较高,夜晚则倾向于在上层水(0m和0.5m)中活动。相反,卵形盘肠溞白天在上层水中密度较高,象鼻溞则在11:00和15:00时各水层中的密度显著高于夜晚。汤匙华哲水蚤和广布中剑水蚤白天倾向于在下层水中活动,夜晚则逐渐迁移到上层水中。许水蚤在夜晚和凌晨3:00时各水层中的密度显著高于白天。中华窄腹剑水蚤昼夜垂直变化不明显。微囊藻与短尾秀体溞密度呈显著的负相关,而与象鼻溞和卵形盘肠溞呈显著的正相关(p<0.01)。     

Vertical distribution and diel migration of flagellated phytoplankton in a small humic lake

The vertical distributions and migrations are described of the most abundant flagellated phytoplankton species from the summer community of a small forest lake in southern Finland. The lake showed a steep and stable thermal stratification with a shallow oxygenated epilimnion. Horizontal variation of phytoplankton distribution within the lake was tested on two scales and found to be statistically significant only in the case of Mallomonas reginae. The vertical distribution of flagellated phytoplankton was assessed by reference to the distribution of a non-motile, neutrally buoyant species Ankyra judayi. Statistically significant, active vertical positioning was demonstrated for all the flagellates examined with the exception of Spiniferomonas bourrellyi. Diel vertical migrations were apparent for all species showing active positioning and the pattern of an evening descent and a morning ascent was ubiquitous. The extent and timing of diel migrations varied between species. The most extensive migrations were by Cryptomonas marssonii which crossed a temperature gradient of 14 °C and penetrated far into the anoxic hypolimnion. Several categories of competitive advantage can be gained by species undertaking such diel vertical migrations.     

The effects of temperature and size on growth and mortality of cod larvae

The optimal temperatures for growth of four groups of hatchery-reared cod larvae (geometric mean weight: 73, 191, 249 and 251 μg), reared on rotifers at four or five constant temperatures between 4 and 16° C for 14, 12, 9 and 16 days were 9.7, 12.3, 12.7 and 13.4° C, respectively. The maximum growth rate also increased with size and was 6.5, 9.6, 11.7 and 11.3% day⁻¹ for the respective size groups. The optimal temperature for survival was 8.5–8.8° C for all size groups. The results indicate an opposite relationship between (1) size and optimal temperature for growth and (2) size and maximum growth rate of cod larvae, to that observed for juvenile and immature cod.     

Movements of ultrasonically tagged brown trout (Sulmo trutta L.) in Lake Constance

In Lake Constance from September 1986 to May 1988 13 adult lake dwelling brown trout ( Sulmo trutta L.) were tagged with ultrasonic transmitters and tracked almost continuously for up to 13 days. Two behaviour types were observed: (a) random movement in locally restricted areas and (b) excursions of up to 40 km distance. Swimming activity during the day was significantly higher than at night in most experiments. In summer swimming depth ranged between 8 and 16 m, and in winter between 0 and 3m. The preferred water temperature was about 14°C in the thermally stratified waterbody. During the experiments mean swimming speed ranged between 0.3 km h⁻¹ (0.1 bodylengths s⁻¹) and 0.9 km h⁻¹ (0.6 bodylengths s⁻¹).