Testing in situ avoidance reaction of vendace, Coregonus albula, in relation to continuous artificial light from stationary vertical split-beam echosounding

Abstract  The in situ avoidance reaction of vendace, Coregonus albula L., in relation to continuous artificial light in terms of a scaring device was tested by split-beam echosounding. The tests were carried out in the mesotrophic Bigge Reservoir (Germany) in November 2007 and 2008 using a Simrad EK 60 echosounder (120 kHz). An underwater lamp with light turned on and off was lowered to 30 m water depth at four sampling sites. No reaction of vendace was observed for the sampled references (light off), but strong avoidance behaviour by swimming mainly downwards was observed when the light was turned on. The light avoidance reaction was confirmed by the volume backscattering strength ( S _v) and calculated fish densities (fish ha⁻¹). The mean fish densities for the samples carried out with light turned on were lower (268 fish ha⁻¹, ±409 SD, n  =   32) than for the references with light turned off (5028 fish ha⁻¹, ±2317 SD, n  =   56). The related median values (33 and 3726 fish ha⁻¹, respectively, n  =   88) differed significantly (Mann–Whitney test, P  = <0.001). In conclusion, artificial light has great potential to scare pelagic vendace from areas near the bottom outlets of reservoirs to prevent entrainment losses.     

Communal pair spawning behaviour of vendace (Coregonus albula) in the dark

Mating in nature is rarely random, and most fish species have refined mating systems. The vendace (Coregonus albula) is a short‐lived, small‐sized, cold‐water‐adapted pelagic schooling species that is known to spawn in groups, but the actual mating system of this species, like many other group‐spawning fishes, has not been described in detail. Vendace typically spawn in the littoral or sublittoral zones of lakes in late autumn, and the hatching of larvae occurs close to icebreak in the following spring. In our large study lake, vendace larvae were caught in 93% of 1,149 random sampling locations lake‐wide. We examined the courtship and mating of vendace under experimental conditions by nonintrusive observation of the natural behaviour, to clarify whether spawning activity is associated with illumination and to assess the postspawning mortality of vendace. Here, we describe and document in detail for the first time the spawning behaviour of vendace: they spawn in the dark, and females release a small portion of their eggs (on average 1% of mean total individual fecundity) when the female and male, side by side, dart from near the bottom up towards the surface, that is perform a spawning rise. Males and females had several spawning rises (on average 1,200). Our results showed high postspawning mortality (56%). The spawning stress seems to be a potential component of mortality regulating the lifespan duration of vendace.     

Effects of UV radiation on the vertical distribution of vendace [Coregonus albula (L.)] larvae in Finnish lakes

Abstract – Positively phototactic vendace larvae may be vulnerable to enhanced springtime UV levels unless they are able to avoid them. Experimental results, however, suggested that UV avoidance can exist. In this study field data from eight Finnish lakes with different characteristics were analysed. The aim of the study was to determine whether vendace larvae stayed deeper in the water column during sunny than during cloudy periods because thick cloud cover considerably reduces UV irradiance. In addition, avoidance behaviour of larvae was studied in acrylic tubes placed in a lake and under laboratory conditions. The avoidance of high UV-B exposure existed both in the littoral and pelagic zone of the lakes and in laboratory with low UV attenuation. In the lakes with high UV attenuation, avoidance behaviour did not exist. Vendace larvae may use visible light as an indirect indicator of harmful UV-B irradiance.     

Recruitment of sympatric vendace (Coregonus albula) and whitefish (C. lavaretus) is affected by different environmental factors

Vendace and whitefish in Lake Osensjøen (boreal south‐east Norway) were studied by means of gillnet test fishing (1976–2013) and hydro acoustic acquisition (1986–2011). Vendace increased in number between 1998 and 2009 while growth and size at maturity decreased. The relative density of whitefish decreased in the pelagic habitat, whereas growth and size remained the same. Both species exhibited varying year‐class strength. Whereas strong year‐classes of both species became less frequent after 1980 than before this, this seemed to change after 2000, especially for vendace. Generalised additive models suggested a strong positive relationship between July/August mean air temperature and year‐class strength of both vendace and whitefish. Whitefish recruitment was also negatively affected by the new regulation regime implemented since 1981, and positively correlated with water level after hatching and by late ice off. The reason for the diverging impacts of environmental factors on the two species may be that vendace spawn in deeper waters that whitefish do. The results indicate that increasing summer temperatures benefit recruitment of both species, whereas low water level and early ice off will harm whitefish recruitment. Both trends are caused by climate warming. It may be speculated that increased density of the specialised plankton feeding vendace may affect the algal community through increased predation on herbivorous zooplankton and potentially affect the trophic state of the lake.     

Polar light regime and piscivory govern diel vertical migrations of planktivorous fish and zooplankton in a subarctic lake

Abstract –  Seasonal variation in light intensity has strong impacts on invertebrate and vertebrate habitat selection creating trade-offs between foraging gain and risk of death. Diel vertical migration (DVM) has received a particularly interest, but multitrophic level studies in lakes under polar light regime have not been conducted. Here, we examined habitat selection of pelagic zooplankton, planktivores and piscivores in subarctic Lake Muddusjärvi with polymorphic whitefish ( Coregonus lavaretus (L.)). Seasonal change in light was hypothesized to be the most important abiotic factor inducing DVM, whereas predation was considered as an ultimate biotic factor. During period of mid-night sun in June, no DVM was observed at any trophic level, whereas during normal day and night light in September planktivores and zooplankton migrated. DVM was top-down controlled, where piscivorous brown trout ( Salmo trutta L.) used pelagic habitat continuously inducing DVM of its main prey, pelagic whitefish morph, which cascaded to reverse DVM of zooplankton. Top-down control of lower trophic level DVMs by piscivores might be more general pattern in lakes than previously considered.     

Bioenergetic evaluation of diel vertical migration by bull trout (Salvelinus confluentus) in a thermally stratified reservoir

Many species living in deeper lentic ecosystems exhibit daily movements that cycle through the water column, generally referred to as diel vertical migration (DVM). In this study, we applied bioenergetics modelling to evaluate growth as a hypothesis to explain DVM by bull trout (Salvelinus confluentus) in a thermally stratified reservoir (Ross Lake, WA, USA) during the peak of thermal stratification in July and August. Bioenergetics model parameters were derived from observed vertical distributions of temperature, prey and bull trout. Field sampling confirmed that bull trout prey almost exclusively on recently introduced redside shiner (Richardsonius balteatus). Model predictions revealed that deeper (>25 m) DVMs commonly exhibited by bull trout during peak thermal stratification cannot be explained by maximising growth. Survival, another common explanation for DVM, may have influenced bull trout depth use, but observations suggest there may be additional drivers of DVM. We propose these deeper summertime excursions may be partly explained by an alternative hypothesis: the importance of colder water for gametogenesis. In Ross Lake, reliance of bull trout on warm water prey (redside shiner) for consumption and growth poses a potential trade‐off with the need for colder water for gametogenesis.     

Long-term dynamics of pelagic fish density and vendace (Coregonus albula (L.)) stocks in four zones of a lake differing in trawling intensity

Abstract – An 11-year time series of hydroacoustic fish density estimates and fisheries statistics of vendace ( Coregonus albula (L.)) from four zones of a lake differing in trawling intensity was analyzed in order to test the hypothesis that intensive trawling has detrimental effects on pelagic fish stocks, especially vendace recruitment. The standardized fish density estimate in trawled zones showed no decrease in comparison to the non-trawled zone. No signs of recruitment failure associable with trawling intensity were found. The growth of vendace at the end of the study period was slower than that at the beginning, indicating a higher density, most clearly so in the zone with highest trawling intensity. No significant correlations were detected between 3-year mean trawling intensity and yield per unit effort of over-1-year-old vendace or fish density in any zone. Thus, no evidence to support the hypothesis was found. This was suggested to be due to density-dependent compensatory processes in recruitment and/or natural mortality effectively counteracting the population change induced by exploitation. ^Note     

Diel feeding periodicity in larvae of the vendace (Coregonus albula L.) and influence of food availability and environmental factors on food intake

Abstract— Samples were taken every 2 h for 10 days to determine diel foraging periodicity of vendace larvae in the littoral zone of Lake Lentua, northern Finland and to assess environmental factors affecting food intake. Feeding was associated with daylight conditions, so feeding ceased in the dark. Soft-bodied, rapidly digested rotatorians indicated the periodicity more clearly than hard-shelled prey such as cladocerans and copepodites. Wind events and waves in the littoral zone were the major factors affecting food abundance and feeding by the larvae. Waves induced by strong inshore winds reduced zooplankton density, increasing the occurence of larvae with empty guts to 80%. Reduced food availability was reflected in the guts of larvae within 6 h. The density of suitably-sized zooplankton, together with wind effects and photoperiodism, explained most of the variation in gut content in the larvae.     

An experimental approach to estimating vulnerability of European whitefish (Coregonus lavaretus) ova to predation by invasive ruffe (Gymnocephalus cernuus)

Abstract – The complexity of substrate covering spawning grounds of Coregonus lavaretus is assumed to have a role in the protection of incubating ova from predation. It is believed that the Loch Lomond population of C. lavaretus is adversely affected by invasive ruffe (Gymnocephalus cernuus) predation on eggs. To discover the protective ability of substrate commonly found on whitefish spawning grounds, predation experiments of ruffe on artificial eggs were conducted. These were presented to ruffe over different substrates: sand, gravel, pebbles and cobbles. It was found that the greatest protection was provided by pebbles and gravel. Eggs are exposed on sand, but are protected by small gaps between pebbles and gravel, while in cobbles the gaps between substrate particles are large enough to sometimes allow ruffe to foraging within the substrate. Using these results, a comparison between the potential protective ability of substrates of spawning grounds in four Scottish whitefish sites was attempted.     

Plasticity in the diel vertical movement of two pelagic predators (Prionace glauca and Alopias vulpinus) in the southeastern Indian Ocean

Management and conservation of marine predator species relies on a fundamental knowledge of their movements and behaviour. Pop‐up satellite archival tags were used to investigate the vertical movement patterns of five blue sharks (Prionace glauca) and one thresher shark (Alopias vulpinus) within the southeastern Indian Ocean. Sections of similar depth distribution, identified using a split moving window analysis, were investigated in relation to the thermal structure of the water column and activity rates. Minimum horizontal displacement of between 66 and 5,187 km for blue sharks and 16 km for the thresher shark were recorded over 863 tracking days. Maximum depths ranged from 540 to 807 m for blue sharks and 144 m for the thresher shark. All sharks displayed plasticity in their depth distribution, with diel vertical movements and surface‐oriented movements the two most common patterns. Diel movement of prey is the most likely explanation for diel vertical movements of thresher and blue sharks. This study has improved our understanding of the vertical movement patterns of these pelagic predators and the relationship between their depth distribution, temperature, and activity.     

Secretion of hatching enzyme and its proteolytic activity in coregoninae (Coregonus albula L andC. lavaretus L) embryos

After the electrial stimulation Coregoninae embryos secreted the hatching enzyme (chorionase) within 0.1–0.5 h, and the dissolution of their chorions lasted 1.2–2.0 h, depending on embryo's developmental stage (DS 13 or DS 14) and water temperature (5.2 or 9.6–9.8°C).Crude chorionase (hatching liquid) ofCoregonus albula andC. lavaretus was collected in large quantities by means of the electric stimulation of eggs. In both species the temperature optimum of proteolytic activity of the crude chorionasc was 30°C; the activity was lost at temperatures < 3-2°C and > 35–40°C. The maximal proteolytic activity was observed at pH 8.5; a rapid decrease in enzyme activity was evident at pH < 7.0, and the activity was zero at pH 6.The temperature-activity curve of chorionase may reflect the adaptation of Coregoninae to hatching immediately after the ice cover recedes from lakes, whereas the rapid decrease of enzyme activity at pH 7 -pH 6 can affect adversely the process of hatching in acidified lakes.     

Patterns of movement and habitat use by tigerfish (Hydrocynus vittatus) in the Upper Zambezi River (Namibia)

Abstract – During 6 November to 24 December 2000, 23 tigerfish [( Hydrocynus vittatus ), 30–54 cm] were tagged with radio transmitters in the Zambezi River (Namibia) to record habitat utilisation during low, rising and high water levels. The fish were tracked, on average, every fourth day during 23 November to 18 May. Two movement patterns were detected. Approximately 50% of the fish moved <1000 m among tracking surveys, staying within 'defined' home ranges. The remaining fish showed consistent site fidelity for periods, with long distance movements (>1000 m) to new areas among residency periods. Overall, mean distance moved between tracking surveys was 1447 m. Home range size varied among individuals, with a 95% probability of localisation within an average area of 276,978 m². The fish utilised a mean river stretch of 18,836 m (range = 90–71,840). All the fish were recorded in the main stem, and on average, 95% of the fixes were in the main stem during low water. However, the fish used temporary flooded areas to an increasing extent during the rising and high water period, but did not undertake long-distance migrations into the floodplains. Fish were sometimes near vegetation, but were never recorded into or under vegetation.     

Seasonal differences in the diel movements of Macquarie perch (Macquaria australasica) in an upland reservoir

Knowledge of the movement patterns of threatened fishes aids their conservation management and informs sustainable development of natural resources including freshwater. Prior to the expansion of a temperate upland reservoir in Australia, we used radio telemetry to compare seasonal differences in the diel movements of adult endangered Macquarie perch (Macquaria australasica) (235–414 mm total length) in Cotter Reservoir, Australian Capital Territory. Macquarie perch exhibited a diel range of 389 ± 46 m, a diel mobility of 769 ± 93 m and diel area use of 24008 ± 5595 m² among four seasons. Diel range was significantly higher in winter compared with other seasons, and diel mobility was negatively correlated with fish size among four seasons. Macquarie perch inhabited deeper water in summer across the diel cycle (7.2 ± 0.5 m) in comparison with other seasons (spring: 3.8 ± 0.3 m, autumn: 3.0 ± 0.2 m, winter: 3.0 ± 0.3 m). Within seasons, both remote and manual telemetry identified that diel activity was predominantly crepuscular. Prey availability and the threat of avian predation represent the most likely explanations for the differences in seasonal movements. In a reservoir where expansion will likely result in a loss of critical habitat (emergent macrophytes) for this endangered species, we use site‐specific knowledge of the spatial ecology of Macquarie perch to inform the placement of shelter habitat prior to filling the enlarged reservoir.     

Interspecific competition between restocked 0+ salmon parr (Salmo salar) and native fish species in a large European river

Atlantic salmon populations have experienced strong declines in most large European rivers. In the Loire Basin, France, a major restocking effort has been carried out on the Allier River to reduce the wild population decline. This study aimed at investigating the trophic insertion of hatchery‐reared salmon parr and whether they compete for resources with native fish species. For this purpose, two riffles located in the upstream and downstream parts of the restocked area have been surveyed monthly following the initial 0+ salmon fry release. 0+ parr densities remained high upstream, with autumnal values greater than those recommended to maximise smolt production. In contrast, downstream densities declined to weak values despite a faster increase in size, probably mainly due to summer temperatures rising above the discomfort threshold. By analysing the vulnerability to predation of ingested invertebrate prey, we highlighted the role of habitat constraints in driving most of the different fish species to a likely opportunistic utilisation of available food resources. As a result, species such as stone loaches and barbel juveniles exhibited narrow trophic niches and high overlaps with 0+ parr. Nonetheless, both abundant prey resources and spatial or temporal habitat segregation have limited interspecific competition, enabling competitive coexistence of sympatric species, except for barbel juveniles in certain circumstances. Therefore, our findings suggest that future restocking programmes should carefully consider the thermal and hydraulic conditions regarding further smolt production as well as their general trophic context, according to the potential inconvenience of restocked salmon fry arrival relative to native species.     

Effects of day and night on swimming,grazing and social behaviours of rohu Labeo rohita (Hamilton) and common carp Cyprinus carpio (L.) in simulated ponds

Diel rhythmicity of grazing, swimming, resting and social interactions of rohu (Labeo rohita) (weight 66.5–68.3 g) and common carp (Cyprinus carpio) (79.9–82.0 g) were observed in 1 m² simulated ponds using video images. Fish behaviour was monitored during a full 24‐h period, starting at 08:00 hours with a 15‐min recording, which was repeated every 3 h. Rohu spent more time grazing during the day than at night. Grazing activity peaked at the beginning and at the end of the day. Grazing and resting of rohu were negatively correlated. Common carp grazed day and night, showing no distinct grazing peak. Considering the grazing time of rohu and common carp, a feeding schedule with two feeding times at 07:30 and 16:30 hours might be appropriate for better food intake and conversion in rohu–common carp polyculture system.     

Size-related habitat use by bitterling (Rhodeus sericeus) in a regulated lowland river

Abstract  – Habitat use by four size-classes of bitterling ( Rhodeus sericeus ) in a regulated lowland river was studied at two spatial scales. Mesohabitat scale was represented by the bank type (boulder, beach and steep eroded bank). Microhabitat associations with substrate types, vegetation, woody debris and position in the stream were examined at each mesohabitat scale. Three size cohorts of young-of-the-year (0+) fish and a group of adult bitterling were detected. Univariate ( χ ²-tests) and multivariate (Outlying Mean Index analysis) methods were applied to determine size-related habitat associations. All the size-classes of bitterling avoided the steep eroded bank and most 0+ bitterling were distributed along the boulder bank. The smallest fish had the most pronounced habitat requirements and occurred mostly in the depositional areas, whereas the larger 0+ fish occurred in deeper water. The size-related change in the habitat use coincided with a change in diet and morphological state.     

Plasticity in vertical behaviour of migrating juvenile southern bluefin tuna (Thunnus maccoyii) in relation to oceanography of the south Indian Ocean

Electronic tagging provides unprecedented information on the habitat use and behaviour of highly migratory marine predators, but few analyses have developed quantitative links between animal behaviour and their oceanographic context. In this paper we use archival tag data from juvenile southern bluefin tuna ( Thunnus maccoyii , SBT) to (i) develop a novel approach characterising the oceanographic habitats used throughout an annual migration cycle on the basis of water column structure (i.e., temperature-at-depth data from tags), and (ii) model how the vertical behaviour of SBT altered in relation to habitat type and other factors. Using this approach, we identified eight habitat types occupied by juvenile SBT between the southern margin of the subtropical gyre and the northern edge of the Subantarctic Front in the south Indian Ocean. Although a high degree of variability was evident both within and between fish, mixed-effect models identified consistent behavioural responses to habitat, lunar phase, migration status and diel period. Our results indicate SBT do not act to maintain preferred depth or temperature ranges, but rather show highly plastic behaviours in response to changes in their environment. This plasticity is discussed in terms of the potential proximate causes (physiological, ecological) and with reference to the challenges posed for habitat-based standardisation of fishery data used in stock assessments.     

Need for speed: Preference for fast‐flowing water by the endangered semi‐aquatic Pyrenean desman (Galemys pyrenaicus) in two contrasting streams

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Evaluating spatio‐temporal variability in the habitat quality of Atlantic cod (Gadus morhua) in the Gulf of Maine

Reduced abundance and contracted spatial distribution of Atlantic cod, Gadus morhua, in the Gulf of Maine (GOM) may indicate large spatio‐temporal variation in their habitat quality. Season‐specific Habitat Suitability Index (HSI) models were developed to quantify such variation in the offshore GOM management area. Data used were non‐zero cod catch rates with calibrations from the Northeast Fisheries Science Center (NEFSC) spring and fall bottom trawl surveys over the period 1982–2013 and key physical environmental variables including depth, bottom temperature, bottom salinity and sediment types. Significant declines were found in the average HSI across the study area in the springs of early 2000s and 2010s. These low average HSI values coincide with reduced age‐1 recruitment of GOM cod stock after the mid‐1990s. Moreover, the western coastal areas of the GOM generally exhibited higher average HSI values than the eastern coastal areas, whereas the offshore areas always had the lowest average HSI. Relatively higher cod survey catch rates in the western GOM may imply positive influences of environmental controls on the distribution of GOM cod.     

Preliminary telemetry data on the movement patterns and habitat use of European catfish (Silurus glanis) in a reservoir of the River Ebro, Spain

Abstract –  Knowledge of the movements and diel behaviour of the European catfish ( Silurus glanis ), the largest European freshwater fish, is limited to anecdotal information. In a preliminary telemetry study of European catfish, the spring diel movement patterns of five adult catfish were examined. After intraperitoneal insertion of the acoustic tags, the positions of the fish were recorded automatically in the Flix Reservoir (River Ebro, NE Spain). A marked nocturnal mobility pattern was observed throughout the study. During daytime, the catfish were consistently located in the littoral zone and spent extended periods of the day hidden in concealed habitats. Catfish movements were in a radial pattern, with upstream and downstream excursions followed by returns to a previously occupied location. Significant individual variations in movement pattern were observed among the tagged fish and within the 24 h cycle for each fish. Mean instantaneous swimming speed was 0.17 body lengths per second (BL·s⁻¹) at night but 0.09 BL·s⁻¹ during the daytime.