Effect of larval ontogeny, turbulence and light on prey attack rate and swimming activity in herring larvae

The effects of ontogeny (larval size), light and turbulence on the attack rate and swimming activity (proportion of time swimming and duration of swimming bout) of herring larvae (15-28 mm TL) have been investigated. Emphasis was put on the experimental design in order to create a set-up where the turbulence intensity distribution could be accurately measured as well as controlled in the entire experimental tank.Both larval size (ontogeny) and light had a significant positive effect on prey attack rate. Likewise, an intermediate increase in turbulence had a positive effect on prey attack rate, but this effect was dependent of light intensity and larval size.At low light (1.5 μE m² s⁻¹) intermediate turbulence increased the prey attack rate significantly for larger larvae (26 and 28 mm), while at high light (18 μE m² s⁻¹) intermediate turbulence had only a significant positive effect on the attack rate of smaller larvae 20 and 23 mm.In general, our data show a dome-shaped response of turbulence on attack rate and a U-shaped response of turbulence on swimming activity.For herring larvae >20 mm, the maximum (attack rate) and minimum (swimming activity) response of turbulence were found at intermediate turbulence intensities (energy dissipation rates between 7∗10⁻⁸ and 1∗10⁻⁶ W/kg). The highest turbulence level tested (8∗10⁻⁶ W/kg) showed only negative effects, as attack rates where at the lowest and swimming activity at the highest.Swimming activity increased with larval size or light, and decreased at intermediate turbulence. Compared to turbulent intensities under natural conditions this implies that larger herring larvae at 10 m depth have to be exposed to wind speeds of more than 17 m/s before negative effects on attack rate and swimming activity occurs.     

Dispersion and mortality of yolk-sac herring (Clupea harengus L.) larvae from a spawning ground to the west of the Outer Hebrides

A patch of herring larvae located within 24 h of hatching froma spawning bed on the shelf west of the Outer Hebrides, wasfollowed for 7 days. Dispersion of the larvae was modelled usingsimple turbulent diffusion theory. The mortality rate, estimatedfrom the rate of decrease in the total population size, was–2% day^–1 over the tracking period which is verymuch less than rates calculated for herring larvae in otherareas by previous investigators. Successive observations ofthe length, weight, and the incidence of food in the intestinesof larvae, indicated that feeding commenced 5 days after hatchingand that the growth rate was 0.17 mm day^–1.     

Ecology of larval herring in relation to the oceanography of the Gulf of Maine

The dynamics of larval herring in the north-eastern Gulf ofMaine are reviewed with reference to the important physicaland biological oceanographic processes of the region. Particularattention is given to the apparently conflicting hypothesesof larval drift from the tidally well-mixed spawning areas tonursery areas, and larval retention in spawning areas for aperiod of several months. Both processes have been reportedfor the eastern Maine-Grand Manan spawning area of the Gulfof Maine, but the relative importance of each to larval survivalthrough the winter and recruitment to the juvenile stage isnot clear. Both transport and retention are interpreted in lightof oceanographic processes that might impart variability, especiallybetween years, in the proportion of larvae transported awayversus that retained. Results of recent survey cruises in theGulf of Maine show both hypotheses to have merit. It is suggestedthat (i) interannual differences in slope water intrusions intothe Gulf of Maine as they affect the residual circulation, (ii)the lunar periodicity in the intensity of tidal mixing in relationto hatching times, and (iii) the potential for variable spawninglocations relative to the tidal fronts, may affect the distributionsof herring larvae immediately after hatching in the fall, andmay control the proportion of larvae that are advected awaywith the residual currents versus that retained in the vicinityof spawning. It is also suggested that those processes thataffect larval distributions and survival in the fall are importantin determining the overwintering distributions of larvae inthe Gulf of Maine, where the subtle influences of variable foodregimes and water temperatures could potentially exact largeinterannual differences in winter survival and recruitment tothe juvenile stage.     

The role of copepods in the planktonic ecosystems of mixed and stratified waters of the European shelf seas

The European shelf seas can be divided into regions which have tidally mixed waters and thermally stratified waters. The tidally mixed near shore environments support zooplankton communities dominated by smaller copepods and having large meroplankton contributions. These small copepods (Centropages spp., Temora spp., Acartia spp., Paral Pseudo/Microcalanus spp.) together with the microzooplankton component form a different and more complex food web than the larger copepod/diatom link associated with thermally stratified waters. The copepods Calanus finmarchicus and C. helgolandicus account for over 90% of the copepod dry weight biomass in stratified waters. Although occurring in lower numbers in mixed waters they can still make significant contributions to the biomass. A 31 year time series from the European shelf shows the inter- and intea-annual variability of these species. The basic biology and food web that these two systems support, and the transfer of energy, can result in marked differences in quantity and quality of particulates available as food for fish larvae. Calanus dominated systems allow the primary production to be directed straight through the trophic food chain (diatoms/Calanus/fish larvae) while the near shore communities of smaller copepods limit the amount of energy being transferred to the higher trophic levels. Eighty-two Longhurst Hardy Plankton Recorder hauls were used as the data base for this study. In all cases the zooplankton was dominated by copepods both in numbers and biomass accounting for > 80% of total zooplankton dry weight in the Irish Sea, Celtic Sea, shelf edge of the Celtic Sea and the northern and southern North Sea in Spring.     

Copepod grazing in turbulent flow: elevated foraging behavior and habituation of escape responses

When exposed to non-turbulent followed by turbulent conditions,the copepod Centropages hamatus initially responded with numerousescape reactions and increased foraging behavior. However, whenthe cycle of non-turbulent followed by turbulent flow was repeatedfor several consecutive cycles, the two behaviors followed distinctlydifferent patterns. Foraging effort increased during the firsttwo cycles, and then remained at high levels during both turbulentand non-turbulent periods (period durations of 12.5 and 25 min).In contrast, escape behavior habituated rapidly during eachturbulent period and dishabituated during each non-turbulentperiod. These response patterns are suited to the strongly intermittentnature of oceanic turbulence and allow C.hamatus to utilizethe benefits of enhanced encounter rate, while minimizing theexpense of unnecessary escape responses.     

Turbulence disturbs vertical refuge use by Chaoborus flavicans larvae and increases their horizontal dispersion

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Length changes in herring (Clupea harengus) larvae: effects of capture and storage in formaldehyde and alcohol

The effects on standard length of storing laboratory-rearedlarval herring (9–19 mm live length) in 4% buffered formaldehydeor 70% buffered ethanol with and without simulated capture bytowed net were assessed. Lengths during storage were consideredstable after 15 days for larvae placed directly in formaldehyde,and after 30 days for larvae placed directly into alcohol orfor larvae treated by net-capture simulation before storage.The following linear regressions described the relationshipsbetween live length and the stored lengths of larvae after thesetimes: for larvae stored directly in formaldehyde, L = 1.765+ 0.867 x X₁ for larvae stored directly in alcohol, L = 0.564+ 0.971 x X₁; for larvae subjected to net-capture simulationthen stored in formaldehyde, L = 0.984 + 0.993 x X₁; and forlarvae subjected to net-capture simulation then stored in alcohol,L = 0.532 + 0.989 x X₁ where L = live standard length and X₁= standard length after storage. The non-linear regression formulaparameterized by Theilacker (Fish. Bull US, 78,685–692,1980) for northern anchovy larvae provided a good fit to thedata for herring larvae subjected to net-capture simulationand then stored in formaldehyde. However, the model had to bere-parameterized to provide a good fit for larvae stored inalcohol. The precision achieved in length measurements usinga computer-aided measuring system is also discussed.     

Effects of small-scale turbulence on development time and growth of Acartia grani (Copepoda: Calanoida)

To test the effects of small-scale turbulence on developmentand growth of marine copepods, a series of 10 1 labortory microcosmswere used to follow the development, under turbulent and calmconditions, of a cohort of Acartia grani, a common coastal planktoniccopepod of temperate zones Aside from possible indirect effectsdue to differences in food availability, turbulence significantlyshortened development times and modified growth rates Theseinfluences seem to vary throughout its life history, late naupliiand early copepodites being more affected.     

Turbulence, watermass stratification and harmful algal blooms: an alternative view and frontal zones as “pelagic seed banks”

Watermass stratification has been considered the essential physical condition that dinoflagellates require to bloom because of their relative inability, unlike diatoms, to tolerate the elevated shear-stress associated with water-column mixing, turbulence and high velocity, coastal currents. The swimming speeds of 71 flagellate taxa, with a focus on dinoflagellates, are compared to the turbulence fields and vertical velocities that develop during representative wind conditions, upwelling and at frontal zones. The results suggest that the classical stratification–dinoflagellate bloom paradigm needs revision. Tolerance of turbulence, growth within well-mixed watermasses and survival and dispersal while entrained within current systems are well developed capacities among dinoflagellates. Their secretion of mucous, often copious during blooms, is suggested to be an environmental engineering strategy to dampen turbulence. Biophysical tolerance of turbulence by dinoflagellates is often accompanied by high swimming speeds. Motility speeds of many species exceed in situ vertical current velocities; this also allows diel migrational patterns and other motility-based behavior to persist. Species belonging to “mixing-drift” life-form assemblages can increase their swimming speeds through chain formation, which helps to compensate for the increased turbulence and vertical water-column velocities of their habitats. The ability of dinoflagellate species to tolerate the vertical velocities of offshore, frontal zones, where abundant populations often develop, suggests that fronts may serve as “pelagic seed banks”, occurring as pelagic analogues of nearshore seed beds, from which seed stock is dispersed. The different ecologies associated with the hypothesized, “pelagic seed banks” of vegetative cells and the “seed beds” of resting stage cells deposited onto sediments are discussed. There is a contradiction in the stratification–HAB paradigm: the quiescent conditions of a stratified watermass, with its characteristic nutrient-poor conditions are expected to promote stasis of the population, rather than growth and blooms. The analyses suggest that dinoflagellate blooms do not preponderate in stratified watermasses because the bloom species are biophysically intolerant of the higher velocities and turbulence of more mixed watermasses. The watermass stratification that often accompanies flagellate blooms is probably a secondary, parallel event and less essential than some other factor(s) in triggering the observed bloom.     

Impact of turbulence on riverine zooplankton: a mesocosm experiment

1. With increases in river discharge over time and space, zooplankton generally encounter increased turbulence, turbidity, hydraulic forces, downstream advection and food limitations, all of which should affect species diversity and densities. Of these factors, the role of turbulence on the distribution of zooplankton is least known along longitudinal and lateral dimensions in river networks. 2. We tested the factorial effects of turbulence and grazing level on Ohio River potamoplankton in spring and summer using twelve 1600‐L, outdoor mesocosms. Turbulence was calculated using the Froude number for equal depths but with current velocities of 0.064 and 0.32 m s^?1. Grazing levels corresponded to a high density treatment (=ambient river densities of rotifers, copepods and cladocerans) and a low density treatment (initially no zooplankton >64 μm). All tanks had the same water residence time, and hydraulic stress was minimized by circular flow patterns. 3. Zooplankton densities and population growth rates were significantly affected by turbulence level and season. In general, rotifer populations grew faster in high turbulence tanks (though Keratella and Brachionus populations flourished in both treatments in summer) and microcrustaceans thrived better in low turbulence environments. The larger, calanoid copepods handled more turbulent conditions much better than cyclopoids or nauplii. Zooplankton had no detectable effects on particulate organic carbon concentrations in either month (values were higher in spring), but rotifers reduced chlorophyll concentrations in both months. 4. The relative importance of turbulence in controlling potamoplankton is probably to vary not only on a longitudinal basis in river networks but also with both the hydrogeomorphic complexity of river reaches and the type and amount of river regulation. Plans for river rehabilitation and management should incorporate non‐turbulent habitats in large rivers as a means of enhancing zooplankton populations and providing an important food web component for planktivores.     

A model for Acartia tonsa: effect of turbulence and consequences for the related physiological processes

A mathematical model of the energy budget of the coped Acartiatonsa is proposed. It takes into account the processes of ingestion(encounter, capture, ingestion sensu stricto), digestion (assimilation,gut transit and egestion), catabolic expenses and biomass production.In order to represent the potential effects of small-scale turbulenceon the whole physiological processes of a copepod, some processsubmodels already published are combined. A major assumptionof the model is a satiety effect resulting from midgut filling,which leads to a decrease in the feeding activity. The modelpermits the simulation of the short-term dynamics of ingestionunder different food and turbulence conditions, as well as anintegrated physiological balance over 24 h. The model is validatedthrough comparison with data at both scales Simulations showthat turbulence increases ingestion rates and gut contents,and causes a decrease in gut passage time and assimilation efficiency.As a consequence, the dependent physiological processes areaffected differently by turbulence, which preferentially increasesegestion and egg-production rates Simulated daily ingestionrates of A.tonsa, for suspension feeding on. Thalassiosira weissflogiiand for ambush predation on Strombidium sulcatum, are in goodagreement wth the available experimental observations. The concurrentdirect effect of turbulence on the copepod's metabolism, dueto increased escape reactions, is also simulated. Results ofthe model show that a switch from suspension feeding on diatomsin calm condition to ambush predation on ciliates in turbulentconditions, might allow A.tonsa to maintain its gross growthefficiency at the same level. It is suggested that a dynamicrepresentation of processes occurring over a time scale of afew seconds is necessary to obtain, once integrated over 24h, the correct simulation of the effect of microscale turbulenceon ingestion and the related physiological processes.     

A model of the predatory impact of larval marine fish on the population dynamics of their zooplankton prey

A simulation model of the population dynamics of two speciesof calanoid copepods (Calanus.r pacificus and Pseudocalanussp.) was forced with predation pressure from a genetic, hypotheticalpopulation of larval marine fish. Results of the model are sensitiveto changes in parameters describing the dynamics of both predatorand prey populations, including initial numbers, fecundity,growth, mortality, size of prey organisms and feeding selectivityof the predators; the relative importance of these parametersis tested by way of a brute-force sensitivity analysis. Usingresults from recent ichthyoplankton surveys in Dabob Bay, WA,USA, the model was also forced with predation from populationsof larval Pacific herring (Clupea harengus pallasi) and Pacificwhiting (Merluccius productus). Results of the various simulationruns lead to the conclusion that marine fish larvae can significantlyimpact the population dynamics of their calanoid copepod prey,but that the magnitude of this impact is highly dependent onspecies-specific values of various population parameters.     

Hydrodynamic disturbances produced by small zooplankton: case study for the veliger larva of a bivalve mollusc

Zooplankton produce hydrodynamic disturbances during swimmingand feeding that enlarge their perceptive volume. From the standpointof both prey and predators, fluid disturbances increase theprobability that an organism is detected, identified and reactedto within appropriate time and space scales. Morphology andkinematics dictate the magnitude, symmetry and attenuation ofdisturbances in the fluid medium. Therefore, fluid disturbancesmay be species and age (size) specific. Normal and high-speedvideo microscopy was used to study flow-field generation byfree-swimming and tethered bivalve larvae. These organisms swimand feed using many highly coordinated and symmetrically distributedappendages (i.e. cilia). Larvae tethered in flow at variousfree stream velocities (U₀), simulating swimming activity, inducedparticle trajectories approximately parallel to the organism'sdorso-ventral axis. Velocity (v) and acceleration (a) were symmetricalin the transverse plane and asymmetrical in the vertical plane.Greatest velocity magnitudes ({small tilde}7, 3 and 6 mm s^–)occurred dorsal to the velum and attenuated with source distance(r) as 1/r, 1/r^1.9 and 1/r^2.9 at 10 s^– U₀ =, 3.1 and 6.4mm s^–1, respectively. For a larva in flow, but with velumretracted, simulating sinking, velocity attenuated at Mr towardsthe organism. Mean velocity gradients were on the order of 3,8 and 10 s^–1 for swimming, sinking and hovering larvae,respectively. The high-frequency (22 Hz) component of particlevelocity past free-swimming larvae was due to beat frequencyof the velar cilia. This attenuated rapidly with r leaving onlylow-frequency (1–3 Hz) disturbances 0.1 mm beyond thetips of the cilia. Comparisons of the kinetic energy dissipationrate for turbulence in coastal waters with the kinetic energyof laminar flow fields implied possible dominance of the flowfield of hovering, but not swimming, larvae to at least threebody diameters from the organism (–1 mm). These differencesin flow fields have important implications for larval survival.The perceptive volume of a hovering larva will be 40-fold greaterthan that of a swimming or sinking larva. However, a hoveringlarva U also more likely to be detected by a potential predatorthat uses mechanosensory organs to locate prey.     

Effects of environmental factors on ichthyoplankton communities in the Miramichi estuary, Gulf of St Lawrence

Ichthyoplankton in 20 taxa (17 identified to species, threeto genus) representing 14 families were collected in 10 surveysof the Miramichi estuary between May and September 1992. Thetaxonomic composition was typical of other estuaries in theGulf of St Lawrence and Gulf of Maine. Larvae of three anadromousspecies [rainbow smelt (Osmerus mordax), alewife (Alosa pseudoharengus)and blueback herring (Alosa aestivalis)] were several ordersof magnitude more abundant than any other taxon. The upper estuary(Miramichi River and its tributaries) probably serves as a nurseryground for larvae of these species and others such as Atlantictomcod (Microgadus tomcod). The species composition of the lowerestuary (Miramichi Bay) was dominated by typically marine formsand probably serves as a nursery ground for winter flounder(Pleuronectes americanus), smooth flounder (Pleuronectes putnami).sculpin (Myoxocephalus sp ) and sand lance (Ammodytes sp) Ofthe environmental factors investigated, salinity was the mostuseful predictor of larval distribution in the estuary.     

Environmental determinants in morphospace and diet of the larval blenny Calliclinus geniguttatus from an upwelling ecosystem

The effects of two contrasting environmental conditions in nearshore waters off central Chile on the diet and morphospace of two cohorts of larval labrisomid blenny Calliclinus geniguttatus were studied using geometric morphometrics and gut content analysis. The two environmental conditions corresponded to (a) a cold period with upwelling-favourable southwesterly winds and a mixed water column of cooler water and (b) a warm period with calm winds and stratified warmer water. During the cold period, fish larvae had a more hydrodynamic head shape, longer jaws and a higher feeding incidence, suggesting a greater food supply due to upwelling events and a possible increase in encounter rates in the turbulent environment. In contrast, the larvae from the warm period had a more robust head shape with smaller jaws and a lower feeding incidence, which was related to higher water temperatures and lower wind intensities. The present study suggests that larvae have a rapid response to environmental changes on a short time scale (i.e., from weeks to months), showing a link between environmental conditions and changes in the phenotypic traits and diet of the larval stages of this cryptobenthic species.     

Grid-generated turbulence in a mesocosm experiment

In two mesocosm experiments designed to study the effect of nutrients and turbulence on trophic levels below fish larvae in the ecosystem, turbulence was created by oscillating grids. Utilizing two different frequencies, two levels of turbulence (`high' and `low') were generated in a total of eight mesocosms in both experiments. In each mesocosm a two-layer density structure was created. We succeeded in generating turbulence that mimics turbulence found in natural ecosystems, with turbulent eddies on larger scales than those relevant for organism interactions. Energy dissipation rates () were calculated from the frequency spectrums of the measured turbulent velocities. In the second experiment the average energy dissipation rates in the upper layer were 1.9×10^–7 W kg^–1 and 5.5×10^–8 W kg^–1 in the `high' and `low' enclosures, respectively. Finding `background' turbulence levels between 5.3×10^–9 and 3×10 ^–8 W kg^–1, we suggest that future experiments consider turbulent measurements as a standard variable.     

Growth, development and condition of Dendraster excentricus (Eschscholtz) larvae reared on natural and laboratory diets

Feeding invertebrate larvae may be food limited while developingin the ocean. If they are, then their time in the plankton isprolonged, which likely increases mortality. Food limitationcould be due to the quantity and/or quality of the food available.In an effort to answer how food type influences larval nutrition,we compared growth, development and lipid deposition for Dendrasterexcentricus larvae reared in natural seawater from two depths(1 and 20 m) and in filtered seawater on a monoculture laboratorydiet of 6 cells µL^–1 of the green alga Dunaliellatertiolecta (Butcher). Five days post-fertilization, larvaereared on the laboratory diet had developed to the latest stage,were the largest and had lipid deposits. Larvae reared on naturalsurface water were intermediate in size and developmental stage,and larvae reared in the water from 20 m depth were the smallestand developed the slowest. This trend continued at 8 days post-fertilizationwhen surface water diet larvae were similar in size to laboratorydiet larvae, but their juvenile rudiments were significantlysmaller. To assess food availability in each food treatment,we compared the concentration of chlorophyll (Chl) a, b andc in natural seawater from each depth and in D. tertiolectaculture in filtered seawater. Natural seawater collected fromthe surface had the highest concentration of Chl a and c, whereasChl b was not significantly different between treatments. IncreasedChl concentrations in the surface water are likely due to higherconcentrations of diatoms and dinoflagellates, which are typicallynot high-quality food items for echinoid larvae. Our resultssupport a hypothesis that echinoid larvae in the water columnmay be limited by food quality.     

Feeding ability and survival during starvation of marine fish larvae reared in the laboratory

The larvae of Clyde and Baltic herring (Clupea harengus L.), cod (Gadus morhua L.) and flounder (Platichthys flesus L.) were reared and fed to examine the changes in feeding ability and survival during progressive starvation. The time to initial feeding for yolk-sac larvae and to the point-of-no-return (PNR, when 50% of the larvae, although still alive, are no longer strong enough to feed) for both yolk-sac and older larvae were determined. The yolk-sac larvae of Clyde and Baltic herring, cod and flounder begin to feed on days 6, 3, 5 and 6 post-hatching at rearing temperatures of 7.5, 9.2, 6.9 and 9.5°C, respectively. The time to reach the PNR for yolk-sac larvae of these species is only 3–5 days after yolk resorption. From the onset of starvation in older larvae the time to reach the PNR is 6–7 days for 36-and 60-day-old Clyde herring at 9.6 and 10.5°C and for 46-day-old Baltic herring at 13.1°C but it is 23 days for 32-day-old flounder at 12.3°C. In yolk-sac larvae the peak of feeding rate and intensity usually occurred on the day that the yolk became exhausted, or 1 day later. Older larvae could withstand longer periods without food than yolk-sac larvae, especially in flounder. While the feeding rate during starvation of older larvae slowly decreased the feeding intensity first increased significantly and then decreased. Survival of larvae remained high up to the PNR.     

Genetic diversity of coastal Northwest Atlantic herring populations: implications for management

Analysis of nine tetranucleotide microsatellite loci for Atlantic herring at five locations in the Northwest Atlantic including the Bras d'Or Lakes shows considerable genetic variation and significant population structure within the Coastal Nova Scotia management component, and among coastal populations and herring collected from Georges Bank. However, results are also consistent with gene flow across the Gulf of Maine. The magnitude of differentiation between the Bras d'Or Lakes sample and all others considered was sufficient to warrant further investigation. These data support the precautionary spawning-ground based management approach implemented in this area.     

Effects of larvae ontogeny, turbidity, and turbulence on prey attack rate and swimming activity of Atlantic herring larvae

The effect of turbulence, light level, and ontogeny on herring larva's attack rate and swimming activity was tested in a previous study. However, during larval seasons (spring and autumn), water clarity is frequently impaired by alga blooms, which also most probably will affect larva feeding rate. Therefore, this study was to investigate the effects of turbidity, turbulence, and ontogeny on the attack rate and swimming activity of herring larvae. By adding diatomaceous earth (DE) to the water, three turbidity levels were established: 0, 35, and 80 Jackson Turbidity Unit [JTU; which coincide with a beam attenuation (c) of 0.1, 2.5, and 4.8 m⁻¹, respectively]. An unfavourable (8×10⁻⁶ W/kg) and a favourable turbulence level (1×10⁻⁶ W/kg) were chosen based on results from the earlier study. The results show that intermediate turbidity (35 JTU) has a positive effect on the attack rate of smaller larvae (20 mm), while high turbidity (80 JTU) has a negative effect on attack rate of all tested larvae size groups. In general, attack rate was lower at the highest turbulence compared to the low level, independent of turbidity level. However, there was one exception, when turbidity was at the highest, the largest larvae (29 mm) seemed to gain from feeding in the highest turbulence level. The overall activity level was higher in the presented study than in the earlier study without turbidity. The favourable turbidity level (35 JTU) coincides with turbidity levels normally found at the equivalent depth during spring and autumn blooms in the area of where the experimental larvae originate. In addition, turbidity's effect on light absorbtion and how it influences the maximum feeding depth of the larva are discussed.