Vertical Distribution Of Eggs Research Articles

Ontogenetic changes in the buoyancy and salinity tolerance of eggs and larvae of polar cod (Boreogadus saida) and other gadids

Climate change in Arctic and sub-Arctic seas is leading to rapid changes in the dispersal potential of marine organisms. In Alaskan waters, loss of sea ice and salinity changes associated with warming may have a strong effect on the distribution and survival of eggs and larvae of key fish species at the Pacific–Arctic interface, such as polar cod (Boreogadus saida) in the Chukchi Sea and walleye pollock (Gadus chalcogrammus) in the northern Bering Sea. We conducted laboratory experiments to investigate how changes in ontogeny and salinity influenced the buoyancy and survival of eggs and larvae of these and two other gadids (Pacific cod, Gadus macrocephalus and saffron cod, Eleginus gracilis). Polar cod egg density varied among family groups, but eggs were more buoyant than those of walleye pollock overall. Eggs of both species followed a typical pattern of increasing density early in development. After hatching, polar cod and walleye pollock larvae were similar in density, and were notably more buoyant than the larvae of species with demersal eggs (Pacific cod and saffron cod). Larvae of the two Arctic species (polar cod and saffron cod) were less sensitive to salinity challenges than the sub-Arctic gadids, but polar cod became more sensitive at 6 weeks post-hatch. These results highlight possible adaptations of polar cod to ensure that their eggs are positioned under or near ice after spawning so that larvae hatch in productive surface waters during ice break-up. A comparison of modeled seawater densities to egg densities during the post-spawning period suggests that walleye pollock eggs would only be in contact with seasonal sea ice on the northern edge of their distribution at latitudes above ~ 63 °N. The synergic effects of environmental variables on vertical distribution of eggs and their survival potential will be important biogeographic mechanisms to consider with climatic warming and continued loss of sea ice.

Egg size and density estimates for three gadoids in Icelandic waters and their implications for the vertical distribution of eggs along a stratified water column

Abstract The vertical distribution of fish eggs can have important consequences for recruitment through its influence on dispersal trajectories and thus connectivity between spawning and nursery locations. Egg density and size are key parameters for the modelling of vertical egg distributions, both of which show variation at the species level, as well as between and within individuals (i.e., through ontogeny). We conducted laboratory experiments on the eggs of wild-spawning cod, haddock and saithe from Icelandic waters to estimate these parameters throughout ontogeny. Subsequently, this information was used in a 1-dimensional model to generate vertical distributions for each species along a stratified water column. Saithe eggs were significantly smaller and less dense than cod and haddock eggs. Cod eggs were slightly denser than haddock eggs in the first ontogenetic stage but statistically similar in the later stages. No significant differences were found between the egg diameters of cod and haddock. For each species, both parameters changed significantly through ontogeny. Yet despite these significant results, the 1-d model suggests that neither the interspecific nor ontogenetic differences would have a significant impact on the vertical egg distributions. Only under highly stratified conditions, when buoyancy is minimised due to the freshwater layer, do distributional differences become evident. In such situations, incorporating intraspecific variation in egg density into the model substantially reduced the distributional differences and this is highlighted as an important consideration for the modelling of pelagic vertical egg distributions.

Vertical distribution of eggs and larvae of <i>Maurolicus japonicus</i> (Sternoptychidae, Pisces) in Tosa Bay, Japan

Vertical distribution of eggs and larvae of <i>Maurolicus japonicus</i> (Sternoptychidae, Pisces) in Tosa Bay, Japan

Helicoverpa zea (Lepidoptera: Noctuidae) Oviposition and Larval Vertical Distribution in Bt Cotton Under Different Levels of Nitrogen and Irrigation

In some Bt cotton (Gossypium hirsutum L.) varieties, bollworm (Helicoverpa zea Boddie) larval behavior differs from non-Bt varieties. Laboratory assays indicate bollworm larvae can detect Bt proteins, which may cause behavioral differences. Plant stress from factors including fertility and water availability causes changes in plant physiology and Bt expression. Our objective was to determine whether nitrogen and irrigation influenced bollworm behavior in Bt cotton by recording the vertical distribution of eggs and larvae over time. We conducted small plot experiments with Cry1Ac + Cry1F cotton in 2016 and 2017 with three nitrogen rates, along with irrigated and nonirrigated treatments during 2017. Bollworm locations were determined by in-field examination of 10-20 cotton plants per plot over 6-8 wk. The location of each egg and larva was recorded by node, with instar estimation of each larva. Oviposition was higher in in plots receiving nitrogen; first and second instars were also more common in plots receiving nitrogen or irrigation, whereas older instars had similar numbers among treatments. Oviposition was more evenly distributed throughout the canopy earlier in the sampling period than during later weeks, with more eggs in the top third of the canopy in only three of 14-wk. Early instars were also evenly distributed throughout the canopy. Later, instars moved to the middle portions of the canopy, away from bottom nodes, and did not move toward the terminal. Understanding bollworm behavior can inform both crop scouting and resistance management decisions.

Comparison of Size, Terminal Fall Velocity, and Density of Bighead Carp, Silver Carp, and Grass Carp Eggs for Use in Drift Modeling

AbstractInvasive Asian carp established in the United States spawn in the turbulent water of rivers, and their eggs and early larvae develop while drifting in the current. The eggs, which are believed to perish if they settle before hatching, are slightly denser than water and are held in suspension by water turbulence. It is possible to use egg drift modeling to assess the capability of a river to support the survival of Asian carp eggs. Detection of spawning and estimation of egg abundance in the drift are typically assessed by ichthyoplankton trawls. Correct sampling design and interpretation of trawl data require knowledge of the vertical distribution of eggs in the drift, which can be accomplished with particle transport models. Data that are required to populate models of egg drift and vertical distribution include physical properties of assessed rivers and information on egg size, density, and terminal fall velocity, but data on these egg characteristics have not been previously available. Physical characteristics of the eggs are presented as a function of postfertilization time. We recorded mean egg diameter and terminal fall velocity for eggs from each Asian carp species during the first 5 h of development and at approximately 12 and 22 h postfertilization. Eggs of all species reached their maximum size before 4 h. Water‐hardened eggs of Silver Carp Hypophthalmichthys molitrix and Grass Carp Ctenopharyngodon idella were similarly sized in our trials, and water‐hardened eggs of Bighead Carp H. nobilis were the largest. After water hardening, Silver Carp eggs sank slowest, and Bighead Carp eggs sank fastest. For a given species, smaller‐diameter eggs generally had faster terminal fall velocities and higher specific gravity than larger eggs. We provide regression models of egg density and diameter for all three species, discuss usage of these data in modeling the drift and dispersion of Asian carp eggs, and discuss implications for egg sampling design.Received December 1, 2015; accepted March 20, 2017 Published online June 20, 2017

Egg buoyancy of flounder, Platichthys flesus, in the Baltic Sea—adaptation to salinity and implications for egg survival

Vertical distribution of eggs as determined by the egg buoyancy, i.e. the difference in specific gravity between the egg and the ambient water, have profound implications for the reproductive success and hence recruitment in fish. Here variability in egg specific gravity of flounder, Platichthys flesus, was studied along a salinity gradient and by comparing two reproductive strategies, spawning pelagic or demersal eggs. Egg characteristics of 209 egg batches (covering ICES subdivisions (SD) 22–29 in the brackish water Baltic Sea) was used to reveal the significance of egg diameter and egg dry weight for egg specific gravity (ESG), subpopulations, and egg survival probabilities of pelagic eggs following a major saline water inflow event. As an adaptation to salinity, ESG (at 7°C) differed (p<0.001) between areas; three subpopulations of flounder with pelagic eggs: 1.0152±0.0021 (mean±sd)gcm−3 in SD 22, 1.0116±0.0013gcm−3 in SD 24 and 25, and 1.0096±0.0007gcm−3 in SD 26 and 28, contrasting to flounder with demersal eggs, 1.0161±0.0008gcm−3. Egg diameter differed (p<0.001) between subpopulations; from 1.08±0.06mm (SD 22) to 1.26±0.06mm (SD 26 and 28) for pelagic eggs and 1.02±0.04mm for demersal eggs, whereas egg dry weight was similar; 37.9±5.0μg (SD 22) and 37.2±3.9μg (SD 28) for pelagic, and 36.5±6.5μg for demersal eggs. Both egg diameter and egg dry weight were identified as explanatory variables, explaining 87% of the variation in ESG. ESG changed during ontogeny; a slight decrease initially but an increase prior to hatching. Egg survival probabilities judged by combining ESG and hydrographic data suggested higher egg survival in SD 25 (26 vs 100%) and SD 26 (32 vs 99%) but not in SD 28 (0 and 3%) after the inflow event, i.e. highly fluctuating habitat suitability. The results confirm the significance of ESG for egg survival and show that variability in ESG as and adaptation to salinity is determined mainly by water content manifested as differences in egg diameter; increase in diameter with decreasing salinity for pelagic eggs, and decreased diameter resulting in demersal eggs.

Vertical distributions of the early life stages of walleye pollock (Theragra chalcogramma) in the Southeastern Bering Sea

The present study examines vertical distributions of the early life stages of walleye pollock (Theragra chalcogramma) in the Southeastern Bering Sea to assess ontogenetic and diel vertical migration in relation to development and habitat. Walleye pollock demonstrated a decrease in the depth of occurrence following hatching, indicating an ontogenetic change in vertical distribution. Eggs occurred deepest in the water column and early juveniles occurred shallowest. Vertical distributions were related to the date of collection, water column depth, and thermocline depth. Non-feeding stages (eggs and yolksac larvae, <4.5mm standard length [SL]) did not exhibit diel vertical migration. Feeding larvae exhibited diel vertical migration, although patterns varied between two feeding stages. Preflexion stage larvae (4.5–9.9mm SL) were concentrated between 10 and 20m during the day and deeper at night. Postflexion stage larvae (flexion and postflexion, 10.0–24.5mm SL) underwent regular diel migrations (0–20m, night; 10–40m, day). Low sample sizes limited our ability to assess diel vertical migration in early juveniles, but this stage tends to occur in the upper 20m of the water column, regardless of time of day. These results suggest that vertical distributions and diel migration potentially are driven by prey availability at sufficient light levels for preflexion larvae to feed and a trade-off between prey access and predation risk for postflexion larvae. Vertical distributions of eggs and preflexion larvae varied with habitat examined (on the continental shelf versus over the continental slope). Vertical distributions of walleye pollock eggs, yolksac larvae, and preflexion larvae in the Bering Sea are different from distributions in other ecosystems, which can impact transport and modeling efforts.

Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) in cotton: vertical distribution of egg masses, effects of adult density and plant age on oviposition behavior

The objective of this work was to determine the most suitable density of adults per plant, ideal plant age, and vertical distribution of eggs in different parts of the plant in a greenhouse investigation for future resistance studies and improvement of S. frugiperda management practices in cotton. The experiments of Spodotera frugiperda oviposition in relation to adult density, relationship between cotton plant age and Spodoptera frugiperda oviposition, Spodoptera frugiperda non-preference for oviposition on cotton varieties were conducted with plants of cotton variety BRS Ita 90. The experiments were conducted with plants of cotton variety BRS Ita 90. Non-preference (antixenosis) for oviposition was studied in FiberMax 966, FiberMax 977, DeltaOpal, DeltaPenta, Acala 90, Coodetec 408, Coodetec 409, Coodetec 410, BRS Cedro, BRS Ipê, BRS Aroeira, IPR 96, IPR 20, BRS Araçá, IAC 24, and BRS Ita 90 varieties. We concluded that the fall armyworm S. frugiperda prefers to oviposit on plants with approximately 60 days of age, on the lower surface of leaves located in the upper third of the plant tests under greenhouse conditions. A density of at least three pairs of S. frugiperda adults per plant was sufficient to conduct non-preference-for-oviposition tests under greenhouse conditions. Coodetec 408, BRS Aroeira, BRS Araçá, BRS Ita 90 and DeltaPenta varieties showed non-preference-for-oviposition resistance to S. frugiperda. In regions with high infestations of S. frugiperda, it would be prudent for the cultivation the use of BRS Ita 90 cotton variety.

Changes in egg buoyancy during development and its effects on the vertical distribution of anchovy eggs

Small pelagic fish populations exhibit reproductive strategies resulting from past natural selection pressure, by which certain traits become more or less common in a population, allowing them to adapt and become better suited to certain habitats. One such adaptation is the buoyancy of eggs, which is observed as density changes during development. This is an important issue in fisheries and modeling science, as it affects the vertical distribution of eggs and, therefore, egg transport. Recently, individual-based models for anchovies in the Mediterranean have focused on developing adequate biological algorithms to simulate realistic spatial variations of eggs and larvae. Some models that include movements of particles according to Stokes’ law also assume a constant value of egg density during egg development. However, field observations show differences in the vertical distribution of eggs when egg density during development is considered. We address the problem of egg density and its vertical distribution within a biological context. In Mediterranean waters, the incubation time for anchovy eggs during peak spawning is approximately 48–70h; during these first hours, egg density has an influence on the horizontal and vertical trajectories of eggs, as well as their routes and hatching zones. In this study, we introduce an algorithm describing the egg density of European anchovy eggs throughout development. Egg density measurements were carried out in a density gradient column (DGC). We fitted a polynomial model that estimated egg density, as a function of time from fertilization and that was dependent on temperature. Simulations to study the vertical transport of eggs in the Mediterranean were carried out using ICHTHYOP/MARS3D. The vertical distribution of pelagic eggs was determined by a set of interacting biological and physical parameters related to eggs (density, diameter) and ambient seawater (density, viscosity, turbulence), respectively. The egg buoyancy model introduced here was validated and will provide insight for the design of anchovy egg surveys, as the vertical position of the eggs in the water column during development can be inferred by the hydrographic structure of seawater.

Vertical distribution of eggs and larvae of the Baltic Sprat Sprattus sprattus balticus (Clupeidae) in relation to seasonal and diurnal variation

Vertical distribution of eggs and larvae of the Baltic sprat Sprattus sprattus balticus was studied. Diurnal variation was revealed in the distribution of eggs of the first stage of development and larvae with a length of more than 5 mm, which is related to dynamics of spawning and vertical migrations of sprat larvae. Seasonal variation in the distribution of eggs manifested itself in: (1) widening of the range of occurrence from the zone of halocline at the beginning of spawning (winter) to the entire water column in spring-summer and (2) emergence in the surface layer in July of all stages of egg development unlike the presence from May to June mainly of the first stage of development. It is suggested that the ability of early ontogenetic stages of sprat to maintain in the surface layer depended on the extent of development of vertical gradients of density in the thermocline.

Vertical distributions of Japanese sardine (Sardinops melanostictus) eggs: comparison of observations and a wind‐forced Lagrangian mixing model

AbstractTo understand how Japanese sardine eggs vertically disperse in water, we acquired a series of depth‐resolved samples of eggs while tracking a drogued buoy to follow the water mass in which the eggs were spawned. This survey was conducted in early March of 1991, when the sardine population was abundant and actively spawning in the Pacific off Kyushu. Buoyancy of the eggs was estimated by measuring the specific gravity, and was 1–4 × 10−3 less than that of the water in which they were spawned, giving a rising speed of 1–2 mm s−1. In an attempt to diagnose the dynamics underlying the observed vertical profiles of the eggs, a one‐dimensional egg distribution model incorporating the egg rising speed and wind‐ and depth‐dependent vertical diffusivity was formulated. Observed eggs were mainly in the surface mixed layer, and their vertical distribution changed with wind‐induced surface mixing: during strong winds, the eggs were distributed relatively evenly over the upper 60–75 m, whereas during weak winds and shallow pycnocline, the eggs formed a subsurface peak around 25 m depth. The model accurately reproduced these variations in the vertical distributions of eggs, suggesting that the gradients of egg concentration are sensitive to the strength of wind and the structure of underlying hydrography. Further tests with the model showed the importance of positive buoyancy: neutrally buoyant eggs formed subsurface maxima below the food‐rich euphotic zone. The balance of vertical mixing and floating, which allows the eggs to remain in the euphotic zone but away from the near‐surface waters, may be an adaptation to minimize surface‐enhanced predation, while allowing the eggs to hatch in the food‐rich euphotic zone.

Vertical distribution, population density, and natural egg parasitism of cotton leafworm on cotton under IPM

Alabama argillacea is one of the most important pests of cotton crops in Brazil, and therefore it is necessary to research its behavior and natural biological control in areas under IPM conditions to improve the management of its population. In the pursuit of this objective, experiments were carried out on cotton crops in Dourados, Mato Grosso do Sul, in the 1997-1998 growing season. With the object of determining the population density and vertical distribution of eggs, larvae, and pupae, the total number of eggs, small, medium and large sized larvae and pupae found on the upper, middle, and lower sections of the plants were weekly recorded. This was conducted in an area where pest management was done according to IPM techniques and tactics. In the same area, eggs of this species were also gathered to determine the proportion of parasitism. The pest lays the majority of eggs on the upper part of the plants where the majority of midand large-sized larvae were also found. Small larvae were generally found on both middle and lower parts of the plants. A growing percentage of parasitism by Trichogramma pretiosum was found from the beginning through the end of the crop season, reaching almost 100% parasitized eggs by the end of plant development. The average number of eggs found per plant was high during almost the whole crop cycle, reaching about 30 eggs per plant by the end of the cycle. The total number of small caterpillars was high and constant during the whole evaluation period, while medium and large ones were more abundant in the final phase of the crop cycle. The total caterpillar population reached peaks of around four caterpillars per plant.

The distribution of life cycle stages of two deep-water pleuronectids, Dover sole ( Microstomus pacificus) and rex sole ( Glyptocephalus zachirus), at the northern extent of their range in the Gulf of Alaska

Dover sole ( Microstomus pacificus) and rex sole ( Glyptocephalus zachirus) are both commercially valuable, long-lived pleuronectids that are distributed widely throughout the North Pacific. While their ecology and life cycle have been described for southern stocks, few investigations have focused on these species at higher latitudes. We synthesized historical research survey data among critical developmental stages to determine the distribution of life cycle stages for both species in the northern Gulf of Alaska (GOA). Bottom trawl survey data from 1953 to 2004 (25 519 trawls) were used to characterize adult distribution during the non-spawning and spawning seasons, ichthyoplankton data from 1972 to 2003 (10 776 tows) were used to determine the spatial and vertical distribution of eggs and larvae, and small-meshed shrimp trawl survey data from 1972 to 2004 (6536 trawls) were used to characterize areas utilized by immature stages. During the non-spawning season, adult Dover sole and rex sole were widely distributed from the inner shelf to outer slope. While both species concentrated on the continental slope to spawn, Dover sole spawning areas were more geographically specific than rex sole. Although spawned in deep water, eggs of both species were found in surface waters near spawning areas. Dover sole larvae did not appear to have an organized migration from offshore spawning grounds toward coastal nursery areas, and our data indicated facultative settling to their juvenile habitat in winter. Rex sole larvae progressively moved cross-shelf toward shore as they grew from April to September, and larvae presumably settled in coastal nursery areas in the autumn. In contrast with studies in the southern end of their range, we found no evidence in the GOA that Dover or rex sole have pelagic larval stages longer than nine months; however, more sampling for large larvae is needed in winter offshore of the continental shelf as well as sampling for newly settled larvae over the shelf to verify an abbreviated pelagic larval stage for both species at the northern end of their range.

Patterns in rotifer diapausing egg banks: Density and viability

Here we present a quantitative study on the density, age and viability of the diapausing egg banks of the rotifer species complex Brachionus plicatilis in the sediments of 15 water bodies from Eastern Spain. Sampled ponds, located in coastal and inland areas, varied in salinity and ranged in size, depth and permanence. By identifying ‘hatched’, ‘deteriorated’ and ‘viable’ diapausing eggs in the sediment samples, we estimated production, hatching and deterioration in relation to the habitat properties of each pond. Our results indicate the presence of large numbers of diapausing eggs in the sediments of almost all of the ponds studied (2–115 eggs cm − 2 ). Inland ponds tended to have higher densities than coastal lagoons. The vertical distribution of eggs in the sediments frequently showed a non-decreasing pattern, which suggested a high among-year variation in egg production. Despite maximum age of eggs of 60–80 years, the median age (3–30 years) suggests that rotifer egg banks are young in the studied ponds. Egg senescence is suggested by the declining abundance of ‘healthy-looking’ eggs with depth. The proportion of ‘deteriorated’ eggs ranged 75–99% suggesting that deterioration rates in the sediments are high and vary among habitats. Hatching and deterioration rates, as estimated from the counts of ‘hatched’, ‘deteriorated’ and ‘healthy-looking’ eggs in the sediments, largely varied among ponds. An association between hatching and deterioration rates is suggested by our data. This is in agreement with the hypothesis that hatching rates of diapausing eggs depended not only on the risks associated with the water column, as initially expected by the general theory on diapause, but they are also related to the incidence of deterioration processes in the sediment.

Modelling the effects of physical–biological interactions and spatial variability in spawning and nursery areas on transport and retention of sardine Sardinops sagax eggs and larvae in the southern Benguela ecosystem

A particle-tracking individual-based model (IBM) coupled with a 3-D hydrodynamic model was used to investigate how spatial variability in spawning and nursery grounds may influence transport and retention of sardine ( Sardinops sagax) eggs and larvae in the southern Benguela ecosystem. A temperature-dependent Bělehrádek development model based on laboratory growth data was used to determine the duration of one egg and two larval stages. Successful transport/retention depended on each particle's stage of development rather than its age. Results show that recruitment could occur in two distinct nursery areas on the west and south coasts of South Africa. Three viable recruitment ‘systems’ were identified: two are retention-based (spawned and retained on the west coast (WC–WC) or the south coast (SC–SC)) and one is transport-based (spawned on the south coast and transported to the west coast (SC–WC)). In the WC–WC system, the vertical distribution of eggs influenced retention; at intermediate depths of spawning (25–50 m) eggs avoided both offshore Ekman drift and deep cold water. In the SC–SC system, the area of spawning was important; > 50% of eggs from the eastern Agulhas Bank (EAB) were retained in the south coast nursery grounds, whereas very few eggs were transported there from west of Cape Agulhas. In the SC–WC system, area of spawning was also important; 40% of the eggs spawned on the western Agulhas Bank (WAB) were transported to the west coast nursery ground. Sardine life history strategy could be divided between two main systems: the west coast system (spawning on the WAB and WC, and recruiting to the WC) and the Agulhas Bank system (spawning on the central and eastern Agulhas Bank, and recruiting to the SC).

Influence of oceanographic fronts and low oxygen on the distribution of ichthyoplankton in the Benguela and southern Angola currents

The study focuses on ichthyoplankton populations in the southern Angola Current, the Angola-Benguela Front and the coastal upwelling area of the northern and central Benguela Current. The horizontal and vertical distributions of eggs and larvae of sardine Sardinops sagax, anchovy Engraulis encrasicolus and horse mackerel Trachurus trachurus capensis were examined in relation to distribution patterns of temperature, salinity and dissolved oxygen. Samples were collected during February–March 2002 using a Bongo net and a multiple opening/closing net in five depth layers (0–30m, 30–60m, 60–100m, 100–140m and 140–220m). Eggs and larvae were restricted mainly to the shelf areas in the Angola-Benguela Font and southern Angola Current waters. Their vertical distributions in the Angola-Benguela Front and northern Benguela upwelling region showed a sharp decline in abundance at about 60m depth. This decline appeared to be correlated to the distribution of dissolved oxygen in the water column It is hypothesised that low oxygen concentrations have a strong impact on the development and survival of the early life-cycle stages of fish, and that recruitment of sardine and other pelagic species relies more on the upward extension of the oxygen minimum layer than was previously thought.

Stage-dependent vertical distribution of pelagic fish eggs on the western Agulhas Bank, South Africa

The vertical distributions of eggs of anchovy Engraulis encrasicolus, sardine Sardinops sagax and round herring Etrumeus whiteheadi over the western Agulhas Bank, South Africa, were sampled between 0m and 100m, using an opening-closing WP-2 net, to investigate differences in developmental stages of the eggs. Distributions varied ontogenetically, and vertical dispersion indices showed that early-stage eggs of all species were less aggregated than middle- and late-stages. Most of early-stage anchovy eggs were distributed in the upper 40m, whereas those of sardine and round herring were more widely distributed down to 60m and 80m respectively. Results suggest that anchovy spawn nearer to the surface than do sardine and round herring.

Population structure and fecundity of the microcopepod Oncaea bispinosa in the Red Sea—a challenge to general concepts for the scaling of fecundity

General concepts of reproductive traits in pelagic marine copepods have been developed largely without data from the microcopepod family Oncaeidae. For a very abundant oncaeid species in the Red Sea, Oncaea bispinosa Bottger-Schnack, reproductive parameters are provided and used for testing how the copepod family Oncaeidae may fit into these concepts. Total number and vertical distribution of eggs of this species have been calculated for populations in the northernmost Red Sea and the Gulf of Aqaba during spring, based on counts of detached egg sacs retained in fine mesh net samples. Reproduction is indicated to take place in the meso- and bathypelagic zones, with maximum abundances of eggs occurring within the deep population centres of females at 250 to 450 m depth. The epipelagic population appeared to be non-reproductive. By calculating the biomass of the eggs and the females using published data on size/weight relationships, a first estimate of the reproductive potential of O. bispinosa was obtained and compared with various other oncaeid species. The presently available data for Oncaeidae in total suggest a relationship between single egg (Ec) and female (Fc) carbon weight (Ec = 0.012 × Fc0.48) that differs from corresponding regressions reported in the literature for other marine pelagic copepods. In addition, regressions are given for minimum and maximum clutch carbon related to female carbon weight. Potential egg production rates of O. bispinosa have been calculated from the observed fractions of egg-bearing females and published data on egg developmental times, as available for larger oncaeids. These production rates appear to be lower than have been observed in experimental studies for different pelagic microcopepod taxa. Remaining uncertainties in generalizing concepts for the scaling of fecundity in pelagic marine copepods are discussed, and the need for more comparable data, especially for the numerically abundant oncaeids, is emphasized.

Buoyancy measurements and vertical distribution of eggs of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus)

Measurements were made of the density and settling velocity of eggs of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus), using a density-gradient column. These results were related to observed vertical distributions of eggs obtained from stratified vertical distribution sampling in the Bay of Biscay. Eggs of both species had slightly positive buoyancy in local seawater throughout most of their development until near hatching, when there was a marked increase in density and they became negatively buoyant. The settling velocity of anchovy eggs, which are shaped as prolate ellipsoids, was close to predictions for spherical particles of equivalent volume. An improved model was developed for prediction of the settling velocity of sardine eggs, which are spherical with a relatively large perivitelline volume; this incorporated permeability of the chorion and adjustment of the density of the perivitelline fluid to ambient seawater. Eggs of both species were located mostly in the top 20 m of the water column, in increasing abundance towards the surface. A sub-surface peak of egg abundance was sometimes observed at the pycnocline, particularly where this was pronounced and associated with a low-salinity surface layer. There was a progressive deepening of the depth distributions for successive stages of egg development. Results from this study can be applied for improved plankton sampling of sardine and anchovy eggs and in modelling studies of their vertical distribution.

Modelling the vertical distribution of eggs of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus)

AbstractUnderstanding and modelling the vertical distribution of fish eggs in the water column is a major challenge for future use of underway‐continuous egg samplers as estimators of the total egg abundance. This study presents modelling of field data of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) egg vertical distribution obtained from LHPR sampling in the Bay of Biscay. Starting from Sundby's model [Deep‐Sea Res.30 (1983) 645], improvements were achieved through successive modifications concerning egg buoyancy and vertical propagation of wind‐induced turbulence. In addition, measurements of egg settling velocity and buoyancy by stages were included as inputs for the model. The best model fitting was achieved through the adoption of a gradual turbulence vertical decay model (proportional to the inverse of the water density profile), a Gaussian variability of egg density and adaptability of the egg density to the surrounding water by means of permeability of the chorion. This led to improvements over the Sundby original model. The coefficient of determination (R2) of the modelled egg abundance profiles compared with the observed ones was around 80% for both sardine and anchovy. The model described successfully the vertical distribution of eggs for waters of high surface salinity (R2 of almost 90%), but less so for waters of low surface salinity (R2 of about 70%).