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

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.

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.

Variations in the distributions of the Centropages chierchiae and Temora stylifera (Copepoda: Calanoida) in the north-eastern Atlantic Ocean and western european shelf waters.

Centropages chierchiae and Temora stylifera occurred rarely in the Continuous Plankton Recorder (CPR) survey in the Bay of Biscay, Celtic Sea, and English Channel before 1988. By 2000 they were found frequently and in abundance. The seasonal cycles of abundance of these species differ, C. chierchiae occurring mainly in the summer while T. stylifera was found most frequently in late autumn or winter towards the northern limits of its distribution. The increase in abundance of both species is related to temperature. However, in the years when it was found in the samples, the frequency of occurrence of C. chierchiae was correlated positively with the strength of the shelf edge current and negatively with the North Atlantic Oscillation (NAO) while the reverse was true for T. stylifera.

Nutrient Distributions In An Estuary - Evidence Of Chemical Precipitation Of Dissolved Silicate And Phosphate

Continuous autoanalytical recordings of the axial distributions of dissolved nitrate, silicate and phosphate in the influent freshwater and saline waters of the Tamar Estuary, south-west England have been obtained. Short-term variability in the distributions was assessed by repetitive profiling at approximately 3-h intervals on a single day and seasonal comparisons were obtained from ten surveys carried out between June 1977 and August 1978. Whereas nitrate is always essentially conserved throughout the upper estuary, the silicate- and phosphate-salinity relationships consistently indicate a non-biological removal of these nutrients within the low (0–10%) salinity range. Attempts to quantify precisely the degree of removal and to correlate this with changes in environmental properties (pH, turbidity, chlorophyll fluorescence, salinity, freshwater composition) were mainly inconclusive due to short-term fluctuations in the riverine concentrations of silicate and phosphate advected into the reactive region and to the rapid changes in turbidity brought about by tidally-induced resuspension and deposition of bottom sediment.

Vertical Distributions Of Calanus-Finmarchicus And Calanus-Helgolandicus (Crustacea Copepoda)

The vertical distributions of the spring populations of Calanus finmarchicus (Gunnerus) and C. helgolandicus Claus are described and compared. The differences we observed between the two species have probably confused the understanding of the vertical distribution and development of the populations of Calanus spp. in the shelf seas around the United Kingdom where the species occur together. The results imply that these two congeneric species have different behaviour patterns which minimise interspecific competition where the species have sympatric distributions. C. finmarchicus has its younger development stages overlying the older stages in the water column. In C. helgolandicus the converse is true; i. e., the majority of the populations of Stage I and II copepodites of the first spring generations are found below the thermocline. It is also suggested that the different behaviour patterns lead to different feeding regimes and strategies.