The relationship between target strength frequency response and vertical swim velocity: a new approach for fish discrimination

In-situ identification of fish species using acoustic methods is a key issue for fisheries research and ecological applications. We propose a novel approach to fish discrimination based on the relationship between target strength frequency response (TS(f)) and vertical swim velocity (VSV), as a proxy of fish body orientation. The measurements were carried out with a wideband echosounder on live fish of five species confined in a net cage. The data show a large dependence of TS(f) on VSV. To compare the variability of frequency responses of different fishes, we calculated ΔTS(f, VSV) as the difference between the TS(f) at given VSV and the TS(f) at VSV = 0, i.e. when the fish was swimming horizontally. We demonstrated that the relationships between ΔTS and VSV were similar for fish of the same species but dissimilar for different species. This implies that the acoustic fish discrimination in nature might be performed when the variations of the VSV can be measured from acoustically tracked fish. This can be a promising method for remote fish discrimination, for instance, for fish with diurnal vertical migrations. Further validation of this approach for fish recognition is required.

Influence of dinoflagellate diurnal vertical migrations on dimethylsulfoniopropionate and dimethylsulfide distribution and dynamics (St. Lawrence Estuary, Canada)

The influence of the diurnal vertical migration of the dinoflagellates Alexandrium tamarense and Scrippsiella trochoidea on dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) dynamics was studied during a 34-h Lagrangian experiment in the St. Lawrence Estuary in July 2000. Particulate DMSP (DMSPp), dissolved DMSP (DMSPd), and DMS exhibited diel patterns with minimum concentrations during the night and maximum concentrations around noon. DMSPp concentrations were correlated with the abundance of the two vertically migrating DMSP-rich dinoflagellates. The DMSPp:Chl a ratio exhibited similar diel variations, suggesting a light-induced de novo DMSP synthesis during the day. Diel variations of the DMS:Chl a ratio suggest that the accumulation of DMS around noon resulted from physiological responses of the algae and (or) bacteria to light. Biological gross DMS production and bacterial DMS consumption were decoupled, leading to rapid fluctuations in DMS. These results show that in systems dominated by DMSP-rich dinoflagellates containing DMSP lyases, DMS concentrations may vary by as much as a factor of 10 over a 24-h period. Such diel variations must be considered when estimating the contribution of such systems to the DMS sea to air flux.

Observations on swimming, posture and buoyancy in the giant oceanic ostracods, Gigantocypris mulleri and Macrocypridina castanea

Gigantocypris is a good swimmer, capable of moving fast enough to overtake a variety of zooplanktonic Crustacea and chaetognaths. It swims smoothly without violent accelerations and decelerations. Previous reports of unstable weak swimming in this species stem from the study of overheated animals; Gigantocypris is stenothermal, adversely affected at temperatures above about 15°C. Gigantocypris is nearly neutrally buoyant (sinking speed 0·2 mm s−1) and is capable of hovering for long periods with the aid of slight movements of the propulsive antennae. When swimming quickly Gigantocypris achieves an average speed of 164 mm s−1 (9·6 mm body length; 391 antennal beats min−1) at 12·5°C. It is unlikely that Gigantocypris performs significant diurnal vertical migrations. The heart of Gigantocypris is large and beats at ca 100 beats min−1. Macrocypridina is a faster swimmer (<52·3 mm s−1 (8·3 mm body length) at 12·5°C with an antennal frequency of 583 beats s−1), but each beat is accompanied by great accelerations and decelerations. Macrocypridina is dense, sinking at 40 mm s−1. Macrocypridina is eurythermal, its swimming speed increasing steadily between 11·5 and 20°C.

Diurnal Variations in the Grazing of Planktonic Copepods

Records were made of the presence or absence of food in the guts of Calanus finmarchicus caught in three different depths, at intervals of 4 hr. through 24 hr. It was found that 80-100 % of the Calanus caught at the surface were full of food at all hours of the day.The number of Calanus containing food in deeper water was distinctly less. This may be correlated with the abundance of its food close to the surface. Diurnal vertical migrations took place on some occasions when samples were taken, but not on all.In the absence of vertical migration Calanus was abundant at the surface and feeding continuously at all hours of the 24 hr. Where vertical migration took place feeding was mostly at the surface and was restricted to the hours of darkness, i.e. in summer to a period distinctly less than 8 hr. round midnight. Laboratory observations confirm the absence of any feeding rhythm.