Diel variations in schooling patterns and spatial dynamics during spawning were studied in Norwegian spring-spawning herring (Clupea harengus) off south-western Norway by acoustic surveying, diel cycle experiments and school tracking by sonar, and bottom gillnet sampling. Herring formed horizontally extensive, loosely packed demersal layers shortly after darkness. At night, the fish disappeared in the acoustic dead zone, but lifted off the bottom early in the following mornings. At daytime the herring reorganised into dense pelagic schools. The evening descent to the spawning habitat was considered as part of a precautionary strategy towards visual predators, as the bottom is a high-risk zone for archetypal pelagic fish like herring. Large numbers of gadoids, which are potential herring predators, were present in the area. Herring not ready to spawn dominated the bottom samples in 4 out of 5 days, suggesting that pre-spawning herring followed the descent of ripe herring. The herring spawning layers shifted in a south-easterly direction from day to day in diel spawning waves.

This study investigates whether pelagic fish schools of different species or groupings (e.g. herring, "surface herring", "gadoids", mackerel, sprat and sandeels) were positively or negatively associated with each other in time and space. To do this, statistical models were fitted to pre-processed acoustic fisheries data to reveal how pelagic school prevalence varied with respect to spatial (latitude and longitude) and temporal (time of day) information. The model outputs, which take the form of probabilities fitted to the presence or absence of schools, were then used to calculate correlation coefficients, which are useful for measuring association between pairs of variables. Results depended upon the specific species pairs under investigation. Herring and "surface herring" were, for example, very generally sympatrically associated with each other in both space and time, while herring and gadoid schools, on the other hand, had allopatric distributions.

A hydroacoustic monitoring technique to quantify and assess the ecological requirements for migration of the anadromous clupeid, Alosa fallax fallax (twaite shad) was developed, and its effectiveness studied, on the River Wye in Wales. The acoustic monitoring technique was a side aspect application, with two transducers fixed permanently to the riverbank and the acoustic beam from each aimed horizontally across the river towards the opposite bank, perpendicular to flow. Two split-beam echo sounders and transducers were deployed, each operating at different frequencies (200 and 420 kHz). Using a combination of these two frequencies it was possible to demonstrate that shad show bold avoidance behaviour to sound transmitted at 200 kHz and would not pass the monitoring site when sound was transmitted at this frequency. They remained unaffected by sound transmitted at 420 kHz and were observed migrating upstream in large, loosely aggregated shoals. From visual observations above and below the water, shoals were estimated to comprise of many hundreds of individuals, covering a size range of between 30 and 45 cm. Only a few individuals could be resolved by the acoustic system operating at 420 kHz, and it was therefore, not possible to obtain a count of fish by "target tracking" single shad. However, by transmitting 200 kHz sound pulses on a 50% duty cycle the seasonal and daily patterns of shad migration were derived from the analysis of data gathered by the acoustic system operating at 420 kHz.

In populations of herring (Clupea harengus) or sprat (Sprattus sprattus), one typically observes a pattern of schools forming at dawn and dispersing at dusk, usually combined with vertical migration. This behaviour influences interactions with other species; hence a better understanding of the processes could contribute to deeper insight into ecosystem dynamics. This paper reports field measurements of the dispersal at dusk and examines two hypotheses through statistical modelling: that the vertical migration and the dissolution of schools is determined by decrease in light intensity, and that the dissolution of schools can be modelled by diffusion, i.e. active repulsion is not required. The field measurements were obtained during 3 days in March at one location in the Baltic Sea and included continuous hydroacoustical monitoring, trawl samples, and hydrographical CTD data. Echogram patterns were analysed using the school detection module in Echoview® and local light intensities were calculated using a model for surface illuminance. The data and the analysis support that schools migrate upwards during dusk, possibly trying to remain aggregated by keeping the local light intensities above a critical threshold, that schools initiate their dissolution when ambient light intensity drops below this critical threshold, and that fish subsequently swim in an uncorrelated random walk pattern.

The fish and invertebrate behavior of the Ubol Ratana Reservoir, Thailand, were monitored using up- and downlooking split beam sonar located at a fixed location. In the same area and period, ichthyoplankton nets and multimesh gillnets were used. The bulk of targets, recorded by acoustics and direct capture, consisted both of fish 3–4 cm long and insect larvae 0.2–1 cm long. Diurnal patterns of behavior were very distinct: during the daytime, invertebrates were hidden in the bottom and most fish stayed in compact shoals. Time course of acoustic fish biomass and abundance was very variable due to shoaling. Only the largest fish were recorded as solitary targets. At night, the whole acoustic range was filled with targets and the time course of fish biomass (5–15 kg ha–1) and abundance (20–45 thousand individuals ha–1) were more constant. The biomass increased mostly at surface layers. Fish appeared in the evening in the water column 1 h earlier and stayed there in the morning 1 h longer than invertebrates. Dawn and dusk are good periods for studying fish before invertebrates outnumber them. Apart from fish, according to the target strength, swimming speed and depth distribution, at least four groups of water invertebrates were distinguished acoustically, some with extremely fast vertical movement (7–9 cm s–1 vertical speed). Comparison of up- and downlooking observations gave comparable results in midwater layer outside the near-field of the transducer. The uplooking approach can be more suitable for night records; downlooking for the day.

The development of acoustics tools and methods for monitoring anthropized ecosystems represents a new field for the application of acoustics. Monitoring such an environment was not possible with single vertical echo sounders, due to the fact that the artificial structures and the natural targets were not distinguishable. Monitoring data were collected along the French Mediterranean coastline, during five short surveys of mussel culture longline areas. Both the Reson Seabat 6012 multibeam sonar (455 kHz) and the Simrad SR 240 omnidirectional sonar (23.75 kHz) were used for target detection. The former tools allow accurate allocation of the different types of echoes to artefacts, fish schools and scattered fish. The school characteristics collected included morphological, geographical (GPS, school location), and behavioural (connections with the longlines). An acoustic survey undertaken with the same hardware near the study area allowed the comparison of fish schools and the TS distribution of individual fish in the open sea and in the mussel area. These data permitted us to evaluate the ecological impact of a mussel culture on the ecosystem, in a context of predation behaviour of fish on these longlines. Finally, the acoustic data revealed the configuration of each concession and the level of charge of each line. We discuss the applicability of this technology for in situ real time monitoring for joint management of such ecosystems. The information can allow littoral cooperative management or incorporating it into an ecosystem approach.