Acoustic survey of a jellyfish-dominated ecosystem (Mljet Island,Croatia)

Acoustic techniques have been proposed as a new tool to assess jellyfish populations. However, the presence of mixed echoes from jellyfish and other organisms that share their distribution often prevent accurate estimates of their abundance and distribution being obtained. The isolated population of Aurelia inhabiting the Veliko Jezero (Big Lake-BL) of Mljet Island, in the South Adriatic Sea, offered a good opportunity to employ acoustic techniques to assess an entire jellyfish population. During October 2–5, 2006, combined video and acoustic methods were used in BL to determine the vertical distribution of medusae. Two synoptic acoustic surveys were performed during the day and night. In the daylight echograms, medusae were clearly discernible from the acoustic data, and their presence verified by video camera images, as forming a layer of varying density at and below the thermocline (15–30 m). The depth of the jellyfish layer also coincided with the depth of maximum dissolved oxygen concentration. The echointegration of these daylight data enabled quantification the Aurelia population, at a frequency of 120 kHz. In the night echograms, the acoustic signals of Aurelia were at least partially masked by pelagic and demersal fish, which disaggregated from schools and formed a layer associated with a strong thermocline at 15 m. An average target strength (TS) of −76.4 dB was obtained in situ corresponding to a mean length of 10.8 cm and a mean wet weight of 134 g measured from sampled medusae. These results were combined with echo-integration values to provide an estimate of 4,238,602 individuals and a biomass of 568 tons of Aurelia in BL. This study provided a synoptic view of Mljet Lake and illustrated the potential of acoustic surveys of jellyfish populations to contribute to ecosystems studies. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Schooling behaviour of arctic cod,Boreogadus saida,in relation to drifting pack ice

Synopsis Concentrations of arctic cod were detected with a hydroacoustic system in Resolute Bay, NWT during 2 weeks in August of 1986. Fish biomass within the bay was about 30 t. The fish were feeding primarily on amphipods, which were abundant. When the daily location of the schools was examined in relation to the extent and position of drifting pack ice, a pattern emerged suggesting that the distribution of the fish was influenced by the amount and location of ice cover. If the bay was relatively ice-free, the density of schooling cod was high and the size of the schools, as 2-dimensional surface area, was generally small. When ice covered the bay, density within the schools was lower and they occupied more area. Arctic cod were most dispersed after the bay had been filled with pack ice for several days. It is postulated that this behaviour is a response to potential predation by seabirds and marine mammals.     

The Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) populations of Windermere,UK: population trends associated with eutrophication,climate change and increased abundance of roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis>)

The north and south basins of Windermere in the English Lake District, UK, support autumn- and spring-spawning populations of Arctic charr, Salvelinus alpinus, which have been studied since the 1930s. Continuous investigations of the population dynamics of Arctic charr at this lake have involved gill netting since 1939, collection of fishery catch-per-unit-effort data since 1966, and hydroacoustic surveys since 1990. Analysis of these and associated long-term data on the abiotic environment and other components of the fish communities revealed recently contrasting fortunes of the Arctic charr populations of the north and south basins, the latter of which has been significantly impacted by eutrophication while both basins have shown elevated water temperatures and increasing roach, Rutilus rutilus, populations. Despite the introduction of phosphate stripping in 1992 and some subsequent initial improvement, the hypolimnion of the south basin still remains significantly eutrophicated and the fishery catch-per-unit-effort in this basin is now at record low levels. In addition, the spatial distribution of roach has expanded to form significant components of the fish communities of inshore and offshore surface habitats, where this cyprinid may compete with Arctic charr for zooplanktonic prey. It is concluded that the Arctic charr populations of Windermere, particularly those of the south basin, currently face a number of significant environmental pressures and continued management action is required to ensure their survival.     

Hydroacoustic observation of fish spatial patterns and behavior in the ship lock and adjacent areas of Gezhouba Dam, Yangtze River

The split-beam echo sounders were used in the No. 1 ship lock and the adjacent areas of the Gezhouba Dam Reach separately, to observe fish spatial distribution and the behavior of local fish about the Gezhouba Dam, and figure out whether the ship lock could served as a channel for fish migration in a way. The resulted showed that there were significance difference in both fish size (converted from the TS of individual fish) and abundance in the five marked sub-areas. Beyond that, there was an analogical fish distribution format in the analogical environmental condition. It might well be that fish species distributed in this area with the regioselectivity, which might resulted in the spatial differentiation. The horizontal fixed-location detection of the No. 1 ship lock revealed that, there were many ups and downs in the fish density with significant difference during a diurnal period, which might be resulted by the law of diel vertical distribution. There were all together 38 fishes moving up- or downstream the ship lock were detected during the period of ship-lock opening. In spite of the fact that, we cannot regard that the ship lock can served as the channel for fish migration. Because the evidence originated from the fish spatial differentiation and behavior in preference to that, it was the general behavior of that the fish swam backwards and forwards across the ship lock.     

Hydroacoustic observation of fish spatial patterns and behavior in the ship lock and adjacent areas of Gezhouba Dam, Yangtze River

The split-beam echo sounders were used in the No. 1 ship lock and the adjacent areas of the Gezhouba Dam Reach separately, to observe fish spatial distribution and the behavior of local fish about the Gezhouba Dam, and figure out whether the ship lock could served as a channel for fish migration in a way. The resulted showed that there were significance difference in both fish size (converted from the TS of individual fish) and abundance in the five marked sub-areas. Beyond that, there was an analogical fish distribution format in the analogical environmental condition. It might well be that fish species distributed in this area with the regioselectivity, which might resulted in the spatial differentiation. The horizontal fixed-location detection of the No. 1 ship lock revealed that, there were many ups and downs in the fish density with significant difference during a diurnal period, which might be resulted by the law of diel vertical distribution. There were all together 38 fishes moving up- or downstream the ship lock were detected during the period of ship-lock opening. In spite of the fact that, we cannot regard that the ship lock can served as the channel for fish migration. Because the evidence originated from the fish spatial differentiation and behavior in preference to that, it was the general behavior of that the fish swam backwards and forwards across the ship lock.     

Acoustical estimates of fish and zooplankton distribution in the Piaseczno reservoir, Southern Poland

Hydroacoustical surveys in the Piaseczno reservoir were performed in May and September 2002 using a Biosonics 101 dual beam echo sounder. They have revealed very scarce fish populations in pelagic waters with twice-higher abundance in autumn (530 fish ha⁻¹) as compared with spring (280 fish ha⁻¹). Small and very small fish (below 10 cm length) dominated. Apart from fish, Chaoborus larvae were producing acoustical echoes of the TS similar or slightly weaker than that of small fish. Invertebrates formed a thin layer, less than 2 m thick at the border of an anoxic zone, and were changing their depth position between 6 and 16 m, both diurnally and seasonally.     

A new direct method of stock assessment of the loliginid squid

Hydroacoustic research conducted on chokka squid (Loligo reynaudi d’Orbigny, 1845), off the east coast of South Africa from 1994–2005, has led to the development of an innovative stock assessment technique, perhaps applicable to all loliginids that migrate inshore to spawn. This technique combines hydroacoustic biomass estimates made on the spawning concentrations inshore, and minimum biomass estimates made both inshore and offshore using demersal surveys employing the swept-area method. The hydroacoustic estimate uses an improved method to obtain target strength measurements, and squid concentrations are individually mapped from a small boat with a towed transducer. This method may be used even during intense fishing operations because of the manoeuvrability of the small boat inside a tight cluster of fishing vessels. Biomasses of the individual concentrations are then summed. The inshore biomass, also includes dispersed, mature squid migrating between concentrations, this is assessed using a concentration stability factor. The biomass of dispersed squid offshore is again calculated using the swept-area method, a well known demersal survey methodology. The biomass of concentrated (spawning) squid offshore is calculated using the same proportions between concentrated and dispersed squid which were found inshore. All four components are then summed to calculate the total biomass. The result obtained is subject to the effect of complex temporal dynamics, as new animals are recruited to the adult pool and those recently assessed migrate to other sectors of the distribution area.