An evaluation of the bottom trawl surveys in the Benguela Current Large Marine Ecosystem

Demersal fish, shrimp and cephalopod assemblages on the continental shelves and slopes off Angola, Namibia and the southern and western coasts of South Africa have been monitored in terms of fisheries-independent trawl surveys since the 1980s. The time series have provided vital input to stock assessments and are widely used in studies of ecology and biodiversity. The objectives of this study were to evaluate the technical specifications of the vessels and trawls used, to examine effects of modifications on catching efficiency, and to assess implications of these modifications over time. We find that the demersal trawl data collected in South Africa are not comparable with those of Namibia and Angola, and that the time series of Angola and Namibia contain inherent differences in terms of catchability of bottom dwellers. The introduction of smaller bobbins gear on the RV Dr. Fridtjof Nansen in 1994 increased the catchability of bottom-dwelling species, and catch rates of monkfish and sole were higher in surveys with commercial vessels than the RV Dr. Fridtjof Nansen. We recommend that temporal trends are interpreted with caution and that time series for the three countries are viewed in isolation

Impact of herring spawning behaviour on acoustic abundance estimates

Spawning dynamics of Norwegian spring spawning herring was studied in south western Norway 29 March to 3 April 2000 using hydroacoustics. The horizontal distribution of the spawning layers shifted in a south-easterly direction during the study period indicating directional spawning. A diurnal spawning pattern was found, with layers of spawning herring recorded at night from 18 to 24 UTC (= local time - 2 hours), few herring recordings during the night from 24 to 06, and most herring recorded pelagically during the day. Recorded fish density was highest in the period 15 to 18 and lowest from 21 to 03, and schools staying pelagically had higher density than bottom layers. The observed behaviours are likely to influence acoustic abundance estimates, particularly when surveying during the night towards the end of the spawning season

Spatial and temporal overlap and trophic interactions between Atlantic Saithe (Pollachius virens) and Norwegian spring spawning herring (Clupea harengus) on herring spawning grounds

Atlantic saithe (Pollachius virens) and Norwegian spring-spawning herring (Clupea harengus) represent two of the most abundant and ecologically important fish species in the Northeast Atlantic. The two species co-occur in the Vesterålen/Lofoten area during the wintering of herring, but the temporal and spatial overlap between species in different scales as well as their trophic interactions is otherwise largely unknown. Saithe is a potentially important predator on herring. In the present study we found the commercial catches of saithe off western Norway to be closely related to the migration of herring into the coastal spawning grounds, indicating that saithe follow the high concentrations of migrating herring. Acoustic surveys within a defined study area demonstrated that the species overlapped spatially and temporarily. Degree of overlap varied both diurnally and with the maturity status of the herring (pre-spawning, spawning or spent). Concurrent catches of saithe and herring from trawling and gillnets supported an overlap also on the meso-scale. Observations on more fine-scaled behaviour obtained from an underwater rig with highsensity cameras positioned in high-density areas of herring demonstrated that the distribution and behaviour of herring and saithe was closely linked to the diel light cycle and spawning of herring. When looking at the actual outcome of the interactions between the species on a kind of temporally, integrated spatial microscale, it was observed that stomachs of large saithe (>50 cm) on average contained 2–3 herring, while the stomachs of smaller saithe were filled with herring eggs. This suggests a considerable predation pressure by saithe on different life stages of herring

Distribution and diversity of fish species along the Sudanese Red Sea coast based on three combined trap and gillnet surveys

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Schooling dynamics of spawning herring (Clupea harengus L.) in a bay in South-Western Norway

The schooling dynamics of hening (Clupea harengta L.) was investigated during spawning in Raunefjord, south-westem Norway in 1994. A single school of Nonvegian spring spawning hening WSS) was tracked during daytime over a five day period in the spawning season, using multibeam sanning sonar and echosounder. Gillnet samples of the herring were coilected daily fiom the school. The school completed spawning in 3-4 bys. It remained one unit throughout the period, but when spawning was initiated, the school divided vertidy into two components, one pelagic and one derned. DiBlering individual choices in the trade-off between su~val and reproduction hr fish prior to, during and aikr spawning may have caused vertical gradients of key h r s such as food and predaton to act as dividing forces. Prior to spa*, Late matured and ripened individuals seeking d o w n d and early matured and spent individuals searching upwards probably caused the vertical school shape to be cyhdricai. As spawning proceeded, fish from the demersai component spred outwards at the bottom, causing this component to take on the shape of a carpet, whereas the pelagic unit condens.e d. into a somewhat tighter baU. Wben spa+ was completed, the two to fonn a loose flake dose to the suhce. Possible effects of timing and duration of the spa* period on acoustic survey estimates of spawning herring are discussed

In situ of Cape horse mackerel (Trachurus capensis)

The acoustical target strength (TS) of Cape horse mackerel (Trachurus capensis) was measured in situ at 38 kHz using a submersible split beam transducer in combination with a SIMRAD EK 500 echosounder. The transducer head was lowered to scattered aggregations of horse mackerel close to the bottom in order to resolve single echo targets at short range (5-30 m), Echograms were printed for a range intervai of 5-50 meters from the transducer head, Time of reception, compensated and uncompensated TS, range and alongship and athwardship offset angle were recorded from the EK 500 serial port and stored in ASCII- files at an IBM- compatible computer. Single fish targets were tracked using especially developed software and selected for analysis. The measurements suggest a lower TS than presently applied, but the literature is inconclusive on the matter, and the results should therefore be interpreted with caution

Pinging down the food web: multi-frequency acoustic discrimination of jellyfish and fish

Jellyfish are gaining increasing prominence in many pelagic marine ecosystems worldwide. It has been argued that jellyfish-dominated communities will be the end-point in ecosystems perturbed by high fishing effort, and that increases in jellyfish abundance could be indicative of, and consequences of, climate-induced changes and/or regime shifts in pelagic ecosystems. Jellyfish are difficult to sample using conventional netting techniques, and data on changes in distribution and abundance are consequently sparse. Recent field observations and modelling studies have however shown that jellyfish can be detected acoustically at frequencies used routinely for fisheries surveys (18, 38, 120 and 200 kHz). Acoustic surveys might therefore provide a means for monitoring jellyfish populations but, prerequisite to this, echoes arising from jellyfish must be distinguishable from echoes returned by other scatterers. This paper presents multi-frequency acoustic data for two jellyfish that are common in the Namibian Benguela; Chrysaora hysoscella and Aequorea aequorea, and explores how characteristic acoustic signatures of jellyfish may enable these organisms to be discriminated acoustically from pelagic fish including horse mackerel (Trachurus trachurus capensis) and pilchard (Sardinops ocellatus). The ability to discriminate jellyfish and fish using multi-frequency acoustic data may lead to improved acoustic estimates of pelagic fish biomass (by reducing bias due to jellyfish echoes) and aid ecological investigations of jellyfish