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1.
Although acoustic tracking has been used to study the movement of several species of clawed and spiny lobsters, only recent technological advances have provided sufficiently small transmitters to examine the utility of using acoustic tracking as a means to analyse the movement of relatively small spiny lobsters, such as Jasus lalandii. The effect of the transmitter on the mobility of J. lalandii was tested in aquarium experiments and was shown to have no influence on movement in three separate experiments. Thereafter, adult male rock lobsters (86–98mm carapace length) were tracked in field trials for up to 32 days at Betty's Bay (n = 4) and Hermanus (n = 5) off the Western Cape, South Africa. Tracking J. lalandii in the field using acoustic tags was successful, even in areas with dense kelp beds and rocky outcrops. The signal from the transmitters was readily detectable from the surface and subsequent use of underwater tracking equipment enabled visual confirmation of the position of the rock lobsters. Lobsters moved significantly longer distances (>45m day?1) in the first two days following tagging than during any subsequent time period (<10m day?1). This suggests that transmitter attachment and/or handling altered the movement pattern for the first 72 hours after tagging. During the period of observation, J. lalandii displayed classical nomadic behaviour.  相似文献   
2.
Hydro-acoustic surveys have been used to provide annual estimates of May recruitment and November spawner biomass of the South African sardine Sardinops sagax and anchovy Engraulis encrasicolus resources since 1984. These time-series of abundance estimates form the backbone of the assessment of these resources, and consequently the management of the South African sardine and anchovy is critically dependent on them. Upgrades to survey equipment over time have resulted in recent surveys providing more accurate estimates of abundance, yet in order to maintain comparability across the full time-series, estimates of biomass mimicking the old equipment were used for a number of years. In this paper we develop a method to revise the earlier part of the time-series to correct for receiver saturation in the older generation SIMRAD EK400 and EKS-38 echo sounders and to account for attenuation in dense sardine schools. This is applied to provide a revised time-series of biomass estimates for the South African sardine and anchovy resources with associated variance–covariance matrices. Furthermore, the time-series presented here are based on updated acoustic target strength estimates, making this the most reliable time-series currently available for both resources.  相似文献   
3.
The validity of abundance estimates from hydroacoustic surveys relies, inter alia, on the ability of the fishing gear on the research vessel to sample non-selectively. This study compares the length frequencies of Cape horse mackerel Trachurus trachurus capensis taken in Namibian waters by the R.V. Welwitchia and commercial midwater trawlers. Results indicate that the length distributions of catches taken by the Welwitchia were significantly different from those from commercial trawlers, with a greater proportion of fish >25 cm being sampled by the latter. Also, the biomass estimated per length-class from hydroacoustic surveys over the period 1994–2000 was compared with those from annual landings of the commercial fishery. The length distribution of horse mackerel in purse-seine catches compares favourably with those from the research vessel. However, comparisons with the midwater trawl catches indicated that the length frequencies obtained from research vessels during the years 1994–1997 underestimated the number of large fish in the population and biased the biomass in those years. From 1998 to 2000 the bias was negligible. The magnitude of the error varied between years, but it remained fairly low. To compensate for this bias, length distribution data from midwater trawlers should be integrated into the acoustic biomass calculation procedure.  相似文献   
4.
A study was undertaken to investigate the spatial, diurnal and interannual dynamics of the pelagic component of Namibian hake stocks. Data were derived from trawl surveys conducted on board the R.V. Dr Fridtjof Nansen during the period 1991–1999. In addition to trawl surveys, acoustic methods were applied concurrently to quantify fish biomass off the bottom (above the headline). The pelagic component of hake was variable in both time and space – it was denser in the north and at depths of 200–500 m. Pelagic densities also correlated well with the density of adult fish (≥30 cm) at the bottom, but not with that of young hake (>18 and <30 cm) at the bottom. Pelagic densities were lowest at midday and highest at night, early morning and late afternoon densities being intermediate between the two. Incorporation of acoustic methods allows for at least partial account to be taken of diurnal variation in determining catchability. The fraction of the biomass off the bottom was not the same each year, thus affecting assessment and management advice for the stock, even if the survey results are treated only as relative indices of abundance. More field studies are needed to investigate the size range and species of hake active in vertical migration.  相似文献   
5.
The feasibility of using telemetry equipment to study the movements of estuarine-associated fish in South Africa was investigated by conducting a tracking experiment on spotted grunter Pomadasys commersonnii in the East Kleinemonde Estuary. The telemetry equipment comprised two VEMCO V8 transmitters and a VEMCO VR60 receiver linked to a directional hydrophone. Field experiments demonstrated that the equipment's maximum detection range was 400m. The transmitters were tuned to different frequencies, and interference between these was found to be negligible. The accuracy of locating the transmitter equalled the previously determined Global Positioning System (Garmin GPS 12) accuracy of approximately 5m. Two fish were tracked over a seven-day period. The fish preferred the lower reaches of the estuary where they made repeated and prolonged use of some areas. A tank experiment was conducted to examine the effects of the transmitter implantation. Fish recovered quickly after the surgical procedure, and no differences in swimming behaviour and growth rates were found between these fish and the controls.  相似文献   
6.
From 2001 to 2006, two new designs of flipper bands made from silicone rubbers were tested on African penguins Spheniscus demersus at 365 nests on Robben Island, South Africa. We compared, over six years, the breeding success, from hatching to fledging, of three different groups of penguins: those with rubber bands (117 nests), with conventional stainless-steel bands (103 nests) and without bands (145 nests). There were no significant differences in breeding success between the three groups, suggesting that neither the currently used steel bands, nor either of the new rubber-band designs, were harmful during the seasons investigated. The rubber bands caused less wear of feathers and less drag on a model penguin than the steel bands. In captivity, the behaviours of African penguins fitted with rubber bands were not noticeably different to those of unbanded birds.  相似文献   
7.
The results of the first seasonal survey of the fish of the Kavango River flood-plain along the Angola/Namibia border are reported. The river experiences peak flooding from February through June, with the 375-km long floodplain extending up to 5 km across. The floodplain was sampled five times in 1992 by seine, fish traps and rotenone. The data indicated a pronounced structural and functional response of the fish community in relation to the alternating flood and drought conditions in the river. Catch per unit effort and diversity were highest during months of peak flooding (May and June), and lowest during the month of least flow (November). The reproductive strategies of K-selected piscivorous cichlids and tigerfish were in advance of flooding. Many r-selected invertivores, especially cyprinids, were in relative synchrony with flooding and the stimulation of littoral zone plant growth, while other invertivores lagged the cyprinids. Herbivores had lowest relative abundance during peak flooding; this seemingly inverse relationship with the invertivores should not be interpreted as replacement, but rather the swamping of the system with young-of-the-year r-selected invertivores. The data support the Flood Pulse Concept, which hypothesizes that flooding is the major "driver" of productivity in lowland or floodplain rivers.  相似文献   
8.
Between 1966 and 1989,255 shortfin mako sharks Isurus oxyrinchus were caught in the gill nets which protect the tourist beaches of Natal. The catch rate showed no trend, fluctuating about a mean of 0,34 sharks·km-net?1·year?1. Catches were highest between May and November, when the water tends to be cooler. The sex ratio was 1,4 males to 1 female. Specimens ranged in size from 84 to 276 cm precaudal length, with a mode of 191–195 cm for males and 251–255 cm for females. Males matured at 160-170 cm and females at approximately 220 cm. Catches included two recently fertilized females and two with well-developed embryos. External injuries were found on 14 per cent of males examined and 41 per cent of females. Elasmobranchs were the most common prey category, occurring in 60 per cent of stomachs containing food, followed by teleosts at 40 per cent. Most of the elasmobranch prey were sharks shorter than 1 m.  相似文献   
9.
The vertical distribution and migration of the Cape horse mackerel Trachurus trachurus capensis were investigated on the basis of midwater and bottom trawl collections taken over diel cycles at two sites on South Africa's south coast. Attempts were also made to investigate the relationship between bottom trawl catch rates and acoustic abundance estimates. During the first cycle, horse mackerel were dispersed in midwater during the night, and towards sunrise they coalesced into schools and migrated to the seabed where they remained throughout the day. During that cycle, horse mackerel did not appear to feed, despite the prevalence of potential food, mainly Calanus copepods and euphausiid larvae, in the water column. In contrast, during the second cycle, horse mackerel remained near the seabed during the 24-h experiment, but they exhibited a marked feeding periodicity, with most feeding taking place in the late afternoon. They appeared to select for larger copepods in the water column. Several reasons are advanced to explain the difference in the vertical migratory behaviour of horse mackerel between the two sites, on the basis of the vertical distribution of their potential prey. Diel differences in acoustic estimates of horse mackerel abundance were significantly reduced after applying an acoustic deadzone correction factor. Trawl catches, however, could not be reconciled with acoustic estimates during the day, when horse mackerel were too close to the bottom for effective echo-integration.  相似文献   
10.
As it is not always possible to analyse nutrient samples on board ship during research cruises, it is necessary to preserve them for analysis ashore, and freezing is by far the simplest and seemingly the most accurate means of preservation. To verify this statement, a series of experiments on samples analysed both at sea and later ashore (after freezing) was conducted. Freezing is apparently a viable method for preserving samples destined for analysis of nitrate and low silicate (<30–40 mmol·m?3), but phosphate samples are seemingly not well preserved when frozen. In the latter case, the deficits between analysis of fresh samples aboard ship and frozen duplicates ashore vary randomly. To determine normal trends in nutrient distribution it seems feasible to store samples in the frozen state, but for highly accurate work, as would be necessary for WOCE studies, it is advisable to analyse samples at sea immediately after collection.  相似文献   
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