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1.
A trawl catches only a portion of the fish in its path. The term catchability therefore refers to the fraction of the available fish caught. A method was developed and tested to establish catchability constants for trawl surveys of Namibian hake Merluccius spp. A catchability constant can be expressed as a simple relationship between hake area densities calculated from trawl catches and acoustic biomass estimates. Initial values were on an order of magnitude of 0.8, meaning that the catch takes 80% of the hake available to the trawl. The methodology depends on careful area selection (flat bottom, homogenous fish distribution), following the same trawl lane during subsequent hauls in an area, and thorough acoustic post-processing. A pronounced and repetitive pattern in catchability within surface daylight hours was found. Early morning and early afternoon catches were low, and the best catches of the day were made around noon, a result that may influence stock assessments based on trawl data, because morning and afternoon data would under-represent actual abundance.  相似文献   
2.
A simulation model is used to investigate possible ecological effects of up- and downwelling on nitrogen flows through a kelp-bed food web off the Cape Peninsula, South Africa. The model depicts the flow of nitrogen, which is often limiting in marine ecosystems, from kelps, other macrophytes and phytoplankton, through filter-feeders to carnivores, with a feedback loop via faeces and bacteria to detritus and filter-feeders. When modelled as a closed system, bacteria associated with detritus and animal faeces form a large component of the particulate nitrogen available to filter-feeders, and the faeces feedback loop dominates nitrogen flow. When measured rates of water transport are incorporated into the model, bacteria have little opportunity to accumulate before being transported out of the system. Animal faeces and kelp detritus are the dominant filter-feeder food components under upwelling conditions, whereas phytoplankton is the major contributor to particulate organic nitrogen under downwelling conditions. When realistic pulses of upwelling/downwelling derived from wind indices are used as model input, filter-feeders are shown to decline during the summer upwelling season when much potential food is advected out of the system, and they increase during the winter when downwelling conditions are more prevalent, bringing in nitrogen-rich phytoplankton from the blooms developing offshore.  相似文献   
3.
The pathogens of an eye infection affecting capture-stressed pelagic fish were isolated and identified. Prophylactic use of veterinarian tetracycline was found to be an effective treatment.  相似文献   
4.
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   
5.
Orange roughy Hoplostethus atlanticus are unusual fish. They form dense aggregations that have fuelled lucrative fisheries at great depths (600–1 400 m), especially off Namibia, New Zealand and Australia. They are thought to be very long-lived (>100 years, maturity at 22 – 40 years), and to have exceptionally low natural mortality (M = 0.045–0.064 year?1) and slow growth rates (K = 0.055–0.070 year?1). In addition, they spawn large eggs and have low fecundity. These factors combine to make orange roughy highly susceptible to overfishing; most stocks are below 30% of pristine levels. Assessments are obtained from indices of catch rate and trawl, acoustic and egg surveys. Acoustic estimates are the most direct, but are confounded by the species' low target strength (?50 to ?53 dB)–attributable to the wax-filled swim bladder. Extracellular wax esters are stored in abundance and comprise mostly mono-unsaturated fatty acids, with low concentrations of the ω-3 fatty acid family. This unusual composition (resultant from the species' diet) ensures neutral buoyancy. Stock separation has been inferred mainly from biological studies, but genetic studies have also found differences among stocks within New Zealand and Australia. Deep-water habitat may be damaged by trawling operations and may take many years to recover, so in some quarters there is a call for a portion of suitable habitat to be set aside for preservation. Although Namibian orange roughy are shallower, smaller and younger than those in other stocks, the Namibian fishery sustained high catches for only a few years before quotas were reduced, from 12 000 to 1 875 tons. Three management lessons are suggested for developing orange roughy fisheries based on the Namibian experience: (1) imposition of catch limits during exploratory fishing; (2) starting the acoustic surveys earlier in the fishery, if possible; (3) greater reliance on trends in catch rate until a survey series has been established.  相似文献   
6.
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   
7.
During 1997, considerable scientific differences arose about the status of the Namibian hake (Merluccius capensis and M. paradoxus) resource, and as to whether the hake Total Allowable Catch (TAC) should be substantially decreased or increased. These differences revolved primarily around whether or not abundance estimates from the swept-area trawl surveys by the Norwegian research vessel Dr Fridtjof Nansen should be considered as reliable measures of biomass in absolute (as distinct from relative) terms. The paper relates the computations underlying the Interim Management Procedure (IMP) approach that was put forward at that time as a basis to resolve this impasse. The anticipated performance, in terms of catch and risk of resource depletion, of a number of simple candidate IMPs for the Namibian hake resource is evaluated. The IMPs depend on two parameters, whose values are to be chosen by decision-makers, and adjust the TAC up or down from one year to the next according to whether trends in recent commercial catch rate and survey indices of abundance are positive or negative. Performances are evaluated across the then current wide range argued for resource abundance and status. Trade-offs in performance across the candidates considered are discussed. One of the candidates was subsequently selected by a joint meeting of scientists, industry and Ministry officials in February 1998 and served as the basis for scientific recommendations for the TAC for the hake resource for the following three years.  相似文献   
8.
Following independence of Namibia in 1990, abundance of that country's hake stocks was monitored by trawl surveys conducted by the Norwegian F.R.V. Dr Fridtjof Nansen using a systematic survey design. Precision from such designs is considered to be better than with a random design, but it cannot be quantified through standard analysis of variance. In 1998 and 1999, the trawl surveys were duplicated in full using a commercial trawler that operated in parallel with the Dr Fridtjof Nansen. Both vessels had the same fishing gear and rigging. In both years, biomass and distribution patterns estimated by the vessels were similar. The paired datasets collected from the surveys were used to analyse variability in the point samples. Absolute differences between pairs of catches were, in general, proportional to the catch, but they varied randomly following a normal distribution around the catch level. Local variability was analysed as random noise, modelled and later reapplied on single vessel data series to evaluate the effect of sample size and replicates on the survey mean. In a survey with 200 stations, local sample noise accounted for about 4% of the variability in the survey mean. Alternatively, running series of simulated surveys by bootstrapping on pairs of catch data gave similar results, when a small systematic vessel effect was adjusted for. The main statistical techniques applied were less susceptible to outlier catches than straightforward correlations or regressions and could therefore, perhaps with some advantage, also be used to estimate the vessel factor when intercalibrating trawl survey vessels.  相似文献   
9.
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   
10.
An assessment of catch and effort in the Eastern Cape shore and skiboat linefisheries was undertaken between 1994 and 1996 by means of roving creel and access point surveys. Catch-and-effort data were obtained from direct observation of 3 273 shore-fishers, 172 recreational and 223 commercial skiboat outings. Total effort in the region was high at 903 186 fisher-days year?1 in the shore fishery and 64 266 and 24 357 fisher-days year?1 in the commercial and recreational skiboat sectors respectively. The fisheries are multispecies in nature. The shore fishery consisted of 66 species, the recreational skiboat fishery 44 species and the commercial skiboat fishery 48 species. Just 10 species accounted for 75, 83 and 90% of the catch of the shore, recreational and commercial skiboat fishery respectively. The average catch per unit effort (cpue) was low in all sectors, 1.15 kg fisher?1 day?1 in the shorefishery, 9.4 kg fisher?1 day?1 in the recreational skiboat fishery and 21.5 kg fisher?1 day?1 in the commercial skiboat fishery. Catch data showed that professional and club anglers are more successful fishers. Comparisons with historic records for Port Elizabeth revealed that the cpue in the shore fishery had declined markedly, whereas the total effort increase was negligible (0.1%). In addition, the species composition of the fishery has changed.  相似文献   
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