Measurement of the rate of depletion of benthic fauna by prawn (shrimp) otter trawls: an experiment in the Great Barrier Reef, Australia

We carried out a trawl-depletion experiment for benthic fauna in an area closed to commercial trawling. A 12-fathom prawn (shrimp) trawl net having a swept path of about 18 m was towed 13 times along each of six tracks. Each track was 2.7 km long and known from previous surveys to contain several relatively dense patches of megabenthos. The depletion rate was assumed to vary between trawls on a given track, and was modelled by a beta distribution defined by a mean rate and a dispersion parameter. Maximum likelihood analysis of the sequence of catches on each track suggests that a single trawl removes a small, but non-negligible, fraction of the benthos. The combined effect of 13 trawls on each track, assuming the trawls were aligned, was to remove a substantial proportion of the initial biomass. Gastropods suffered the greatest impact (an estimated 95% removed, on average). Ascidians, sponges, echinoids, crustaceans and gorgonians were depleted by an estimated 74–86%, and all other taxonomic groups except algae (27% removed) were reduced by at least 54%. Using differential global positioning systems and careful navigation, we aimed to achieve a high degree of overlap between successive trawls. Nonetheless, records show that the trawler deviated from this path and it is likely that the net did not sweep exactly the same path during the 13 trawls on each track. A simulation study was conducted to examine the effect on parameter estimates of ignoring the probable non-alignment of trawls. There was little bias in the estimate of the dispersion parameter. With the level of dispersion found on these tracks, the estimated mean depletion rates would, however, have been biased (upwards when the true mean depletion rate was below 15%, downwards when the true mean depletion rate was above 15%). This means that most mean depletion rates estimated from the catch data are probably biased by 2–3%. The depletion rates estimated from this study may be acceptable in areas that are trawled infrequently or sparsely. However, the cumulative effect of frequent trawls in an area of high intensity trawling is likely to be substantial both in terms of organisms directly affected by trawling and indirectly due to attracting scavengers and removing refuge habitat for fish and other mobile organisms.     

Reducing risks of Salmonella infections in poultry

3预防沙门氏菌传播的措施 3.1卫生和消毒 在引进新一批鸡群前,所有鸡舍和器具必须进行清洁消毒,特别是在前一批鸡群为沙门氏菌阳性的情况下,清洁消毒就更有必要.卫生措施的有效性可通过严格的细菌学定量检测验证,要求肠道细菌计数小于103个/25 m2.     

Economic analysis of an epizootic of pseudorabies and subsequent production following the institution of a vaccination program in a Pennsylvania swine herd

The economic impact of pseudorabies was examined in a commercial swine herd. At the onset of clinical signs, a modified-live virus vaccine was administered to the sow herd and repeated at 3-month intervals. According to production data from the 320-sow farrow-to-feeder unit, preweaning mortality increased twofold, and subsequently, the number of pigs weaned per litter decreased by 19% (P less than 0.005) during the 5-week epizootic. Also, the number of pigs born alive decreased by 6% during the epizootic (P less than 0.05). No significant differences in production were observed between the 6-month periods before and after the epizootic. Actual cash flow analysis for the farm under isomarket conditions revealed a decreased net return of $2.40/inventoried sow/week, which was attributed to the disease during the epizootic, and a $0.46 decrease in net return/inventoried sow/week in the postepizootic period. Most seropositive herds have clinical signs less severe than those described in this herd, and the cost of eradication of the virus from a swine herd can be in excess of $200/inventoried sow. Thus, we believe that sufficient financial incentives are not available to all swine producers to ensure their enthusiastic cooperation in the effort to eradicate pseudorabies from the US swine population.