A Study to Relate Shrimp Trawl Tow Duration to the Quality of White Shrimp During an Outbreak of Black Gill Disease - Year II
Commercial shrimp trawlers typically tow their nets for two to four hours, with most averaging about three hours. Experience has shown that fishermen using a tow duration not exceeding three hours does not result in a damaged harvest. However, in recent years, since black gill has been common, a number of shrimpers have complained that shrimp quality has suffered during periods when the incidence of black gill is high. This study was undertaken to determine if shrimp quality is compromised by long tows and if this can be ameliorated by shorter tow durations. Additionally, direct observations by DNR biologists would help determine if mortalities occurred during tows of longer durations.
It is presumed that melanized gills exchange gases at rates lower than normal gills. If this is true, black gill shrimp are more vulnerable to stress and die early in a tow. There has also been supposition that shrimp with black gill may have exoskeletons that are thinner than those of normal shrimp. Immediate, direct observations will offer additional data to address this question.
A commercial trawler was contracted for four days to provide data for DNR biologists during late September 2006. DNR biologists were onboard to examine catch and collect data. The St. Helena Sound area was chosen as the sampling site because high incidences of black gill have been noted in this area and sea conditions are generally better than the open ocean (reducing the likelihood of lost days to bad weather). Market quality of shrimp was compared for shrimp collected in tows having durations of 30 minutes, one hour, two hours and three hours. Four tows were made each day including each of the four durations. The order of tow durations was randomly selected on the first day and shifted by one each day, allowing each tow duration to be the first tow. This helped the biologists determine if shrimp quality is affected by time of day. As shrimp came aboard, a randomly picked sub-sample of shrimp was examined to estimate the number of live (active), dead and moribund shrimp. The total number and weight of shrimp from each tow was determined. A randomly collected sub-sample of about 100 shrimp from each tow was examined for quality. This included body color, black gill infection, damage to and apparent thinness of the exoskeleton, and other metrics. Other ancillary data collected included identification and quantification of bycatch.
After analyzing the black gill frequency data provided by shrimpers, we believe that we missed the height of the black gill infection period by one week. We will be conducting this project again next year with hopes of better timing to coincide with the peak black gill infection period.
If collaborating researchers wish to have fresh tissues for more detailed examination of the parasites or shrimp health, samples will be collected and preserved as required. Please contact us for details.