This project evaluated the prevalence and intensity of an ectoparasite Eubrachiella antarctica as a marker for stock discrimination of Antarctic toothfish in the Ross Sea. New Zealand Ministry of Fisheries Observers on four toothfish longline vessels recorded the number of E. antarctica on the fins and in the buccal cavity of 621 large D. mawsoni (mostly 120-150 cm). Up to five D. mawsoni per set were examined for parasites in Small Scale Research Units (SSRUs) 88.1C, 88.2E and 88.2F; and two fish per set in SSRUs 88.1H - 88.1J. Up to 15 E. antarctica, one from each of the first fifteen sets in each SSRU, were dissected out and stored in ethanol for laboratory identification. All of the preserved copepod parasites were identified as female or male E. antarctica. D. mawsoni from 88.1H had a higher prevalence of E. antarctica compared to D. mawsoni from 88.1C, 88.1I, and 88.1J (and to 88.2E) and higher intensity compared to D. mawsoni from 88.1C, 88.1I, and 88.1J. However, neither intensity nor prevalence were significantly different between 88.1H and 88.2F. These area differences do not appear to be sampling artefacts produced by differences in host length structure, sex ratio, seasonality, or observer experience, but represent a regional difference in prevalence and intensity of E. antarctica on D. mawsoni. The small-scale regional differences suggested by the parasite marker are inconsistent with the genetic and tagging studies which suggest homogeneity at small spatial scales. Genetic, tagging, and parasite studies measure different biological parameters and results from these independent studies provide a picture of toothfish movement over different time scales. The stability of this regional difference in prevalence and intensity of E. antarctica needs to be tested with large scale sampling in future years.
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