A sidescan survey of schooling fish off the Otago Pensinsula (Marine Science)

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O'Driscoll, Richard

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Abstract:

The spatial distribution of pelagic schooling fish off the coast of Otago, New Zealand was studied using a Klein 595 side-scan sonar with an effective frequency of 130kHz. Sidescan sonar allows mapping of near-surface schools of fish, but it has seldom been used for this purpose. Acoustic data were collected off Otago on 21 days during the late summer and autumn of 1994, 1995 and 1996. Two thousand one hundred and ninety eight schools were detected. Of these, 348 schools of barracouta (Thyrsites atun), 67 schools of jack mackerel (Trachurus murphyi), and 17 schools of slender tuna (Allothunnus fallai) were identified. Schools of barracouta (median area, a= 31m2) were significantly smaller than schools of jack mackerel (a = 350m2) or slender tuna (a = 250m2). Many of the unidentified schools may have been sprat (Sprattus antipodum or S. muelleri). Schools were detected throughout daylight hours, but fish dispersed at night. Detection of schools was limited by surface reverberation, bottom backscatter and the automatic gain of the instrument. Statistical analyses based on Ripley' s K function were developed, and showed schools of barracouta occurred in clusters with a patch length of 525-3230m. The spatial distribution of schools of barracouta and jack mackerel at these scales was related to the distribution of surface swarms of the euphausiid, Nyctiphanes australis. Over 75% of schools were detected· in areas with surface densities of krill higher than the median ( 121 N. australis km-1). N. australis made up more than 98% of the wet mass of stomach contents of jack mackerel, slender tuna, and schooling barracouta. Barracouta captured away from schools had also fed on krill, but the proportion was much lower than in schooling barracouta (51.7% of wet mass of stomach contents). Barracouta are facultatively schooling fish, and may form schools to help locate patches of krill. The distribution of krill was influenced by spatial structure in the physical environment. Net catches of krill were highest in a band of low salinity surface water which was present in the study area following periods of high river flow. On days when this feature was present, mean densities of krill were 3.5 times higher in the low salinity band than in surrounding waters. There was no clear association between the occurrence of seabirds and fish schools. The distribution of planktivorous seabirds was determined by the distribution of krill, but the spatial structure of seabird aggregations was related to physical hydrography. Patches of ii .... sitting birds contained more birds and were longer along transects with high salinity structure (fronts) .

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vii. Ill. Maps in text, 30cm.

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1997ODriscoll

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http://download.otagogeology.org.nz/temp/Abstracts/1997ODriscoll.pdf

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Citation

O'Driscoll, Richard, “A sidescan survey of schooling fish off the Otago Pensinsula (Marine Science),” Otago Geology Theses, accessed February 8, 2025, https://theses.otagogeology.org.nz/items/show/335.

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