Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day
A series of pelagic trawl sampling trials on pelagic species distributed near surface off Angola were conducted by R/V «Dr. Fridtjof Nansen» operating a medium sized pelagic trawl with a vertical opening of about 20 m. The results have been analysed with respect to effects such as vessel and gear av...
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ftimr:oai:imr.brage.unit.no:11250/105769 2023-05-15T16:18:27+02:00 Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day Luyeye, Nkosi Coetzee, Janet Boyer, D. Misund, Ole Arve 1997 application/pdf http://hdl.handle.net/11250/105769 eng eng ICES ICES CM Documents;1997/W:12 This report is not to be cited without prior reference to the authors http://hdl.handle.net/11250/105769 18 s. trawl trål pelagic fish pelagisk fisk sampling prøvetaking stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Catch: 925 VDP::Mathematics and natural science: 400::Geosciences: 450::Oceanography: 452 VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Fish health: 923 Working paper 1997 ftimr 2021-09-23T20:15:19Z A series of pelagic trawl sampling trials on pelagic species distributed near surface off Angola were conducted by R/V «Dr. Fridtjof Nansen» operating a medium sized pelagic trawl with a vertical opening of about 20 m. The results have been analysed with respect to effects such as vessel and gear avoidance, time of day, towing speed and tow duration. Numerous trawls targeting on dense sardinella shoals failed to yield large catches. Shoals that were at the trawl depth as the vessel passed over were often observed far below the foot rope of the trawl, indicating vertical avoidance. Often shoals in the net mouth were recorded by the net sonde for long periods. It was assumed that these fast swimming fish were able to swim along with the trawl, and escaped out of the trawl when slowing down the towing speed during hauling. No correlations were, however, found between towing speed in the range 2.7 - 5.0 knots and the total catch of sardinella. Increasing the tow duration during the day to more than 120 minutes also failed to yield consistently higher catches. At night, there was a negative correlation between tow duration in the range 15 - 60 min and the catch size, simply because the trawl was towed for a longer time to obtain proper samples in areas with scattered recordings. During the day sardinella was caught both inshore and further offshore, while horse mackerel was caught further offshore only. Both species were well represented in the catches made at night. There were substantial day and night differences in the length distributions of sardinella with small sardinella being caught only at night. Report Fridtjof Nansen Institute for Marine Research: Brage IMR Fridtjof ENVELOPE(-56.717,-56.717,-63.567,-63.567) |
institution |
Open Polar |
collection |
Institute for Marine Research: Brage IMR |
op_collection_id |
ftimr |
language |
English |
topic |
trawl trål pelagic fish pelagisk fisk sampling prøvetaking stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Catch: 925 VDP::Mathematics and natural science: 400::Geosciences: 450::Oceanography: 452 VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Fish health: 923 |
spellingShingle |
trawl trål pelagic fish pelagisk fisk sampling prøvetaking stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Catch: 925 VDP::Mathematics and natural science: 400::Geosciences: 450::Oceanography: 452 VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Fish health: 923 Luyeye, Nkosi Coetzee, Janet Boyer, D. Misund, Ole Arve Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day |
topic_facet |
trawl trål pelagic fish pelagisk fisk sampling prøvetaking stock assessment bestandsberegning VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Catch: 925 VDP::Mathematics and natural science: 400::Geosciences: 450::Oceanography: 452 VDP::Agriculture and fishery disciplines: 900::Fisheries science: 920::Fish health: 923 |
description |
A series of pelagic trawl sampling trials on pelagic species distributed near surface off Angola were conducted by R/V «Dr. Fridtjof Nansen» operating a medium sized pelagic trawl with a vertical opening of about 20 m. The results have been analysed with respect to effects such as vessel and gear avoidance, time of day, towing speed and tow duration. Numerous trawls targeting on dense sardinella shoals failed to yield large catches. Shoals that were at the trawl depth as the vessel passed over were often observed far below the foot rope of the trawl, indicating vertical avoidance. Often shoals in the net mouth were recorded by the net sonde for long periods. It was assumed that these fast swimming fish were able to swim along with the trawl, and escaped out of the trawl when slowing down the towing speed during hauling. No correlations were, however, found between towing speed in the range 2.7 - 5.0 knots and the total catch of sardinella. Increasing the tow duration during the day to more than 120 minutes also failed to yield consistently higher catches. At night, there was a negative correlation between tow duration in the range 15 - 60 min and the catch size, simply because the trawl was towed for a longer time to obtain proper samples in areas with scattered recordings. During the day sardinella was caught both inshore and further offshore, while horse mackerel was caught further offshore only. Both species were well represented in the catches made at night. There were substantial day and night differences in the length distributions of sardinella with small sardinella being caught only at night. |
format |
Report |
author |
Luyeye, Nkosi Coetzee, Janet Boyer, D. Misund, Ole Arve |
author_facet |
Luyeye, Nkosi Coetzee, Janet Boyer, D. Misund, Ole Arve |
author_sort |
Luyeye, Nkosi |
title |
Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day |
title_short |
Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day |
title_full |
Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day |
title_fullStr |
Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day |
title_full_unstemmed |
Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day |
title_sort |
trawl sampling of small pelagic species off angola. effects of avoidance, towing speed, tow duration and time of day |
publisher |
ICES |
publishDate |
1997 |
url |
http://hdl.handle.net/11250/105769 |
long_lat |
ENVELOPE(-56.717,-56.717,-63.567,-63.567) |
geographic |
Fridtjof |
geographic_facet |
Fridtjof |
genre |
Fridtjof Nansen |
genre_facet |
Fridtjof Nansen |
op_source |
18 s. |
op_relation |
ICES CM Documents;1997/W:12 This report is not to be cited without prior reference to the authors http://hdl.handle.net/11250/105769 |
_version_ |
1766004629113405440 |