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Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean

Standard weight (Ws) equations were established for yellowfin tuna (Thunnus albacares) in the Western Indian Ocean and used as a baseline for calculating relative weight (Wr) indices. Length-weight data for yellowfin tuna were obtained from the Indian Ocean Tuna Commission (IOTC) for 1984 to 1991 an...

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Main Author: Harikishun, Ameil
Other Authors: Moloney, Coleen
Format: Thesis
Language:English
Published: Department of Biological Sciences 2014
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access_status_str Open Access
author Harikishun, Ameil
author2 Moloney, Coleen
author_browse Harikishun, Ameil
Moloney, Coleen
author_facet Moloney, Coleen
Harikishun, Ameil
author_sort Harikishun, Ameil
collection Thesis
description Standard weight (Ws) equations were established for yellowfin tuna (Thunnus albacares) in the Western Indian Ocean and used as a baseline for calculating relative weight (Wr) indices. Length-weight data for yellowfin tuna were obtained from the Indian Ocean Tuna Commission (IOTC) for 1984 to 1991 and 2003 to 2013. Four quadratic standard weight equations were established (based on mean, median, first and third quartile statistics for each length class) using log10-transformed weight (W, in grams) and fork length (FL, in mm) data. The median Ws equation (log10(W) = -3.903 + 2.425(FL) + 0.103(FL)2) was the preferred baseline for the Wr analysis. Relative weight estimates for individual fish ranged from 92 to 110. Annual and monthly Wr estimates ranged from 99 to 101 and 99.9 to 100.5 respectively. A GLM was fitted to separate the effects of year, month and sex on the response variable Wr. Year, month, sex and the interaction terms Year:Sex and Month:Sex all contributed significantly to the variability in Wr explained. Correlations between environmental variability and yellowfin tuna condition were observed. Concentrations of suitable prey in 2003 and 2004 correlated to above average condition, although 2005 and 2006 were below average condition during similar prey availability and environmental conditions. Relative weight followed an increasing trend from 2008 to 2013 despite lower biological enrichment. Mechanisms such as a shallower thermocline and reduced fishing pressure are proposed explanations. This study provided a reference study for body condition studies of yellowfin tuna in the Indian Ocean. It also provided support for the application of the Wr index to other tuna species in the Indian Ocean managed by the IOTC.
format Thesis
id oai:open.uct.ac.za:11427/7632
institution University of Cape Town (South Africa)
language eng
last_indexed 2026-06-10T12:32:12.136Z
license_str Not specified — see source repository
provenance_str_mv Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository
publishDate 2014
publishDateRange 2014
publishDateSort 2014
publisher Department of Biological Sciences
publisherStr Department of Biological Sciences
record_format dspace
source_str UCTD — University of Cape Town Open Access Repository
spelling oai:open.uct.ac.za:11427/7632 Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean Harikishun, Ameil Moloney, Coleen Marsac, Francis Ndjaula, Hilkka O N Standard weight (Ws) equations were established for yellowfin tuna (Thunnus albacares) in the Western Indian Ocean and used as a baseline for calculating relative weight (Wr) indices. Length-weight data for yellowfin tuna were obtained from the Indian Ocean Tuna Commission (IOTC) for 1984 to 1991 and 2003 to 2013. Four quadratic standard weight equations were established (based on mean, median, first and third quartile statistics for each length class) using log10-transformed weight (W, in grams) and fork length (FL, in mm) data. The median Ws equation (log10(W) = -3.903 + 2.425(FL) + 0.103(FL)2) was the preferred baseline for the Wr analysis. Relative weight estimates for individual fish ranged from 92 to 110. Annual and monthly Wr estimates ranged from 99 to 101 and 99.9 to 100.5 respectively. A GLM was fitted to separate the effects of year, month and sex on the response variable Wr. Year, month, sex and the interaction terms Year:Sex and Month:Sex all contributed significantly to the variability in Wr explained. Correlations between environmental variability and yellowfin tuna condition were observed. Concentrations of suitable prey in 2003 and 2004 correlated to above average condition, although 2005 and 2006 were below average condition during similar prey availability and environmental conditions. Relative weight followed an increasing trend from 2008 to 2013 despite lower biological enrichment. Mechanisms such as a shallower thermocline and reduced fishing pressure are proposed explanations. This study provided a reference study for body condition studies of yellowfin tuna in the Indian Ocean. It also provided support for the application of the Wr index to other tuna species in the Indian Ocean managed by the IOTC. 2014-09-22T12:00:17Z 2014-09-22T12:00:17Z 2013 Bachelor Thesis Honours BSc http://hdl.handle.net/11427/7632 eng application/pdf Department of Biological Sciences Faculty of Science University of Cape Town
spellingShingle Harikishun, Ameil
Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
thesis_degree_str Bachelor's / Honours
title Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
title_full Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
title_fullStr Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
title_full_unstemmed Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
title_short Establishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean
title_sort establishing a weight length baseline for evaluating changes in yellowfin tuna thunnus albacores body condition in the western indian ocean
url http://hdl.handle.net/11427/7632
work_keys_str_mv AT harikishunameil establishingaweightlengthbaselineforevaluatingchangesinyellowfintunathunnusalbacoresbodyconditioninthewesternindianocean

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