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Spatial and temporal variability in lipid content and gonadosomatic index of sardine (sardinops sagax) in the Southern Benguela ecosystem
Author(s)
De Goede, Johannes
Date Issued
2004
Type
Thesis
Publisher
Cape Technikon
Abstract
Determining lipid content for sardine (Sardinops sagax) in the Southern Benguela
Ecosystem (SBE), through applying a method of fat staging, is a relatively easy method
to use as a biological time-series to assess the condition of sardine. The condition of fish
is an important indicator for fluctuations in the population size of post-recruit fish.
Depending on the amount of fat associated with the stomach, one of seven fat stages was
assigned to each sardine. Visually assigned fat stages were then converted to a lipid
content value, given as a percentage of wet body mass, by applying a conversion equation
to these fat stages.
Time-series of lipid content and gonadosomatic index (GSI) were generated using general
linear models (GLM). Results from these GLM's explained 34% and 39% of the
observed variance in lipid content and GSI respectively. Monthly and annual least square
means (LS mean) were derived from these GLM's to show seasonal variability in lipid
content and GSI, for the period 1996 to 2003. Clear seasonal patterns in both lipid content
and GSI were observed. Lipid content showed a decrease, but an interannual trend in GSI
could not be observed, for the time series.
The study area, which ranged from the Orange River mouth (28°40'S and 16°30'E) on
the west coast to Port Elizabeth (25°37'E and 33°57'S) on the east coast was divided into
four smaller zones to allow for spatial tests. Zone 1 being on the west coast and Zone 4
on the east coast. Both lipid content and GSI showed strong spatial differences, with lipid
content being at highest levels in Zone 4 throughout the time series, and the highest GSI
levels being observed in Zone 1.
Ecosystem (SBE), through applying a method of fat staging, is a relatively easy method
to use as a biological time-series to assess the condition of sardine. The condition of fish
is an important indicator for fluctuations in the population size of post-recruit fish.
Depending on the amount of fat associated with the stomach, one of seven fat stages was
assigned to each sardine. Visually assigned fat stages were then converted to a lipid
content value, given as a percentage of wet body mass, by applying a conversion equation
to these fat stages.
Time-series of lipid content and gonadosomatic index (GSI) were generated using general
linear models (GLM). Results from these GLM's explained 34% and 39% of the
observed variance in lipid content and GSI respectively. Monthly and annual least square
means (LS mean) were derived from these GLM's to show seasonal variability in lipid
content and GSI, for the period 1996 to 2003. Clear seasonal patterns in both lipid content
and GSI were observed. Lipid content showed a decrease, but an interannual trend in GSI
could not be observed, for the time series.
The study area, which ranged from the Orange River mouth (28°40'S and 16°30'E) on
the west coast to Port Elizabeth (25°37'E and 33°57'S) on the east coast was divided into
four smaller zones to allow for spatial tests. Zone 1 being on the west coast and Zone 4
on the east coast. Both lipid content and GSI showed strong spatial differences, with lipid
content being at highest levels in Zone 4 throughout the time series, and the highest GSI
levels being observed in Zone 1.
Additional information
Thesis (MTech (Oceanography))--Cape Technikon, Cape Town, 2004
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Spatial and temporal variability in lipid content and gonadosomatic index of sardine (sardinops sagax) in the Southern Benguela ecosystem.pdf
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