Browsing by Author "Ndjaula, Hilkka O N"
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- ItemOpen AccessEstablishing a baseline for evaluating changes in fish body condition and population dynamics of Cape hake (Merluccius capensis and M. paradoxus) in South Africa(2014) Boyd, Danielle Winona; Moloney, Coleen; Ndjaula, Hilkka O N; Fairweather, TraceyStandard weight (Ws) equations were developed and relative weight (Wr) indices calculated for both shallow-water (Merluccius capensis) and deep-water (M . paradoxus) Cape hake. The aim was to provide fixed baselines and Wr indices to aid understanding of temporal and spatial variability in fish condition . Baselines were calculated using the empirical percentiles (EmP) method applied to length (L) – weight (W) data collected by research surveys on the South African west and south coasts, from 1983 to 2013 . Four quadratic Ws equations for each species were generated, based on the following weight statistics for each size class j : first quartile, median, third quartile and mean. Median Ws equations were chosen for the baseline for shallow-water hake (log₁₀ Ws=-2.491 + 3.33 8 *(log₁₀ L)-0.065 *(log₁₀ Lj)²) and deep-water hake (log₁₀ Ws =- 2.161 + 2.930*(log₁₀ L) + 0.0456 *(log 10Lj)²). Shallow-water hake was found to be the heavier of the two species at the same length. Wr indices display good condition values (>100%) for both species throughout all analyses . Fish condition for both species was best between 1988-2009, ranging from 50-1 65% for individual fish and 100 – 109% for annual averages. Monthly mean Wr indices peaked in June and October for shallow-water hake and July and October for deep-water hake. Mean Wr values were different for shallow-water hake on the south (105%) and west (103%) coasts. Deep-water hake showed no spatial variability in mean Wr values. There were no differences between mean Wr values of males and females between, and within, species. Future hake data may be compared to these Ws equation baselines and Wr indices to show changes in body condition for this commercially important stock.
- ItemOpen AccessEstablishing a weight-length baseline for evaluating changes in yellowfin tuna (Thunnus albacores) body condition in the Western Indian Ocean(2013) Harikishun, Ameil; Moloney, Coleen; Marsac, Francis; Ndjaula, Hilkka O NStandard 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.