Browsing by Author "Durholtz, M D"

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    Open Access
    Growth and mortality rates of sardine (Sardinops sagax) pre-recruits in the Southern Benguela region during early 2001
    (2006) Weni, Elliot; Field, John G; Durholtz, M D
    The main objectives of this study were: (1) to develop a method of accurately and consistently counting daily increments; (2) to use the increments in otoliths counts to estimate age of the individuals; (3) to back-calculate hatch-dates to identify cohorts, and, (4) to analyze growth and mortality of sardine larvae and juveniles in the Southern Benguela region during early 2001.
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    The effects of ageing biases on stock assessment and management advice: a case study on Namibian horse mackerel
    (2008) Wilhelm, M R; Durholtz, M D; Kirchner, C H
    We explore the influence of age-estimation errors on the results of the age-structured production model (ASPM) used for horse mackerel stock assessment in Namibia for the period 1961–2003. The analysis considered age data from eight readers collected during an otolith-reading workshop. Four scenarios of age-estimation errors were assumed: Case 1 — a reference age computed as the modal age of estimates obtained by the four most experienced readers; Case 2 — age readings from a precise and experienced (Namibian) reader of horse mackerel otoliths; Case 3 — age estimates from a reader that displayed positive bias compared with the reference ages; and Case 4 — age estimates from a reader that displayed negative bias compared with the reference ages. The age–length key of each case was applied to length distributions of survey, pelagic fleet and midwater fleet landings (1991–2003) to obtain catch-at-age data. These data were then used in the ASPM. Results obtained from Case 3 differed most significantly from the others and appeared to be unrealistic in terms of the state of the stock and negative log-likelihood estimates. The conclusion is that more resources need to be directed towards age determination, because management recommendations are highly sensitive to errors in ageing. Most effort should be placed into age estimation of age groups 3–5 (20–30 cm total length), but significant effort needs to be devoted to age estimation of midwater commercial samples. Finally, the extent of sampling and the raising strategy of length frequencies should be improved.
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    Open Access
    An updated assessment of the Agulhas sole resource, Austroglossus pectoralis
    (2016-09) Butterworth, Doug S; Glazer, J P; Fairweather, T P; Durholtz, M D
    This analysis updates that of Butterworth and Glazer (2014), which considered two hypotheses of decreasing catchability and of decreasing productivity to account for a recent large drop in CPUE. Two further years of data reflect some increase in CPUE. For the most pessimistic scenario (a decrease in productivity, which remains at its current reduced level into the future), projections are somewhat more positive than previously, with recent biomasses estimated higher and projected to decrease more slowly if the 2013 effort level is maintained.