Browsing by Subject "M. paradoxus"

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    Results from sensitivity tests related to the models used to standardize the offshore commercial trawl CPUE data of M. capensis and M. paradoxus respectively
    (2009) Glazer, Jean Patricia; Butterworth, Doug S
    General Linear Models (GLMs) are applied to the catch per unit effort (CPUE) data from each of the two hake stocks, Merluccius capensis and M. paradoxus, in order to derive standardized indices of abundance which are then input into the stock assessment models and the Operational Management Procedure (OMP). Since it is difficult to distinguish between the two hake species at the landing site catches are reported for both species combined. An algorithm that makes use of size and depth information is thus used to split the catches into the two species (Gaylard and Bergh, 2004). In recent years, for various reasons, a large proportion of the drag records do not have the catches reported by size rendering them unusable; hence an extensive exercise was undertaken to allow for the inclusion of these data in the analyses (Fairweather et al, 2009). The results reported here are for various sensitivities related to GLM3 of Fairweather et al (2009). This is the version of the data that corrects for the errors encountered in the historic extracts used previously, and also includes those data that were previously excluded given that size information was not reported. A procedure was applied to interpolate the size information from data for which there is size information within a given cell which, for each year, is defined by a depth range, latitude range (for the West Coast) or longitude range (for the South Coast), and quarter (Jan-Mar, Apr-Jun, July-Sept and Oct-Dec).
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    Open Access
    Why does the current hake assessment indicate the extent of depletion of the M. paradoxus population to be high, but that of M. capensis to be much less?
    (2008) Rademeyer, Rebecca A; Butterworth, Doug S
    The relatively high extent of depletion estimated for the M. paradoxus population is found to be robustly determined, with all five sources of data contributing to the assessment suggesting that both this extent and current fishing mortality are relatively high. However these fives sources lead to appreciably differently perceptions for the extent of depletion of the M capensis population. The GLMstandardised CPUE series commencing in 1978 is found to be the most influential of the five in leading to present estimates of both a relatively low extent of depletion and fishing mortality for this population. However if there is a trend in bias over time in this index as a measure of abundance, irrespective (almost) of the direction of this bias, the extent of depletion of the M. capensis population would be estimated to be notably higher. Research priorities indicated by this analysis are a focus on ageing and sex-differentiation for M. paradoxus, and on the possibility that factors responsible for an increase in catching efficiency may have been omitted from the M. capensis CPUE GLM standardisation.