A robust method for generating separate catch time-series for each of the hake species caught in the Namibian trawl fishery

E. Johnsen, Johannes N. Kathena

Research output: Contribution to journalJournal articleResearchpeer-review


Merluccius capensis and M. paradoxus are morphologically very similar and cannot be registered separately by species in the Namibian commercial hake catches. This prevents a biologically plausible single-species stock assessment from being carried out. Here, species-separated data from an observer programme and scientific surveys are used to model the spatio-temporal overlap of the two species, which are then used to predict the catch by species in each commercial trawl. The study presents a method that compensates for both the escapement and codend retention differences in the survey and commercial trawls. The accuracy with which species were identified was found to be considerably higher during scientific surveys compared with that obtained from observers, whereas the seasonal coverage of the observer data was much better than that of the surveys. The estimated spatiotemporal model parameters from each of these two data sources were similar, however, despite these differences. In support of previous findings, M. capensis had a shallower and more northerly distribution than M. paradoxus with depth and latitude together explaining 51% and 85% of the residuals in the models produced from observer and survey data respectively. Model outputs suggest that during the period 1998-2007, M. paradoxus has dominated the annual hake catches. Even though our method is unable to account for abrupt and unexpected changes in the species' geographical distribution, it does open the way for the establishment of a single-species hake assessment in Namibia.
Original languageEnglish
JournalAfrican Journal of Marine Science
Issue number1
Pages (from-to)43-53
Number of pages11
Publication statusPublished - 2012
Externally publishedYes


  • COD
  • assessment
  • landings
  • Merluccius
  • Namibia
  • single species
  • Ecology, Evolution, Behavior and Systematics
  • Aquatic Science
  • accuracy assessment
  • catch statistics
  • commercial species
  • escapement
  • fishery survey
  • fishing gear
  • gadoid
  • geographical distribution
  • spatiotemporal analysis
  • stock assessment
  • time series
  • trawling
  • Merluccius capensis
  • Merluccius paradoxus
  • Rexea solandri
  • Mathematical biology and statistical methods
  • Ecology: environmental biology - General and methods
  • Ecology: environmental biology - Animal
  • Ecology: environmental biology - Wildlife management: aquatic
  • Biophysics - Biocybernetics
  • Animal distribution
  • Animals, Chordates, Fish, Nonhuman Vertebrates, Vertebrates
  • species distribution
  • trawl fishery
  • geographical position
  • catch composition
  • model output
  • bottom depth
  • catch time-series
  • species spatio-temporal overlap
  • codend retention difference
  • escapement difference


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