Towards a catchability constant for trawl surveys of Namibian hake

Abstract

A trawl catches only a portion of the fish in its path. The term catchability therefore refers to the fraction of the available fish caught. A method was developed and tested to establish catchability constants for trawl surveys of Namibian hake Merluccius spp. A catchability constant can be expressed as a simple relationship between hake area densities calculated from trawl catches and acoustic biomass estimates. Initial values were on an order of magnitude of 0.8, meaning that the catch takes 80% of the hake available to the trawl. The methodology depends on careful area selection (flat bottom, homogenous fish distribution), following the same trawl lane during subsequent hauls in an area, and thorough acoustic post-processing. A pronounced and repetitive pattern in catchability within surface daylight hours was found. Early morning and early afternoon catches were low, and the best catches of the day were made around noon, a result that may influence stock assessments based on trawl data, because morning and afternoon data would under-represent actual abundance.

Keywords: acoustics, catchability, hake, trawl surveys

African Journal of Marine Science 2001, 23: 375–383