Spatial distribution patterns, abundance and population structure of deep-sea crab Chaceon macphersoni, based on complementary analyses of trap and trawl data

Abstract

Marine species such as deep-sea geryonid crabs often exhibit high spatio-temporal variability in abundance and size over depth, substratum type and season, therefore data collected from a single gear type may not represent the whole population. Complementary data from trawl (soft substratum) and trap (hard substratum) fisheries were analysed within a general linear modelling (GLM) framework to assess distribution, abundance and population structure of Chaceon macphersoni off eastern South Africa. Catch rates, mean size, maturation size and sex ratio were modelled relative to year, month, depth, latitude and gear effects. Trap and trawl analyses indicated higher abundance as depth increased up to 500 m, and during the austral spring and summer. The mean size of crabs remained constant at all depths sampled, and sex ratios were skewed towards females. Females were smaller than males, and achieved maturity at a smaller size. A standardised index based on trawl data (1988–2010) showed a long-term decline in abundance, with some recovery after 2002, whereas the trap index showed recent local depletions on hard substrata. Using data from two gear types confirmed broad gradients in abundance, but also emphasised subtle trends, such as local depletions on hard substrata, that would not have been apparent from trawl data only.