Chondrichthyan occurrence and abundance trends in False Bay, South Africa, spanning a century of catch and survey records

Abstract

Commercial fishing in False Bay, South Africa, began in the 1600s. Today  chondrichthyans are regularly taken in fisheries throughout the bay. Using a  combination of catch, survey and life history data, the occurrence and long-term changes in populations of chondrichthyans in False Bay are described. Analyses of time series data from five fishing methods between 1897 and 2011 provided trends in relative abundance. Of the 37 species found in False Bay, 25 showed no  significant trends for any fishing method. Of the 12 species and two genera with catch trends, four showed a common trend across methods, two increasing  (Mustelus mustelus and Carcharhinus brachyurus) and two decreasing (Galeorhinus galeus and Raja spp.), while another two species (Triakis megalopterus and Isurus oxyrinchus) showed highly significant declines in one method only. A productivity index was used in conjunction with information on species distribution, catch frequency and trends in abundance to assess the vulnerability of False Bay chondrichthyans. Among the various habitats and depth strata, the surf-zone has been most impacted by exploitation, whereas the conservation of deep-water, soft-sediment species has benefited from the ban on trawling in False Bay implemented in 1928. Continued monitoring using catch data from beach-seine, recreational shore-angling and the commercial linefishery is recommended, along with improved species identification.

Key words: chondrichthyans, elasmobranchs, exploitation, vulnerability, population trends, conservation.