Molecular identification of croaker dried swim bladders (maw) on sale in Hong Kong using 16S rRNA nucleotide sequences and implications for conservation

Hong Kong is a major import and transhipment centre for seafood inclusive of a wide range of dried products. Dried fish swim bladder (commonly referred to as 'maw') has long been a significant and valuable component of dried seafood sold in the city. Although the species and provenance of swim bladder are poorly documented the major target taxon involved has historically been the croakers (Sciaenidae). This study is the first to examine the possible provenance and species supplying dried fish swim bladder on sale in Hong Kong using DNA barcoding based on the partial sequence of the 16S rRNA gene. A reference sequence database was constructed from available Genbank fragments and sequences obtained from freshly identified fish specimens. Seventy-nine individual dried fish swim bladders, consisting of four commonly sold morphotypes, were examined with sequences successfully obtained from ∼46% of the samples. Five taxa were identified, four of which matched known species in the sequence database, including three sciaenids and one latid. Confidence in species identifications was high due to low within-group divergence (0.0-1.3%) and high between-group divergence (≥2.9%). None of the identified species is local to Hong Kong waters and none are cultured. Several croakers exploited for swim bladder are depleted or threatened, two of them globally endangered. Interest in fish swim bladder in Chinese markets remains high and it has recently become a replacement for sharks' fin in banquets with decline in fin use. These findings highlight the value of barcoding for species verification of processed fish swim bladders and its potential application in the monitoring of their international trade. The findings also highlight data gaps for species coverage in the available DNA sequences and the need for better trade monitoring of dried swim bladder.