Modelling the bycatch of Anguilla marmorata using a generalized depletion model with an example from the Taiwanese glass eel fisheries for Anguilla japonica

Aquaculture of freshwater eels (genus Anguilla) relies solely on the glass eel fisheries for production stock. The countries where multiple anguillid species occur have become important sources for supplying glass eels. In East Asia, the giant marble eel Anguilla marmorata was a bycatch species in the glass eel fisheries for A. japonica, whose population has declined dramatically over the recent three decades. We quantify the impact of glass eel fisheries on the bycatch of A. marmorata using a generalized depletion model with transit stock structure and examine the possible ecological origin of the bycatch. The catch and effort data from the glass eel fisheries targeting A. japonica in the estuary of the Shuang River, northeastern Taiwan, from 1981 to 1994 was used as an example. Compared to A. japonica, A. marmorata had lower catches, lower estimates of abundance, and higher variability in abundance among recruitment waves. The number of recruitment waves, exploitation rate, and relative escapement for A. marmorata were generally similar to those for A. japonica, implying similar impacts from the glass eel fisheries. The abundance estimates did not correlate with minimum annual sea surface temperature. Similarities in the early life history characteristics of A. marmorata and A. japonica could indicate possible ecological origins for the bycatch of A. marmorata, such as a similar splitting mechanism controlled by eddies during larval-stage oceanic migration. Lower abundance and higher variability in A. marmorata might result from a year-round opportunistic spawning pattern, or a strategy corresponding to the distribution edge. The generalized depletion model applied to anguillid glass eel fisheries for multiple vulnerable species showed that similar exploitation pressures may occur for both targeted and non-targeted species. This finding may be applicable to countries in Southeast Asia and Africa where multiple species of anguillid eels occur.