Chondrichthyan Bycatch: Risk Assessment, Spatiotemporal trends and Contributions to Management

Abstract

Bycatch is one common aspect of fisheries worldwide and recognized as one of the significant impacts over protected and threatened species, chondrichthyans among them. This thesis aims to help mitigate and provide tools to aid the conservation of chondrichthyans. In the first data chapter I developed a model of risk assessment to quantify relative vulnerability of the group based on spatial overlap of natural distributions and fishing events and related to the species resilience. In this study I used commercial fisheries data provided by the Australian Fisheries Management Authority and applied the method to bycatch chondrichthyans and target species. Results were consistent with IUCN status of the species, including cases with regional differences from global status. In the second data chapter, spatial and seasonal distribution of chondrichthyan bycatch events in southern and eastern Australia were analysed. Results indicate differences in rates between gears and also the importance of oceanographic factors to bycatch ratio as well to catch per unit of effort whilst diversity is more influenced by gear type. The spatial analyses also indicated hotspots for bycatch in the Great Australian Bight and Bass Strait. In the third data chapter, I estimated the age, growth and reproductive parameters of the Eastern Fiddler Ray Trygonorrhina fasciata and the Sydney Skate Dipturus australis, two endemic species often caught as bycatch. Age estimates, based on vertebrae bands, were used to calculate growth parameters, which were similar to related species. Moreover, based upon the somatic and gonad indexes, reproductive biology information of these species is provided. In the fourth data chapter I describe the diet and estimated trophic levels of these batoids. Results indicated a diet consisting largely of crustaceans and place the species within the range of trophic level for closest relatives and support their status as mesopredators on the region.