Estimation of natural and fishing mortality using length composition data
Sound estimates of natural and fishing mortality form the basis of fishery stock assessment and modelling. Without these data, the extent to which the spawning biomass has been reduced or the effectiveness of management measures in sustaining wild fish stocks cannot be determined. These estimates of mortality are crucial if the commonly-used reference points for fishing mortality and biomass are to be used in managing fisheries. Considerable advances have been made in obtaining estimates of mortality when data on the age composition of the annual catch are available. However, when there are only data on the length composition of the catches, analyses become more complex and the algorithms are not as well defined. Although information is usually available within the biological and fishery data that allows estimation of total mortality, the information on natural mortality must usually be obtained from empirical models that relate natural mortality to characteristics of the life history for other fish species or by estimating the relationship between total mortality and fishing effort, thereby obtaining an estimate of natural mortality by subtraction. Furthermore, there is a need to understand how natural mortality varies with size. Although the study of mortality deserves to be a primary focus of fishery research, it is difficult and thus often set aside. For many of Australia’s finfish stocks, there is a need to develop length-based methods to estimate mortality, which can be used to monitor the stock status of the recreational fisheries, and to obtain improved estimates of natural mortality that can be used in assessing sustainability. The project falls within FRDC’s Natural Resources Sustainability Program and is intended to increase and apply knowledge of stock assessment methods by developing methods of estimating mortality using length composition data which, compared with age composition data, are relatively inexpensive to collect.
Final report
Tactical Research Fund: Assessing alternative fishing seasons for red-legged banana prawns in the Joseph Bonaparte Gulf and future directions for collaborative research for NPF Industry Pty Ltd
Tactical Research Fund: Reducing dolphin bycatch in the Pilbara Finfish Trawl Fishery
The bycatch of dolphins and other protected/listed species in the PFTIMF has been recognised as being too high by the WA Minister of Fisheries and the Department of Environment, Water, Heritage and the Arts. In 2005, the Minister wrote to industry participants expressing his concern for the “…real or perceived lack of adequate action being undertaken to address this serious bycatch issue…”. He indicated that he was prepared to close the fishery to protect the industry’s reputation should progress not be made. Semi-flexible exclusion grids reduced the dolphin catch rate by almost 50% in 2006, but the Minister has stated that further reductions are necessary if the fishery is to continue.
In 2007, the need for a renewed approach to resolving the PFTIMF bycatch issues were further highlighted in reports from DoF to DEWHA and the Draft Bycatch Action Plan: There are significant differences in bycatch reporting between trips with and without independent observers on board; Observer coverage was set at a minimum 22% to gain data that would prove statistically robust, but only 19% was achieved in the first half of 2007; Temporal/spatial data on bycatch has been collected, but the data has proven inconclusive or has not been analysed in detail; A deck-mounted electronic observer system designed to replace on-board observers has proven unsuitable thus far; and, bycatch continues.
There are both biological and political needs for an immediate effort to further reduce bycatch. Common themes to successful implementation of bycatch reduction include: Collaborations between industry, scientists and resource managers; Pre- and post-implementation monitoring; and, compliance via enforcement and incentives. This project has been designed to conform to these themes and to meet the need of assisting industry in moving from ‘Interim Managed’ to ‘Managed’ fishery status and fulfilling the requirements of the EPBC Act.