Project Title:

Final Report - 2009/031 - Taking female mud crabs (Scylla serrata) assessment of risks and benefits

Project Number: 2009/031
Published Date: Dec 2010 Year:
ISBN: 978 0 7345 0419 7 ISSN:
Description: A series of analyses of catch-effort data from compulsory commercial logbooks and from the Department’s Long-Term Monitoring Programme (LTMP) were conducted after the Workshop. Although not part of the Project plan, these were initiated as a result of questions arising from the Workshop participants about the reliability of the data used in the simulation modelling. Exploration of the logbook data and results of the analyses suggest that biases in the data (from a variety of sources, but principally the widespread use of more than the permitted number of pots) may be giving an over-optimistic view of the status of the East Coast mud crab resource. In particular, if a significant proportion fishers are reporting that catches are greater than they actually are, this will result in an overestimate of the productivity of the stock, and an underestimate of the impact of removing the SSHB. Declining stock density trends in the fishery-independent LTMP data series over the past decade are consistent with the commercial data trends, although the time series is too short to rule out the possibility that this could be part of a decadal-scale environmental cycle. Reliable commercial and recreational crab fishers also report that legal-sized mud crabs are becoming more and more difficult to catch.

 

Principal Investigator: Ian Brown

 

Key Words: mud crab, blue swimmer crab, management strategy, modeling, stock size, sex harvest policy, Ovigerous female, Scylla serrata.

 

Summary:

A series of analyses of catch-effort data from compulsory commercial logbooks and from the Department’s Long-Term Monitoring Programme (LTMP) were conducted after the Workshop. Although not part of the Project plan, these were initiated as a result of questions arising from the Workshop participants about the reliability of the data used in the simulation modelling. Exploration of the logbook data and results of the analyses suggest that biases in the data (from a variety of sources, but principally the widespread use of more than the permitted number of pots) may be giving an over-optimistic view of the status of the East Coast mud crab resource. In particular, if a significant proportion fishers are reporting that catches are greater than they actually are, this will result in an overestimate of the productivity of the stock, and an underestimate of the impact of removing the SSHB. Declining stock density trends in the fishery-independent LTMP data series over the past decade are consistent with the commercial data trends, although the time series is too short to rule out the possibility that this could be part of a decadal-scale environmental cycle. Reliable commercial and recreational crab fishers also report that legal-sized mud crabs are becoming more and more difficult to catch.

 

In accordance with the Communications Plan agreed to at the Workshop, the Minister was informed of these facts. Although no public announcement has yet been made, it is understood that the Minister’s view is that before any changes can be made to the SSHP, the issues relating to over-potting need to be addressed satisfactorily. Along with other issues relating to improving our ability to confidently monitor changes in the mud crab resource, finding solutions to the over-potting problem will represent a significant part of the review of the mud and blue swimmer crab fisheries.
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