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Small bivalve survey, assessment and stock status update: 2016. Ostrea angasi - Georges Bay Venerupis largillierti - Northern Zone, Georges Bay

Citation

Jones, H and Gardner, C, Small bivalve survey, assessment and stock status update: 2016. Ostrea angasi - Georges Bay Venerupis largillierti - Northern Zone, Georges Bay, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, June (2016) [Contract Report]


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Abstract

The four commercial small bivalve fisheries in Georges Bay and Ansons Bay are surveyed every two to three years for the purposes of estimating total biomass and assessing fishery status in order to assist with the allocation of quota for the forthcoming fishing year. In 2016, a stock assessment with total allowable commercial catch recommendations (TACC) was conducted for the Flat Oyster Ostrea angasi fishery in Georges Bay. A limited pilot survey was also conducted for the Georges Bay Northern Zone Venus Clam Venerupis largillierti fishery in response to a voluntary TACC limit being agreed prior to the assessment period.

The O. angasi survey recorded an increased estimated area of fishable reef than previously used for assessments and a best estimate of total biomass of 391.3 tonnes ( s.e. 362.6 420.0). Best estimates of total abundances from survey data across the fishing area suggested that there was a 90% chance the total abundance exceeded 5.3 million oysters. The current harvest fraction for O. angasi is set at 10% of the estimated total biomass. TACC recommendations are provided in the form of probability tables calculated from estimated total biomass and converted to counts per dozen at the 50th, 20th and 10th quantile of the estimated total biomass mean. The TACC allocations presented in the probability table provide a choice in certainty that the estimation of total biomass is greater than the probability value. The low proportion of annual catch to TACC recorded in the O. angasi fishery since 2008 would suggest that the current level of fishing pressure is unlikely to cause the stock to become recruitment overfished. In accordance with national stock status definitions, this fishery is assessed as sustainable. Recommendations to changes in the harvest strategy for this species are given in light of the DPIPWE shellfish rules document review for 2017. These recommendations follow Commonwealth Harvest Strategy guidelines for the inclusion of limit reference points with harvest strategies.

The V. largillierti area survey for 2016 recorded an estimated fishery area of 99,713 m2, which represented a slight enlargement to the estimated area in 2014 but lower than previous survey records in 2003, 2009. There were also substantial changes in the location of the V. largillierti beds in 2016 compared to previous years. No formal biomass survey was conducted in 2016 due to the voluntary reduction in TACC to 3 t agreed by licensees for fishing years ending August 2016 and August 2017. The large reduction in catch and geometric mean catch per unit effort (CPUE) data from fishing years ending 2013 to 2016 are interpreted as a decline in biomass which may have been caused by change in environmental conditions affecting recruitment. With total catches for 2013-2014 at <8 % of the estimated fishable biomass fishing mortality is unlikely to be responsible for biomass decline. Cohorts of undersize clams identified in 2014 would have been expected to reach the LML in 2015 - 2016 but have not been sufficient to rebuild stock. Based on national stock status definitions this species is assessed as environmentally limited. The current low TACC and subsequent minimal level of fishing pressure may be expected to allow the stock to recover provided suitable environmental conditions exist. A small pilot survey in one of the fishing beds indicated evidence of pre-recruits and marketable size animals which may provide a basis for the recovery of the fishery in the future. Recommendations to changes in the harvest strategy for this species are given in light of the DPIPWE shellfish rules document review for 2017. These recommendations follow Commonwealth Harvest Strategy guidelines for the inclusion of limit reference points, which will assist in reducing the risk of recruitment overfishing in the future.

Item Details

Item Type:Contract Report
Keywords:bivalves, stock assessment, oysters, clams
Research Division:Agricultural and Veterinary Sciences
Research Group:Fisheries Sciences
Research Field:Aquatic Ecosystem Studies and Stock Assessment
Objective Division:Animal Production and Animal Primary Products
Objective Group:Fisheries - Wild Caught
Objective Field:Wild Caught Edible Molluscs
Author:Jones, H (Mr Hugh Jones)
Author:Gardner, C (Professor Caleb Gardner)
ID Code:112000
Year Published:2016
Deposited By:Sustainable Marine Research Collaboration
Deposited On:2016-10-19
Last Modified:2016-10-24
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