83_SCA1_09

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83_SCA1_09.pdf

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83_SCA1_09.pdf (200.5 kb)

Abstract
Estimates of current biomass for the Northland fishery are available (from a 2007 survey) but there are no estimates of reference biomass with which to compare them. A substantial increase in biomass was observed between 2003 and 2006, which resulted in the 2006 biomass estimate being the highest recorded for Northland. In 2005 and 2006, estimates of biomass were considerably higher than those in 2003 for some beds (notably Bream Bay), but similar or lower in others. There appeared to have been a “shift” in biomass away from the Far North and towards Bream Bay and Mangawhai/Pakiri Beach. This was the “reverse” of the shift towards the Far North that occurred in the early 1990s. However, the 2007 survey results suggest the biomass in Bream Bay and Mangawhai/Pakiri has declined markedly since 2006, and, consequently, the overall fishery biomass is far lower in 2007 than in recent years.

Substantial uncertainty stemming from assumptions about dredge efficiency during the surveys, rates of growth and natural mortality between survey and season, and predicting the average recovery of meatweight from greenweight remain in these stock assessments. Future research should be aimed at reducing this uncertainty, and could include a modelling study of dredge efficiency using existing data, and more field studies of scallop growth and mortality. Managing the fisheries based on the number of recruited scallops at the start of the season as opposed to recruited biomass (the current approach) could remove the uncertainty associated with converting estimated numbers of scallops to estimated meatweight.

We do not understand the processes that have resulted in such large fluctuations in scallop abundance. To get sustainable yield from such a variable stock it is necessary to alter the catch every year. Recent management of Northland scallops has been based on a Current Annual Yield (CAY) approach using F0.1 as an appropriate reference point, which is considered both appropriate and conservative. Annual pre-season research (dredge) surveys are required to estimate recruited biomass and for stock assessment to estimate CAY. Commercial catch limits are adjusted following a review of the survey results and stock assessment, and after consultation with fishery stakeholders.

Document date
Sunday, 31 May 2009

Document type
V 1.3

File format
Adobe PDF

File size
200.5 kb

Reference number
2009

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Uploaded from
M:\SCIPOL\Working Groups 2009\Plenary 2009\FINAL\MFish 2009 May Plenary\May 2009 - PDF\83_SCA1_09.pdf

Uploaded date
Friday, 12 June 2009

Search tags

Species: SCA;
Stock: SCA1;

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