85_SCA-CS_09

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85_SCA-CS_09.pdf

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85_SCA-CS_09.pdf (199.7 kb)

Abstract
Estimates of current biomass for the Coromandel fishery are available from the 2007 dredge survey but the only reference biomass that might be calculated is average recruited biomass. Scallop biomass can be expected to vary from one year to the next, so the long-run average is difficult to estimate and not necessarily a good indicator. However, biomass estimates around the turn of the century (2000) were consistently at or near the lowest on record and it seems reasonable to conclude that the population was, for unknown reasons, at a very low ebb. In contrast, following reasonable increases in biomass, catch rate, and condition of scallops in 2003 and, especially, 2004, the biomass in 2005 (almost regardless of what was assumed about dredge efficiency) was the highest on record and probably higher than in the mid-1980’s when not all of the beds were surveyed. This remarkable resurgence was strongest at the Mercury Islands location, but most beds showed some increase in density. The 2006 survey results suggested a maintenance of the high biomass observed in 2005. The 2007 survey results show there has been a decline in the overall biomass since 2006, although thebiomass estimate is still high compared with historical records.

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 Coromandel 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 each year following a review of the survey results and stock assessment, and after consultation with fishery stakeholders. In recent years, the agreed catch limits have been substantially less than the estimated CAY.

Document date
Sunday, 31 May 2009

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V 1.3

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Adobe PDF

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199.7 kb

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2009

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M:\SCIPOL\Working Groups 2009\Plenary 2009\FINAL\MFish 2009 May Plenary\May 2009 - PDF\85_SCA-CS_09.pdf

Uploaded date
Friday, 12 June 2009

Search tags

Species: SCA;
Stock: SCACS;

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