A new direct method of stock assessment of the loliginid squid |
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Authors: | Marek R Lipinski Michael A Soule |
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Institution: | (1) Marine and Coastal Management, DEAT, Private Bag X2, Roggebaai, 8012 Cape Town, South Africa;(2) Fisheries Resource Surveys, P.O. Box 31306, Tokai, 7966 Cape Town, South Africa |
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Abstract: | Hydroacoustic research conducted on chokka squid (Loligo reynaudi d’Orbigny, 1845), off the east coast of South Africa from 1994–2005, has led to the development of an innovative stock assessment
technique, perhaps applicable to all loliginids that migrate inshore to spawn. This technique combines hydroacoustic biomass
estimates made on the spawning concentrations inshore, and minimum biomass estimates made both inshore and offshore using
demersal surveys employing the swept-area method. The hydroacoustic estimate uses an improved method to obtain target strength
measurements, and squid concentrations are individually mapped from a small boat with a towed transducer. This method may
be used even during intense fishing operations because of the manoeuvrability of the small boat inside a tight cluster of
fishing vessels. Biomasses of the individual concentrations are then summed. The inshore biomass, also includes dispersed,
mature squid migrating between concentrations, this is assessed using a concentration stability factor. The biomass of dispersed
squid offshore is again calculated using the swept-area method, a well known demersal survey methodology. The biomass of concentrated
(spawning) squid offshore is calculated using the same proportions between concentrated and dispersed squid which were found
inshore. All four components are then summed to calculate the total biomass. The result obtained is subject to the effect
of complex temporal dynamics, as new animals are recruited to the adult pool and those recently assessed migrate to other
sectors of the distribution area. |
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Keywords: | Stock assessment Squid biomass Hydroacoustics Demersal surveys |
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