New Zealand Journal of Marine and Freshwater Research abstracts

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Utility of morphological data for key fish species in southeastern Australian beach-seine and otter-trawl fisheries: predicting mesh size and configuration

Matt K. Broadhurst

NSW Department of Primary Industries
Fisheries Conservation Technology Unit
P.O. Box J321 Coffs Harbour
NSW 2450, Australia
email: mbroadhurst@nmsc.edu.au

Koenraad K. P. Dijkstra

National Marine Science Centre
P.O. Box J321 Coffs Harbour
NSW 2450, Australia

Dennis D. Reid
Charles A. Gray

NSW Department of Primary Industries
Cronulla Fisheries Centre
P.O. Box 21 Cronulla
NSW 2230, Australia

Abstract Samples of 16 key fish species targeted by oceanic prawn trawlers and oceanic and estuarine beach seiners in southeastern Australia were measured for up to 10 morphological dimensions that included: total (Wt) and eviscerated weights (EWt); total (TL), fork (FL), natural (NL), standard (SL), dorsal standard (DSL), and anal standard lengths (ASL); and maximum body height (MH), width (MW), and girth (MG). All partial lengths were indexed against TL to provide linear relationships and, where there were sufficient data, tested for intra-specific sexual dimorphism. Nearly all species that were sexed demonstrated at least some morphological divergence, and especially for transverse measures; probably owing to different reproductive conditions. Sex-specific or combined regressions of DSL and ASL and transverse measures versus TL were used to identify the most appropriate: (1) partial length for compliance purposes; and (2) mesh sizes and configurations for the different fishing gears used. Using relevant data describing the smallest commercial-sized fish species (stout, Sillago robusta and red spot whiting, S. flindersii—150 mm TL) retained in prawn trawls, a new square-mesh codend (made from 35-mm mesh hung on the bar) was designed and constructed. The selectivities of this square-mesh codend and a conventional diamond-mesh design (40-mm mesh) were then modelled and compared in a case study. The conventional codend was demonstrated to be non-selective for the sizes of eastern king prawns and red spot whiting targeted. In contrast, the square-mesh codend selected red spot whiting across narrow selection ranges (18.88–15.67 mm) and at lengths of 50% retention (143.39–146.32 mm TL) that closely corresponded to the estimated maximum mesh opening. The square-mesh codend also improved selection for eastern king prawns, without impacting on commercial catches. Based on these results, similar predictions were made for appropriate mesh sizes and configurations in oceanic and estuarine beach seines; both of which were demonstrated to comprise minimum mesh sizes that would be non-selective for the sizes of all targeted fish.

Keywords selectivity; trawl; seine; bycatch reduction; mesh size; fisheries management

New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40: 259–272
0028–8330/06/4002–0259     © The Royal Society of New Zealand 2006
M05052; Online publication date 10 April 2006. Received 10 August 2005; accepted 3 October 2005

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