512 



Abstract — The western butterfish 

 iPcntapodus vitta) is numerous in the 

 bycatch of prawn trawling and recre- 

 ational fishing in Shark Bay, Western 

 Australia. We have thus determined 

 crucial aspects of its biological char- 

 acteristics and the potential impact of 

 fishing on its abundance within this 

 large subtropical marine embayment. 

 Although both sexes attained a maxi- 

 mum age of 8 years, males grow more 

 rapidly and to a larger size. Maturity 

 is attained at the end of the first year 

 of life and spawning occurs between 

 October and January. The use of a 

 Bayesian approach to combine inde- 

 pendent estimates for total mortality, 

 Z, and natural mortality, M, yielded 

 slightly higher point estimates for Z 

 than M. This result indicates that 

 P. vitta is lightly impacted by fishing. 

 It is relevant that, potentially, the 

 individuals can spawn twice before 

 recruitment into the fishery and that 

 73% of recreationally caught individu- 

 als are returned live to the water. 



Biological characteristics and mortality 

 of western butterfish iPentapodus vitta), 

 an abundant bycatch species 

 of prawn trawling and recreational fishing 

 in a large subtropical embayment 



Jason C. Mant 



Michael J. Moran 



Stephen J. Newman 



Western Australian Fisheries and Marine Research Laboratories 

 Department of Fisheries, Government of Western Australia 

 39 Northside Drive 

 Hillarys (Boat Harbour), Western Australia 6025, Australia 



S. Alex Hesp 



Norman G. Hall 



Ian C. Potter (contact author) 



Centre for Fish and Fisheries Research 



School of Biological Sciences and Biotechnology 



Murdoch University 



South Street 



Murdoch, Western Australia 6150, Australia 



Email address for I C. Potter: i.potter@murdoch,edu,au 



Manuscript submitted 14 October 200.5 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



14 November 2005 by the Scientific Editor. 



Fish. Bull. 104:512-520(2006). 



The family Nemipteri(iae (threadfin 

 breams) is found in tropical and sub- 

 tropical waters of the Indo-west Pacific 

 (Nelson, 1994). The western butter- 

 fish iPentapodus vitta) (also known as 

 "striped whiptail"') is a small benthic 

 nemipterid, which is endemic to West- 

 ern Australia (Russell, 1990) and very 

 numerous in Shark Bay on the west 

 coast of this state (Heithaus, 2004). It 

 is also very abundant in the bycatch 

 of the prawn trawl and recreational 

 fisheries in this large subtropical envi- 

 ronment. However, as with the vast 

 majority of bycatch species in embay- 

 ments, no estimate has been made of 

 the mortality of P. vitta due to fishing. 

 Furthermore, although P. vitta is very 

 abundant in certain environments, 

 there are no data on the age composi- 

 tion, growth, and reproductive biol- 

 ogy based on validated age data for 

 a population of this species. Indeed, 

 previous studies of the Nemipteridae 

 have focused almost entirely on spe- 



cies ofNemiptei'is and Scolopsis, which 

 are widely distributed throughout 

 the Indo-Pacific, where they are an 

 important component of commercial 

 and artisinal fisheries (Sainsbury and 

 Whitelaw, 1984; Young and Martin, 

 1985; Murty et al., 1992; Zacharia, 

 1998). 



The present study elucidates key 

 aspects of the biology of P. vitta and, 

 in particular, length-weight relation- 

 ships, size and age compositions, 

 growth, size and age at maturity, 

 duration of the spawning period, and 

 mortality. Focus is also placed on 

 ascertaining whether this species is 

 hermaphroditic, as is the case with 

 certain other nemipterids (Young 

 and Martin, 1985). The question of 

 whether the mortality of a species 

 with the "characteristics" of P. vitta 

 is particularly attributable to sub- 



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