...large subtropical marine embayment. Although both sexes attained a maximum 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 independent 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 individuals are returned live to the water.

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The family Nemipteridae (threadfin breams) is found in tropical and subtropical waters of the Indo-west Pacific (Nelson, 1994). The western butterfish (Pentapodus vitta) (also known as "striped whiptail" (1)) is a small benthic nemipterid, which is endemic to Western 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 environment. However, as with the vast majority of bycatch species in embayments, 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 composition, growth, and reproductive biology based on validated age data for a population of this species. Indeed, previous studies of the Nemipteridae have focused almost entirely on species of Nemipteris 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 relationships, 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 substantial trawling and recreational fishing in an embayment has also been explored. For this purpose, we have used a model that reconciles the often imprecise and conflicting estimates of total and natural mortality that are obtained by using traditional approaches (Hall et al., 2004).

Materials and methods

Sampling regime

766 P. vitta, ranging from 90 to 198 mm in fork length (FL), were collected by trawling throughout Shark Bay in November and December 1997. Trawling was conducted at night by using twin 11-m prawn trawl nets with 50mm mesh in the panels and 44-mm mesh in the codends, as are employed by commercial prawn trawl operators in Shark Bay. Each trawl lasted 20 minutes. The samples collected were used for estimating the mortality of western butterfish in Shark Bay. Pentapodus vitta (n=339), ranging from 31 to 215 mm in fork length, were also collected in many months from 1999 to 2004 by using trawls, hook-and-line fishing, and beach seines. These samples were used to obtain aging, growth, and reproductive data in the study.

Relationships of total length and weight to fork length

The fork length (FL) of each fish, and the total length (TL) of each fish whose caudal fin was not damaged, were measured to the nearest 1 mm. The relationship between TL and FL of each sex was then calculated by using the equation

TL = a + b x FL,

to enable the TL of any fish with a damaged caudal fin to be estimated from its Ft. The weight of each fish (W) was recorded to the nearest 1 g. The relationships between the fork length and weight of females and males were described by the equation

log W = log a + b log FL.

Analysis of covariance (ANCOVA, [alpha]=0.05) was used to determine whether the relationships between TL and FL and between weight and FL of the males and females of P. vitta were significantly different (Zar, 1999).

Validation of the aging method and age determination

The sagittal otoliths of each P. vitta were removed, cleaned, and stored in labelled paper envelopes. Because a preliminary examination demonstrated that the growth zones were often easier to discern in sectioned than in whole otoliths, all fish were aged by using sectioned otoliths. Transverse...

NOTE: All illustrations and photos have been removed from this article.



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