Mant et a\ Biological characteristics and mortality of Pentapodus vilta 



513 



stantial trawling and recreational fishing in an em- 

 baynient 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 50- 

 mm 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 west- 

 ern butterfish in Shark Bay. Pentapodus vitta {/i = 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 repro- 

 ductive 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 FL. 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 + 6 log FL. 



Analysis of covariance (ANCOVA, c<=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). 



film. These sections were then mounted on glass slides 

 and viewed with transmitted light under a dissecting 

 microscope. 



Validation that a single growth zone is formed each 

 year in otoliths of P. vitta was undertaken by using 

 marginal increment analysis. The distance between the 

 outer edge of the outermost opaque zone and the periph- 

 ery of the otolith section (the marginal increment) was 

 measured on the sectioned otoliths of each fish caught 

 between June 1999 and May 2000. The marginal incre- 

 ment was then expressed either as a proportion of the 

 distance between the primordium and the outer edge of 

 the opaque zone, when only one opaque zone was pres- 

 ent, or as a proportion of the distance between the outer 

 edges of the two outermost opaque zones, when two or 

 more opaque zones were present. All measurements 

 were made with the Leica Image Manager 1000 (Leica 

 Microsystems AG, Wetzlar, Germany). 



Ages were assigned to each P. vitta on the basis of the 

 number of opaque zones (annuli) in sectioned otoliths 

 and by taking into account the dates of capture and 

 the birth date of the fish. The birth date was desig- 

 nated as 1 December because this date corresponded 

 approximately to the mid-point of the period when, on 

 the basis of gonadosomatic indices and the trends shown 

 by gonadal and oocyte development, P. vitta spawn (see 

 "Discussion" section). The opaque zones were counted in 

 the region between the primordium and the proximal 

 surface along the ventral and dorsal margins of the 

 sulcus acousticus, where the opaque bands were usu- 

 ally well defined and thus easily identified. The opaque 

 zones in each otolith were counted without knowledge 

 of the length or date of capture of the fish from which 

 that otolith had come. 



The precision of the counts from 175 P. vitta otolith 

 sections made by the senior author (JCM) on two sepa- 

 rate occasions and between JCM's second count and a 

 count made by another experienced reader, was as- 

 sessed by using the index average percent error (lAPE) 

 of Beamish and Fournier (1981). The lAPE values were 

 low (i.e., 0.7%) for the two counts made by the senior 

 author (JCM)) and for JCM's second count and that of 

 the independent reader (i.e., 3.9%), demonstrating a 

 high level of precision for the otolith readings. Because 

 the senior author was more experienced in reading the 

 otoliths of P. vitta, his second round of counts (and 

 these were almost invariably the same as in the first 

 round of counts) was used for determining the ages of 

 individual fish. 



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 sections of each otolith, 0.3-0.5 mm 

 thick, were cut through the primordium by using a low- 

 speed jewellery saw and then ground using 9-|um lapping 



Growth 



Fish caught by all methods in all months (except those 

 obtained during extensive trawling in November and 

 December 1997) — fish that collectively covered the full 

 size range of P. vitta, were used to provide data for con- 

 structing growth curves. The large samples collected by 

 trawling in November and December 1997 were excluded 

 because they would have introduced an excessive bias 

 towards the larger fish at younger ages (see Ricker 



