Initial assessment of age, growth and reproductive ... - PanamJAS

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322C. IZZO & B. M. GILLANDERScatch rates of T. fasciata in the south east trawlfishery between 1992 and 2002 (Reardon 2003), thisspecies was ranked the eighth most abundant amongbenthic chondrichthyan by-catch species caught intrawl operations in the lower west coast of Australia(Hyndes et al. 1999). Trawl fishing has been shownto significantly alter the relative abundances ofchondrichthyans caught as both target and by-catchspecies (Graham et al. 2001, Kennelly 1995).Marshall et al. (2007) previously describedthe dietary composition and reproductive cycle of T.fasciata in Western Australia; however, to date,there have been no previous investigations into thegrowth of T. fasciata, and the time to sexualmaturity still remains unknown. Thus, here wedetermine rates of growth and reproduction whichwill be valuable in order to assess the susceptibilityof T. fasciata to fishing impacts.Materials and MethodsSpecimens of T. fasciata were collectedfrom the by-catch of commercial prawn trawlvessels undertaking normal fishing operations in thegulf waters of South Australia. Specimens weresexed and the total length (TL) and disc width (DW)of all specimens were measured to the nearestmillimetre. TL and DW had a significant linearrelationship:TL (mm) = 2.4578 × DW - 9.6382(n = 43, r 2 = 0.95, p < 0.001)DW (mm) = 0.3863 × TL + 16.19(n = 43, r 2 = 0.95, p < 0.001)Specimens were also weighed (BW: bodyweight) to the nearest gram. Body weight increasedpredictably with TL:BW (g) = 7 × 10 -6 × TL 2.96(n = 43, r 2 = 0.99, p < 0.001)Age estimates were based on counts ofbands in the vertebrae of specimens. Vertebral centrawere cleaned of excess tissues, placed in a 5%sodium hypochlorite solution for 10 – 20 min,followed by a rinse in freshwater and dried in anoven. Individual vertebral centra were thenembedded in a clear setting epoxy resin and cut into(approximately) 300 µm sections with a diamondtipped lapidary saw. These sections were thenmounted onto a microscope slide and examinedunder a dissecting microscope with a transmittedlight source (Fig. 1).Marginal increment ratio (MIR) analysiswas attempted in order to determine the time of bandformation (Liu et al. 1998). However, growth bandformation could not be fully validated as specimenswere obtained between November and April eachyear. Therefore, age estimates presented fromhereon are based on counts of growth bands,with the explicit assumption that the periodicity ofband formation in T. fasciata is annual (i.e., oneband = one year). In order to verify the suitabilityof the vertebral centra as an ageing structure forT. fasciata (i.e., the vertebral centra developed inproportion to body size), the centrum diameter(CD) was measured to the nearest millimetreusing digital calipers for each specimen beforesectioning.Figure 1. Vertebral centra section of an 860 mm total lengthmale southern fiddler ray (Trygonorrhina fasciata) showing 11growth bands.Counts of bands were made from twovertebrae from each specimen. Growth bandswere defined as a pair of opaque (outer) andtranslucent (inner) circuli observed in the corpuscalcareum of the sectioned vertebrae (Fig. 1).Counts were made without prior knowledgeof the size, sex or previous count of the specimen.If the counts varied between the two vertebrae,a third was sectioned and counted. If the countof the third vertebra matched either of the previoustwo it was taken as the count. If there was noagreement between any of the three samples thenthat specimen was excluded from the analysis.This counting procedure was repeated and theestimated ages between readings compared.Errors in the counts of growth bands in the vertebralcentra were analyzed using the Index of AveragePercentage Error (IAPE) as described by Beamishand Fournier (1981).Age estimates based on counts of growthbands were fitted to the von Bertalanffy (1938)growth function. Growth parameters were calculatedusing FISHPARM version 3.0S (Prager et al. 1987).Pan-American Journal of Aquatic Sciences (2008) 3(3): 321-327
Age, growth & reproduction in T. fasciata.323We scored the male reproductive status of T.fasciata on three criteria: (1) the degree of claspercalcification (none, partial, or fully calcified); (2) thedegree of vas deferens coiling (none, partial, orcomplete coiling); and (3) the presence, or absenceof semen. Males were considered sexually maturewhen the claspers were rigid and the vas deferensshowed partial or complete coiling and semen waspresent. Inner clasper length (CL) was also measuredto the nearest millimetre.The female reproductive status of T. fasciatawas based on two criteria: (1) ova diameters; and (2)uterine width and development. Females wereconsidered mature when gravid or with ova > 10 mmin diameter in their ovaries, and if the uteri weredifferentiated from the oviducts and measured > 10mm in width at their widest point (Neer & Cailliet2001).Results and DiscussionIn total, 43 specimens (26 males and 17females) of T. fasciata were examined to providepreliminary age and growth estimates. Males rangedin length from 275 to 877 mm TL, whereas femaleswere slightly longer, ranging from 251 to 1084 mmTL (Fig. 2).Monthly changes in the vertebral MIR ofT. fasciata appeared to peak in April, whichcoincides with the approximate time of birth,followed by an abrupt decrease in May (Fig. 3).These limited data suggest that growth bandformation occurs once a year between April andMay.Vertebral centra were verified as suitableageing structures as CD had a significant linearrelationship with TL:TL (mm) = 89.553 × CD + 150.58(n = 86, r 2 = 0.95, p < 0.05)CD also showed a significant linearrelationship with DW:DW (mm) = 37.72 × CD + 71.41(n = 86, r 2 = 0.91, p < 0.05)The IAPE for counts of growth bands in thevertebral centra of T. fasciata was 3.56%, wellbelow the accepted 5% error thresholds (Campana2001). Growth bands in sectioned vertebral centrawere easily identifiable in individuals greater than360 mm TL, and as a result no individuals wereexcluded from the study. The calculated vonBertalanffy growth function parameters for males,females and combined sexes are shown in Table I.Females and males possessed similar growth rates(k = 0.16 and k = 0.18, respectively); however,females attained greater maximum lengths thanmales (L ∞ , = 1157, L ∞ , = 930 mm TL, respectively;Fig. 4). The agreement of the band count (age)-atlengthdata to the calculated growth curve suggeststhat the von Bertalanffy growth curve accuratelyforecasts growth in South Australian T. fasciata. Thesmallest specimens which displayed signs of freshumbilical scarring were 251 to 289 mm TL,indicating that this is the approximate size range atbirth of T. fasciata (Table I). This is similar to theestimated size at birth (Y-axis intercept) derived bythe von Bertalanffy growth curve (Fig. 4).The oldest T. fasciata specimen determinedfrom counts of growth bands in vertebrae was a1084 mm TL female that was 15 years old. Thisspecimen provides an initial, conservative estimatefor longevity. Using the equation for theoreticallongevity (t max ) (Skomal & Natanson 2003) weestimated that the t max for both sexes combined was26.6 years (Table I).Figure 2. Length-frequency distribution of the southern fiddlerray (Trygonorrhina fasciata) specimens collected from SouthAustralia.Figure 3. Monthly changes in the marginal increment ratio(MIR) of the southern fiddler ray (Trygonorrhina fasciata).Where the numbers indicate sample size and vertical barsindicate ± the standard error values.Pan-American Journal of Aquatic Sciences (2008), 3(3): 321-327
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