Comparing external and internal dorsal- spine bands to interpret the age and growth of the giant lantern shark, Etmopterus baxteri (Squaliformes: Etmopteridae) Sarah B. Irvine, John D. Stevens, Laurie J. B. Laurenson Environ. Biol. Fish.(2006)77: 比較背棘之內外輪紋進行燈籠鯊，南方烏鯊 ( 棘鮫目 : 烏鯊科 ) 的年齡及成長研究 M 賴衛

Information on Etmopterus baxteri Subclass Elasmobranchii Order Squaliformes Family Etmopteridae Image Description: Compagno 1984

Distribution

Elasmobranchs are typically aged using annuli on the vertebral centra. The centra of most dogfish are poorly calcified and do not produce reliable band counts (Wood et al. 1979), creating the necessity to find an alternate ageing structure. Etmopterus baxteri Etmopterus baxteri possess dentine spines (one anterior to each dorsal fin) that may be used to investigate age. Introduction

Objective The aim of this study was to estimate the age and growth of the giant lantern shark, Etmopterus baxteri. Age estimates from both external and internal dorsal- fin spine bands were used to investigate growth, longevity and age at maturity.

Materials and methods

Fig.1 Topographic map of southeastern Australia indicating the fishing locations where E. baxteri specimens were collected as bycatch. Specimen collection

Biological examination For those dogfishes with a damaged caudal fin, FL was converted to TL using the linear relationship: TL = 1.07FL , (n = 874, r 2 = 0.97). The relationship between weight and length was examined and compared between each sex using an analysis of covariance (ANCOVA).

vertebrae A few discontinuous and irregularly spaced concentric bands were visible, and other stains and treatments had no effect and it was therefore decided that chemically treated whole centra were not suitable for age determination.

Dorsal-fin spines As the second spine is larger and is less often damaged than the first spine. Spine morphometrics: 1. total spine length(TSL) 2. external spine length (ESL) 3. external spine width (ESW)

External base bands were staining with an alizarin red derivative for 3 – 5 days. Spines were embedded and sectioned following Irvine et al. (2006). And section thickness was generally 250 – 350μm. Precision was calculated using the coefficient of variance (CV) across all fish ages (Change 1982).

Growth analysis VBGM is the most commonly used model to describe fish growth in length or weight (Campana2001). L Ψ mean length at reference age Ψ L ψ mean length at reference age ψ L X mean length at reference age (Ψ+ ψ/2)

The Francis parameters are biologically significant and can therefore be directly compared between sexes. In order to present the VBGM equations for each sex, the Francis parameters were related to the conventional VBGM parameters using:

Longevity Longevity was investigated using the equations 1. Taylor(1958), age at which 95% Fabens (1965), age at which 99% Ricker (1979), age at which 95% where t 0 and k are VBGM parameters derived from Francis growth parameters.

Maturity Sexual maturity in males was determined by clasper condition and macro-examination of the testes. Females was based on the condition of the ovaries and uteri adapted from Wetherbee (1996) and Stehmann(2002).

A logistic curve was fitted to the proportion PA of sexually mature individuals by age (A) using: where ln is the natural logarithm.

The relationship between weight (Wt) and total length (TL) differed significantly with sex (ANCOVA;p < 0.05): Results

Fig. 2 Relationship between animal length (TL) and the mean total spine length (TSL) and internal spine length(ISL) for E. baxteri

Spine structure and growth The relationship between TL and TSL for non- damaged spines: TL = 1.15TSL (r 2 = 0.82; n = 362) and did not differ between sexes (ANCOVA; p = 0.399; F = 0.711).

External bands The number of external bands (No.) increased with spine size (TSL) Male = 54(1 – e( – 0.05(No.+7.8))) r 2 = 0.80, n = 74. Female = 69(1 – e( – 0.0.3(No.+7.3))) r 2 = 0.83, n = 137.

Internal bands

Growth Assuming external bands were formed annually, the Francis model fitted the observed length- atage data well for both sexes (male r 2 = 0.78; female r 2 = 0.78) (Fig. 5a)

The von Bertalanffy equations for external bands were: male L t = 60.59(1 – exp( – 0.082(t ))); female L t = 68.13(1 – exp( – 0.040(t ))). For internal bands were: male L t = 59.55(1 – exp( – 0.163(t ))); female L t = 69.25(1 – exp( – 0.116(t ))).

A curvilinear relationship was found between the number of external bands (EB) and internal bands (IB) [IB = – (EB 2 ) (EB) , r 2 = 0.785, n = 101; Fig. 6 ].

Longevity The corrected longevity estimates for a commercially fished population were much higher than the maximum band counts (Table 2).

Age at maturity

Discussion 1. The relationship between internal and external counts suggests that only the first two bands were formed at the same time. 2. The present study also supports the possibility that internal dentine stops forming or that the internal bands become undistinguishable when tightly packed. 3. Verification and validation of the periodicity of band formation is essential, although most traditional techniques are unsuitable for deepwater species.

Age at maturity 1. Female age at maturity using external bands (30 years) differed from the age of the youngest pregnant female (35 years). 2. No difference was found between the size at maturity (L50) and the size of the smallest pregnant female (both 62 cm TL) (Irvine 2004), suggesting that females may not grow during reproductive activity.

The high longevity and late age at maturity of E. baxteri from southeastern Australia are indicators of a low productivity. Non-selective trawl fishing off southeastern Australia collected a wide age range of E. baxteri. The deepwater dogfishes is also of great importance for identifying sensitive habitats, and effective management and conservation may require the collaboration of different States or nations. Prospective

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