Short communicationTotal and partial length–length, mass–mass and mass–length relationships for the piked spurdog (Squalus megalops) in south-eastern Australia
Introduction
Fisheries taking sharks are common throughout the world. Given that commercial shark species are normally beheaded, eviscerated and landed in one of the two forms: with fins attached (carcass) or without fins attached (trimmed carcass), only partial lengths and masses can be recorded after landing (FAO, 2000). Furthermore, due to increase in worldwide demand for shark fins, in many fisheries only the fins are retained whereas the rest of the animal is discarded. Due to these fishing practices, relationships between partial lengths and total length and between partial masses and total mass of shark are needed to determine the length and mass composition of captured sharks. Therefore, conversion to live weight and length equivalent units using appropriate conversion factors is an essential requirement for fisheries monitoring programmes and stock assessments.
Size relationships and size conversion factors have several biological applications and are commonly used in fishery management. Size relationships, particularly total mass–total length relationship, are commonly reported in biological studies of sharks (e.g. Stevens and McLoughlin, 1991, Kohler et al., 1995). Many studies test for differences between sexes in these relationships; in some cases, significant differences are found (e.g. Chiaramonte and Pettovello, 2000, Walker, 2005), whereas other studies show no differences (e.g. Bridge et al., 1998, Francis and Stevens, 2000). Many species of sharks exhibit sexual dimorphism in maximum size, females being larger and heavier than males (e.g. Cortés, 2000). For these species, size relationship comparisons are thus made between groups of different ranges of size so similarities or differences in these relationships may be an artefact of comparing smaller individuals (males) with larger individuals (females).
In the present study, length–length and mass–length relationships of male and female piked spurdogs (Squalus megalops), an abundant shark of southern Australia (Graham et al., 2001), were compared using different ranges of size. In addition, due to the common fishing practice of eviscerating, beheading and finning sharks, conversion factors from partial lengths and partial masses to total length and total mass were determined.
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Materials and methods
Male and female S. megalops were collected from the by-catch of shark and demersal trawl fishery vessels operating in the Australian Southern and Eastern Scalefish and Shark Fishery during October 2002–April 2004. Total (TL), fork (FL) and carcass (CL) lengths were measured to the nearest millimetre. Fork length was measured from the tip of the snout to the caudal fork and CL was measured from the fifth gill-slit to the precaudal pit. Total (TM), carcass (CM), liver (LM), pectoral fins (PFM)
Results
There were no significant differences in the FL–TL, CL–TL, CM–TL, PFM–TL, CFM–TL and CM–CL relationships between males and all females and between males and small females (t-test, P > 0.05 for comparisons of slopes and intercepts). Therefore, sexes and sizes were pooled for calculation of conversion factors, shown in Table 1. The conversion factors estimated are applicable to the size range analysed (270–635 mm TL), which covers most of the population size range, with the exception of neonates (TL <
Discussion
There were no sex or size effects in the FL–TL, CL–TL, CM–TL, PFM–TL, CFM–TL and CM–CL relationships. These length and mass measures reflect structural size and somatic growth with little trade-off between somatic and reproductive growth. Otherwise, the relatively larger increase in reproductive tissue experienced by adult female sharks (e.g. Yano, 1995) would be coupled with a decrease in their somatic tissue, particularly carcass mass, expecting differences in the CM–TL and CM–CL
Acknowledgments
We are grateful to P. Risley, G. Richardson and the crew of the fishing vessel ‘Nungurner’ for help in sample collection. This research was supported by an International Postgraduate Research Scholarship and a University of Adelaide Postgraduate Research Scholarship to JMB and an Australian Fisheries Research and Development Corporation grant (FRDC 2002/033) to TIW. BMG was supported by an Australian Research Council QEII Research Fellowship. Funding for the field and laboratory components was
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