Total and partial length–length, mass–mass and mass–length relationships for the piked spurdog (Squalus megalops) in south-eastern Australia

Abstract

Common commercial fishing practices of eviscerating, beheading and finning sharks create the need for using conversion factors from partial lengths to total length and from partial masses to total mass. In the present paper, these conversion factors were calculated for Squalus megalops. In addition, total and partial length–length and mass–length relationships of male and female S. megalops were compared using different ranges of size. There was no effect of size range on measurements reflecting only somatic growth (fork and carcass lengths; carcass, pectoral fin and caudal fin masses) but for variables reflecting somatic and reproductive growth (total and liver masses), different outcomes can be expected when different ranges of size are compared.

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.