Length–Length and Width–Length Conversion of Longnose Skate and Big Skate Off the Pacific Coast: Implications for the Choice of Alternative Measurement Units in Fisheries Stock Assessment

Abstract

Most traditional fisheries models have size or stage–age relationships as their foundation. Two long‐established metrics used are FL and TL. Body length measurements are linked to age, maturity, and fecundity. Obtaining FL or TL measurements are not always convenient, especially for larger fish (>100 cm TL). Two skate species, Longnose Skate Raja rhina and Big Skate R. binoculata, were collected from commercial fishery landings along the coast of Washington State and length–length or width–length conversions were investigated for four metrics of size: TL, tail only length (TOL), interspiracular width (ISW), and interorbital width (IOW). Relationships between TL and alternative measures were examined using model II regression analyses and confirmed a strong linear relationship in all cases (R 2 > 0.94). It is likely that both TL and TOL measures have errors due to worn‐off tail portions; conversely, the ISW is nonlaborious and convenient to obtain. The ISW metric has a smaller coefficient of variation compared with IOW if we assume that both metrics have the same distribution of measurement errors. We recommend ISW as the preferred alternative metric in the assessment of large skate. The ISW will probably yield measurements that are equivalent, or potentially superior, to traditional metrics because the measurement is taken from body parts devoid of worn‐off portions. Received December 30, 2011; accepted June 13, 2013