An assessment of otoliths, dorsal spines and scales to age the long-finned gurnard, Lepidotrigla argus, Ogilby, 1910 (Family: Triglidae) By D. E. van der Meulen 1,2,3 , R. J. West 3 and C. A. Gray 2 1 NSW Department of Primary Industries, Batemans Bay Fisheries Centre, Batemans Bay, NSW, Australia; 2 NSW Department of Primary Industries, Cronulla Fisheries Research Centre of Excellence, Cronulla, NSW, Australia; 3 School of Biological Sciences, University of Wollongong, Wollongong, NSW, Australia Summary Sagittal otoliths, dorsal spines and scales were critically assessed as structures to potentially determine the age of the long-finned gurnard, Lepidotrigla argus. Counts were made of opaque growth increments and a readability score was assigned to each structure. Comparisons of growth incre- ment counts were made between structures and between readings. All three structures showed some degree of read- ability and quantifiable growth increments, but this varied within fishes and between structures. Initial results showed that whole otoliths were more suitable to determine age esti- mates than dorsal spines and scales. Scales were considered unsuitable due to between reading ageing bias, variation in age estimates between structures, low precision and poor readability for this species. Dorsal spines showed evidence of loss of growth increments due to hollowing of the vascular core, which resulted in underestimation of older individuals in comparison to whole otoliths. Further analysis showed that growth increment counts from whole otoliths were lower for older individuals in comparison to sectioned otoliths. It is suggested that this is because of decreased clar- ity of growth increments towards the outer margin of whole otoliths in older individuals; this problem was not present with sectioned otoliths. It was concluded that sectioned oto- liths were a more suitable structure from which to estimate age of L. argus than were whole otoliths, dorsal spines and/ or scales. Introduction Age-based information plays an important role in determin- ing the population dynamics and life history characteristics of an organism (Winemiller and Rose, 1992; Worthington et al., 1995; Campana, 2001). In particular, age determina- tion assists in quantifying the rates of key processes such as growth, recruitment and mortality, which are essential for assessing the status of exploited populations, such as com- mercially and recreationally caught fish (Lai et al., 1996; Myers et al., 1997; Campana, 2001; Berkeley et al., 2004; Lewin et al., 2006). Age-based information can therefore support the sustainable management of such organisms (Mace, 1994; Jennings et al., 1999; Reeves, 2003; Berkeley et al., 2004). An important step in any ageing study is to firstly estab- lish a suitable method for age determination. The use of calcified structures to determine the age of fish is a well- accepted method (Lai et al., 1987; Campana, 2001). This involves counting periodic growth increments (alternating opaque and translucent zones) that are formed by accretion as the fish grows (Morales-Nin, 1992). Sagittal otoliths are the most widely used structure for ageing fish as they have been found to provide the most accurate and precise esti- mates of age over the largest age and size ranges (Secor et al., 1995; Campana, 2001). Other bony structures such as vertebrae, dorsal spines and scales have also successfully been used to age fish (Lai et al., 1987; Metcalf and Swearer, 2005; Rifflart et al., 2006). However, all methods of ageing are problematic. For example, small otolith size and/or the clarity of growth increment structures can preclude their use (Ihde and Chittenden, 2002; Isermann et al., 2003; Phelps et al., 2007). Otolith growth and clarity has been shown to be variable within species, between locations and at different life history stages (Fowler and Short, 1996; Caldow and Wellington, 2003; Fischer et al., 2004; Jessop et al., 2004). When assessing the potential of a structure for age determi- nation, it is important to take into account the numbers aged, as well as the efficiency and cost effectiveness of the steps involved (Worthington et al., 1995). In this study we examine the utility of otoliths, dorsal spines and scales to potentially age the long finned gurnard, Lepidotrigla argus (Family: Triglidae). This species forms an important by-catch component of the Ocean Prawn Trawl Fisheries in eastern Australia (NSW D.P.I., 2004). Currently, there exists little life history information for this species, thus hindering the assessment of the potential impacts of fishing. Several studies have assessed age in triglids, but mostly for the larger growing genus Chelidonichthys (Booth, 1997). Like other members of the genus Lepidotrigla, L. argus are of small size and possesses relatively small otoliths which can make age determination difficult. The specific aims of this study were to: (i) examine the presence and utility of growth increments in otoliths, dorsal spine and scales, and (ii) com- pare the amount of variation in the counts and readability of growth increments between structures. This information is used to identify the most suitable structure for ageing L. argus for future demographic studies. Materials and methods Collection of samples Samples of Lepidotrigla argus were collected monthly between April 2006 and April 2007. Fish were collected aboard a chartered commercial prawn trawl vessel at loca- tions ranging from 20 to 100 m depths offshore from the Clarence River, NSW (Lat. 29.424°S; Long. 150.373°E). U.S. Copyright Clearance Centre Code Statement: 0175-8659/2013/2904–815$15.00/0 J. Appl. Ichthyol. 29 (2013), 815–824 © 2013 Blackwell Verlag GmbH ISSN 0175–8659 Received: May 20, 2012 Accepted: January 17, 2013 doi: 10.1111/jai.12181 Applied Ichthyology Journal of