Spots on Sides of Giant Sea Bass (Stereolepis gigas Ayres, 1859) are Likely Unique to Each Individual

Abstract

The giant sea bass (Sterolepis gigas Ayres, 1859; hereafter GSB) is a large and presumably keystone nearshore reef species found from California, USA to southern Mexico, including the Gulf of California. Giant sea bass were severely overfished in the twentieth century leading to local extinctions. Restrictions on harvest off California have led to at least a mild resurgence in the population (Pondella and Allen 2008; House et al. 2016) and, based on a genetics study, an effective population size of perhaps 500 individuals off southern California and northern Baja California (Chabot et al. 2015). However, there has been no direct assessment of their numbers off California. From at least spring through fall, it is likely that most, if not all, GSB inhabit nearshore waters to depths of perhaps 30 m (Love 2011). We are interested in determining if recreational divers can assist us in determining the abundance of this species in California waters as divers can often easily approach and photograph these fish. If there was a way of identifying individuals underwater from still or video images, images from divers could help us determine the number of individuals in the sampling area. This process would require individuals to have unique markings. Giant sea bass of all ages have dark spots or blotches on their heads and flanks and these might be idiosyncratic and thus useful in identifying individuals. Among fishes, markings of various sorts, including spots and stripes, have been shown to be unique to individuals and thus may be of value in determining species abundances (Meekan et al. 2006; van Tienhoven et al. 2007; Claydon et al. 2010; Giglio et al. 2014). To determine if we could use the spotting on the flanks of GSB as unique markers, we made preliminary observations of GSB spot patterns at the Aquarium of the Pacific in Long Beach. Here we took photographs of the three GSBs (two adults and one likely subadult) on 18 June, 1 July and 3–4 July 2014. We photographed these fish at least hourly for 12 hours (18 June, 1 July) or over 24 hours (3–4 July). Over the past few years, pattern recognition software, such as the Individual Identification System (I3S Spot 4.02; www.reijns.com/i3s, hereafter referred to as I3S), have been developed to partially automate the process of reviewing images and helping to identify individuals (van Tienhoven et al. 2007). We used I3S to compare spots on the sides of each individual and between individuals. Using both this software and visual comparisons, we found: 1) the spot patterns did not change over the 12 or 24 hours surveyed, 2) the spot patterns were unique to an individual, and 3) each pattern was unique to a side of each individual (i.e., the spot patterns were asymmetric). Soon after these analyses, we acquired from Ms. Sandy Trautwein (Aquarium of the Pacific) images taken in 2003 of two of these Aquarium of the Pacific fish. Again, using I3S and a visual inspection, we determined that the spot patterns of these individuals were identical to those observed on these