# Automatic Classification of Morphologically Similar Fish Species Using Their Head Contours
## 1 Introduction  
Being able to determine the differences in shape between closely related fish species not only has an impact on fisheries and their management but also is essential for a variety of studies on marine ecology, genetics, phylogeny, biological invasion, and anthropogenic impact on the environment, among others [1]. All of these studies require the correct identification of a wide range of species, and the automatic classification of such species from images is still an open problem. The correct identification of many specimens of these sympatric species requires an experienced observer and is a highly time-consuming task; otherwise, many misclassification errors occur. This paper focuses on two cases in which this problem appears. The first case deals with the separation between Red Mullets, (_Mullus barbatus_ and _Mullus surmuletus_), and it will be treated as a binary classification problem. The second case is a multiclassification problem in which different species of Gurnard _Chelidonichthys cuculus_, _Chelidonichthys lucerna_, _Chelidonichthys obscurus_, _Eutrigla gurnardus_, _Lepidorigla cavillone_, _Lepidorigla deiuzeidei_, _Peristedion cataphractum_, _Trigla yura_, and _Trigloporus lastoviza_ are involved. Red Mullets and Gurnards are commonly caught and sold in fish markets as a mixture of species (Figure 1). The observation is that experts who can correctly classify individuals of these species of fish by using shape characteristics--instead of considering the entire silhouette, represented as a closed contour- identify them by focusing on a specific contour fragment. They disregard the remaining part of the contour as they realise that it is irrelevant for classification purposes. This is because the discarded contour part provides information on variations within individuals of the same species with respect to age, size, sex or body condition, but does not provide information on the intrinsic characteristics involved in the distinction between species. The challenge for automated identificationis to take these species-specific open contour fragments pointed out by the specialists to develop features that correctly identify the species automatically. In both cases considered in this work, experts mark the segment of interest of the profile as being the part that goes from the nose to the beginning of the dorsal fin of the fish. The first case addressed deals with the binary classification of the _Mullus_ network. Red Mullets (_M. barbatus_ and _M. surmuletus_) are a major target as demersal fish species for the Mediterranean fish industry [2; 3]. There are many comparative studies of these two species focusing on a range of topics, such as pollution bio-indicators, and fisheries or studies on age composition, growth, life history, feeding, genetics, ecology, and distribution [4; 5; 6; 7; 8; 9]. As mentioned above, the evaluation of living resources requires the correct identification of species. Searches for morphological features to help with the task of identifying these sympatric species have focused on characteristics, such as head and body measurements [10; 11], barbells [12], dentition [13] or otoliths [14]. Amongst all of these biometric features, the angle of the head has been found to be one of the best diagnostic characteristics to distinguish between the two species, as the angle that the _M. barbatus_'head contour forms is more marked than that of _M. surmuletus_[10; 15]. Even in fishery studies, however, the individual variability within Red Mullets causes identification problems and the only reliable distinction is the color pattern of their bent dorsal fin. However, it is not easy to obtain images of this feature from boat moorings in fishing harbours as the fish's dorsal fin is folded. Furthermore, in most cases, the information on the fish caught that is given upon the arrival of the boat does not discriminate between both species of Red Mullets.