Jaws from the deep: biological and ecological insights on the kitefin shark Dalatias licha from the Mediterranean Sea

Abstract

Due to their late maturation, extreme longevity, low fecundity and slow growth rates, deep-sea Chondrichthyes are extremely vulnerable to human impacts. Moreover, assessing the impact of deep-sea fisheries is difficult, as many species (including sharks) are part of the bycatch and are often discarded at sea, and/or landed under generic commercial-species codes. The lack of this information on fishery data sets and the limited availability of species-specific life history data make challenging the management of deep-sea Chondrichthyes. The kitefin shark Dalatias licha is a cosmopolitan elasmobranch, mainly found on continental and insular shelf-breaks and slopes in warm-temperate and tropical waters. This species is a common by-catch of the deep-sea trawling, considered as “Endangered” by the IUCN Red List for all European waters, Mediterranean Sea included. Here we present the results of a study based on a total of 78 specimens of kitefin shark collected over 3 years in the Ligurian Sea (NW Mediterranean) as by-catch from deep-water fisheries. Total length ranged from 380 to 1164 mm, and individual weight ranged from 198 to 8000 g. Immature and mature individuals showed a sex ratio dominated by males. Adult males were observed throughout the year, while mature females were observed only in spring-summer. These data lead to hypothesise a spatial segregation between genders. The kitefin shark diet was dominated by bony fish (mainly Macrouridae) and other small sharks (e.g., Galeus melastomus and Etmopterus spinax), but their gut included plastic items and parasites. Data reported here underline the rarity, complex ecology and the threat for this shark species and support the urgency of promoting initiatives for their monitoring and conservation.

Ontogenetic Shifts in Body Morphology of Demersal Sharks’ Species (Order: Squaliformes) Inhabiting the Western-Central Mediterranean Sea, with Implications for Their Bio-Ecological Role

“…Indeed, changes in fish body length have often been related to an increase in swimming efficiency and capacity as an adaptation for a better predation capability, thus influencing feeding habits [41], or as a means to escape from predators [42]. In this regard, the positive allometric growth of the caudal fin of the Squaliformes investigated here, which is in agreement with that reported in other Mediterranean regions for D. licha, S. blainville, and S. canicula [43], might represent an additional clue that points towards this latter hypothesis, as a wider caudal fin area with respect to body size could provide an extra boost, increasing the shark's chances to successfully hunt bigger and faster nektonic organisms [43][44][45]. Consequently, the ability to feed on these kinds of prey could represent a shift in the trophic level of these species during their ontogenetic growth, thus also modifying the role they play in their ecosystems [44], a pattern which has already been described in several other elasmobranch species [2].…”