The incidence of bent dorsal fins in free‐ranging cetaceans

Abstract

Laterally bent dorsal fins are rarely observed in free-ranging populations of cetaceans, contrary to captivity, where most killer whale Orcinus orca adult males have laterally collapsed fins. This topic has been poorly explored, and data/information on its occurrence and possible causes are limited. The present study: (i) undertakes a review of the available information on bent dorsal fins in free-ranging cetaceans, and updates it with new records, (ii) reports on the proportion of bent fins in different study populations, and (iii) discusses possible causes. An empirical approach based on bibliographic research and compilation of 52 new records collected worldwide resulted in a total of 17 species of cetaceans displaying bent dorsal fins. The species with the highest number of records (64%) and from most locations was O. orca. On average, individuals with bent dorsal fins represent < 1% of their populations, with the exception of false killer whales Pseudorca crassidens and O. orca. While line injuries associated with fisheries interactions may be the main cause for P. crassidens, and the vulnerability to health issues caused by the evolutionary enlargement of the fin may be the cause for O. orca adult males, factors contributing to this abnormality for other species are still unclear. The occurrence of bent dorsals could be influenced by a set of variables rather than by a single factor but, irrespective of the cause, it is suggested that it does not directly affect the animals' survivorship. While still rare in nature, this incident is more common (at least 101 known cases) and widespread (geographically and in species diversity) than hypothesized, and is not confined only to animals in captive environments. Investigation into the occurrence of bent fins may be an interesting avenue of research.

Incidence of Odontocetes with Dorsal Fin Collapse in Maui Nui, Hawaii

“…This is the first published instance of a bent dorsal fin in either a pantropical spotted dolphin or a spinner dolphin. Neither of the previous studies addressing bent dorsal fins in odontocetes of the Hawaiian Islands (Baird & Gorgone, 2005;Alves et al, 2017) have included pantropical spotted dolphins in their analyses. Baird & Gorgone (2005) report no bent dorsal fins in spinner dolphins; however, it is possible that they assessed a different island-associated group (e.g., Andrews et al, 2010).…”

“…Photographs of marked individuals were visually inspected by two independent researchers and categorized as bent or not bent. Bent dorsal fins were defined as a dorsal fin that is completely or partially bent over laterally (Luksenburg, 2014;Alves et al, 2017). Animals that were known to be neonates or calves were not included due to concerns that the presence of any fetal folds in the dorsal region may affect assessment of bent dorsal fin injuries.…”

“…Dorsal fin collapse is rare in wild populations of odontocetes, with published rates generally less than 1%, if at all present (Baird & Gorgone, 2005). Exceptions to this are wellstudied populations of killer whales (Orcinus orca) and the main Hawaiian Islands' population of false killer whales (Pseudorca crassidens) (Visser, 1998;Alves et al, 2017). A recent publication by Alves et al (2017) documented 17 species of free-ranging cetaceans with collapsed or bent dorsal fins worldwide.…”

“…Exceptions to this are wellstudied populations of killer whales (Orcinus orca) and the main Hawaiian Islands' population of false killer whales (Pseudorca crassidens) (Visser, 1998;Alves et al, 2017). A recent publication by Alves et al (2017) documented 17 species of free-ranging cetaceans with collapsed or bent dorsal fins worldwide. Prior to that publication, bent dorsal fins were under-reported in the literature and only documented anecdotally for wild populations of killer whales (e.g., Visser, 1998;Baird & Stacey, 1989;Matkin et al, 2008), common bottlenose dolphins (Tursiops truncatus; Wilson et al, 1997;Baird & Gorgone, 2005), false killer whales (Baird & Gorgone, 2005), white-beaked dolphins (Lagenorhynchus albirostris; Higdon & Snow, 2008), and dusky dolphins (Lagenorhynchus obscurus; Kugler & Orbach, 2014).…”

“…The collapse of a dorsal fin likely results from breakdown of these supportive cartilaginous fibers. Such breakdown can occur due to acute injury of the leading edge of the dorsal fin or, in cases with no visible injury, breakdown of fibers due to illness, stress, or age (Visser, 1998;Alves et al, 2017).…”