Elasmobranch stock assessment studies are usually made through fisheries surveys data. However, in large marine protected areas (MPAs) the use of destructive techniques must be dismissed in order to avoid population impacts. In 2005, while conducting a marine habitat survey in two marine Special Areas of Conservation (Sebadales de Playa de Inglés and Franja Marina de Mogán) in south Gran Canary Island (Canary Islands, Spain) with underwater towed video (UTV) and underwater visual census (UVC) transects, we recognized the opportunity rose to assess elasmobranch populations through UTV. Number of observed species and specimens, overall field work effort and total surveyed area were determined and compared between methods. Mean observations per day per unit of time (MOPUT) and mean observations per day per unit of surveyed area (MOPUA) were also compared through Mann-Whitney rank sum statistical test (α=0.05). Data analysis demonstrated that UTV is a very useful tool to rapidly assess elasmobranch populations in large MPAs in good visibility underwater environments. It can assess larger areas than UVC with the same effort (statistically significant difference found for the MOPUT; p=<0.001), leading to more observed species (5 vs 2) and specimens (46 vs 3) per day of work, with no loss in resolution power (MOPUA values were not significantly different between UTV and UVC; p=0.104).
Background: A variety of abnormalities have been described for sharks, rays and skates across different ecoregions. Morphological and functional anomalies in these species, however, were not yet documented in distributions from the Canary Islands, the spiny butterfly ray Gymnura altavela (Linnaeus, 1758) and from in situ observations. The aim of the present study is to fill these knowledge gaps.Results: A female spiny butterfly ray G. altavela with an unfused anterior part of the right pectoral fin to the neurocranium was observed in the port of Sardina del Norte (Gran Canaria Island) during a visual scuba diving census. Conclusion: The size and the observation of the activities swimming, burying and preying confirmed the adaption of the specimen for the anomaly and underdeveloped electrosensory system in its survival. The limited knowledge of teratogens and their triggering factors, and the striking similarity with an anomaly reported for G. poecilura (Shaw, 1804) from South India, suggest genetic expression aberrations or mechanical obstructions during gestation as origin for the disorder.
The present communication documents the first observation of a morphologic anomaly in a juvenile Spiny Butterfly Ray Gymnura altavela. This observation is interesting for its similarity with a rostral abnormality in an adult G. altavela from the Canary Islands 10 years earlier and with the one from a juvenile Long-tailed Butterfly Ray G. poecilura from the Western Indo-Pacific. Although no firm conclusions can be drawn from these records, it reinforces the hypothesis of a congenital disorder for this type of malformation within the genus. The individual appeared to have successfully adapted for its anomaly, suggesting that these instances are not of major conservation concern. Hence, long-term studies are required to monitor anomaly occurrences and to evaluate their potential threats.
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