Insights about depth distribution, occurrence and swimming behavior of Hexanchus griseus in the Cantabrian Sea (NE Atlantic)

Abstract

The bluntnose sixgill shark, Hexanchus griseus, is a relatively common, wide-spread and deepwater shark that occurs along outer continental shelves and upper slopes. However, very little information exists about its distribution patterns, migrations and behavior. Data on occurrence and behavior of sixgill sharks inhabiting waters north of Spain (Galicia and Cantabrian Sea, NE Atlantic) were obtained from yearly oceanographic trawl surveys, commercial fishing boat records, and a pop-up satellite archival tag that was deployed for 75 days on a juvenile specimen. Most of the specimens were caught in Galician waters, and nearly all of those from surveys were juveniles (length range, 60–121 cm). Catch depth from surveys and commercial vessels ranged from 90 to 800 m and 90 to 1750 m respectively. Combining all data, a moderate relationship was observed between catch depth and size of the sharks (). The sex ratio was 1:1.86 in favor of females. September and February were the months with more catches reported from commercial vessels. Data obtained from one electronic pop-up tag (Mini PAT), provided information about depth and temperature preferences over 75 days. Mean depth obtained during that period was 913 m (depth range 727–1247 m), and the mean temperature was 10.3 °C, (range 8.0–10.8 ° C). Movements up and down in the water column within a single day ranged from 50 to 385 m. No cyclic diel vertical migration was however observed, the shark moved smoothly without a defined pattern. Vertical movements at semidiurnal frequency were intermittently observed. The thereby differences observed in the swimming behavior of H. griseus compared to other areas could be attributed to size, foraging activity or atypical behavior.