Environmental cycles and individual variation in the vertical movements of a benthic elasmobranch

Abstract

Trends in depth and vertical activity reflect the behaviour, habitat use and habitat preferences of marine organisms. However, among elasmobranchs, research has focused heavily on pelagic sharks, while the vertical movements of benthic elasmobranchs, such as skate (Rajidae), remain understudied. In this study, the vertical movements of the Critically Endangered flapper skate (Dipturus intermedius) were investigated using archival depth data collected at 2 min intervals from 21 individuals off the west coast of Scotland (56.5°N, −5.5°W) in 2016–17. Depth records comprised nearly four million observations and included eight time series longer than 1 year, forming one of the most comprehensive datasets collected on the movement of any skate to date. Additive modelling and functional data analysis were used to investigate vertical movements in relation to environmental cycles and individual characteristics. Vertical movements were dominated by individual variation but included prolonged periods of limited activity and more extensive movements that were associated with tidal, diel, lunar and seasonal cycles. Diel patterns were strongest, with irregular but frequent movements into shallower water at night, especially in autumn and winter. This research strengthens the evidence for vertical movements in relation to environmental cycles in benthic species and demonstrates a widely applicable flexible regression framework for movement research that recognises the importance of both individual-specific and group-level variation.

First report of an egg nursery for the Critically Endangered flapper skate Dipturus intermedius (Rajiformes: Rajidae)

“…The depth distribution of egg cases documented in this study is consistent with previous studies of vertical movement in adult flapper skate. The water depths of the egg nursery are towards the shallower limit of the depth range of adult flapper skate studied on the west coast of Scotland (Neat et al, 2015; Thorburn et al, 2021; Lavender et al, 2021b). Mature female flapper skate spend more of their time in water shallower than 50 m during the winter (November–April) and it is possible that this is linked to egg laying (Thorburn et al, 2021; Lavender et al, 2021b).…”

“…The Loch Sunart to the Sound of Jura Nature Conservation Nature Conservation MPA (NCMPA), covering an area of 741 km 2 , was designated for the protection of the common skate complex in 2016 based on the high level of site attachment documented in adult flapper skate in the area from mark-recapture records and telemetry (Neat et al, 2015). Flapper skate demonstrated high occupancy of the deep water (100-150 m) within the NCMPA (Lavender et al, 2021a) in the summer months with more movement into shallower areas during the winter (Thorburn et al, 2021;Lavender et al, 2021b). A second, smaller (6.05 km 2 ) urgent NCMPA The split of the species complex into flapper and common blue skate (Iglésias, Toulhoat & Sellos, 2010) has resulted in difficulty in interpreting earlier literature regarding the reproductive biology of the species.…”

“…The Loch Sunart to the Sound of Jura Nature Conservation Nature Conservation MPA (NCMPA), covering an area of 741 km 2 , was designated for the protection of the common skate complex in 2016 based on the high level of site attachment documented in adult flapper skate in the area from mark–recapture records and telemetry (Neat et al, 2015). Flapper skate demonstrated high occupancy of the deep water (100–150 m) within the NCMPA (Lavender et al, 2021a) in the summer months with more movement into shallower areas during the winter (Thorburn et al, 2021; Lavender et al, 2021b). A second, smaller (6.05 km 2 ) urgent NCMPA was designated in March 2021 (Red Rocks and Longay urgent NCMPA, Figure 1) 70+ km to the north of Loch Sunart to the Sound of Jura NCMPA following initial reports from scallop divers and recreational divers of high densities of flapper skate egg cases, and subsequent additional dive surveys and a review of drop‐down video (DDV) camera footage collected during previous surveys of the area.…”

Benthic animal-borne sensors and citizen science combine to validate ocean modelling

“…Given available data, the explanation(s) for this trend remain(s) uncertain. One possibility is that flapper skate exploit fine-scale variation (not captured by the model mesh) in temperature for activities such as resting, which is thought to increase during summer when vertical activity is lower 43 and known to occur in cooler-than-average habitats in other elasmobranchs 44 . Another possibility is a seasonal bias in the influence of the model’s temperature forcing in the study site, but further data are required to evaluate this possibility.…”

“…Since the 1970s, a wealth of citizen-science mark-recapture data has been assembled from a recreational catch-and-release sport fishery that principally targets skate from anchored charter vessels over areas of relatively deep (> 100 m) water 34 – 40 . More recently (2016–17), the flapper skate became the focus of a major electronic tagging and tracking project that deployed passive acoustic telemetry and archival tags 39 – 41 , 43 . Collectively, these data provide a novel opportunity to develop the use of benthic species as animal oceanographers and support model validation across a central portion of the WeStCOMS domain.…”