Seasonal and Ontogenetic Variation in Depth Use by a Critically Endangered Benthic Elasmobranch and Its Implications for Spatial Management

Abstract

Seasonal and ontogenetic variations in depth use by benthic species are often concomitant with changes in their spatial distribution. This has implications for the efficacy of spatial conservation measures such as marine protected areas (MPAs). The critically endangered flapper skate (Dipturus intermedius) is the designation feature of an MPA in Scotland. This species is generally associated with deeper waters >100 m; however, little is known about its seasonal or ontogenetic variation in habitat use. This study used archival depth data from 25 immature and mature flapper skate tagged in the MPA over multiple years. Time series ranged from 3 to 772 (mean = 246) days. Generalised additive mixed models and highest density intervals were used to identify home (95%) and core (50%) highest density depth regions (HDDRs) to quantify depth use in relation to time of year and body size. Skate used a total depth range of 1–312 m, but home HDDRs typically occurred between 20 and 225 m. Core HDDRs displayed significant seasonal and ontogenetic variations. Summer core HDDRs (100–150 m) suggest high occupancy of the deep trenches in the region by skate of most size classes. There was an inverse relationship between body size and depth use and a seasonal trend of skate moving into shallow water over winter months. These results suggest that flapper skate are not solely associated with deep water, as skate, especially large females, are frequently found in shallow waters (25–75 m). The current management, which protects the entire depth range, is appropriate for the protection of flapper skate through much of its life history. This research demonstrates why collecting data across seasonal scales and multiple ontogenetic stages is needed to assess the effectiveness of spatial management.

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

“…For flapper skate, this study provides the first evidence that interactions between environmental cycles affect vertical movement, extending previous research that has mainly focused on the independent effects of specific predictors (Wearmouth and Sims 2009;Pinto and Spezia, 2016). In line with recently documented seasonal trends in average depth use (Thorburn et al 2021), there is a substantial seasonal shift in diel vertical movement patterns, with normal DVM and elevated nocturnal activity weakening in summer when skate tend to be less active and spend more time in deeper water. This points towards solar light as an important correlate of vertical movements and may have implications for their vulnerability as bycatch in other parts of their range (Bendall et al 2017), as noted for Arctic skate (Amblyraja hyperborea) (Peklova et al 2014) and other elasmobranchs (Gilman et al 2019;Siskey et al 2019;Arostegui et al 2020).…”

“…This points towards solar light as an important correlate of vertical movements and may have implications for their vulnerability as bycatch in other parts of their range (Bendall et al 2017), as noted for Arctic skate (Amblyraja hyperborea) (Peklova et al 2014) and other elasmobranchs (Gilman et al 2019;Siskey et al 2019;Arostegui et al 2020). There is also evidence for a role for individual variation in movement, alongside ontogenetic segregation in depth use documented previously (Thorburn et al 2021).…”

“…While there are other correlated drivers of seasonal trends, such as temperature, photoperiod is the least variable across the study area and the most strongly associated with diel cycles through solar light levels. Furthermore, recent work suggests that depth use is not driven by temperature over seasonal timescales (Thorburn et al 2021). The expression of these covariates at Oban assumes that skate remained around this area over their time at liberty and that the values of the covariates remained relatively stable across this area at any one time, which is supported by analyses of passive acoustic telemetry data (Lavender et al in review) and environmental conditions (see 'Study site').…”

“…The movement ecology of flapper skate is poorly understood, but several studies have examined their vertical distribution and depth preferences (Neat et al 2015;Thorburn et al 2021). Modelling suggests a preference for depths from 100-300 m in close proximity to the coast .…”

“…Within the LStSJ MPA, Neat et al (2015) reported a daily depth range of 50-180 m for three tagged individuals. More recently, Thorburn et al (2021) aggregated data from multiple studies to examine long-term (seasonal and ontogenetic) variation in depth use in terms of how well the MPA covers skate depth preferences. They showed that skate used depths from 1-312 m but core depth ranges (20-225 m) varied seasonally and across size classes, with shallow-water (25-75 m) use increasing in winter, especially by large females.…”