Abstract
AbstractIdentifying the mechanistic drivers of migration can be crucial in shaping conservation and management policies. The cownose ray (Rhinoptera bonasus) is a relatively poorly understood elasmobranch species that occurs along the U.S. Atlantic coast and undergoes large‐scale seasonal migrations. To better understand the drivers and timing of cownose ray seasonal migration in order to inform potential management measures, we analyzed telemetry detections of 51 mature cownose rays (38 female, 13 male) tagged with acoustic transmitters in the Maryland and Virginia portions of Chesapeake Bay. Detections within their summer habitat in Chesapeake Bay and winter habitat in the vicinity of Cape Canaveral, Florida, were matched with publicly available sea surface temperature (SST) data recorded by data buoys near the areas of tag detections and with local photoperiod and day of year. These variables were used in boosted regression tree models of ray presence (all rays combined, females only, and males only) in each seasonal habitat. Models were developed for presence during the entire summer and winter season, and for the time periods of arrival and departure from both summer and winter habitats. Seasonal presence in both summer and winter habitats was associated with distinct temperature, photoperiod, and date ranges, with temperature as the most influential variable in seasonal models. In models of arrival and departure periods, southward migration (departure from Chesapeake Bay and arrival off Cape Canaveral) was strongly associated with SST for all rays and arrival in the Chesapeake Bay region after northward migration was most strongly associated with day of year. The most influential variable during the period of northward departure from Cape Canaveral differed between males (day of year) and females (SST). This suggests that mature female northward migration may be driven by temperature while male northward migration may be driven by endogenous cues. These findings provide detailed information on the timing of cownose ray arrival at, presence in, and departure from seasonal habitats and provide potential justification for including the species in cross‐taxa comparative studies on migratory behavior.
Highlights
Mechanisms that signal the seasonal phenology of animal migrations have interested researchers for centuries
Departure of cownose rays from summer habitat in Chesapeake Bay and adjacent shelf waters was associated with declining sea surface temperature (SST) across all groups, while northward migration from overwintering areas off Cape Canaveral, FL was associated with day of year for males and increasing SST for females
The differences observed for departure from overwintering habitat suggest male migration is driven more by endogenous cues than SST or by environmental cues not included in the analysis
Summary
Mechanisms that signal the seasonal phenology of animal migrations have interested researchers for centuries. Debate on proximate influences was historically as diverse as animal migration itself, but was typically based on discriminating between exogenous and endogenous factors. An “internal clock” mechanism was proposed to influence bird movement as early as 1702 (reviewed in Gwinner 1996), while early research on fishes primarily focused on temperature (Goode 1879, Cunningham 1895, Gurley 1902, Chidester 1924). It is widely accepted that round-trip, seasonal migration is adaptive in nature, involving both internal mechanisms and environmental factors (Secor 2015, Shaw 2016) and often generalizes across taxa (Dingle and Drake 2007). Sex-specific differences in departure and arrival at breeding and feeding sites have been seen across taxa (reviewed in Morbey and Ydenberg 2008), with the potential for variation between fall and spring migrations (Sharma et al 2018)
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