Decadal-scale phenology and seasonal climate drivers of migratory baleen whales in a rapidly warming marine ecosystem

Abstract

Species' response to rapid climate change can be measured through shifts in timing of recurring biological events, known as phenology. The Gulf of Maine is one of the most rapidly warming regions of the ocean, and thus an ideal system to study phenological and biological responses to climate change. A better understanding of climate-induced changes in phenology is needed to effectively and adaptively manage human-wildlife conflicts. Using data from a 20+ year marine mammal observation program, we tested the hypothesis that the phenology of large whale habitat use in Cape Cod Bay has changed and is related to regional-scale shifts in the thermal onset of spring. We used a multi-season occupancy model to measure phenological shifts and evaluate trends in the date of peak habitat use for North Atlantic right (Eubalaena glacialis), humpback (Megaptera novaeangliae), and fin (Balaenoptera physalus) whales. The date of peak habitat use shifted by +18.1 days (0.90 days/year) for right whales and +19.1 days (0.96 days/year) for humpback whales. We then evaluated interannual variability in peak habitat use relative to thermal spring transition dates (STD), and hypothesized that right whales, as planktivorous specialist feeders, would exhibit a stronger response to thermal phenology than fin and humpback whales, which are more generalist piscivorous feeders. There was a significant negative effect of western region STD on right whale habitat use, and a significant positive effect of eastern region STD on fin whale habitat use indicating differential responses to spatial seasonal conditions. Protections for threatened and endangered whales have been designed to align with expected phenology of habitat use. Our results show that whales are becoming mismatched with static seasonal management measures through shifts in their timing of habitat use, and they suggest that effective management strategies may need to alter protections as species adapt to climate change.

Keywords: Gulf of Maine; North Atlantic right whale; climate change; endangered species; fin whale; humpback whale; ocean warming; phenology.

FIGURE 1

Cape Cod Bay study region located in Massachusetts, USA. East‐west aerial survey tracklines are shown as thin black lines. Tracklines are separated by 2.8 km. Shaded contour regions, from light to dark, are shown for 0–10, 10–25, 25–50, 50–75, 75–100, and >100 m depths. Inset shows the Gulf of Maine with our study area outlined in black. Green and orange areas show the western and eastern Gulf of Maine regions, respectively

FIGURE 2

Modeled changes in occupancy within each year, 1998–2018 for right whales (a), humpback whales (b), and fin whales (c). Occupancy was estimated for each half‐month sampling period; only posterior means are plotted for clarity. Earlier years in the time series are represented by cooler colors and later years are represented by warmer colors and. Whale illustration credit: NOAA Fisheries

FIGURE 3

Mean occupancy of right whales in Cape Cod Bay from 1998 to 2018 within sampling periods: 1–15 January (a), 16–31 January (b), 1–15 February (c), 16–29 February (d), 1–15 March (e), 16–31 March (f), 1–15 April (g), 16–30 April (h), and 1–15 May (i). Vertical lines show 50% CrI. Solid and dashed fitted lines are inversely weighted by the variance in each estimate of mean occupancy. Dashed fitted line in (h) indicates a significant trend in mean occupancy as a function of year, for all years in which mean occupancy for this sampling period was ≥0.005. Mean occupancy < 0.005 is indicated by hollow circles. Shaded regions represent 95% CIs. Bold p‐values indicate statistical significance (p < .05)

FIGURE 4

Day of maximum occupancy for right whales (a), humpback whales (b), and fin whales (c) in Cape Cod Bay from 1988 to 2018. Vertical lines show 50% CrIs. Fitted lines are inversely weighted by the variance in each estimate of mean occupancy. Shaded regions represent 95% CIs. Bold p‐values indicate statistical significance (p < .05)

FIGURE 5

Relationship between the day of maximum occupancy (day‐of‐year) for right (a, b), humpback (c, d), and fin (e, f) whales with the spring thermal transition date (day‐of‐year) for the western (a, c, e) and eastern (b, d, f) Gulf of Maine. Error bars show the 50% CrIs for dates of maximum occupancy. Fitted lines are linear model fits inversely weighted by the variance in each estimate of mean occupancy. Shaded regions represent 95% CIs. Bold p‐values indicate statistical significance (p < .05). Note that y‐axes differ between species