Vol. 394: 289–302, 2009 MARINE ECOLOGY PROGRESS SERIES Published November 18 doi: 10.3354/meps08267 Mar Ecol Prog Ser OPEN ACCESS Seasonal and interannual correlations between right-whale distribution and calving success and chlorophyll concentrations in the Gulf of Maine, USA Brittan L. Hlista1, Heidi M. Sosik1,*, Linda V. Martin Traykovski1, Robert D. Kenney2, Michael J. Moore1 1Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 2Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882-1197, USA ABSTRACT: The North Atlantic right whale Eubalaena glacialis is one of the most endangered spe- cies of large whales. Although human-caused mortality is the primary factor contributing to poor recovery of E. glacialis, variability in reproductive success may also play a role. The present study evaluates the idea that seasonal distributions and reproductive success in E. glacialis are linked to food availability or related environmental conditions that can be assessed by treating satellite- derived sea-surface chlorophyll (chl) concentration as a proxy. Sea-surface chl time series in the major high-use feeding habitats and whale-sightings data were compared. Whale transition between habitats reflects a pattern in the seasonal distribution of peak concentration in satellite-derived chl. A regionally and seasonally weighted chl index was calculated to reflect aspects of average potential food condition. We found a significant correlation between the number of whale calves born and the weighted chl averaged over the prior 2 yr. These findings are consistent with the view that food avail- ability during and just before the gestation period may be a critical factor regulating reproductive success, with low food years contributing to delays in conception. Longer time series are necessary to examine the predictive relationship between weighted chl concentration and calf production. Although ecological interactions and whale reproductive biology are certainly more complex than can be encompassed by emphasizing only food availability, analysis of satellite-derived surface chl concentrations provides a practical means to monitor a level of ecosystem variability that affects right-whale distributions and productivity. KEY WORDS: Right whale · Eubalaena glacialis · Calving · Seasonal patterns · Ocean color · SeaWiFS · Chlorophyll · Remote sensing Resale or republication not permitted without written consent of the publisher INTRODUCTION 5.3% in 1980 to –2.4% in 1994 (Caswell et al. 1999). Overall, the E. glacialis population growth rate is As a consequence of the whaling industry, the North extremely low (<1% since 1992; Fujiwara & Caswell Atlantic right whale Eubalaena glacialis population 2001). The 2 major anthropogenic sources of right- was reduced to very low levels by the end of the 19th whale mortality include vessel strikes and entangle- century and its current population is estimated at ment with fixed fishing gear (Knowlton & Kraus 2001). <400 ind. (IWC 2001, Kraus et al. 2005). Despite inter- Although anthropogenic mortality compromises the national protection from commercial whaling since recovery of this population, strong evidence suggests 1935, the population is showing slow signs of recovery, that a variable reproductive rate (Knowlton et al. 1994, at best. Demographic estimates have indicated a grad- Kraus et al. 2001) and a calving interval that fluctuates ual decline in the population growth rate from about from 3 to 6 yr (Kraus et al. 2007) also contribute to the *Corresponding author. Email: [email protected] © Inter-Research 2009 · www.int-res.com 290 Mar Ecol Prog Ser 394: 289–302, 2009 reduced population growth rate. The factors that instance, an anomalous winter bloom across the Gulf of could contribute to decreases in reproductive rate Maine in late February 1999 may have allowed an include environmental contaminants, body condition/ extra generation of C. finmarchicus to develop, prece- nutritional stress, loss of genetic variability, infectious ding the usual March-April spring bloom, thus leading disease, and marine biotoxins (Reeves et al. 2001). to a substantial accumulation of the population (Durbin The present study examines the idea that reproductive et al. 2003). success in E. glacialis is linked to food availability and Food limitation due to low availability of phytoplank- evaluates satellite-derived sea-surface chlorophyll ton has also been hypothesized as a source of Calanus (chl) concentration as an environmental proxy for the finmarchicus mortality (Campbell et al. 2001), specifi- nutritional potential of right-whale feeding grounds. cally in the naupliar stages that are more susceptible to The relationship between nutrition and reproduction starvation. Starvation in these early stages may ulti- in female mammals has been well documented for a mately limit the availability of high-quality right-whale number of species. In terrestrial mammals, body fat food (Stage 4 and 5 copepodites) later in the season. impacts fertility by supplying the necessary energy for Thus, those phytoplankton bloom conditions that favor reproduction and for sexual maturation (Frisch 1984, the accumulation of the right whale’s food source Thomas 1990). In the North Atlantic fin whale Bal- (C. finmarchicus) may in fact be directly beneficial for aenoptera physalus, Lockyer (1986) found that an the whales, particularly for females, who must meet apparent body-fat enhancement paralleled an increase the demands of pregnancy and nursing, through either in food supply and appeared to be associated with higher food intake or larger metabolic reserves. increased fecundity. For Eubalaena glacialis, blubber Data on the variability in phytoplankton biomass thickness can be measured acoustically in the field provide a means to explore the hypothesis that food (Moore et al. 2001) and poor body condition, as indi- limitation of Calanus finmarchicus impacts the growth cated by low blubber thickness, has been correlated and survival of Eubalaena glacialis. Surface concentra- with poor reproductive success. Angell (2005) found tion of chl, the dominant photosynthetic pigment in that blubber thickness in reproductively active female phytoplankton, is widely used as an index of phyto- right whales decreased during lactation, increased plankton abundance and biomass. Because the right after weaning, and was thickest several months prior whale’s zooplankton food source, C. finmarchicus, is to the start of pregnancy. This result emphasizes that only 1 trophic level above phytoplankton, the distribu- females utilize their blubber reserves for energetic tion and concentration of phytoplankton may reflect support during reproduction, and argues for the impor- potential C. finmarchicus production over scales re- tance of evaluating nutritional factors in regulating the lated to life-cycle progression and advective processes reproductive success of this species. in the Gulf of Maine (~10s to 100s of kilometer spatial A thorough evaluation of right-whale population scales and weekly to monthly time scales). Food avail- dynamics requires a better understanding of the spa- able during and before the right whale’s gestation tial and temporal variability of the whales’ food supply. period may be a critical factor regulating their repro- Numerous studies (Murison & Gaskin 1989, Mayo & ductive success, but this is difficult to assess directly Marx 1990, Kenney & Wishner 1995, Wishner et al. with conventional sampling approaches because of the 1995, Beardsley et al. 1996, Woodley & Gaskin 1996, large spatial ranges and extended periods of time Baumgartner & Mate 2003) conducted in each of the involved in whale feeding behavior. Remote sensing major feeding habitats in the western North Atlantic approaches for examining the spatio-temporal distrib- have demonstrated that a single species of zooplank- ution of phytoplankton biomass provide a practical ton, the 2 to 3 mm long calanoid copepod Calanus fin- means to explore a possible link between reproductive marchicus, particularly late-stage copepodites, is the success in whales (calf production) and environmental primary prey. conditions that may regulate their food supply. Recent studies have demonstrated that the abun- Remotely sensed ocean-color data provide unique dance and distribution of phytoplankton can directly synoptic views of marine phytoplankton biomass over affect egg production rates (Campbell et al. 2001) and long periods of time and large horizontal expanses of development (Crain & Miller 2001) of Calanus finmar- the surface ocean, far beyond what can be assessed chicus. For example, Campbell et al. (2001) showed from shipboard measurements. Data acquired by the that in areas of higher chl concentration, early cope- Sea-viewing Wide Field-of-view Sensor (SeaWiFS) podite stages were larger and in better condition than since its launch in 1997 can be used to quantify the copepodites found in nearby areas with lower chl con- spatial and temporal variability of phytoplankton chl (a centration. Phytoplankton blooms can have a direct proxy for biomass) (e.g. O’Reilly et al. 1998). Chl esti- effect on the late copepodite stages as well, especially mated from ocean color is an imperfect proxy for total if they persist for some time (Durbin et al. 2003). For phytoplankon biomass for reasons ranging from vari- Hlista et al.: Right whales and chlorophyll 291 ability in the ratio of phytoplankton carbon to chl to the patterns