Evidence for density-dependent changes in body condition and pregnancy rate of North Atlantic fin whales over four decades of varying environmental conditions

Abstract

Williams, R., Vikingsson, G. A., Gislason, A., Lockyer, C., New, L., Thomas, L., and Hammond, P. S. 2013. Evidence for density-dependent changes in body condition and pregnancy rate of North Atlantic fin whales over four decades of varying environmental conditions. – ICES Journal of Marine Science, 70: 1273–1280. A central theme in ecology is the search for pattern in the response of a species to changing environmental conditions. Natural resource management and endangered species conservation require an understanding of density-dependent and density-independent factors that regulate populations. Marine mammal populations are expected to express density dependence in the same way as terrestrial mammals, but logistical difficulties in data acquisition for many large whale species have hindered attempts to identify population-regulation mechanisms. We explored relationships between body condition (inferred from patterns in blubber thickness) and per capita prey abundance, and between pregnancy rate and body condition in North Atlantic fin whales as environmental conditions and population size varied between 1967 and 2010. Blubber thickness in both males and females declined at low per capita prey availability, and in breeding-age females, pregnancy rate declined at low blubber thickness, demonstrating a density-dependent response of pregnancy to prey limitation mediated through body condition. To the best of our knowledge, this is the first time a quantitative relationship among per capita prey abundance, body condition, and pregnancy rate has been documented for whales. As long-lived predators, marine mammals can act as indicators of the state of marine ecosystems. Improving our understanding of the relationships that link prey, body condition, and population parameters such as pregnancy rate and survival will become increasingly useful as these systems are affected by natural and anthropogenic change. Quantifying linkages among prey, fitness and vital rates will improve our ability to predict population consequences of subtle, sublethal impacts of ocean noise and other anthropogenic stressors.

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“…However, it should be noted that due to the logistical difficulty of observing births for cetaceans, and the fact that the highest risk of mortality occurs during the neonatal period (Stolen & Barlow 2003, Mann & Watson-Capps 2005, estimates of fecundity or birth rate likely integrate early calf (neonatal) survival rate to some degree. N onetheless, relationships among prey abundance, body condition, and fecundity (which may include some aspect of calf survival) have been documented for cetaceans (Ward et al 2009, Williams et al 2013, Meyer-Gutbrod et al 2015, suggesting a process by which DD responses in fecundity could occur. Specifically in bottlenose dolphins, shorter birth intervals were noted in a population from the US Atlantic coast following a significant depletion from a morbillivirus outbreak (Thayer 2008), and increases in calf/group ratios were documented in the years following a major hurricane in the northern Gulf of Mexico, presumably in response to increased prey availability following fishery closures or potential calf losses during the hurricane (Miller et al 2010).…”

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“…For large whales, the most extreme variability in energetic demands and body condition are in females; body condition and blubber thicknesses increase significantly prior to pregnancy, and are depleted through to the end of lactation (e.g., Lockyer, 1986;Miller et al, 2012b;Williams et al, 2013). Models (Klansjcek et al, 2007;Villegas-Amtmann et al, 2015) and observations (Lockyer, 1978(Lockyer, , 1986(Lockyer, , 1987Williams et al, 2013) suggest plasticity in calving intervals depending on a female's nutritional demands and environmental conditions; small changes in energy availability have large impacts on calving interval and age at first parturition.…”

“…Major changes in nutritive condition and reproduction can occur in response to good or bad prey years or environmental conditions, being natural, unforeseen circumstances (Williams et al, 2013;Rolland et al, 2016). Human-induced factors may also contribute.…”

“…Blubber and lipid stores can be considered an energy currency with which to evaluate the energetic consequences and sublethal impacts of various stressors in cetaceans and many other mammal species (e.g., Miller et al, 2011;Williams et al, 2013). Excess energy is deposited in lipid stores which become the primary energy source during fasting (Lockyer, 1986;Worthy and Lavigne, 1987), illness (Koopman et al, 2002;Dunkin et al, 2010), or increased nutritional demand (e.g.…”

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“…Body condition, a metric used to estimate relative energetic reserves, has been correlated with survival and reproductive success of both terrestrial (Young 1976, Schulte-Hostedde et al 2001) and marine mam -mals (Lockyer 1986, 1993, Pitcher et al 1998, Hall et al 2001, Pettis et al 2004, Rolland et al 2007b, Miller et al 2011 and is often used to assess and monitor the overall health of animal populations (Beck et al 1993, Gerhart et al 1996, Pitcher et al 1998, Bradford et al 2012, Williams et al 2013. In large whale species, energetic reserves are stored in, and mobilized from, the blubber layer, visceral tissues, and muscles (Lockyer et al 1985, Aguilar & Borrell 1990, Niaess et al 1998, and thus characteristics of these tissues, including thickness and chemical composition, are considered to be primary indicators of overall body condition (Lockyer et al 1985, Miller et al 2011.…”