Abstract
Evaluating how predators metabolize energy is increasingly useful for conservation physiology, as it can provide information on their current nutritional condition. However, obtaining metabolic information from mobile marine predators is inherently challenging owing to their relative rarity, cryptic nature and often wide-ranging underwater movements. Here, we investigate aspects of energy metabolism in four free-ranging shark species (n=281; blacktip, bull, nurse, and tiger) by measuring three metabolic parameters [plasma triglycerides (TAG), free fatty acids (FFA) and cholesterol (CHOL)] via non-lethal biopsy sampling. Plasma TAG, FFA and total CHOL concentrations (in millimoles per litre) varied inter-specifically and with season, year, and shark length varied within a species. The TAG were highest in the plasma of less active species (nurse and tiger sharks), whereas FFA were highest among species with relatively high energetic demands (blacktip and bull sharks), and CHOL concentrations were highest in bull sharks. Although temporal patterns in all metabolites were varied among species, there appeared to be peaks in the spring and summer, with ratios of TAG/CHOL (a proxy for condition) in all species displaying a notable peak in summer. These results provide baseline information of energy metabolism in large sharks and are an important step in understanding how the metabolic parameters can be assessed through non-lethal sampling in the future. In particular, this study emphasizes the importance of accounting for intra-specific and temporal variability in sampling designs seeking to monitor the nutritional condition and metabolic responses of shark populations.
Highlights
Nutrition and energy metabolism in wild animals are both conceptualized and measured by a variety of methods, ranging from techniques that span behavioural approaches, morphological and developmental measurements and ecological interactions (Raubenheimer et al, 2009)
The TAG were highest in the plasma of less active species, whereas free fatty acids (FFA) were highest among species with relatively high energetic demands, and CHOL concentrations were highest in bull sharks
These results provide baseline information of energy metabolism in large sharks and are an important step in understanding how the metabolic parameters can be assessed through non-lethal sampling in the future
Summary
Nutrition and energy metabolism in wild animals are both conceptualized and measured by a variety of methods, ranging from techniques that span behavioural approaches, morphological and developmental measurements and ecological interactions (Raubenheimer et al, 2009). The demand for energy constrains the behaviour of animals (Speakman, 1997), which is fundamental to life-history variation and intrinsically. Lipids, including fatty acids and sterols, are a diverse group of metabolites (small molecule intermediates and products of metabolism) that play a critical role in almost all aspects of biological life. Three lipid classes play crucial roles in energy transport and are commonly used to study metabolism: triglycerides (TAG), free fatty acids (FFA) and total cholesterol (CHOL; Han, 2016; Orešic, 2009). Strategies of lipid metabolism, including absorption and depositional processes, appear different between the different animal classes, reflecting distinct evolutionary paths (SpeersRoesch et al, 2006; Mazurie et al, 2010)
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