Prey consumption estimates for salmon sharks

Abstract

Top predators, such as salmon sharks (Lamna ditropis), can influence the abundance and population structure of organisms at lower trophic levels through direct effects, such as predation mortality, and indirect interactions. As a first step towards better understanding the average annual prey consumption for individual adult salmon sharks, we bracketed consumption estimates using three methods: (1) daily ration requirement; (2) bioenergetic mass balance; and (3) a Bayesian model of shark growth. In the first method, we applied ration estimates for related lamnid shark species that yielded salmon shark estimates of 1461 and 2202kgyear–1. The second method used a mass–balance technique to incorporate life history information from salmon sharks and physiological parameters from other species and produced estimates of 1870, 2070, 1610 and 1762kgyear–1, depending on assumed diet. Growth modelling used salmon shark growth histories and yielded estimates of 16900 or 20800kgyear–1, depending on assumed assimilation efficiency. Of the consumption estimates, those from the mass–balance technique may be the most realistic because they incorporated salmon shark life history data and do not produce extreme values. Taken as a whole, these estimates suggest that salmon sharks have similar energetic requirements to piscivorous marine mammals.

Resurgence of an apex marine predator and the decline in prey body size

“…California sea lions ( Zalophus californianus ) and Steller sea lions ( Eumetopias jubatus ) may target adult Chinook salmon, although consumption from these predators is thought to be a fraction of that consumed by fish-eating killer whales, in part because Chinook salmon represent a small fraction of sea lions’ diets. Increasing abundances of other marine predators such as salmon sharks ( Lamna ditropis ) could also be compounding the effects of predation by killer whales if their selectivity patterns are similar and their abundance is large enough to significantly reduce the survival of large Chinook salmon (33–35). Importantly, killer whales are highly selective for the largest fish and are estimated to consume several times more Chinook salmon biomass than other predators (23).…”

Flexibility for fuelling reproduction in a pelagic ray (Mobula eregoodoo) suggested by bioenergetic modelling

“…Manishin et al . (2019) estimated an annual reproductive cost for female L. ditropis in birth years to be 136 MJ compared to M. eregoodoo here at 30 MJ. Nonetheless, L. ditropis is a larger, endothermic species that maintains its body consistently at c .…”

“…This study provides the first estimates of relative energy use (including costs of reproduction) in a batoid species and complements the few studies of relative energy use in elasmobranchs ( e.g ., Manishin et al ., 2019). Aplacental viviparity with histotroph enrichment (the reproductive mode of Mobula ) is rare among batoids and may require different energy expenditure to other reproductive methods in elasmobranchs.…”

“…This study presents energy content data of a batoid, the longhorned pygmy devil ray Mobula eregoodoo , and interprets its energetics and reproductive strategy in relation to other elasmobranchs. To authors’ knowledge there is just one estimate of the energetic cost of reproduction as a proportion of total energy use in any elasmobranch (Manishin et al ., 2019) and none for batoids. Further, it is not appropriate to extrapolate estimates of reproductive costs across elasmobranch species given their wide variety of reproductive modes.…”

Conservation implications of forage base requirements of a marine predator population at carrying capacity

“…To describe the energy required by an individual gray reef shark on a given day C i,j (kJ), we used a gamma model, suitable for a continuous, non-negative variable. The equation to describe the mean daily energy requirements of each individual was based on an energy balance equation previously used for other shark species ( Manishin et al., 2019 ; Schindler et al., 2002 ; Wood et al., 2009 ): Here, A i,j was daily routine metabolism (as gray reef sharks are obligate ram ventilators and are required to swim in order to obtain sufficient oxygen), D i,j was daily energy used for digestion (specific dynamic action), E i,j was energy that was lost daily to waste (excretion and egestion), G i,j was energy allocated daily towards growth and R i,j was energy allocated towards reproduction.…”