A revised estimate of daily ration in the tiger shark with implication for assessing ecosystem impacts of apex predators

Abstract

NOAA, National Marine Fisheries Service, Southeast FisheriesScience Center, 3500 Delwood Beach Road, Panama City, Florida 32408, USAApex predators may impact ecosystem structure and func-tion through trophic cascades (Estes, Tinker & Williams1998). Studies of diets, feeding patterns and food webdynamics can contribute to an understanding of commu-nity structure and ecological interactions (Winemiller 1989;Krebs 1998). Thus, understanding trophodynamics isimportant for developing ecosystem models for predictingcommunity responses to anthropogenic changes (Walters,Christensen & Pauly 1997). Studies of this kind are espe-cially critical in the case of large sharks, given that severalspecies are experiencing varying levels of populationdeclines on a global scale (Dulvy et al. 2008; Camhi et al.2009). Accordingly, there is mounting concern – andincreasing effort – to predict the consequences of largeshark declines for food web dynamics (Estes et al. 2011).In a recent study, Hammerschlag et al. (2012) investi-gated the movement patterns of an apex marine predator,the tiger shark (Galeocerdo cuvier), in relation to an eco-tourism provisioning site. The authors rejected the nullhypothesis that sharks displayed restricted long-term habi-tat use at the site. However, they speculated that theobserved long-term and large-scale shark movements maybe related to cryptic, lesser-understood aspects of Atlantictiger shark life history (i.e. reproduction, mating, foragingforays).As part of their discussion, Hammerschlag et al. (2012)considered the daily ration of 210-kg adult female tigersharks. However, no published studies exist that havequantified this aspect of tiger shark food-consumptiveneeds. To get a broad approximation of daily ration, theauthors averaged values derived from two other species ofsharks found in the subtropical Atlantic: the lemon shark(Negaprion brevirostris), daily ration estimate of 2 1%body weight (Cortes & Gruber 1990); and shortfin makoshark (Isurus oxyrinchus), daily ration estimate of 4 6%body weight (Wood et al. 2009). The species used in thecalculation here were chosen in part because of theirbehavioural and geospatial overlap with the tiger sharkmovements found in Hammerschlag et al. (2012). How-ever, the resulting approximation of the daily ration valuefor an adult tiger shark (3 7%) is likely inflated for twoprimary reasons: (i) using values derived from juvenile spe-cies, although daily ration in sharks is known to decreasewith size (Wetherbee & Cortes 2004) and (ii) averaging val-ues from two species, one of which, the shortfin mako, isregionally endothermic, while tiger sharks are ectothermic(Carlson, Goldman & Lowe 2004).Here we present a different, more traditional approachto estimate the daily ration for tiger sharks followingWinberg (1956). Accordingly, daily ration (kcal day