Does a bigger mouth make you fatter? Linking intraspecific gape variability to body condition of a tropical predatory fish

Abstract

In gape-limited predators, gape size restricts the maximum prey size a predator is capable to ingest. However, studies investigating the energetic consequences of this relationship remain scarce. In this study we tested the hypothesis that gape-size variability influences individual body condition (a common proxy for fitness) in one of the largest freshwater teleost predators, the barramundi. We found that individual barramundi with larger gapes relative to body size had higher body condition values compared to conspecifics with smaller gapes.Body condition was highest soon after the wet-season, a period of high feeding activity on productive inundated floodplains, and body condition decreased as the dry-season progressed when fish were restricted to dry season remnant habitats. The increased condition obtained during the wet season apparently offsets weight loss through the dry season, as individuals with large gapes were still in better condition than fish with small gapes in the late dry season. Elucidation of the links between intraspecific variability in traits and performance is a critical challenge in functional ecology. This study emphasizes that even small intraspecific variability in morphological trait values can potentially affect individual fitness within a species' distribution.

When and where are catfish fat fish? Hydro‐ecological determinants of energy reserves in the fork‐tailed catfish, Neoarius graeffei, in an intermittent tropical river

“…Previous isotopic food web research found little evidence of remote floodplain carbon in fish inhabiting dry season pools in the Fitzroy River (Jardine, Pettit, et al., 2012); however, that study did not examine large‐bodied fish species. We posit that wet season floodplain inundation is most important for the dry season energetics of large species, such as fork‐tailed catfish and barramundi ( Lates calcarifer ), which have the capacity to store large amounts of fat in their body cavity (see Luiz et al., 2019). These species move into floodplain habitats to forage in other northern Australian rivers (Crook et al., 2019; Pusey et al., 2020), and in the Fitzroy River, we expect them to take advantage of wet season production and potentially link floodplain and main‐channel food webs (Jardine, Pusey, et al., 2012).…”

“…This study is one of the few to investigate the link between fish energy stores and flow/water availability in northern Australian rivers (but see Luiz et al., 2019 and Lear et al., 2020); however, its inference is limited by the short term nature of the datasets examined. The association between energy stores and wet season flow in the Fitzroy River is indirect and asserted via inter‐annual patterns.…”

“…Ecologically, changes in flow that diminish the capacity of fork‐tailed catfish to acquire and store energy may reduce survival and recruitment of the species and impact the ability of large individuals to transfer energy from the floodplain to main‐channel refuge pools. The importance of flow, including antecedent flows, for fish abundance has received considerable attention in Australia (Arthington et al., 2014; Beesley et al., 2014; King et al., 2009), but far fewer studies have examined the relationship with other measures of health, such as condition or energy reserves (but see Balcombe et al., 2012 and Luiz et al., 2019). This study contributes empirical data that can assist in assessments of flow‐ecology relationships (Poff & Zimmerman, 2010) and supports the conceptual understanding between flow and fish (Bunn & Arthington, 2002).…”

“…These stored fats can be mobilised to meet energetic needs for metabolism/catabolism (Tocher, 2003), movement (Tocher, 2003), or reproduction (Balcombe et al., 2012; McBride et al., 2015; Pusey, 1990). In rivers with seasonal or variable flow regimes, fish energy reserves typically peak during or soon after periods of high flow (Balcombe et al., 2012; Luiz et al., 2019) because the inundation of previously dry habitat, including floodplains, creates a boom in food production (Junk et al., 1989; Leigh et al., 2010). This is particularly the case in tropical or dryland regions where flooding coincides with warm water (Junk et al., 1989; Puckridge et al., 2010), which promotes primary production and the subsequent transfer of energy to higher trophic levels.…”

“…They are also characterised by high interannual variability in flows (Kennard et al., 2010). Both ecological research and Indigenous knowledge suggest that fish energy reserves are linked to the magnitude of wet season flow and seasonal changes in flow (Jackson et al., 2011; Lear et al., 2020; Luiz et al., 2019), although the flow‐ecology linkages are generally poorly known. In particular, in the intermittent Fitzroy River in the Kimberley region of north‐western Australia, localised research on river ecology is limited and the impacts of altered river flows must be inferred from elsewhere (Douglas et al., 2019).…”