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The large amount of data needed to construct bioenergetic models often results in the use of parameters from other species and life stages, which may lead to unrealized errors (Bartell et al., 1986, Post, 1990, Ney, 1993, Klumb et al., 2003).
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Metabolic costs are a large part of the energy budget (Adams and Breck, 1990), and sensitivity analyses of bioenergetic models indicate the importance of parameters related to metabolism (Bartell et al., 1986). These models define the individual-level mass balance between the amount of food consumed, growth, energy expenditures, and waste production (Kitchell et al., 1977). However, our understanding of the energetics of reef fishes has lagged behind that of freshwater and other marine fishes, and little is known about the physiological cost (energetic response) of habitat selection by reef fish that use nearshore and estuarine habitats as juveniles (Jones and McCormick, 2002).īioenergetic models can predict fish growth potential under different environmental conditions (Brandt et al., 1992, Brandt and Kirsch, 1993), as well as examine prey production/predator demand relationships (e.g., Rudstam et al., 1994, Demers et al., 2000, Ault et al., 1999, Ault et al., 2003) and predict fish habitat use (Hill and Grossman, 1993). Modeling is one approach to incorporate the physiological response of fishes to abiotic factors such as temperature and salinity, and can be used to evaluate the effects of heterogeneous environments resulting from either natural events or human activities on fish growth (see Brandt et al., 1992, Demers et al., 2000). For reef species that do not settle directly to the reef, the consequences of inhabiting different abiotic environments for juvenile growth, survival and production remain uncertain (Gillanders and Kingsford, 1996, Jones and McCormick, 2002). Ultimate production of juveniles and recruitment to the adult population is a function of the relative contribution of juveniles from different habitats and environments (Beck et al., 2001). The increased metabolic costs in high salinities (∼7% at the high temperature) represent a significant energy cost for juveniles, that would need to be balanced by lower predation risk or greater food availability to result in similar juvenile production compared to lower salinity environments. At the lowest temperatures (18 ☌), salinity did not have a significant effect on oxygen consumption. At intermediate temperatures (24-26 ☌), the increase due to salinity from 5 to 45 psu was less dramatic, equivalent to a 2 ☌ increase in temperature. A polynomial equation describing oxygen consumption as a function of temperature and salinity indicated the increase due to salinity from 5 to 45 psu at high temperatures (30-33 ☌) was equivalent to a 3 ☌ increase in temperature. Oxygen consumption was significantly higher at high salinities, and the salinity effect was temperature dependent. Analysis of covariance, using fish weight and mean activity rate as covariates, indicated significant temperature and salinity effects on oxygen consumption. Video recordings of fish in the respirometer chambers were analyzed to quantify the spontaneous activity rate of individuals. An open, flow-through respirometer was used, enabling trials to be run for long periods (∼16 h), while maintaining water quality (dissolved O 2 >70% saturation), and providing fish sufficient time to habituate to the chambers undisturbed. To quantify the energetic cost of inhabiting these different habitats, routine metabolism of individual gray snapper was measured in the laboratory at 20 combinations of temperature (18, 23, 28, and 33 ☌) and salinity (5, 15, 25, 35, and 45 psu). Juvenile gray snapper ( Lutjanus griseus ) occupy a wide range of estuarine and nearshore habitats that differ in physico-chemical properties.