Energy allocation in juveniles of a warmtemperate

Authors: Christopher D Stallings Felicia C Coleman Christopher C Koenig Daniel A Markiewicz

Publish Date: 2010/05/20

Volume: 88, Issue: 4, Pages: 389-398

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Abstract

During the first year of life organisms are faced with competing demands for energy between growth and storage Most research on energy allocation in young fishes has focused on coldtemperate species which are subjected to strong seasonal fluctuations in productivity while few studies have considered those at lower latitudes where seasonality is less pronounced Gag Mycteroperca microlepis of the northeastern Gulf of Mexico settle in coastal seagrass beds in the spring as juveniles and emigrate to offshore reefs in the fall Upon settlement these young fish grow at remarkably fast rates but their growth slows considerably before emigration Slowed growth can be explained by one of three hypotheses 1 sizespecific emigration times 2 reduced feeding efficiency associated with declines in primary and secondary productivity or 3 energetic shifts in allocation from growth to storage Gag emigrate essentially as a cohort so slowed growth does not result from differential emigration patterns based on fish size They also emigrate before seasonal declines in primary and secondary productivity thus food remains abundant and feeding efficiency constant The more plausible hypothesis is that there is an energetic shift from growth to storage The liver serves as the primary site of lipid storage and the hepatosomatic index of juvenile gag increases coincident with reduced growth The overall effect of increased energy stores is presumably for use during offshore migration and/or for overwinter survivalOrganisms at all life stages have limited energy budgets Glazier 1999 Kozlowski and Teriokhin 1999 and thus allocate resources among competing demands for maintenance growth reproduction and storage Perrin 1992 Reznick and Yang 1993 McManus and Travis 1998 When energetic demands shift there is a tradeoff energy diverted to reproduction for instance is no longer available for growth Reproduction is not the only developmental trading point for young fish Indeed youngofyear YOY fishes must balance demands for energy between growth and storage to overcome two major sources of mortality during their first year predation and starvation Vulnerability to gapelimited and sizedependent predators decreases with size so selection for fast somatic growth is common among young fishes Sogard 1997 Biro et al 2005 However many species must also endure periods of low food resource availability at some point during their first year eg due to seasonal declines in productivity movement into food depauperate habitats by relying on stored energy to avoid starvationOne strategy used by YOY fishes to meet these competing demands is to allocate energy to growth initially and then divert that allocation at least in part to storage in anticipation of seasonal resource shortages Post and Parkinson 2001 This strategy allows fish to attain a size above that at which they are most vulnerable to predators while building reserves for future periods when food is lacking Achieving a large size before food becomes scarce is also advantageous because per mass metabolic demands decrease with increasing size Shuter and Post 1990 Schultz and Conover 1999 Competing demands of growth and storage are therefore strongest for the smallest youngest fish Post and Parkinson 2001 Most of the evidence for fast growth followed by increased storage comes from species at high coldtemperate latitudes where productivity varies greatly on seasonal scales Far less is known about energy allocation of juvenile fishes at lower warmtemperate latitudes where growing seasons are longer periods of low winter resources are shorter and temporal patterns of productivity are less variableThe first hypothesis that decreasing water temperature may lead to declines in growth rates Ross and Moser 1995 is based on the wellstudied effects of ambient temperature on metabolic rates of poikilotherms Houlihan et al 1993 However Koenig and Coleman 1998 observed late season declines in growth of juvenile gag from three Florida seagrass beds despite no significant changes in water temperature range 28–31°C We therefore dismiss the temperature hypothesis as an explanation for decreased growth at least for gag inhabiting coastal regions in the northeastern Gulf of MexicoThe second hypothesis suggests that reported slowed growth may represent a sampling artifact caused by early emigration of fast growing individuals Studies documenting declines in growth of late seagrassstage juvenile gag used cohortlevel growth rates to measure the change in mean size of gag between temporally separated sampling events Withincohort settlement to seagrass occurs over a relatively short period of time Fitzhugh et al 2005 hence youngofyear in a local seagrass population are nearly equivalent in age and size It is also accepted that juvenile gag undergo mass emigration from seagrass meadows to offshore reefs thus supporting use of cohortlevel measurements However there is evidence that some individuals move out of the seagrass before the mass emigration Hastings 1979 Ross and Moser 1995 Koenig and Coleman 1998 If early emigration occurs and is composed of large ie fast growing individuals the use of cohortlevel growth rates might be obscured by comparing the mean size of the cohort before loss of large individuals through early emigration equaling the “inflated” mean size with the mean size of the remaining cohort after early emigration This would suggest an apparent rather than actual decline in growthThe third hypothesis that feeding efficiency of juvenile gag may decrease late in their seagrass stage is based on the observation that productivity and therefore potential prey availability declines in the late summer and autumn months No prior study has directly examined feeding efficiency of juvenile gag relative to changes in growth but our own observations in seagrass meadows in the northeaster Gulf of Mexico suggest that prior to emigration productivity and potential prey availability are not limitingThis leads us to the objectives of this study which were to more carefully examine the second and third hypotheses and posit a fourth which has not been previously suggested that energy allocation just prior to egress shifts from growth to storage Specifically we ask the following questions 1 Do growth rates of late seagrassstage juvenile gag decline relative to their earlier stages and 2 if declines in growth rates are detected are they due to changes in feeding efficiency energy allocation or both

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