Notes 2881 Ecology, 85(10), 2004, pp. 2881–2887  2004 by the Ecological Society of America SYNERGISTIC EFFECTS OF CONSPECIFICS AND FOOD ON GROWTH AND ENERGY ALLOCATION OF A DAMSELFISH DAVID J. BOOTH 1 Department of Environmental Sciences, University of Technology, Sydney, Westbourne Street, Gore Hill, New South Wales 2065 Australia Abstract. Grouping organisms may suffer reduced growth due to food deprivation in the presence of larger conspecifics, but this cost can be outweighed by advantages of reduced predation risk in larger groups. Here, tagged new juveniles of a small, grouping damselfish Dascyllus aruanus, were added to small coral heads that were either empty or supported small groups of larger conspecifics. Half of the coral heads with conspecifics and half of those without were subjected daily to additions of brine shrimp, similar to natural zoo- plankton food, over a three-week period. Growth was measured as increase in body length, change in body condition (lipid content), and overall energy gain. Body lipid reserves were higher (21%) in recruits sub- jected to added food and in the presence of conspecifics (13%). Total energy was higher (53%) in recruits with added food but was not affected by conspecific presence. There was an interaction between conspecific presence and food supplementation treatment on change in body length, a commonly used growth estimator. These differences suggest that the benefits of extra food and effects of conspecific presence differ depending on the metric used to measure growth, but that presence of larger conspecifics enhanced the increased growth that resulted from higher food intake. Key words: body condition; conspecific density; Dascyllus aruanus; energy allocation; growth; lipids; recruitment; social groups. INTRODUCTION Many organisms live in groups. Some of these groups comprise family members, with inclusive fitness benefits (Hamilton 1971), but most are of unrelated individuals. Benefits, especially to smaller individuals, accrue in terms of reduced predation risk, while en- hanced food gathering and consequent growth may also ensue. However, increased competition for food in larg- er groups is more common (e.g., Coolen 2002) and so a tradeoff between survival benefits and growth costs may result in optimal group sizes well below maxima. In several species of coral reef fishes, new arrivals usually attempt to join the largest groups of conspe- cifics available (Sweatman 1988, Booth 1991, 1992), despite apparent costs associated with food acquisition as the lowest-ranked group members. Most benthic ma- rine fishes have a pelagic larval phase which ends upon arrival at suitable benthic habitat (settlement: Doherty 1991, Caley et al. 1996). Rates of mortality are highest Manuscript received 11 August 2003; revised 15 February 2004; accepted 29 February 2004; final version received 14 April 2004. Corresponding Editor: P. T. Raimondi. 1 E-mail: David.Booth@uts.edu.au during the pelagic stage, however the first weeks post- settlement is a period of high and variable mortality, and migration is rare, and events at this time may have a disproportionate effect on population dynamics. Therefore, choices of group size at settlement can have a disproportionate effect on size-based attainment of maturity and fitness (Booth 1995). Major sources of mortality of coral reef fish new settlers are predation and aggression of competitors (Caley 1993, Carr and Hixon 1995, Hixon and Carr 1997, Connell 2000a). Both can be spatially and tem- porally variable on reefs (Connell 2000b), and may influence a new settler’s choice of habitat (Booth and Wellington 1998). Factors affecting mortality, partic- ularly density-dependent mechanisms, have been well- recognized recently as having an effect on population dynamics (Hixon and Carr 1997, Hixon and Webster 2002). While mortality is potentially a major source of spatial and temporal variation in population dynamics, factors affecting growth rate of reef fish are less stud- ied. Variation in growth may indirectly affect popu- lation dynamics through causing longer residence time in small, predator-vulnerable sizes, or through inhibi- tion of attainment of maturity, which is often size-based