Mortality of juvenile damselfish: implications for assessing processes that determine abundance

Ecology, Jan, 1999 by Russell J. Schmitt, Sally J. Holbrook

INTRODUCTION

Replenishment of demographically open populations has been examined extensively in marine reef fishes since, like most benthic marine species, they typically have early developmental stages that disperse in the plankton (Doherty and Williams 1988, Mapstone and Fowler 1988, Olafsson et al. 1994, Booth and Brosnan 1995). Despite considerable attention, there currently is little agreement regarding the relative contributions of a planktonic supply of propagules and of subsequent demographic processes in determining densities of local populations (Doherty 1991, Jones 1991, Caley et al. 1996). Clearly, some potentially large fraction of the spatial and temporal variation observed emanates from variability in the number of planktonic stages that settle (Doherty 1981, 1991, Victor 1983, 1986, Mapstone and Fowler 1988, Doherty and Fowler 1994a, b). Less obvious is the degree to which postsettlement processes alter patterns originating at settlement (Jones 1991, Caley et al. 1996). To resolve this uncertainty requires knowledge of larval supply relative to potentially limiting reef resources, as well as the magnitude and form of postsettlement mortality.

One current model for marine reef fishes is that the supply of propagules typically is too low to saturate reef resources with the consequence that local population densities reflect a balance between the external supply of colonists and subsequent density-independent mortality (Doherty 1981, 1991). In such "recruitment limitation" (e.g., Doherty 1981) or "recruitment determination" (Forrester 1990) models, per capita loss rates after settlement are unaffected by the density of individuals present (Doherty and Williams 1988, Forrester 1995). If mortality is density independent, the effect of postsettlement loss on density of a cohort mostly will be quantitative (though perhaps large), and there will be relatively little alteration of the pattern of variation that originates at settlement (Doherty 1991, Jones 1991). Although variance will be reduced by density-independent loss (due to a reduction in mean abundance), such loss will not modify qualitatively a linear relationship between the size of a settlement cohort and the subsequent number that survives to an older life stage. By contrast, when the fraction of a cohort that reaches an older life stage declines with density, the relationship between the densities of successive life stages of a cohort will deviate from linear. The existence and strength of density dependence are crucial because temporal density dependence regulates a local population by bounding fluctuations.

There have been two general ways the recruitment limitation model has been tested using reef fishes. The first has been to determine from natural variation in densities of a species the relationship in abundance of different life stages (e.g., Victor 1983, 1986, Jones 1990, Milicich et al. 1992, Robertson 1992, Meekan et al. 1993, Doherty and Fowler 1994a, Schmitt and Holbrook 1996), and to infer from these patterns whether mortality likely scaled with density. Studies like these can indicate that postsettlement processes were not strong enough to eradicate the "supply" signal, but they do not necessarily eliminate the possibility that postsettlement losses involved density dependence. Indeed, the density of a cohort will remain positively correlated with its initial density except under the stringent conditions of complete or overcompensation of all density-independent losses (Caley et al. 1996, Nisbet et al. 1996; also see Warner and Hughes 1988). In fact, in several studies (Forrester 1990, Jones 1990, Schmitt and Holbrook 1996, Steele 1997) density-dependent losses in a cohort did not obscure a positive relationship between initial and final densities.

The second approach has been to test directly whether per capita mortality rates of reef fishes are dependent on density, which typically has involved experimental manipulations of fish densities. Although density dependence in growth rates of individual fish has been found commonly (e.g., Doherty 1982, 1983, Jones 1984, 1987a, b, 1988, Forrester 1990, Anderson 1993, Booth 1995), evidence of compensatory mortality in the juvenile stages of reef fishes has been found less frequently despite a number of attempts (Doherty 1982, 1983, Jones 1984, 1987a, Victor 1986, Behrents 1987, Pitcher 1988, Sale and Ferrell 1988, Robertson 1992, Anderson 1993, Booth and Beretta 1994, Levin 1994, Connell 1997; but see Jones 1987a, 1988, Robertson 1988, Forrester 1990, 1995, Tupper and Hunte 1994, Carr and Hixon 1995, Tupper and Boutilier 1995, Schmitt and Holbrook 1996, Hixon and Carr 1997, Steele 1997). The failure to detect routinely density dependence in mortality of juvenile reef fishes has been viewed as support for recruitment limitation (Doherty 1991, Robertson 1992, Doherty and Fowler 1994a, b).

Our ability to draw meaningful conclusions regarding the nature of postsettlement mortality in juvenile reef fishes, and thus its potential role in population regulation, is hampered by a number of issues. Among these is the fact that the assessment of density dependence is a notoriously difficult task for a number of logistical and statistical reasons (Hassell 1986, Jones 1991, Murdoch 1994). There is, however, a shortcoming that is relatively specific to the study of reef fishes: a frequent failure to examine patterns of mortality of newly settled individuals (Caley et al. 1996, Steele 1997). To date most experimental and observational studies of postsettlement mortality have focused on juveniles that had settled weeks to months earlier (Williams et al. 1994; see review by Caley et al. 1996). Density-dependent processes that are important for local regulation may occur at or just following settlement, yet this possibility cannot be evaluated fully until very early postsettlement mortality is explored more consistently.