Experimental Analysis of the Role of Emigration in Population Regulation of Desert Pupfish

Abstract

Pupfishes (genus Cyprinodon) have existed for long periods in small desert springs and streams that often lack competitors, major predators, and large environmental changes. How their populations are regulated in these systems is poorly known. We experimentally examined the role of emigration in regulating populations of pupfish by (1) comparing the dynamics of populations held in four pools open to emigration with those of populations held in four pools closed to emigration over a 22—mo period and (2) comparing the rates of emigration from open pools having different densities of pupfish in relation to resources. When pupfish populations were prevented from emigrating, they exhibited symptoms of overpopulation similar to those well documented for fenced populations of small mammals. Their numbers increased more than populations in open pools, mortality was high, and body condition and recruitment were low. Again, as in small—mammal populations, these responses were absent in pupfish populations in open pools, where 84% of the total numbers of fish produced emigrated. The timing and magnitude of pupfish emigration was similar in all four open pools and was related to seasonal temperature changes. More males emigrated than females, and emigrants had significantly lower condition factors than residents. When densities of pupfish in relation to resources were doubled in two open pools, the percentages of pupfish that emigrated (41.8 and 42.2%) were almost equal to the 50% drop in resources. In contrast, only 15.5 and 16.0% emigrated from two control pools, where pupfish densities in relation to resources were left unchanged. Our results, demonstrating that emigration is a potentially significant mechanism for regulating populations of desert pupfish, are consistent with data from a variety of other animals (small mammals, hydra, roe deer), suggesting that population regulation via a behavioral spacing mechanism may be common among mobile animals.

Coarse-scale movement patterns of a small-bodied fish inhabiting a desert stream

“…In an experimental study assessing emigration of C. macularius, McMahon and Tash (1988) documented that small male fish were more likely to emigrate compared to large and/or female fish. In our study, however, we did not detect any differences in sex ratio or length of movers compared to non-movers.…”

“…Specifically, because limited resource availability often promotes emigration (McMahon and Tash 1988;McMahon and Matter 2006), we hypothesized that the upstream perennial regions are a source habitat that supplies individuals to downstream ephemeral sink habitats. Additionally, we hypothesized that movement rates will decrease throughout the course of the study, as pupfish establish breeding territories.…”

“…Additionally, we hypothesized that movement rates will decrease throughout the course of the study, as pupfish establish breeding territories. Lastly, based on the results of McMahon and Tash (1988), we hypothesized that fish that move will be preferentially comprised of small males relative to fish that remain sedentary. Small males might be the most mobile demographic group if they are frequently displaced from high-quality breeding habitat by larger males.…”

Effects of food limitation and emigration on self‐thinning in experimental minnow cohorts

“…If social behaviour has an e ect on rates of emigration, then it must certainly have an e ect on self-thinning. In the case of territorial species like pup®sh (MacMahon & Tash 1988) or salmonids (Grant et al 1998), self-thinning may be strongly a ected by territorial behaviour. Unlike territorial salmonids or pup®sh, however, patterns of self-thinning we observed in cohorts of juvenile fathead minnows may have resulted, at least in part, from conspeci®c attraction.…”

“…The mechanism by which individuals are lost from a cohort or population, either via mortality or emigration, may have important e ects on population structure and dynamics (MacMahon & Tash 1988). The design of our experiments was similar to the study reported by MacMahon & Tash (1988 Finally, we were interested in contrasting patterns of self-thinning observed in a species with social behaviour that di ers dramatically from more commonly studied salmonid ®sh (Grant et al 1998). Unlike territorial salmonids, fathead minnows often exhibit strong conspeci®c attraction, such as schooling behaviour in response to predation risk (Theodorakis 1989;Hager & Helfman 1991).…”