FISHERY BULLETIN: VOL. 78, NO. 3 



vulnerable (Butler 1961; Gotshall 1978). The be- 

 havior of mathematical models describing such 

 multiple adult feedback mechanisms, termed 

 "multistage" recruitment processes, has been re- 

 cently investigated by McKelvey et al. (in press). 

 Collection of data suitable for analysis of these 

 more complex models would seem to pose difficul- 

 ties far in excess of those required for the simple 

 models. In such cases experimental laboratory 

 studies may be more effective and more effi- 

 cient than "field" investigations for providing 

 insight into the fundamental problem of stock and 

 recruitment. 



Laboratory studies of stock and recruitment 

 have many advantages over similar studies of 

 natural wild populations. Strict environmental 

 control (of food supply, temperature, dissolved 

 oxygen, photoperiod, habitat dimension and 

 type), elimination of sampling error through com- 

 plete enumeration, and opportunity for unique 

 behavioral observations in an aquatic environ- 

 ment are the most obvious. Less obvious but 

 perhaps more significant advantages include the 

 investigator's unrestricted manipulative control 

 over population size and age structure, so that 

 the chief modes of regulation are likely to be 

 endogenous population mechanisms, causally den- 

 sity-dependent, and the opportunity for replica- 

 tion in quantity. 



From the perspective of statistical analysis, the 

 influence of exogenous, density-independent fac- 

 tors is reduced, observations may be collected over 

 as wide a range of population states as desired, 

 and (through replication) the inherent variability 

 of the dynamic population processes themselves 

 may be examined (Royama 1977). 



Among fishes the study of laboratory popula- 

 tions has centered on a single species, the guppy, 

 Poecilia reticulata. The characteristics of small 

 size, rapid maturity, frequent and repeated repro- 

 duction, and a relatively short life span render the 

 guppy an ideal subject for laboratory study of the 

 stock-recruitment relation. Basic information has 

 been gathered regarding reproduction (Felin 1935; 

 Purser 1938; Turner 1937; Rosenthal 1952). fecun- 

 dity (Felin 1935; Hester 1964), and growth (von 

 Bertalanffy 1938), and there have been many, 

 often long-term, studies of population behavior 

 (Breder and Coates 1932; Shoemaker 1944; Silli- 

 man 1948, 1968; Silliman and Outsell 1958; Laak- 

 so 1959; Warren 1973; Yamagishi 1976). While the 

 focus of these studies has not been the stock- 

 recruitment relation per se, it is often possible to 



interpret observed population dynamics in part as 

 a reflection of an overcompensatory function re- 

 lating numerical population addition to densities 

 of adult fish. 



The guppy is a viviparous member of the family 

 Poeciliidae and exhibits strong sexual dimorph- 

 ism. The maximum weight for mature males is 

 perhaps 0.2-0.4 g as compared with a maximum 

 weight for females of perhaps 1-2 g. Sexual matu- 

 rity is normally reached by both males and females 

 at 12-16 wk at weights of 0.1-0.15 and 0.2-0.3 g. 

 Average longevity under laboratory conditions 

 might be 12-18 mo. Young are produced in discrete 

 broods, ranging in size from 2 or 3 to 50, at roughly 

 monthly intervals at 25° C. Brood size depends on 

 female size and, as a result, the appropriate 

 measure of adult reproductive potential is not the 

 number of adults but, since fecundity is nearly 

 proportional to female weight, the total adult 

 female biomass. Males mate freely and indiscrim- 

 inantly with available females, and sperm from a 

 single mating may remain viable and produce 

 successive broods for periods up to 6 mo (Winge 

 1937). Females are clearly the overwhelmingly 

 important component of the adult stock. 



At birth fry weigh 7-8 mg and, immediately 

 after ejection by females, are extremely vulner- 

 able to predation by adults. Cannibalism has been 

 regarded as the primary feedback mechanism 

 controlling population size (Breder and Coates 

 1932; Laakso 1959). Recent experiments have 

 suggested that cannibalism is chiefly a function of 

 contacts between individuals rather than of re- 

 sponses to limitations in food supply (Silliman 

 1968; Warren 1973). 



Laboratory environments for guppy population 

 study have differed significantly in two respects: 

 food supply and provision of refuge areas. Food has 

 been delivered at either fixed and limiting or 

 "to excess" ration levels, and refuge areas have 

 only rarely been provided. These studies have 

 shown that total population biomass is strongly 

 influenced by food supply (Silliman 1968), but 

 numerical densities and dynamics are strongly 

 affected by refuge area provision and only slightly 

 influenced by food supply. 



Cycles of abundance have been observed in all 

 long-term studies where refuge areas have been 

 absent (Breder and Coates 1932; Shoemaker 1944; 

 Laakso 1959). Characteristics of abundance cycles 

 (begun with small numbers of adults) include 

 initial increases in numbers, reduction of such 

 increases to near zero as adult predator density 



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