SILLIMAN: EXPERIMENTAL EXPLOITATION OF FISH POPULATIONS 



Within this general objective, it was desired to 

 determine which combination of exploitation 

 rates applied on the two species would provide the 

 maximum sustainable yield. To approach this 

 problem, test and control pairs of populations were 

 established in aquariums and allowed to grow 

 several months under controlled conditions. 

 Various combinations of exploitation rates were 

 then applied to the test pair to determine popula- 

 tion interactions and total yields. A "base line" for 

 evaluating the results was obtained by growing 

 and exploiting each of the competing species 

 independently. 



APPARATUS AND PROCEDURES 



Experimental Animals 



A lengthy fund of experience (Silliman 1948, 

 1968; Silliman and Outsell 1958) built up with the 

 guppy, Poecilia reticulata, dictated this as one of 

 the experimental fishes. For the other, it was 

 desired to have a species that was similar to the 

 guppy in size, reproduction, and feeding habit but 

 readily distinguishable from it in a mixed popula- 

 tion. 



A fish that met these requirements fairly well 

 was the red swordtail hybrid, Xiphophorus 

 maculatus x X. helleri, which will be referred to 

 simply as "swordtail." It is somewhat larger than 

 the guppy, but lived in the same sized aquarium. It 

 is a live-bearer, like the guppy, and will readily eat 

 the foods commonly fed guppies. Its brilliant red 

 color permits easy distinction in the adults, and 

 even the newborn young are pink or orange and 

 may be distinguished from newborn guppies. In 

 both species, adult males can be distinguished 

 from adult females by external inspection. Dis- 

 tinguishing male characters are the modified anal 

 fin (gonopodium) and fin and body color in the 

 guppy and the elongated lower caudal fin 

 ("sword") in the swordtail. 



Aquarium Tanks 



Fish were grown in four conventional glass- 

 walled aquariums, each with a water surface of 44 

 cm by 24 cm, a water depth of 19 cm, and a volume 

 of 20 liters. Inside each tank was an air-stone and 

 a fiber-charcoal filter. Tanks were placed in a row 

 with their longer axes parallel, and lettered A, B, 

 C, D from left to right. 



Tanks A and C were for juvenile and adult fish 

 together. They each had a refuge in the left front 

 corner for the escape and subsequent removal of 

 recently born fish, or "fry." This refuge was 

 formed with a fence consisting of 21 cm by 3 mm 

 glass rods placed 1.5 mm apart, enclosing a right 

 isosceles triangular space of 15 cm hypotenuse. 



Although guppies could survive and achieve 

 population growth in the above-described tanks, 

 preliminary experiments showed this not to be 

 true for the swordtails. Tanks B and D were 

 therefore provided as "nurseries" for the tem- 

 porary relocation of newborn young from tanks A 

 and C, respectively. The juveniles were placed 

 back in tanks A and C when they had grown to 

 such size that they would no longer pass through a 

 sieve consisting of 3-mm plastic rods placed 2 mm 

 apart, thus making them recruits to the fishable 

 stock. 



Food and Feeding 



A diet previously developed for guppies 

 (Silliman 1968) was fed to all fish (Table 1). The 

 food Artemia nauplii, however, requires special 

 mention. The original intention was to feed the 

 fish in the nursery tanks one-half the amount fed 

 those in the adult tanks. The weight of nauplii 

 produced was mistakenly believed to be directly 

 proportional to amount of Artemia eggs placed in 

 the culture beakers and, therefore, one-half the 

 amount of eggs placed in the beakers for the adult 

 tanks was placed in those for the nursery tanks. 

 Production tests (Table 2) based on duplicate 

 hatchings produced under standard conditions, 

 however, showed production not to be proportional 

 to the amount of eggs. Amounts of eggs inserted 

 were kept the same, nevertheless, on the chance 

 that unhatched eggs were eaten (observed on one 

 occasion). 



Artemia nauplii provided so small a proportion 

 (1/100%) of the total diet that the lack of propor- 

 tion noted above would have no significant effect 

 on total food intake. The small amount of living 

 food provided by the nauplii was regarded in the 

 same sense as vitamins in human nutrition: as 

 something required in small amounts for good 

 health, but not furnishing a significant proportion 

 of total food intake by weight. It is pertinent to 

 note that the smallest number of nauplii (1,500) 

 indicated by any of the tests (Table 2) would 

 provide over four nauplii per fish for the largest 



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