364 



Fishery Bulletin 92(2). 1994 



tion size-frequency shifted again towards 

 smaller size classes as adult individuals 

 were rapidly removed from the population. 

 Clearly, for a successful fishery, a delicate 

 balance exists between sufficient mortality 

 to permit the fishery to exist and too much 

 mortality which will reduce the harvest- 

 able yield. 



Food supply is a complicating factor. In- 

 creased food supply will not always result 

 in increased population density or in- 

 creased harvestable yield. The timing of 

 the food supply interacts in subtle ways 

 with the timing and intensity of mortality, 

 sometimes producing higher densities and 

 sometimes lower ones. The simulations 

 show that the effect of variations in food 

 supply is complex; no simple rules apply 

 and a number of feedback mechanisms 

 exist. In one case, for example, lower popu- 

 lation density resulted from increased food 

 supply because increased growth permitted 

 more oysters to enter the size classes that 

 were exposed to mortality, thereby result- 

 ing in a population that declined. In an- 

 other case, a one-month change in the tim- 

 ing of the spring and fall blooms changed 

 population density by a factor of 2 at the 

 same mortality rate. In other cases, little 

 impact occurred in the population despite, 

 for example, the complete failure of the 

 spring bloom. 



Population stability and 

 population crashes 



The stability of oyster populations is sen- 

 sitive to several factors, including the tim- 

 ing and intensity of mortality, latitude, and 

 food supply. (We use the term stable in the 

 sense of Underwood [19891 for populations 

 able to recover quickly from perturbation. 

 The terms elasticity and resiliency might 

 also be used. ) Increased mortality reduced 

 population density in every comparison. 

 Oftentimes, a relatively stable equilibrium 

 occurred as recruitment balanced mortal- 

 ity over the long term. In all cases, how- 

 ever, mortality rates sufficient to destabi- 

 lize this equilibrium could be found and a 

 population decline resulted. When mortal- 

 ity extended over a wider range of size 

 classes or affected larval survivorship, 

 population destabilization occurred more 

 easily. In the former case, more oysters 



GALVESTON BAY 



-i — i — i — r 



300 600 900 1200 1500 1800 

 TIME (Days) 



2100 



100000 



10000 



1000 



100 



e 

 z 



B Number ol Individual* 



Mortality (no/month) 



Spawn (kcal) 



Numtxx ol Adults 



10 



10000 



8000 - 



S 6000 - 



S 4000 



H 

 ll 



II 

 I I 



I 



 .'■'  .K)  , J ,i. ( i A i I\/^j-* K 



4000 



3000 



- 1000 



18 24 30 36 42 48 54 60 66 72 

 Julian Month 



c/) 



2000 



3 4 

 Julian Year 



1 2345678910 

 Size Class 



Figure 1 2 



Simulated time development and population distribution of a 

 Galveston Bay Crassostrea virginica population exposed to a 

 continuous mortality rate of 99.9% restricted to size classes 5 

 and larger and in which mortality occurred only during the sum- 

 mer. Compare Figure 11. (A) The number of individuals per size 

 class and reproductive effort per size class. Isolines, for number 

 of individuals, are the logarithms of the number of oysters 

 (log 1() N). Shading for the amount of reproductive effort (spawn) 

 represents the logarithm of cal (logical). (B) Monthly averaged 

 values of the number of individuals, the number of adults (/=4, 

 10), and the monthly reproductive effort in kcal for the 6-year 

 simulation. (C) The yearly reproductive effort (number of kcal 

 spawned). (D) The final size class distribution in the population 

 at day 2,160. Further information in Figure 2 and Table 2, case 29. 



