Biology Question #1723

Ashley, a 16 year old female from Langenburg, Saskatchewan asks on December 4, 2003,

Does population density affect fish growth rates? I'm trying to determine if population density affects the growth rate of fish. I have two containers that are the same size. In one container I have a very low population density and in the other I have a high population density. I make sure I feed all the fish the same amount. Would the population density have an effect on the fish? Does availability of space determine how quickly they can grow?

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The answer

Allen Curry, Professor, University of New Brunswick, Fredrickton answered on March 3, 2005

Yes, the density will affect the growth of the fishes. We know that closed systems like your containers have a limited carry capacity (that is the key element for aquaculture and the aquarium fish business). That means there is only so much biomass (total mass of all living things) that can be sustained in a container of a given size, so the bigger the container the more fish it will support (assuming they are fed and cleaned regularly).

Out in the wild however, the answer is usually no, we don't see density affecting growth. The reason is that wild systems tend to be open with very complex inputs and outputs. When fish biomass starts to increase--that can be the number of fish or the total weight of all fish--the population of fish responds by either reducing its numbers ( leaving the area, suffering greater number of deaths from predators, lower birthrate) or they lower their growth rate, that is, they stop growing. So in the wild, we rarely ever see that density affects growth because many other things happen before these two factors affect each other.

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