The biomass of fish inside an MPA can increase by between 2 to 21 times from its original level. As the fish grow and multiply they move into areas where fishing is allowed. This is called the spill over effect and can significantly increase fish catches around MPAs.

MPAs also allow fish to get bigger, and this has a very substantial effect on their egg production. When a mature fish grows and doubles its length, its egg  production increases 8 times. The quality and fertilisation rate of eggs from bigger fish are also better. The combined effect of the spill-over effect and the increase in egg production is called the reserve effect.

The scale of the reserve effect will vary depending on the size, shape, location and management of an MPA. Networks of MPAs are also thought to produce far greater benefits to fisheries and biodiversity.