Target Fish Populations

Fishing activity can significantly reduce the population size of a targeted fish species. In fact, fisheries management has long been aimed at achieving such a reduction. This because it is thought that ‘fishing down’ a stock will increase its productivity, through removing older, slower growing individuals and freeing up ecological ‘space’ for younger faster growing cohorts. But if fishing levels are not well managed, then sometimes biomass can reduce to a level where the stock has difficulty recovering. This can be a particular problem with long-lived species, which reproduce slowly, such as orange roughy and sharks. It can also occur when fish populations are hard to measure and therefore monitor.

In the main, fishing preferentially removes larger and older fish, which might affect the reproductive potential of the stock. This is because larger fish produce more eggs, in relation to their body weight, than smaller fish. The eggs are also larger increasing their chance of survival. Fishing also changes the size and age structure of exploited populations and potentially reduces their genetic diversity. 

Some fish species, such as orange roughy, have a set of biological characteristics which mean that they are only able to sustain very low exploitation rates. In addition, if such stocks do become depleted, recovery takes a long time and may not occur at all. The characteristics include: ¹⁸³²

• Maturation at relatively old ages
• Slow growth
• Long life expectancies
• Low natural mortality rates
• Spawning that may not occur every year (not all fish may spawn in any one year)

Last updated at 2:59PM on November 23, 2017