Fish Spawning Aggregations: A Reef Resilience Toolkit Module

Overfishing

Fishing in Java at sunset. Photo © Ahmed Fuadi/TNC

Overfishing refers to the removal of marine organisms from the environment at a level that is not sustainable. The level of “sustainability” required in a fishery is difficult to measure, and may vary because overfishing can be defined from a number of different perspectives.

Because certain gear and fishers themselves may select for larger fish, there is often a reduction in the larger size classes of the population. This can occur in spawning aggregation fishing, where the largest and most mature members of the stock are usually being fished. As fishing continues, the average fish size becomes smaller, and fish are caught before they can mature. This results in an overall reduction in the mean size of the adult population, meaning smaller and less fecund fish. The series of graphs below describe how this may occur.

Graph showing the percentage of fish mature for each length class. © TNC	A sequence of graphs showing the effect of fishing the large members of the population at different fishing mortality levels. Note that at very high rates of mortality most of the fish being captured will be immature. © TNC

Growth overfishing occurs when the rate of fishing produces a loss in biomass of the stock that is greater than the biomass gained due to growth. Growth overfishing is also used to describe a situation where too many small fish are being harvested from a stock, and fish are removed before reaching a size at which maximum growth and productivity would be obtained from the stock.

The BOFFF Hypothesis

While in all species of fish, larger females produce more eggs, there is evidence that in some species, larger females can produce exponentially greater quantities of eggs4. The BOFFF hypothesis, or Big Old Fat Fecund Female hypothesis, refers to this phenomenon. Thus, removing a few larger females from a population can have a dramatically greater effect on reproductive output, and recruitment, than the removal of numerous smaller females. Recent studies also show that the condition of larvae also improves with the length or age of the fish, and therefore the larvae produced by large females may have higher survival rates5.

The BOFFF hypothesis. Adapted from Palumbi 2004.

Recruitment overfishing occurs when the spawning biomass of the population is reduced, such that the number of larvae and recruits produced are not great enough to replenish the population. This problem may be exacerbated by the removal of large females, which produce exponentially more eggs than relatively smaller females. Read about the BOFFF Hypothesis in the sidebar to the right.

Ecosystem overfishing 1occurs when organisms are removed at such a rate that the composition of the ecosystem is changed significantly, often leading to system-wide detrimental effects.

Economic overfishing occurs when fishery resources are not being used in the most efficient manner. The limit of economic overfishing is commonly defined as the point at which fishing effort exceeds the total profit of the fishery, i.e., the amount of fish caught does not pay for the costs of fishing.

Malthusian overfishing2 is an expression coined to describe a situation that may occur when fishing levels are too intense for a fishery to be sustainable. As the fishery becomes exploited and catch levels decrease, fishers may take desperate measures, and partake in destructive fishing practices to maintain catches. For example, in the Indo-Pacific, dynamite fishing is a destructive practice often used to increase catch, without increasing effort. Clearly, this type of fishing is detrimental to the ecosystem, and is not sustainable.

The fishing of FSAs may lead to several types of overfishing discussed above3. By removing reproductively active members from the population, FSA fishing has the greatest potential to catalyze recruitment overfishing. As the abundance of spawning fish declines, ecosystem, economic, and Malthusian overfishing may subsequently occur. A number of management strategies are recommended to prevent overfishing, and these are discussed in the toolkit.

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