What’s the Problem?
Examples of the Effects of Fishing on FSAs
- There is a reported diminution of populations and loss of FSAs for a number of commercially important Indo-Pacific reef fish species. A number of FSAs along the Great Barrier Reef, and at least five FSAs in Palau, have disappeared6.
- One of the few viable Nassau grouper spawning aggregations in Belize once had 15,000 fish. Over a period of 25 years, there has been a population decline of more than 80%. Recent fisheries models predict that the protected, but unenforced, aggregation site will disappear by the year 20137.
- A Nassau grouper FSA in Quintana Roo, Mexico, had been fished for more than 50 years and sustained aggregations of 1,000 to 15,000 individuals. The FSA disappeared shortly after fishermen introduced the use of gillnets within migration corridors8.
- In the Gulf of California, unmanaged FSAs of goliath grouper, Epinephelus itajara, black sea bass, Stereolepis gigas, and the gulf grouper, Mycteroperca jordani, have disappeared due to fishing9.
Fish spawning aggregations are extremely important to local ecosystems and economies, but they are not resilient to anything greater than light fishing pressure. In some cases, FSAs have been lost even from subsistence fishing1, which is often considered sustainable2.
The reasons for FSA vulnerability vary, but often it is a result of the predictable nature of FSA formation, and the life history characteristics of many FSA-forming species. Moreover, spawning in FSAs is considered to represent 100% of the total reproductive output for most species studied to date3, such that FSA fishing can have profound effects on populations, often over brief time spans4.
Characteristics of FSA-forming species may include:
- Slow growth rates
- Late sexual maturity
- Small home-range size as adults
- Complex life cycle
- Protogynous hermaphroditism
Slow Growth and Late Maturity
Generally, species with shorter life cycles are more resilient to heavy exploitation, and may recover quickly from low stock numbers. Many FSA-forming species have slow growth rates and delayed sexual maturity, making their stocks less resilient to fishing.
Life cycle of mutton snapper © Rachel T. Graham
Small Home Ranges
Many reef fishes have sedentary life styles as adults and may occupy small home ranges (with the exception of migrations to spawning areas). Site specificity and narrow habitat ranges can make these species easy targets for fishermen5.
Complex Life Cycles
Most reef species exploit a variety of habitats throughout their life cycle. Most often, the larvae of reef species are pelagic, inhabiting the open ocean, whereas the adults are sedentary reef inhabitants.
Many species may also use reef-adjacent habitats, such as mangrove and seagrass beds, during juvenile stages.
Sex Change
Some reef species, such as groupers and wrasses, begin life as functional females, and then transition into functional males later in life. This is termed protogynous hermaphroditism.
Left: Stoplight parrotfish juvenile phase (Females). Middle: Stoplight parrotfish initial phase (Mature females and immature males). Right:Stoplight parrotfish terminal phase (mature males). Photos © Paul Humann
Size selection of either smaller or larger individuals within the population, as can occur with fishing, may alter the sexual composition of the population, reducing reproductive output and, therefore, the number of larvae available to maintain local populations.
1 Hamilton and Kama 2004
2 Kuster et al. 2005
3 Shapiro et al. 1993
4 Sadovy and Domeier 2005, Rhodes and Tupper 2008
5 Morris et al. 2000
6 Johannes et al. 1999
7 Sala et al. 2001
8 Aguilar-Perera 2006
9 Sala et al 2003