Can spawning aggregations be used as indicators of reef fisheries?

Of the wide range of coastal fisheries associated with coral reefs around the world, many exploited species that are valuable and vulnerable to fishing are known to form spawning aggregations. These aggregations have supported traditional fisheries in many areas. Increasingly, certain spawning aggregations are exploited for commercial gain to the extent that some species have been placed at risk and some fisheries compromised. The state of spawning aggregations would appear to be promising candidates as indicators of reef fish condition by indicating (a) pressures on fishery resources caused by human activities (fishing), (b) environmental state (aggregation condition relevant to some baseline), and (c) (societal) response by the degree to which they are managed effectively.

There are a number of pros and cons to consider in assessing the usefulness of aggregations as indicators:

PROS
1. Attracts attention to specific sites and is a good way to stimulate discussion of issues.
2. As an indicator, aggregations are simple to understand and explain.
3. Aggregations are at the more vulnerable end of the spectrum in being easy to overfish as well being characteristic of the biology of many of the more vulnerable and valuable fish species.
4. Use of either catch or underwater visual cencus (UVC) data (although whichever is selected should be consistent over time and conducted according to standard protocols) can give indication of changes over time.
5. Many key species of commercial importance aggregate.
6. An interest and focus on aggregations before too heavy exploitation is desirable, especially in the Pacific.
7. It is widely accepted that declining aggregations is a problem.
8. There is existing monitoring and expertise to assess aggregations.
9. Aggregations address Ecosystem Based Management and conservation objectives.

CONS
1. Using aggregations as indicators inevitably attracts attention to them - the best protected aggregations are the ones that are little known or unknown.
2. UVC surveys of aggregations probably need to be expert surveys - sites are often difficult to dive, timing and methods are simple in concept but challenging in practice. Also dive surveys are expensive.
3. In most countries there are no baselines of aggregation status or condition and in Pacific in many
countries little is understood about them.
4. Aggregations as an indicator are not applicable where aggregations not targeted or not likely to be targeted .
5. There can be natural inter-annual variability in fish number aggregating such that long-term datasets might
be necessary - it is the trend that is important rather than the absolute number of fish.
6. It is not clear how comparable data on aggregations would be across countries because of different social and cultural contexts when establishing baselines on differing natural conditions regionally.
7. Cannot distinguish declines due to aggregation fishing from those due to other causes such as overfishing on the species. However, it appears to be the case that losses of aggregations can be a major factor in declines in fisheries of species with this habit.
8. Potential problem of hyperstability i.e. aggregations can suddenly collapse (Sadovy and Domeier 2005).

I would welcome feedback on the idea of spawning aggregations as indicators of coastal reef fish & fisheries condition.

Yvonne Sadovy de Mitcheson
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Reference: Sadovy Y.J. and Domeier M.L. (2005) Are aggregation-fisheries sustainable? Reef fisheries as a case study. Coral Reef 24:254-262.

May 2009