Migratory Corridors

To reach a spawning aggregation, some individuals may migrate along specific migratory corridors, thus concentrating reproductively active fish in relatively high densities, similar to the aggregation itself. These corridors are often known and frequently exploited by fishers. In many instances, fish are removed prior to reaching FSA sites and thus the reproductive output of the spawning population is diminished1. Because of their linkage to FSAs and because fish utilizing them are often as vulnerable as when in the aggregation, locating and protecting migratory corridors is often critical.

School of jacks. Photo © David Obura

Similar to identifying an FSA, discovering migratory corridors usually involves interviews, and observations of fish around the FSA and fishing activities.

The most likely method of discovering reproductive migratory corridors is through interviews or observations of fishers and fishing activities.
Recreational divers and dive guides may also be aware of these migrations and be a valuable source of information.
When FSAs are located seaward of the reef, fish may move along the reef edge or where migratory passageways are constricted. These areas might provide useful opportunities to conduct visual point monitoring surveys away from the aggregation. Once monitors have an idea of spawning times, monitoring fish when they leave the site might help in identifying migratory corridor(s).
For some target species, such as groupers, movement to the site may be sex-specific and occur in groups. Although the timing may vary, males of some groupers have been shown to move and occupy the FSA site for 2-3 days prior to females2. Males and females may use different patterns of movement to the FSA (e.g., squaretail coralgrouper) or may migrate at different times.
If migration routes can be described, they should be recorded and included in MPA designs.
As more species are investigated, new details are emerging about migratory corridors for a number of FSA-forming species. However, it is too early to say whether all or what proportion of aggregating species use them.

Resources

Some published references on migratory corridors using tag-recapture studies are available online. While the literature on reef fish spawning migrations is relatively sparse, a substantial volume of information exists for turtles, pelagic fishes, such as tuna or cod, salmon or anadromous eels.