Water is in constant motion and transports sediments, nutrients and pollutants. At least during one life stage, most marine organisms move within the water stream, either passively or actively. Connectivity is the word used to describe all these movements.
Connectivity is an important consideration in coastal management and in the design of marine protected areas (MPAs) and MPA networks. When fish larvae and fertilized coral eggs move in water currents from one place to another, these movements become crucial for the location of the new generation of these animals.
Most marine organisms on reefs and in coastal waters are relatively sessile during most of their life. This sedentary lifestyle is abandoned during reproduction: most reef species produce pelagic eggs which become pelagic larvae. Some of these pelagic larvae become fish. When fish grow older, they may travel to another location, while juvenile coral colonies will generate a reef where the coral eggs and larvae end up – in case the marine environment is suitable for reef growth.
Some habitats are critical to the early developmental stages of fish, lobster, and shrimp, while others serve as spawning or feeding grounds. Marine organisms also migrate daily and/or seasonally between habitats. The daily shifts commonly involve nightly feeding migrations between feeding and resting habitat. In some fish species, these daily movements lead to nutrient transfer between seagrass/mangrove areas and the coral reef.
The marine ecosystem is so complex that many connectivity issues are poorly known. Nevertheless, this field of marine ecology is advancing: better understanding is crucial for sound marine management. An example of these advances is the publication in 2010 of “Preserving Reef Connectivity: A Handbook for Marine Protected Area Managers”, which can be found here: Handbook
Special attention in the Handbook (written by P.F. Sale et al., edited by Lisa Benedetti and published by UNU-INWEH), which is the main source for this Coastal Challenges’ article, is given to populational connectivity. This includes
- Evolutionary (genetic) connectivity; and
- Demographic (ecological) connectivity.
In the Handbook, number 1 is said to “be informative when considering long-term (evolutionary) and large-scale biogeographic dispersal patterns of organisms. It can also be useful for managers wanting to assess the genetic uniqueness of populations when making decisions concerning biodiversity preservation.”
Number 2 “involves the extent of linkages that occurs among nearby local populations of a species due to the exchange of individuals”. This type of connectivity is important for the design and management of marine protected areas (MPAs) and no-take fishery reserves (NTRs), and when we want to know the ideal amount of coral reef habitat to protect.
The results of recent investigations are clear: pelagic larvae do not drift aimlessly in the ocean. They use for example sensory capabilities to minimize the extent of dispersal. In many species the larvae have the capability to settle on suitable reef habitat and specific microhabitats.
Connectivity amongst populations of reef species is primarily due to dispersal during larval life; demographic connectivity takes place on scales of up to tens of kilometers. The concept of demographically well connected populations for example across the Caribbean is not true and belongs to the past. Only genetic (evolutionary) connectivity links these habitats far away from each other, when larvae occasionally get transported beyond the usual dispersal range.
When marine parks are intended to function as fisheries management tools, the smaller scale of demographic connectivity should be taken into consideration in the MPA design – and in the design of MPA networks. This type of connectivity is worth remembering also when coral reefs experience massive destruction (hurricanes, bleaching, crown-of-thorns attacks): demographic connectivity defines the limits of natural re-seeding.
Coastal development may damage important inshore areas used by developing fishes and other organisms. For example, pathways between these and offshore habitats may be disrupted. Negative impacts during an organism’s early life stages may also have consequences for the abundance of adults. In addition, linked food webs may be affected. Furthermore, daily or seasonal migration routes could be disrupted. – MPAs and MPA networks should be large enough to encompass the interlinked habitats.
Spawning aggregations of groupers are an exciting phenomenon in many parts of tropical seas. During large scale oceanic movements and gatherings, species behaving in this manner are vulnerable to overfishing. The spawning sites should be part of no-take fishery reserves (NTRs). (More information on NTRs specifically can be found in “Fully protected marine reserves: a guide” by Callum M. Roberts and Julie P. Hawkins, 2000, which can be downloaded from here: Guide)
In general, NTRs promote fish survival and reproduction even when serious overfishing takes place in the surrounding area. Studies have shown that four positive changes inside NTRs take place. These changes, which may benefit fished populations outside reserves, are summarized in the Handbook on reef connectivity. They are:
- Increased reproductive output (increases in fish abundance, spawning biomass, mean age, and body size result in this change)
- Higher net export of juveniles and adults (to surrounding fished areas);
- Higher net export of eggs and larvae (to surrounding fished areas); and
- Protection and recovery of crucial habitats/ecosystems (key underwater areas for the fished species)
The NTR studies indicate that when neighboring NTRs are not more than 10-30 km apart, appropriate levels of populational connectivity exist for most reef species targeted by fishermen. As indicated above, early hydrodynamic models predicted dispersal distances of hundreds of kilometers. Based on new evidence, even relatively small MPAs may be self-sustaining.
In a world of climate change, it is important that coral reefs and other coastal ecosystems are managed as effectively as possible. Their natural resilience can be supported by taking connectivity into consideration.