Sink or Swim?

For a follow-up post for the Keys I tried to find an an article that incorporated both the magrove habitat and the coral reefs that we visited on the trip. That's when I came across this article about the linkages of fish populations between the two habitats.

It is well known that subtropical mangrove areas around the globe are habitats for the juvenile stages of many coral reef fishes. The submerged prop-roots are ideal areas for these young fishes as they provide protection and food. Mangroves found adjacent to coral reefs may be important in maintaining the diversity of reef habitats. That is, of course, if these mangroves are actually acting as nurseries for the reefs (rather than ecological sinks) and if maturing fish migrate out to the reefs to live as adults. Much of the past research has focused on the individual habitats themselves and the individual fish life histories. But, as the threat to mangrove habitats has increased over the years much study has been done on this ecosystem as well as looking at the inter-habitat connectivity between mangroves and reefs. However, this connectivity is difficult to observe and long-term studies are very few. This study focuses on mangroves as nurseries for reef fishes in Biscayne National Park in southeastern Florida. The analysis was performed to evaluate the connectivity of these two habitats and to determine if there were contemporaneous fluxes between magroves and reefs for fish replentishment.

Data was collected and analyzed from two independent, fishery-independent fish survey projects which have been going for more than a decade. The mangrove data was collected by conducting visual surveys along transects running parallel to the shoreline. The coral reef data was collected by placing 15 meter circular quadrants and using a stationary visual method. Both projects recorded things such as abundance and size of each observed taxon. A total of 99 mangrove and 365 reef fish species were found with a total of 68 taxa occuring within both habitats. It was determined that 10 target species from 7 families had the potential to exhibit "ontogenetic shifts" (migration) between the two habitats (Haemulon flavolineatum, H. parra, H. sciurus, Lutjanus apodus, L. griseus, Sphyraena barracuda, Abudefduf saxatilis, Gerres cinereus, Lagodon rhomboides,and Scarus guacamaia). Additionally, three habitat strata were defined: mainland (ML), leeward key (LK), and all reef quadrats (RF). Age classes were determined for each target species by using length-frequencies (basically, the size of the fish), and plots of relative abundances made for each age class in each habitat. These age class abundances were then used to calculate mean annual abundances in each of the three strata. Further abundance indicies also allowed for age classes and habitats allowed for species-specific assessments. This study provides evidence of inter-habitat connectivity for 4 out of the 10 target species (Abudefduf saxatilis, Lutjanus apodus, L. griseus, and Sphyraena barracuda). The authors go into a long discussion about the other 6 of 10 species which came out as non-significant in the connectivity analysis, I'll not discuss them here. The results of the study support the hypothesis that mangrove shoreline habitats serve as nurseries for reef fishes. The authors conclude that (1 )there is a spatial segregation of life history stages in a number of reef fishes in this area with juveniles occupying mangroves in the bay and adults on the reef, (2) that mangrove and reef habitats are connected through ontogenetic migrations, (3) that for some species the linkage results in the propagation of year-class strength of bay populations to reef populations, and (4) that this strength is indicative of the nursery role of mangroves and their role in population replentishment.

Read more here:
Jones, David L. (2010) Connectivity through ontogeny: Fish population linkages among mangrove and coral reef habitats. Marine Ecology Progress Series: 401, 245-258.

(image from maps.statemaster.com)