One of the common reasons given for the importance of mangroves and seagrasses
is that they serve as important “nursery” habitats for many fishes and invertebrates.
That is, juvenile organisms thrive in these systems (e.g., Nassau grouper), and then
often move to other habitats, such as coral reefs, as adults. But why are
these systems serving this important role?
It is simply assumed that nurseries provide a higher food availability (and thus faster growth) and lower predation risk than other habitats. Yet very few studies have actually tested which of these factors is more important. We recently tested which of the two is most important, using french grunts as a model. Here is a link to the paper.
Specifically, we explored both survival and growth rates of grunts on coral reefs, as well as in purported nurseries: bays dominated by seagrasses and channels lined with mangroves). Using otoliths to age the fish (more information on aging fish here), we were able to compare growth rates. As the graph below suggests, small fishes found on reefs were actually larger at a given age, and thus they were growing more quickly.
The survival data were exactly the opposite. Survival rates of small fishes was very low on reefs, and much higher in seagrasses and mangroves. A trade-off was clearly apparent: small can grow faster on reefs, but are likely to be eaten. So it is more beneficial overall to utilize the seagrasses and mangroves until they reach a size that they are less susceptible to predation. The study illustrates how some basic assumptions in marine ecology (e.g., fish use nurseries because they have more food) may not be true in practice. Such studies have important implications for how we justify long-term protection and conservation of these valuable habitats.