Mangroves: Smelly habitats but fantastic nurseries!

Fiddler crabs are mangrove bioturbators, oxygenating the sediments (Image: TalkingNature.com)

People often think of mangroves as smelly muddy places that ‘get in the way’ and block your view of the water.

It’s true they can be smelly, sticky places, but they’re also an important habitat for juvenile fish and crabs which we want to catch when they’re adults. So what is that smell and which fish get a benefit from those mangroves?

What’s the smell?

The ‘rotten egg’ smell that people relate to mangroves is a sulphide based gas, often hydrogen sulphide. It’s a result of decomposing organic matter, like leaves for example, in an environment lacking oxygen. If you’ve ever gone for a walk in the mangroves you’ll know that the mud is usually a very fine and sticky silt. Together with the still waters within the mangrove swamp the fine sediment prevents oxygen from penetrating very far below the surface, so conditions are rife for anaerobic decomposition and the production of rotten egg gas. If you want to get technical have a look here.

Using a liquid nitrogen contact core to sample the mangrove sediments (Image: TalkingNature.com)

Animals like crabs, slugs and worms which live in the mangrove swamps dig burrows and churn through the sediments (bioturbation). By moving the sediment around they help to oxygenate it and reduce but not eliminate anaerobic decomposition. So a healthy population of burrowing crabs and worms within the mangroves helps aerate the sediments, reducing anaerobic decomposition and the amount of rotten egg gas produced.

Scylla serrata is the large mud crab found within the mangroves. Watch your fingers! (Image: TalkingNature.com)

How do the fish benefit?

Many fish species are found in mangroves as juveniles. Some common examples include yellowfin bream, mangrove jack, and moses snapper (Lutjanus russelli) even barracuda!

Why are they there?

Mangroves provide structure in the form of prop roots, fallen trees, channels and other obstructions. These structures give animals shelter because they can hide behind or within them. This shelter is potentially beneficial in two ways:

• Structure provides a place for the juvenile fish to hide from larger predators and,
• Structure could provide a source of food to the juveniles because in turn, smaller fish than them (their prey) hide within the structures as well.

So the habitat potentially provides both shelter and food for juvenile fish; it’s a juvenile food web!

And then there’s outwelling?

Within a patch of mangroves there may be hundreds of thousands of small invertebrates burrowing away, eating algae and detritus. Many of these invertebrates release their eggs and larvae into the water column during flooding tides, so that the larvae are carried away to other mangrove patches…hopefully!

A megalopa, a larval stage of a crab about 5mm long (Image: TalkingNature.com)

This outwelling of larvae and eggs from the mangroves into the adjacent waters provides another source of food for fish and other planktivores. Some fish even time their mangrove foraging forays with big high tides, which is when many crabs spawn.

The quantity of eggs and larvae released over a year from a large mangrove system probably measures in the tonnes! There’s potential there for mangrove ecosystems to be releasing large amounts of nutrition into near shore ecosystems!

Mangroves are probably a vital part of our healthy tropical fisheries and are well worth conserving, even if they are a bit smelly at times.

A mangrove lined creek in southeast Queensland (Image: TalkingNature.com)

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