Dangerous times for Stripey Snappers

Finding a home on the reef can be a lethal time for baby fishes.

The fish

Like many reef fishes, young Stripey Snapper spend the first 4 weeks of their existence in the open ocean. In an attempt to find a reef to call home (a process called settlement), they have to brave a lethal wall of predatory mouths, that is, hungry adult reef fishes. Needless to say life as a baby fish is precarious.

The paper

In their recent fact-packed paper Australian Museum scientists Gaëlle Quéré and Jeff Leis discovered that the young Stripey Snapper - less than 2 cm long - show a surprisingly rich array of behavioural strategies aimed at improving their chances of survival.

Results

Of the 41 observed larvae, 76% reached the reef, 59% settled on the reef and 7% were eaten. The bottom type on which they settled varied, with over half (59%) settled on corals, 29% settled on topographic reef features and only 13% on rubble.

About half of the settling fish interacted with other fishes already established on the reef.  These including predatory attacks and aggressive approaches by residents and aggressive approaches by the settling Stripey Snapper larvae toward other small fish on the reef.

Interestingly, fish swam at different speeds on the windward (28 cm/s) and lee (16 cm/s) sides of the island. They mostly swam in a northerly direction on the lee side of the island but mostly in a southern direction on the windward side.  They avoided the surface, spending most time at depths of 2 m to 15 m.

How they did it

Ok, so how did Gaëlle (now doing postgraduate study in Germany) and Jeff gather the data in their fascinating paper? The answer is a huge amount of hard work and some potentially scary diving in 'blue water'.

Larvae were caught overnight in light traps then released by divers during the day. One diver would follow the released larva at a 'discreet' distance, while a second diver recorded distance, depth and direction every 30 seconds. From 1995 to 2004, teams of divers (including the Australian Museum's own Brooke Carson-Ewart and Amanda Hay) led by Jeff spent hundreds of hours catching and following the larvae of many fish species.

What does it all mean?

Firstly it means that their behaviour is complex, and that these tiny larvae have considerable control over where they disperse and where they settle. It also means that managers of marine parks and fisheries need to take what happens to the larvae into consideration, as just looking after the adult fish does not guarantee their future.

Reference:
Quéré, G. & Leis, J.M., 2010. Settlement behaviour of larvae of the Stripey Snapper, Lutjanus carponotatus (Teleostei: Lutjanidae) Environmental Biology of Fishes 88: 227-238.