Each day we go out collecting data to see if we can determine if fish use the sun to orientate. Collecting data involves putting a fish larva into the 'DISC' which is an acronym for 'Drifting In Situ Chamber'.

Drifting In Situ Chamber
Drifting In Situ Chamber Device (DISC) Image: Lyle Vail
© Australian Museum

To collect data first we must have fish larvae. We collect the larvae using light traps, which have a watertight box which houses a light and a timer that switches the light on at night and off during the day. The traps have a one way opening so larvae can swim in but not out. Much like moths to a flame, some fish larvae are also attracted to light. We set the traps about 10-50 m away from the reef edge

The best part of my day is collecting the fish larvae.  We never know what fishy surprises the light traps will produce. Most importantly we collect Butterflyfishes and Blackaxil Puller to run in the DISC (see the image of an exceptionally cute Butterflyfish larva). The biggest light trap stars for me are the Butterflyfish, Soapfish and Surgeonfishes.

After sorting we load up the tinny with our bucket of larvae and the DISC and head about 700 m offshore. Every 20 minutes we put a larva into the DISC.  We can alter light treatments such as time of day. An 'umbrella' can be attached that blocks the fish's view of the sun and thus its ability to use the position of the sun to navigate. Perhaps some fish larvae can use ultraviolet to navigate and others can't?

An internally-mounted camera takes an image of the larva every 2 seconds. Also mounted inside the DISC are a GPS, a compass and a HOBO (a combined light and temperature meter). The images (together with all of the other data) will hopefully tell us if the larva is using the sun for orientation.  There is huge quantity of data to be analysed, so we won’t know for a while if our hunch about sun cues are correct. We spend about 6-7 hours each day in the tinny deploying and retrieving the DISC, but more about that another day.

More Lizard Fish posts

In the beginning
Baitfish and a large shark
Shark Beaching