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Adventures in Light Trapping

Have you ever thought about how to catch a baby fish? These tiny creatures are all throughout the top layer of the ocean -- but they're so small that you can't use a normal fishing pole to get them. Instead, scientists use various nets. One option is to drag a net with a very fine mesh through the water -- but this takes time and a big boat.

cutting plexiglass for light trap

An alternative is to use a technique called light trapping -- literally enticing tiny fish (and other creatures in the plankton) using a very bright light inside a trap. The trap itself is made of a plankton mesh with holes that are 505 microns in diameter -- so it keeps in all but the smallest pieces of plankton.

Right now, my lab at UC Santa Cruz is on Santa Catalina Island, part of the California Channel Islands off the coast of Los Angeles. We're doing a variety of projects out here, including a long-term population study of California morays (Gymnothorax mordax), a study on how predation influences California two-spot octopus (Octopus bimaculatus), and a study on how the pattern and coloration of giant kelpfish (Heterostichus rostratus) influences feeding success. As an additional project (because we're just that ambitious!) I decided to try out light trapping as a method to obtain larval fish. My two main goals are to 1) see if this method could work in California, where the water is not always very clear, and 2) to see if this method could be used to eventually categorize the species of larval fish in the ocean around Catalina at different times of year. We know of a few species of fish whose populations in southern California depend on larvae arriving from Mexico during occasional climate shifts, and the light trapping could help provide us with further evidence of this and let us see how dependent certain fish populations are on this transport.

light trap for larval fish

I built my own traps with the help of my partner Chris. We based the trap design off of Sponaugle & Cowen (1996), who created a portable and inexpensive trap whose components are all readily available. Building the two traps took about 12 hours in all and required a LOT of sewing and plexiglass cutting. We were lucky that Chris' aunt and uncle have a full, mostly unused workshop at their house with all the tools we needed!

So far, I've been trapping in two locations: Off the pier in the small town of Two Harbors, where we're staying, and off the dock at the Wrigley Marine Science Center, which is 2 miles away and where we do most of our field work. I've checked out a few samples just to make sure the method is working -- and we have fish! Below is a photo I took with the small digital microscope I brought along. I can't wait to look more at the samples and ID all the fish I'm finding!

larval fish


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