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A Fireworm
Photo: Chris Johnson

June 25, 2002
Life Rafts
  Real Audio

Log Transcript

Sailing across the vast expanses of a clear tropical ocean, it is not unreasonable to expect that the marine wilderness might be relatively pristine - relatively untouched and unspoiled by human disturbance. Sadly, as we see over and over again, many view the sea as a garbage dump, a place that hides our refuse from view until ocean currents can disperse it harmlessly and it disappears, well it's out of sight anyway - and isn't that the same thing?

Plastic bags, oil drums, rubber flip-flops, old tyres, fishing net, rope, all make up an armada of man-made rubbish in the oceans that often joins natural debris such as tree trunks and clumps of seaweed that have been ripped by storms and erosion from the coast.

The ocean far from shore, oceanic islands and subsurface seamounts, is a very exposed and dangerous place to live; there are few hiding places. Perversely, marine life can sometimes take advantage of certain types of human refuse. If the object floats, it becomes a shelter for larvae and juvenile fish while they are growing up in the ocean. It offers protection from larger pelagic fish species and from birds that don't see creatures hiding under the flotsam. Sometimes several hunting species will follow a particular piece of debris indefinitely, effectively taking over whatever territory the debris floats into. A single piece of flotsam can be the focal point for several tonnes of fish, including tuna and sharks. In fact the largest haul of tuna ever made came from a purse seine net set under a floating orange crate.

What lives under debris depends on the temperature of the water in which it is drifting. Sailing in the tropics, we expect to find triggerfish, young Rainbow Runners and Dorado (dolphin fish) underneath and a collection of crustaceans and barnacles near the surface.

Today, while on watch, Caleb spotted a white buoy bobbing along on the surface of the sea. As we normally do with what appears to be garbage or debris, we motored over to investigate in case it was something we ought to remove from the water. As it ran down the starboard side of the hull, Caleb hooked the trailing line with the gaff and pulled the buoy aboard. It was unmarked, untethered and trailed some 20 feet of rope beneath it. We noticed a number of small fish scattering in confusion as their shelter was snatched from them.Most made for the protection of the boat, while the silhouettes of larger, unknown animals disappeared into the depths.

It was immediately obvious that this line must have been floating for months or longer, it was covered in an outer layer of algae so dense, that the original size of the rope had swollen to at least four times its original diameter. The algae swarmed with bizarre creatures, we recognized some as crabs and gooseneck barnacles; others were alien to us. Julie, our Scientific Manager, collected some specimens for identification.

Julie Murdoch with the "raft" pulled up by the Odyssey crew for closer examination.
Photo: Chris Johnson

So as to reduce the disturbance and dehydration of the living mass of flotsam, we coiled the entire line into a cooler filled with seawater. The algae appeared to house dozens of crabs of all shapes, sizes and colors. They spilled out over the deck as the line came up, scattering in every direction and scampering frantically for cover. It took the crew hours to ensure that all the escapees were rescued and returned to their home. Large numbers of fireworms were buried snugly beneath the green blanket. Fireworms are omnivorous and feed on invertebrates and algae, some of the worms were huge;, food was obviously not in short supply for this species. However, when handling these animals we used gloves, as they are extremely toxic,. Their long, and numerous chaetae will break off in your skin causing severe irritation which can burn for a month.

After spending a few hours examining the contents of the raft, we decided that the algae had entirely consumed the biodegradable manila rope beneath it, transforming the line into a complex oceanic ecosystem. We put it back into the sea, together with every last microscopic crab that had escaped onto the deck..

Although this life raft had developed it's own thriving multi-species ecosystem, natural rafts such as logs, branches, kelp or seaweed are the most likely types of rafts we encounter in the sea. The effects of manmade refuse, particularly plastics, can be catastrophic for many larger species that may swallow them or become entangled, drifting off to starve later, drown or suffer a lingering death by starvation or from infected wounds .

Log by Genevieve Johnson

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