Bycatch If You Can

Posted by at 10:00 AM, June 11, 2004

Trawler Bycatch: A dead shark is netted, as the bottom trawler moves away. (C) Greenpeace / Walsh

Just like yesterday, we spent the day rocketing around the ocean in pursuit of bottom-trawlers, monitoring their behaviour, and recording the size of their hauls. And again, the weather was fantastic - blue skies and calm seas. Who would have thought it? So much for winter in the Tasman Sea. The folks on board the bottom trawlers probably think we brought the good weather.

We've been spending more time collecting and analysing bycatch - the discarded fish that the trawlers dump back into the sea. There's an assumption that throwing back the fish is 'OK', because the fish can swim off in freedom. Not so - the shock of the kilometre-long journey to the surface, plus the crushing weight and friction inside the net, is more than enough to kill or maim them.

The nets rise to the surface, sometimes visibly full of orange roughy and with little apparent bycatch, while at other times the mesh is bulging with a variety of exotic deep sea animals. Small sharks that have survived the journey from below are desperately trying to thrash free, and have pulled themselves halfway through the net, but are trapped by the other fish. Once on board the trawler, these sharks are removed from the net and often thrown straight overboard, dying, if not already dead. In some hauls they seem to outnumber the roughy. The waiting albatross fall upon them immediately, tearing their stomachs open with hungry beaks and pulling their insides to pieces. We've been finding deflated looking sharks floating on the surface, their internal organs hanging out.

The bycatch we've encountered so far is fairly common to the waters around New Zealand. There have been rattails -large-eyed, pointy-nosed fish that get their names partly from their long, thin tails and rodent-like appearance, and partly from early perceptions that they were scavengers willing to eat anything. We've had ghost sharks, which have long strange cartilaginous snouts, differing from other sharks in having just one gill slit, and in having smooth skin instead of rough dermal denticles (scales to you and me).

There have been lantern sharks - small black sharks with luminous brilliant green eyes - and shovel-nosed sharks. We've come across commercial species, like oreos, floating on the surface, but as they're not a target fish for these trawlers, they are thrown back (dead) - a complete waste. All of these bycatch species have something in common - they are benthopelagic (live on or very near the sea floor) or demersal (living just above the sea floor) - meaning the nets are being towed just above or along the bottom.

Yesterday, we talked about the thousands of discarded orange roughy (Hoplostethus atlanticus) heads floating on the surface, many from fish at least a century old. A few interesting facts about orange roughy: When first captured, roughy was thought to be useless - a fatty layer under their skin caused digestive mayhem in anyone who tried to eat it, earning it the nickname 'diarrhoea fish.' Once a technique was developed for removing this ester layer, however, commercial roughy prospects seemed good, although it remained largely a museum curiosity until the late 1970s. Orange roughy belongs to a group of fish called 'slimeheads' due to the finely sculptured mucous-filled channels around the head. Lovely!

The discarded fish...
Rattails (or grenadiers, family Macrouridae)
Ghost sharks (family Chimaeridae)
Lantern sharks (genus Etmopterus)
Oreos (family Oreosomatidae)

- Kat, marine biologist, and Dave, web editor