Orgy of Eels

Locating an eel spawning ground has been one of the holy grails of ichthyology for many decades. On a recent expedition, we did just that. At a depth of over 1000 metres, in the chilly waters south of Tasmania, we filmed a large aggregation of Basketwork Eels and collected ‘ripe’ specimens.

The story began in February 2007 when an aggregation of Basketwork Eels, Diastobranchus capensis, was filmed at Patience Seamount in the Huon Commonwealth Marine Reserve off southern Tasmania. The survey was guided by reports from knowledgeable commercial fishers who worked in the area before it became a reserve.

Skip forward eight years to April 2015 and you’ll find me and 34 other scientists from eight institutions travelling to Hobart to board the Marine National Facility’s newly commissioned ship, the RV Investigator. We spent one week aboard this 93.9-metre purpose-built research vessel working 12-hour shifts (lucky me, I scored the 2am to 2pm shift).

During the survey, we were particularly keen to follow up the findings of the 2007 expedition, so the ship powered south to Patience Seamount. Would eels be present, and if so, could we capture specimens and assess their reproductive condition?

On the morning of 11 April, an underwater camera was deployed on the seamount. This is an easy thing to say, but think for a moment about the difficulty of filming eels in water over a kilometre deep! Despite the challenges, we obtained excellent footage of a large number of Basketwork Eels gathered on the seamount.

Our next step was to attempt to capture specimens. To our delight, many male and female Basketwork Eels were caught in a beam trawl at a depth just over 1000 m. Careful dissection of these fish revealed that they all had ripe gonads, some of the males shedding milt (sperm). We believe this is the first time that spawning eels have been both filmed in situ and captured. Voyage Leader, CSIRO’s Dr Alan Williams stated “Capture of eels in spawning condition was the missing piece of the jigsaw – it confirmed that we were witnessing a spawning aggregation”.

As usual, answering one question raises many more. Is this an annual event, and how long do these spawning aggregations last? Where do the eels go after spawning? Do they feed during the aggregation? Is Patience Seamount the only place in the region where this species of eel spawns?

Our exciting find was just one outcome of the expedition. Using grabs, trawls, sleds and traps, many specimens of fishes and marine invertebrates were collected and are now lodged in museum collections around the country. They will be available to researchers for years to come.

Confirming that Patience Seamount is a spawning area for Basketwork Eels was really exciting. I can’t help wondering, however, what other strange biological events happen on other seamounts. There are an estimated 30,000 seamounts worldwide, many at depths below 1500 m, and little biological work has been done on the majority of them. I feel like we have only just begun to ‘scratch the surface’.