A Very Large Spawning Aggregation of a Deep-Sea Eel: Magnitude and Status

Alan Williams,Mark Green,Deborah Osterhage,Franziska Althaus,John Pogonoski,Timothy Ryan

doi:10.3390/jmse9070723

Alan Williams, Mark Green + Show 4 more

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https://doi.org/10.3390/jmse9070723

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Abstract

Multiple lines of evidence substantiate the existence of a very large aggregation of the basketwork eel, Diastobranchus capensis, on the small (3 km2) Patience Seamount off southeast Australia. The aggregation appears to be present year-round, but largest in the austral autumn when composed of spawning eels. Twenty eels caught in April 2015 (14 female, 6 male) were all in advanced stages of spawning condition. The eel’s abundance in the aggregation was very high as measured at seamount, local and regional scales. Hydroacoustic measurement of the spawning aggregation’s dimensions (~100 × 1000 m) and conservative counts of 100 s of eels along camera transects of ~1000–2000 m length indicate 10,000 s individual eels may have been present. The absence of other known spawning locations indicates the Patience Seamount is a regional-scale spatial anchor for spawning. The aggregation was protected in a marine park in 2007 following a decades-long impact from bottom trawling, indicating that the population can be expected to stabilise and recover. Monitoring the aggregation’s status, and validating seasonal spawning, provide important opportunities to examine conservation-led recovery in the deep sea as part of Australia’s new national strategy of Monitoring, Evaluation, Reporting and Improvement (MERI) for conservation values within marine parks.

Highlights

Numerous D. capensis were observed in the camera tows on Patience Seamount during each of the three surveys in 2007, 2015 and 2018
More than 25 individual eels were observed in individual images during surveys #1 and #2 (e.g., Figure 2a), and as many as 13 D. capensis individuals counted over a single image polygon during survey #1
The relative abundance of D. capensis along the three camera-transects made in surveys #1 and 2 (March/April) (Figure 3) was very high: standardised counts were 278, 362, 162; counts were lower (17, 123; mean = 70) along two transects made during survey #3 (December) (Figure 3)

Summary

Introduction

Over 1000 species of true eels frequent world oceans [1] from coastal waters to hadal depths [2], but little is known of the spawning ecology of eels. The most studied are the catadromous anguillid eels, known to migrate and spawn in the oceans. Approximate spawning locations, based largely on leptocephalus catches, have been estimated for all temperate and most tropical anguillid species, with the first described over 100 years ago [3]. Marine eels are believed to have a number of spawning strategies indicated by leptocephalus catch, namely long migrations to offshore spawning areas, short migrations to the shelf break or deep water near the continental shelf, or no migration [6]

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