Background and History to the Graveyard Survey

Background and History to the Graveyard Survey

Seamounts, knolls, and hills are prominent features of underwater topography in the New Zealand region. They are often sites of high productivity, and the focus of important commercial fisheries (including the Orange Roughy fishery). However, they can also comprise fragile habitat and it is vital that we start to quanitify what is found there if we are to understand these systems and ultimately conserve and manage them.

Faunal communities on deepwater seamounts in the Orange Roughy depth range are commonly based on extensive coral growths, which are readily impacted by heavy bottom trawl gear. These corals may be long-lived and slow-growing, meaning their recovery from trawling could be slow.

Common changes with trawled ecosystems include a reduction in species diversity, biomass, and number of species. The dominant benthic species can change from large sessile types (e.g. corals, hydroids, sponges) to small opportunistic species, scavengers, and juveniles. The age composition and size structure can change, and there is typically a reduction in habitat complexity.

There have been few studies on the effects of fishing on deepwater seamounts. Koslow et al. (2001) investigated the benthic macrofauna of small seamount features off southern Tasmania, many of which had been trawled in the Orange Roughy fishery. They found strong differences in faunal composition and distribution on fished and unfished seamounts, and concluded that trawling was responsible, although there was a marked depth difference in the fished and unfished seamounts.

In 2001, a similar “compare and contrast” survey was carried out by NIWA on the Graveyard hills. This is a region that has been heavily fished since the early 1990s, but effort has concentrated on a small number of features, which enables study of seamounts in close proximity with similar physical characteristics, that have been fished to varying degrees. Preliminary results of this survey were given by Clark and O’Driscoll (2003), and showed that the unfished seamounts had greater coral cover, and some differences in species composition.