Spatial variability of orange roughy around the Northwest Hills on the Chatham Rise, New Zealand

Abstract

We used research trawl survey data to describe the small-scale distribution and spatial continuity of orange roughy catch rates around the Northwest Hills complex of seamount features on the Chatham Rise to the east of New Zealand. Analyses revealed three distinct spatial patterns. The larger scale spatial structure in the catch rates was defined by the contours running east–west along the Chatham Rise. Orange roughy were caught in a depth band between 850 and 1200 m , a depth distribution that is well known. Superimposed on this larger scale structure was a north–south trend in the highest catch rates at 180 ∘ longitude. This was created by higher catch rates on the seamounts. There were large differences in catch rates at small separation distances due to the seamount effect. However, not all the high catches were associated with seamounts, some being taken on the ‘flat’ areas around the hills. At distances away from the seamounts greater than the decorrelation length scale the spatial distributions of orange roughy catch rates were not random, and did not conform to a Poisson process used to characterise random distributions. Data visualisation of the spatial distributions of orange roughy and groups of bycatch revealed strong patchiness and spatial separation by depth of swimbladder fish, elasmobranchs and squid and oreos. This has important implications for acoustic surveys. Orange roughy overlapped most closely with swimbladder fish distributions, suggesting they may be obscured by species with higher acoustic target strengths. The observed separation of acoustically distinct fish groups by depth makes ground-truthing of acoustic backscatter easier. However, patchiness complicates prediction of where the acoustically distinct fish groups will occur. We conclude that extensive trawling provides a necessary complement to acoustic survey in order to estimate the biomass of orange roughy.