Abstract
We evaluate hypotheses for meso-scale spatial structure in an orange roughy (Hoplostethus atlanticus) stock using samples collected during research trawl surveys off the east coast of New Zealand. Distance-based linear models and generalised additive models were used to identify the most significant biological, environmental, and temporal predictors of variability in diet, proportion of stomachs containing prey, standardised weight of prey, fish somatic weight, fish total weight, and reproductive activity. The diet was similar to that observed elsewhere, and varied with ontogeny, depth, and surface water temperature. Smaller sized and female orange roughy in warmer bottom water were most likely to contain food. Fish condition and reproductive activity were highest at distances more than 20 km from the summit of the hills. Trawl survey catches indicated greater orange roughy densities in hill strata, suggesting hill habitat was favoured. However, analyses of feeding, condition, and reproductive activity indicated hill fish were not superior, despite fish densities on hills being reduced by fishing which, in principle, should have reduced intra-specific competition for food and other resources. Hypotheses for this result include: (1) fish in relatively poor condition visit hills to feed and regain condition and then leave, or (2) commercial fishing has disturbed feeding aggregations and/or caused habitat damage, making fished hills less productive. Mature orange roughy were observed on both flat and hill habitat during periods outside of spawning, and if this spatial structure was persistent then a proportion of the total spawning stock biomass would remain unavailable to fisheries targeting hills. Orange roughy stock assessments informed only by data from hills may well be misleading.
Highlights
Orange roughy is a long-lived, low productivity, vulnerable deep-sea fish that has been targeted by industrial deep-sea trawl fisheries worldwide [1,2,3]
Known spatial structuring in the relative abundance of orange roughy, with greater catches on hills than flat habitats, was not explained by their ecology, at least for the parameters included in this study
Despite obvious differences in the physical attributes of the hill and flat habitats, and the fact that more fishing for orange roughy had occurred on hills, the ecology of the two habitats was not notably different for orange roughy in terms of amount and type of prey they consumed
Summary
Orange roughy is a long-lived, low productivity, vulnerable deep-sea fish that has been targeted by industrial deep-sea trawl fisheries worldwide [1,2,3]. The only large-scale commercial fisheries remaining for orange roughy in 2011 are around New Zealand, with the most recent annual catch being about 9200 t, even though many of the New Zealand stocks have been depleted (biomass fished down to below 20% of initial levels [4]). South Island were depleted and closed in 2000 and 2007 respectively; of these the stock on the Challenger Plateau has been estimated to have recovered and was reopened to commercial fishing, on a small scale, in 2010 [4]. The final stock, on the east coast of the North and South Islands (known as the MidEast Coast), was expected to be rebuilding after being depleted in the mid-1990s [5], but a new assessment, in 2011, indicated the rebuild had not occurred and the stock remains depleted [4]
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