Abstract
On abyssal plains, demersal fish are believed to play an important role in transferring energy across the seafloor and between the pelagic and benthic realms. However, little is known about their spatial distributions, making it difficult to quantify their ecological significance. To address this, we employed an autonomous underwater vehicle to conduct an exceptionally large photographic survey of fish distributions on the Porcupine Abyssal Plain (NE Atlantic, 4850 m water depth) encompassing two spatial scales (1–10 km2) on and adjacent to a small abyssal hill (240 m elevation). The spatial distributions of the total fish fauna and that of the two dominant morphotypes (Coryphaenoides sp. 1 and C. profundicolus) appeared to be random, a result contrary to common expectation but consistent with previous predictions for these fishes. We estimated total fish density on the abyssal plain to be 723 individuals km−2 (95% CI: 601–844). This estimate is higher, and likely more precise, than prior estimates from trawl catch and baited camera techniques (152 and 188 individuals km−2 respectively). We detected no significant difference in fish density between abyssal hill and plain, nor did we detect any evidence for the existence of fish aggregations at any spatial scale assessed.
Highlights
Determining how abyssal fish are distributed would provide valuable data to better understand their role in the ecosystem, and infer which ecological processes may be significant at particular spatial scales
Abyssal hills are estimated to be the dominant landform on Earth[24], with Wessel et al.[25] speculating that there may be as many as 25 million uncharted abyssal hills (>100 m high) in the global ocean based on the size-frequency distributions of 13,000 known seamounts
The fish fauna was dominated by the macrourids Coryphaenoides profundicolus and Coryphaenoides sp. 1 that comprised 41.1% and 37.2% of the total fish density based on the oblique images and 42.4% and 40.5% based on the vertical images
Summary
It is possible that even relatively small hills may enhance local food resources and/or secondary productivity, and so influence the distribution of abyssal fish. The present study was designed to investigate the composition and spatial distribution patterns of the abyssal fish assemblage on the PAP, both in the vicinity of a small 240 m high) abyssal hill, and over the level seafloor of the PAP benthic long-term (30-year) study site[27] These objectives have only recently become achievable through the development of autonomous robotic vehicles capable of full ocean depth operations[28] we employed the Autosub6000 Autonomous Underwater Vehicle (AUV) to conduct a spatially explicit, ultra-large-scale photographic transect survey We use the resultant data to establish: (a) the best estimate of ‘true’ abyssal demersal fish density available to date; (b) the influence of survey method on apparent fish species composition and density; (c) the impact of abyssal hill topography on fish populations; and (d) the first assessment of natural spatial dispersion pattern in abyssal demersal fish
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