Evidence for seasonal cycles in deep-sea fish abundances: A great migration in the deep SE Atlantic?

Rosanna J Milligan,Sofia Pereira Costa,David M Bailey,E Marian Scott,Gilbert T Rowe,Daniel O B Jones,Robert O’Brien,Michael F Vardaro,Jason Chapman,Ken L Smith,Alan J Jamieson,Philippe Susanne,Brian J Bett,Henry A Ruhl

doi:10.1111/1365-2656.13215

Abstract

Animal migrations are of global ecological significance, providing mechanisms for the transport of nutrients and energy between distant locations. In much of the deep sea (>200m water depth), the export of nutrients from the surface ocean provides a crucial but seasonally variable energy source to seafloor ecosystems. Seasonal faunal migrations have been hypothesized to occur on the deep seafloor as a result, but have not been documented. Here, we analyse a 7.5-year record of photographic data from the Deep-ocean Environmental Long-term Observatory Systems seafloor observatories to determine whether there was evidence of seasonal (intra-annual) migratory behaviours in a deep-sea fish assemblage on the West African margin and, if so, identify potential cues for the behaviour. Our findings demonstrate a correlation between intra-annual changes in demersal fish abundance at 1,400m depth and satellite-derived estimates of primary production off the coast of Angola. Highest fish abundances were observed in late November with a smaller peak in June, occurring approximately 4months after corresponding peaks in primary production. Observed changes in fish abundance occurred too rapidly to be explained by recruitment or mortality, and must therefore have a behavioural driver. Given the recurrent patterns observed, and the established importance of bottom-up trophic structuring in deep-sea ecosystems, we hypothesize that a large fraction of the fish assemblage may conduct seasonal migrations in this region, and propose seasonal variability in surface ocean primary production as a plausible cause. Such trophic control could lead to changes in the abundance of fishes across the seafloor by affecting secondary production of prey species and/or carrion availability for example. In summary, we present the first evidence for seasonally recurring patterns in deep-sea demersal fish abundances over a 7-year period, and demonstrate a previously unobserved level of dynamism in the deep sea, potentially mirroring the great migrations so well characterized in terrestrial systems.

Highlights

The deep oceans (>200 m depth) are temporally dynamic and spatially heterogeneous environments that together comprise the largest living space on Earth (Smith, Leo, Bernardino, Sweetman, & Arbizu, 2008)
Our findings demonstrate a correlation between intra-annual changes in demersal fish abundance at 1,400 m depth and satellite-derived estimates of primary production off the coast of Angola
We quantify temporal patterns in demersal fish abundance at two sites on the Angolan continental slope (c. 1,400 m water depth) using 7.5 years of time-lapse photographic data from the Deep-ocean Environmental Long-term Observatory Systems (DELOS; Vardaro et al, 2013). These patterns are compared to corresponding temporal variations in the flux of particulate organic carbon (POC) to the seafloor using methods described in Behrenfeld and Falkowski (1997) and Pace, Knauer, Karl, and Martin (1987)

Summary

| INTRODUCTION

The deep oceans (>200 m depth) are temporally dynamic and spatially heterogeneous environments that together comprise the largest living space on Earth (Smith, Leo, Bernardino, Sweetman, & Arbizu, 2008). The oil and gas industry is moving operations into increasingly deep-ocean regions, for which there are very few baseline datasets available (Cordes et al, 2016; Peterson et al, 2012) Such a lack of ecological data can present a huge challenge to predicting and managing the extent and severity of potential impacts even in relatively well-studied regions. 1,400 m water depth) using 7.5 years of time-lapse photographic data from the Deep-ocean Environmental Long-term Observatory Systems (DELOS; Vardaro et al, 2013) These patterns are compared to corresponding temporal variations in the flux of particulate organic carbon (POC) to the seafloor using methods described in Behrenfeld and Falkowski (1997) and Pace, Knauer, Karl, and Martin (1987)

| MATERIALS AND METHODS

Findings

| DISCUSSION