Natural and anthropogenic drivers of deep-sea fish populations

• Main Author: Milligan, Rosanna J
• Format: Dissertation
• Language: English
• Published: University of Glasgow 2015
• Subjects: QH301 Biology; QL Zoology

Deep-sea demersal fish are likely to be highly important in structuring deep-sea ecosystems, but a paucity of data means that relatively little is known about the spatial and temporal processes that influence their distributions or how these may change at different scales of observation. As human activities continue to expand into deeper waters, the importance of understanding these processes is becoming increasingly urgent. The oil and gas industry are expanding into deeper waters as coastal oil reserves diminish, but potential long-term effects on the benthos are unknown. Time-lapse photographic data collected from within an active oil field on the Angolan continental slope (tropical eastern Atlantic) detected no significant differences in the community composition of fish compared to a reference site. Significant seasonal changes in the total abundance of demersal fish were detected however, with >20-fold increases in abundance recorded from the oil-field observatory. These results suggest that the deep-sea environment in this region is likely to be highly dynamic over seasonal and decadal timescales, and some possible drivers are discussed. Understanding the spatial distributions of fish is important in understanding their potential ecological roles within an ecosystem and how they may benefit from spatially-explicit management measures. In the bathyal NE Atlantic, the demersal fish communities associated with cold-water corals (CWC) appeared to be influenced by processes operating at multiple spatial scales. At the broadest scales (100s km), depth was a significant predictor of community composition, while habitat type was significant at the finest scales (m). These results highlight the need to account for the effects of scale in observational research and may explain why no consensus has thus far been reached regarding the role of CWC habitats for deep-water fish in the NE Atlantic and provide a possible framework for approaching future deep-water community studies. In abyssal depths, the importance of habitat heterogeneity in structuring fish communities has never been previously studied. Investigations of the spatial distributions of demersal abyssal fish around a small (c. 250 m high) abyssal hill showed that the distribution pattern of the total fish fauna and the two dominant taxa were not significantly different from random. Random distributions are unusual in nature and these results suggest that the environment may be essentially homogeneous to abyssal fish at spatial scales between 100 m – 10 km and suggests no influence of the abyssal hill on the fish fauna. The results highlight the great potential value of autonomous vehicles in extending abyssal research over broader spatial scales. Experimental studies investigating the effects of large-scale ecological processes on deep-water ecosystems are often unfeasible. Mathematical models can provide an alternative methodology, but have not been widely applied to the deep sea. In Chapter 5, a simple mathematical model was developed to explore the effects of large, transient resource pulses on the population dynamics of the abyssal holothurian Amperima rosea. Large, apparently transient increases in the biomass of this species have been observed at the Porcupine Abyssal Plain during two “Amperima events”, but the mechanisms leading to the increases remain unclear. The simulations showed that Amperima biomasses within the observed range of the smaller “Amperima event” could be simulated in some circumstances following the addition of a single, large resource pulse (10000 – 20000 g POC ha-1) to the model, which led to a short-term, transient increase in Amperima biomass before a return to previous levels. None of the simulations produced unstable population dynamics. While the scarcity of empirical data from the PAP means that these results must be treated cautiously, they highlight the potential for temporal changes in food supply to rapidly alter the dynamics of abyssal populations.