Variation in the structure and function of deep-sea fish assemblages with depth and over time

The deep sea is the largest environment on Earth, but has remained relatively under-studied due to its inaccessibility. In recent years however, technological advances have increased our understanding of this globally important system. In this thesis, I add to this understanding by examining fish assemblage structure along the environmental gradient of the continental slope at depths of 300–2000 m and over a time period (1998–2014) following a reduction in fishing pressure from previous levels. I show that body size is an important factor in structuring deep-sea assemblages along a depth gradient and that it increases at least up to 1500 m. A new metric, fractional size, builds on our knowledge of size structure by accounting for both intra- and interspecific variation in body size and also increases with depth. The Large Fish Indicator, the slope of the biomass spectrum and fractional size have increased over time, signifying recovery of the size structure of deep-sea assemblages, but this increase is depth-dependent. I reveal other depth-related changes by linking morphological traits that relate to function, such as caudal fin aspect ratio and gape size, to the shifting dominance of feeding guilds and patterns in functional diversity. I show that despite the uniqueness of deep-sea ecosystems, the general macroecological pattern of increasing regional occupancy with increasing local abundance still applies. I incorporate the all-pervading importance of depth into these abundance–occupancy relationships by calculating occupancy based on depth distribution as well as spatial distribution. This thesis reveals some surprising characteristics of deep-sea assemblages, such as high biodiversity and the ability to recover from fishing pressure. It further highlights the importance of body size in the marine environment and of depth resolution in deep-sea ecology.