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

Global Patterns of Extinction Risk in Marine and Non-marine Systems

SummaryDespite increasing concern over the effects of human activities on marine ecosystems [1, 2], extinction in the sea remains scarce: 19–24 out of a total of >850 recorded extinctions [3, 4] implies a 9-fold lower marine extinction rate compared to non-marine systems. The extent of threats faced by marine systems, and their resilience to them, receive considerable attention [2, 4–6], but the detectability of marine extinctions is less well understood. Before its extinction or threat status is recorded, a species must be both taxonomically described and then formally assessed; lower rates of either process for marine species could thus impact patterns of extinction risk, especially as species missing from taxonomic inventories may often be more vulnerable than described species [7–11]. We combine data on taxonomic description with conservation assessments from the International Union for Conservation of Nature (IUCN) to test these possibilities across almost all marine and non-marine eukaryotes. We find that the 9-fold lower rate of recorded extinctions and 4-fold lower rate of ongoing extinction risk across marine species can be explained in part by differences in the proportion of species assessed by the IUCN (3% cf. 4% of non-marine species). Furthermore, once taxonomic knowledge and conservation assessments pass a threshold level, differences in extinction risk between marine and non-marine groups largely disappear. Indeed, across the best-studied taxonomic groups, there is no difference between marine and non-marine systems, with on average between 20% and 25% of species being threatened with extinction, regardless of realm

Sized-based indicators show depth-dependent change over time in the deep sea

Size-based indicators are well established as a management tool in shelf seas as they respond to changes in fishing pressure and describe important aspects of community function. In the deep sea, however, vital rates are much slower and body size relationships vary with depth, making it less clear how size-based indicators can be applied and whether they are appropriate for detecting changes through time. The deep-sea fish stocks of the North Atlantic underwent a period of exploitation followed by management and conservation action that relieved this pressure. We used data from a deep-water bottom trawl survey in the Rockall Trough, at depths of 300–2000 m, to test whether size-based indicators changed over a 16-year period, during which fishing pressure decreased. We applied four indicators to these data: mean body length, mean maximum length, large fish indicator (LFI) and the slope of the biomass spectrum. Patterns were analysed within four different depth bands. The LFI and slope of the biomass spectrum showed positive change over time, suggesting recovery from fishing pressure. This response was generally most apparent in the shallowest depth band, where most fishing activity has been distributed. Values of the LFI were much higher overall than in shelf seas, so the same reference points cannot be applied to all marine ecosystems. These findings imply that size-based indicators can be usefully applied to the deep sea and that they potentially track changes in fishing pressure in the medium term