Functional, size and taxonomic diversity of fish along a depth gradient in the deep sea.

Biodiversity is well studied in ecology and the concept has been developed to include traits of species, rather than solely taxonomy, to better reflect the functional diversity of a system. The deep sea provides a natural environmental gradient within which to study changes in different diversity metrics, but traits of deep-sea fish are not widely known, hampering the application of functional diversity to this globally important system. We used morphological traits to determine the functional richness and functional divergence of demersal fish assemblages along the continental slope in the Northeast Atlantic, at depths of 300-2,000 m. We compared these metrics to size diversity based on individual body size and species richness. Functional richness and size diversity showed similar patterns, with the highest diversity at intermediate depths; functional divergence showed the opposite pattern, with the highest values at the shallowest and deepest parts of the study site. Species richness increased with depth. The functional implications of these patterns were deduced by examining depth-related changes in morphological traits and the dominance of feeding guilds as illustrated by stable isotope analyses. The patterns in diversity and the variation in certain morphological traits can potentially be explained by changes in the relative dominance of pelagic and benthic feeding guilds. All measures of diversity examined here suggest that the deep areas of the continental slope may be equally or more diverse than assemblages just beyond the continental shelf.

The economic implications of changing regulations for deep sea fishing under the European Common Fisheries Policy: UK case study.

Economic impact assessment methodology was applied to UK fisheries data to better understand the implications of European Commission proposal for regulations to fishing for deep-sea stocks in the North-East Atlantic (EC COM 371 Final 2012) under the Common Fisheries Policy (CFP). The aim was to inform the on-going debate to develop the EC proposal, and to assist the UK fishing industry and Government in evaluating the most effective options to manage deep sea fish stocks. Results indicate that enforcing the EC proposal as originally drafted results in a number of implications for the UK fleet. Because of the proposed changes to the list of species defined as being deep sea species, and a new definition of what constitutes a vessel targeting deep sea species, a total of 695 active UK fishing vessels would need a permit to fish for deep sea species. However, due to existing and capped capacity limits many vessels would potentially not be able to obtain such a permit. The economic impact of these changes from the status quo reveals that in the short term, landings would decrease by 6540 tonnes, reducing gross value added by £3.3 million. Alternative options were also assessed that provide mitigation measures to offset the impacts of the proposed regulations whilst at the same time providing more effective protection of deep sea Vulnerable Marine Ecosystems (VMEs). The options include setting a 400m depth rule that identifies a depth beyond which vessels would potentially be classified as fishing for deep sea species and designating 'core areas' for deep sea fishing at depths>400m to minimise the risk of further impacts of bottom fishing gear on deep sea habitats. Applying a 400m depth limit and 'core fishing' area approach deeper than 400m, the impact of the EC proposal would essentially be reduced to zero, that is, on average no vessels (using the status quo capacity baseline) would be impacted by the proposal.

A trait-based metric sheds new light on the nature of the body size-depth relationship in the deep sea.

Variation within species is an often-overlooked aspect of community ecology, despite the fact that the ontogenetic structure of populations influences processes right up to the ecosystem level. Accounting for traits at the individual level is an important advance in the implementation of trait-based approaches in understanding community structure and function. We incorporate individual- and species-level traits into one succinct assemblage structure metric, fractional size, which is calculated as the length of an individual divided by its potential maximum length. We test the implementation of fractional size in demersal fish assemblages along a depth gradient in the deep sea. We use data from an extensive trawl survey at depths of 300-2030 m on the continental slope of the Rockall Trough, Northeast Atlantic, to compare changes in fractional size structure along an environmental gradient to those seen using traditional taxonomic and trait-based approaches. The relationship between fractional size and depth was particularly strong, with the overall pattern being an increase with depth, implying that individuals move deeper as they grow. Body size increased with depth at the intraspecific and assemblage levels. Fractional size, size structure and species composition all varied among assemblages, and this variation could be explained by the depth that the assemblage occupied. The inclusion of individual-level traits and population fractional size structure adds to our understanding at the assemblage level. Fractional size, or where an individual is in its growth trajectory, appears to be an especially important driver of assemblage change with depth. This has implications for understanding fisheries impacts in the deep sea and how these impacts may propagate across depths.

A Scientific Basis for Regulating Deep-Sea Fishing by Depth.

The deep sea is the world's largest ecosystem, with high levels of biodiversity and many species that exhibit life-history characteristics that make them vulnerable to high levels of exploitation. Many fisheries in the deep sea have a track record of being unsustainable. In the northeast Atlantic, there has been a decline in the abundance of commercial fish species since deep-sea fishing commenced in the 1970s. Current management is by effort restrictions and total allowable catch (TAC), but there remain problems with compliance and high levels of bycatch of vulnerable species such as sharks. The European Union is currently considering new legislation to manage deep-sea fisheries, including the introduction of a depth limit to bottom trawling. However, there is little evidence to suggest an appropriate depth limit. Here we use survey data to show that biodiversity of the demersal fish community, the ratio of discarded to commercial biomass, and the ratio of Elasmobranchii (sharks and rays) to commercial biomass significantly increases between 600 and 800 m depth while commercial value decreases. These results suggest that limiting bottom trawling to a maximum depth of 600 m could be an effective management strategy that would fit the needs of European legislations such as the Common Fisheries Policy (EC no. 1380/2013) and the Marine Strategy Framework Directive (2008/56/EC).

Molecular markers reveal spatially segregated cryptic species in a critically endangered fish, the common skate (Dipturus batis).

Many sharks and skates are particularly vulnerable to overfishing because of their large size, slow growth, late maturity and low fecundity. In Europe dramatic population declines have taken place in common skate (Dipturus batis L.), one of the largest demersal fish in regional shelf seas, leading to extirpations from substantial parts of its former range. Here we report the discovery of cryptic species in common skate collected from the northeast Atlantic continental shelf. Data from nuclear microsatellite markers indicated two clearly distinct clades and phylogenetic analysis of mitochondrial DNA sequences demonstrated monophyly of each one of them. Capture locations showed evidence of strong spatial segregation, with one taxon occurring mainly in waters off the southern British Isles and around Rockall, while the other was restricted to more northerly shelf waters. These apparently cryptic species showed overlapping substrate and depth preferences, but distributional limits were closely related to temperature gradients, potentially indicating thermal limits to their distributions. This discovery of hidden diversity within a large, critically endangered marine vertebrate demonstrates how marine biodiversity can be underestimated, even in such a relatively well-studied and heavily exploited region.

Reproductive allocation in Aidablennius sphynx (Teleostei, Blenniidae): females lay more eggs faster when paired with larger males.

Individuals are expected to invest more in current reproductive effort when paired with a partner of higher than average quality. Aidablennius sphynx is an external fertilizing fish with paternal care in which females gain direct benefits from spawning with large males, but often 'make do' with small males. In this study, female reproductive responses to large and small males were investigated. When paired with large males, females spawned more eggs per unit time (i.e., at a faster rate). There was no difference in the size of the eggs spawned by females in relation to partner size. By ovipositing at a faster rate, females may have allocated more reproductive effort to large males. In addition, since small males are known to release far fewer sperm than large males, females may have reduced their spawn rate with small males as a tactic to ensure fertilization.