A Scientific Basis for Regulating Deep-Sea Fishing by Depth

The deep sea is the world’s largest ecosystem [1], with high levels of biodiversity [2, 3] and many species that exhibit life-history characteristics that make them vulnerable to high levels of exploitation [4]. Many fisheries in the deep sea have a track record of being unsustainable [5, 6]. In the northeast Atlantic, there has been a decline in the abundance of commercial fish species since deep-sea fishing commenced in the 1970s [7, 8]. Current management is by effort restrictions and total allowable catch (TAC), but there remain problems with compliance [9] and high levels of bycatch of vulnerable species such as sharks [10]. 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) [11] and the Marine Strategy Framework Directive (2008/56/EC) [12]

Distribution and thermal niche of the common skate species complex in the north-east Atlantic

Temperature is one of the most significant variables affecting the geographic distribution and physiology of elasmobranchs. Differing thermal gradients across a species' range can lead to adaptive divergence and differing developmental times, an important consideration for recruitment rates of exploited species. The Critically Endangered common skate (formerly Dipturus batis) has been divided into 2 species, the flapper skate D. intermedius and blue skate D. batis, both of which have undergone dramatic population declines. Here we examine the environmental thermal and geographic distribution of these species, using observations from scientific trawling surveys and recreational angling around the British Isles. As similar-sized specimens of the 2 species can be confused, we validated species identity using molecular genetic techniques. Both species had more extensive geographic ranges than previously reported and different spatial patterns of abundance. The distribution of the blue skate appears to reflect its partiality to thermally less variable and warmer waters, while flapper skate were found in more variable and notably colder areas. The thermal range and current geographic distribution of these species indicate that future projected climate change could have a differential impact on distribution of flapper and blue skate in the north-east Atlantic

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

Resolving taxonomic uncertainty in vulnerable elasmobranchs : are the Madeira skate (Raja maderensis) and the thornback ray (Raja clavata) distinct species?

Skates and rays constitute the most speciose group of chondrichthyan fishes, yet are characterised by remarkable levels of morphological and ecological conservatism. They can be challenging to identify, which makes monitoring species compositions for fisheries management purposes problematic. Owing to their slow growth and low fecundity, skates are vulnerable to exploitation and species exhibiting endemism or limited ranges are considered to be the most at risk. The Madeira skate Raja maderensis is endemic and classified as ‘Data Deficient’ by the IUCN, yet its taxonomic distinctiveness from the morphologically similar and more wide-ranging thornback ray Raja clavate is unresolved. This study evaluated the sequence divergence of both the variable control region and cytochrome oxidase I ‘DNA barcode’ gene of the mitochondrial genome to elucidate the genetic differentiation of specimens identified as R. maderensis and R. clavate collected across much of their geographic ranges. Genetic evidence was insufficient to support the different species designations. However regardless of putative species identification, individuals occupying waters around the Azores and North African Seamounts represent an evolutionarily significant unit worthy of special consideration for conservation management

Otolith chemistry reveals seamount fidelity in a deepwater fish

There are thousands of seamounts (underwater mountains) throughout the world's deep oceans, many of which support diverse faunal communities and valuable fish stocks. Although seamounts are often geographically and bathymetrically isolated from one another, it is not clear how biologically isolated they are from one another. We analysed the chemical signature of the otoliths of a deepwater fish, the roundnose grenadier (Coryphaenoides rupestris) to test the null hypothesis that there is random exchange between individuals from a seamount and other adjacent areas. The fish were sampled on the Scottish west coast, from the Rosemary Bank seamount and two adjacent locations of similar depth, in the same year at roughly the same time of year. We used flow-injection inductively coupled plasma mass spectrometry to measure trace element concentrations from micro-milled portions of the otolith corresponding to adult and juvenile life history stages. The elemental signatures of the fish from the seamount were distinguishable from the fish from the two other areas during both the juvenile and adult life-history phase. We infer that once juveniles settle on the seamount they remain there for the rest of their lives. Evidence for population structure should be factored into exploitation strategies to prevent local depletion and is an important consideration with respect to Rosemary bank being included in a network of Marine Protected Areas around Scotland