Comparison of deep-water coral reefs and lithoherms off southeastern USA

Two types of deep-water coral bioherms occur off the coast of southeastern United States: Oculina and Lophelia/Enallopsammia. The deep-water Oculina bioherms form an extensive reef system at depths of 70–100 m along the shelf edge off central eastern Florida. These reefs are comprised of numerous pinnacles and ridges, 3–35 m in height. Each pinnacle is a bank of unconsolidated sediment and coral debris that is capped on the slopes and crest with living and dead colonies of Oculina varicosa, the ivory tree coral. In comparison, deep-water reefs of Lophelia pertusa and Enallopsammia profunda corals occur at depths of 500–850 m (maximum 150-m relief) along the base of the Florida-Hatteras slope in the Straits of Florida. On the western edge of the Blake Plateau off South Carolina and Georgia, 54-m high banks of Enallopsammia and Lophelia occur at depths of 490–550 m, whereas on the eastern edge of the plateau the reefs form structures 146 m in height and at depths of 640–869 m. The geomorphology and functional structure of both the Oculina and Lophelia reefs are similar. North of Little Bahama Bank, at depths of 1000–1300 m, a region of bioherms is dominated by the coral Solenosmilia sp.; Lophelia is reportedly absent. This paper summarizes 25 years of submersible studies on the deep-water Oculina reefs, describes submersible reconnaissance of deep-water Lophelia reefs off the southeastern United States, and contrasts these types of bioherms with the deep-water lithoherms in the Straits of Florida west of the Bahamas.     

Coral reefs in the Southern Barents Sea: habitat description and the effects of bottom fishing

Fifteen Lophelia reefs from offshore to coastal areas off northern Norway were studied using video. Health status of the coral habitat (degree of physical impact, % cover of living tissue, colony size), occurrence of trawl marks and lost fishing gear, height of coral colonies and associated fauna were analysed from 44 video-lines. Fishing impact was more frequent on the offshore reefs (36.5% of the observed areas) than those in the coastal reefs (0.6%). The most visible effects of fishing were broken and displaced coral colonies. At some sites only small scattered fragments of live corals were observed, indicating recent impact. The mean colony height of Lophelia and gorgonian corals at impacted sites was around half the size of those at non-impacted sites. Both species richness and abundance was higher at non-impacted coral habitats compared to impacted. The actinarian Protanthea simplex and unidentified brittlestars were the only taxa with higher abundance on impacted compared with non-impacted habitats. The reefs at the offshore location were protected against bottom trawling in 2009 through the establishment of a marine protected area (MPA), but a general ban against trawling on known coral reefs had already been implemented in 1999. In the MPA, signs of regrowth were observed. Most of the observed damage probably occurred over 10 years earlier. Results show that live and non-impacted cold water coral reefs have an important ecological function by enhancing the local biodiversity and fish abundance. Preventing further damage to impacted reefs may lead to full recovery within a few decades.     

Mapping and imaging deep-sea coral reefs off Norway, 1982–2000

The survey and mapping group (SMG) of Statoil is responsible for all seafloor mapping for pipelines and field development in Statoil. During numerous reconnaissance and pipeline route surveys over large portions of our contintental shelf, in the North, Norwegian and Barents Seas, we have only detected deep-water coral reefs in a few specific areas. The first reef we found was in 1982, using a combined side scan sonar and sub-bottom profiler system off Fugløy, northern-Norway. Thereafter, numerous reefs were mapped with similar systems off mid-Norway, during the period 1985–1990 for the reconnaissance and final route mapping of the Haltenpipe project. During 1997, we also mapped some previously known reefs in the Trondheimsfjord area while surveying a route for the Tjeldbergodden - Skogn pipeline project. Between 1997 and 2000, we have mapped more reefs along pipeline routes on the outer mid-Norway continental shelf. These reefs are of a smaller size (less than 5 m high) than those mentioned above (5–31 m high). Although side scan sonar and sub-bottom profilers provide a less ambiguous detection of reefs, we have found that modern multi-beam echosounder data can provide adequate remotely sensed data for deep-water coral reef mapping. The interpretation of the reefs is based on visual documentation by ROV (remotely operated vehicle) or by sampling with gravity corers and grabs (ground-truthing). Based on a limited amount of such ground-truthing, it has been possible to extrapolate and use the specific morphological characteristics of the reefs to map their density and distribution. For the Haltenpipe project, we mapped an offshore route corridor of about 200 km length at a width of 3 km with multi-beam echosounder. On the basis of ground-truthing 14 of the suspected coral reefs, we have found the total number of reefs to be 57 within the 600 km² mapped area. All these reefs are higher than 5 m (the highest is 31 m) and of diameters at their base of more than 50 m. Although they occur in local clusters with up to 10 reefs per km², the mean density of reefs along the entire (200 km long) transect is only 0.09 suspected reefs per km². However, there is a large regional density variation, with the highest regional density being 1.2 reefs per km² in an area of subcropping Palaeocene sedimentary rocks. A brief discussion of why the corals have constructed their reefs in the deep, cool, and generally `hostile' waters of the Norwegian continental shelf and fjords concludes with them probably subsisting on a reliable and steady nutrient source, independent of season and variations in the Atlantic Drift (`Gulf stream'). This positive environmental component is called `hydraulically active substrata'. It is thus speculated that micro-organisms, bacteria etc., utilizing the hydraulically activated chemical porewater gradients, cause a local enrichment on which the cnidarian organisms ultimately depend.     

The deep-water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts

The paper presents documentation on the distribution of, and damages to, deep-water reefs of the coral Lophelia pertusa in Norwegian waters. The reef areas have traditionally been rich fishing grounds for long-line and gillnet fisheries, and the coral habitat is known to support a high diversity of benthic species. Anecdotal reports claim that trawlers often use the gear, wires, chains and trawl doors to crush the corals and clear the area before fishing starts. To get an overview of the situation, information about the distribution and damage were collected from the literature, fishermen, and our own investigations. The results show that the corals are abundant particularly on the mid Norwegian continental shelf between 200 and 400 m depth. In general it seems that the largest densities are distributed along the continental break and at ridges of morainic origin. The reports from fishermen suggested severe damage to the corals and in situ observations using ROV confirmed the presence of mechanically damaged corals located on trawling grounds. A first estimate of the fishery impact indicates that between 30 and 50% of the reef areas are damaged or impacted. Fishermen claim that catches are significantly lowered in areas where the reefs are damaged. Potential ecological consequences of the destruction are discussed.     

Reduced carbon emissions and fishing pressure are both necessary for equatorial coral reefs to keep up with rising seas

A rapid increase in sea-level rise is generating vertical accommodation space on modern coral reefs. Yet increases in sea-surface temperatures (SSTs) are reducing the capacity of coral reefs to keep up with sea-level rise. We use ensemble species distribution models of four coral species (Porites rus, Porites lobata, Acropora hyacinthus and Acropora digitifera) to gauge potential geographic differences in gross carbonate production. Net carbonate production was estimated by considering erosional rates of ocean acidification, increasing cyclone intensity, local pollution, fishing pressure and the projected burdens of increases in SSTs (under Representative Concentration Pathways (RCPs) 4.5, 6.0 and 8.5) through to the year 2100. Our models predict that only 4 ± 0.1% (~60 000 km²) of Indo-Pacific coral reefs are projected to keep up with sea-level rise by the year 2100 under RCP 8.5 – most of which will be located near the Equator. However, with drastic reductions in emissions (under RCPs 4.5 and 6.0 Wm⁻²), we predict that 15 ± 0.3% (~250 000 km²) (under RCP 4.5 Wm⁻²) and 12 ± 0.7% (~200 000 km²) (under RCP 6.0 Wm⁻²) of Indo-Pacific coral reefs, have the potential to keep up with sea-level rise by the year 2100. Yet the burdens of fishing pressure and its cascading effects are projected to be responsible for substantial reef erosion, nearly halving the number of reefs able to keep up with sea-level rise. If action is taken immediately and emissions are drastically reduced to RCPs 4.5 or 6.0 Wm⁻², and reef management reduces the burdens of local pollution and fishing pressure, then our model predicts that 21–27% (~350 000–470 000 km²) of Indo-Pacific coral reefs – most of which will be located near the Equator – would have the potential to keep up with sea-level rise by the year 2100.     

A new laboratory method for monitoring deep-water coral polyp behaviour

Deep-water corals are found along the oceanic margins world-wide and in the north east Atlantic the most abundant species is Lophelia pertusa (L.). There is now growing evidence that deep-water reefs formed by such species are coming under increasing pressures from resource exploitation, principally deep-sea trawling and hydrocarbon exploration. Here a novel and unobtrusive method of recording deep-water coral behaviour in the laboratory is described using time-lapse video to record silhouettes of the polyps under infrared illumination. The polyps of L. pertusa behaved asynchronously and did not show any clear diurnal patterns over a three-day observation period. Conceptually, sessile benthic suspension feeders appear to be vulnerable to smothering by sediments disturbed by bottom trawls or sub-seabed drilling. This method allows deep-water coral polyp behaviour to be continuously monitored in the laboratory and, therefore, the responses of coral polyps to environmental perturbations such as sedimentation can be recorded. Further work is necessary to resolve the sensitivity of deep-water corals to short-term environmental change and the combined approach of in situ monitoring and subsequent laboratory experimentation has great potential to address these issues.     

The effect of scallop dredging on Irish Sea benthos: experiments using a closed area

A 2 km² area off the southwest coast of the Isle of Man (Irish Sea) has been closed to commercial fishing with mobile gear since March 1989. This area was heavily fished for Pecten maximus (Linnaeus, 1758) prior to closure, and the seabed immediately surrounding the closed area is still one of the most heavily dredged in the Irish Sea. Two methods have been used to study the effect of scallop dredging on the benthos in this closed area and adjacent fished areas. Firstly, twice-yearly grab sampling of experimental plots inside and outside the closed area since 1995 has enabled comparisons of the benthic infauna and epifauna of experimentally dredged plots, undredged control plots and plots exposed to commercial dredging. Secondly, divers have carried out visual transect surveys of P. maximus numbers regularly since closure. Communities of experimentally disturbed plots have become less similar to adjacent undisturbed control areas and more similar to commercially dredged areas. At each sampling date, similarity between dredged samples was greater than between undredged samples. Since 1989, there have been increases in the mean numbers of P. maximus in the closed area. The age structure of the closed area P. maximus population is also different to that outside, with a higher mean age due to the presence of large, old individuals.These results present strong evidence that scallop dredging alters benthic communities and suggest that the closure of areas to commercial dredging may allow the development of more heterogeneous communities and permit the populations of some species to increase. A common problem with studying fishing disturbance is the lack of good control sites and this work also demonstrates the value of closed areas to scientific studies of demersal fishing.     

The Biology of Lophelia pertusa (Linnaeus 1758) and Other Deep-Water Reef-Forming Corals and Impacts from Human Activities.

Over the last twenty years, human exploitation has begun to have an impact in the deep sea, especially in the upper bathyal zone. This has mainly taken the form of deep-sea fishing but more recently oil exploration has extended beyond the continental shelf. Deep-water coral reefs occur in the upper bathyal zone throughout the world. These structures, however, are poorly studied with respect to their occurrence, biology and the diversity of the communities associated with them. In the North-East Atlantic the coral Lophelia pertusa has frequently been recorded. The present review examines the current knowledge on L. pertusa and discusses similarities between its biology and that of other deep-water, reef-forming, corals. It is concluded that L. pertusa is a reef-forming coral that has a highly diverse associated fauna. Associated diversity is compared with that of tropical shallow-water reefs. Such a highly diverse fauna may be shared with other deep-water, reef-forming, corals though as yet many of these are poorly studied. The main potential threats to L. pertusa in the North-East Atlantic are considered to be natural phenomena, such as slope failures and changes in ocean circulation and anthropogenic impacts such as deep-sea fishing and oil exploration. The existing and potential impacts of these activities on L. pertusa are discussed. Deep-sea fishing is also known to have had a significant impact on deep-water reefs in other parts of the world.     

Hydroacoustic monitoring of the distribution,density and the mass-removal of pelagic fish in a eutrophic lake

A large scale biomanipulation of pelagic fish by trawling was started in the eutrophic Enonselkä basin (26 km²) of Lake Vesijärvi to improve the water quality which had remained poor in spite of the termination of nutrient loading. The distribution and density of the fish were studied by hydroacoustics before and during the removal. The initial annual fish density varied between 13 000–21 000 fish ha^–1 in the study area in August 1984–89. The mass-removal of the fish by pelagic trawling took place in 1989–1992. The catch varied annually between 64 and 92 kg ha^–1. Roach (Rutilus rutilus (L.)) and smelt (Osmerus eperlanus (L.)) accounted for c. 85% of the weight of the catch. The mass-removal decreased the pelagic fish density in the Enonselkä basin during the trawling. An increase in the density was observed after the trawling ceased in these years, and the initial density level was reached within one month. The density level after mass-removal remained high compared with oligotrophic lakes. The pelagic fish had a diurnal ascending trend with the decreasing light intensity in August, and the fish were significantly (p < 0.01) higher in the water mass in temperature non-stratified water in autumn than in stratified water in summer. The fish were somewhat deeper in the years of mass-removal than before it. Mass-removal did not affect the individual length of the echosurveyed fish. Fish smaller than c. 15 cm (TS < –44 dB; mainly smelt) were numerically dominant throughout the whole study period.     

Comparative demography of commercially important species of coral grouper,Plectropomus leopardus and P. laevis,from Australia's great barrier reef and Coral Sea marine parks

Understanding the spatial and environmental variation in demographic processes of fisheries target species, such as coral grouper (Genus: Plectropomus), is important for establishing effective management and conservation strategies. Herein we compare the demography of Plectropomus leopardus and P. laevis between Australia's Great Barrier Reef Marine Park (GBRMP), which has been subject to sustained and extensive fishing pressure, and the oceanic atolls of Australia's Coral Sea Marine Park (CSMP), where there is very limited fishing for reef fishes. Coral grouper length-at-age data from contemporary and historical otolith collections across 9.4 degrees of latitude showed little difference in lifetime growth between GBRMP and CSMP regions. Plectropomus laevis populations in GBRMP reefs had significantly higher rates of total mortality than populations in the CSMP. Mean maximum lengths and mean maximum ages of P. laevis were also smaller in the GBRMP than in the CSMP, even when considering populations sampled within GBRMP no-take marine reserves (NTMRs). Plectropomus leopardus, individuals were on average smaller on fished reefs than NTMRs in the GBRMP, but all other aspects of demography were broadly similar between regions despite the negligible levels of fishing pressure in the CSMP. Similarities between regions in growth profiles and length-at-age comparisons of P. laevis and P. leopardus suggest that the environmental differences between the CSMP and the GBRMP may not have significant impacts on lifetime growth. Our results show that fishing may have influenced the demography of coral grouper on the GBR, particularly for the slower growing and longer lived species, P. laevis.     

Trawling damage to Northeast Atlantic ancient coral reefs

This contribution documents widespread trawling damage to cold-water coral reefs at 840-1300 m depth along the West Ireland continental shelf break and at 200 m off West Norway. These reefs are spectacular but poorly known. By-catches from commercial trawls for deep-water fish off West Ireland included large pieces (up to 1 m(2)) of coral that had been broken from reefs and a diverse array of coral-associated benthos. Five azooxanthellate scleractinarian corals were identified in these by-catches, viz. Desmophyllum cristagalli, Enallopsammia rostrata, Lophelia pertusa, Madrepora oculata and Solenosmilia variabilis. Dating of carbonate skeletons using (14)C accelerator mass spectrometry showed that the trawled coral matrix was at least 4550 years old. Surveys by remotely operated vehicles in Norway showed extensive fishing damage to L. pertusa reefs. The urgent need for deep-water coral conservation measures is discussed in a Northeast Atlantic context.     

Distribution and size of capercaillie leks in relation to old forest fragmentation

Summary Distribution and size of 38 capercaillie Tetrao urogallus leks were related to amount and configuration of old forest patches in two south-east Norwegian coniferous forests. The smallest occupied patch was 48 ha containing a solitary displaying cock. All patches larger than 1 km² contained leks. Number of cocks per lek increased with increasing patch size. Number of leks per patch increased in a step-wise manner with one lek added for each 2.5–3 km² increase in patch size. In large patches there was one lek per 3–5 km² old forest, and density of lekking cocks was 2–2.5 per km². In small patches density of cocks varied considerably. Density of cocks was not related to patch isolation or patch shape. However, among leks surrounded by 50–60% old forest within a 1 km radius, number of cocks increased with increasing old forest fine-graininess. We argue that when old forests cover more than 50%, a fine-grained mosaic may support higher densities of lekking cocks than a coarse-grained mosaic. Conversely, when old forests cover less than 50%, a fine-grained mosaic is unfavourable, because each old forest patch becomes too small and isolated. Finally, we present a predictive model of how old forest fragmentation influences density of leks, number of cocks per lek, and total density of cocks.     

Age,growth and radiometric age validation of a deep-sea,habitat-forming gorgonian (Primnoa resedaeformis) from the Gulf of Alaska

Sustainable fisheries require (1) viable stock populations with appropriate harvest limits and (2) appropriate habitat for fish to survive, forage, seek refuge, grow and reproduce. Some deep-water habitats, such as those formed by deep-water stands of coral, may be vulnerable to fishing disturbance. The rate at which habitat can be restored is a critical aspect of fishery management. The purpose of this study was to characterize growth rates for a habitat-forming deep-sea coral. Two nearly complete colonies of red tree coral (Primnoa resedaeformis) collected from waters off southeast Alaska were used for an analysis of age and growth characteristics. CAT scans revealed that colonies consisted of multiple settlement events, where older basal structures provided for settlement of new colonies. The decay of ²¹⁰Pb over the length of the colony was used to validate age estimates from growth ring counts. Age estimates were over 100 yr for sections near the heavily calcified base. Based on validated growth ring counts, growth of red tree coral ranged from 1.60 to 2.32 cm per year in height and was approximately 0.36 mm per year in diameter. These growth rates suggest that the fishery habitat created by red tree coral is extremely vulnerable to bottom fishing activities and may take over 100 years to recover.     

Seaweed succession on artificial reefs on different bottom substrata

Artificial rest reefs were set on sandy and rocky bottoms at 5–10 m depth along the coast of southern Japan. Mature thalli ofSargassum, Gelidium and other seaweds were transported from other coastal areas, packed in mesh bags and attached to the reefs to start the beds. After one year, the seaweed flora on the reef on a sandy bottom consisted of more than 20 species, includingSargassum spp. andGelidium amansii, which are important animal food species. Coralline algae were the dominants on the rocky bottom reefs. The lower biomass on reefs on the rocky bottom was due to grazing by urchins. The same number of species was present in the first and second years on reefs on sandy bottoms, but there were moreSargassum thalli the second year.Maximum algal biomass of the artificial reef in May of the second year was 9998 g wet wt m^–2 in sandy areas, 441 g wet wt m^–2 in boulder areas and 228 g wet wt m^–2 in rocky areas. Reefs on rocky bottoms continued to be covered by coralline algae and several species ofCodium andDictyota.     

A preliminary survey of the artisanal fishery on coral reefs of the Tulear Region (southwest Madagascar)

Fish catches from the coral reefs of the Tulear region (southwest Madagascar), are analyzed based on fish landings. This region of the island consists of two barrier reefs, two coral banks, three lagoon reefs and a fringing reef. The total reef area studied was 190 km². Of the whole fishing area, the reef flat was the most frequently used by fishermen. Line catches per unit efort (CPUE) were stable throughout the eight month sampling period (6 to 8 kg/trip to sea), whereas gillnet and seine catches showed differences between the cold period and the warm period. These results appear to be representative of the artisanal fishery catches in the southwest Indian Ocean. Annual fish yield was estimated at 12 t km^-2 yr^-1. Comparable yields have been recorded in certain regions of the Indo-Pacific, that have similar ratios of coral reef area to adjacent shallows and similar fishing practices. Reef species (Lethrinidae, Siganidae) dominated the catches. At present, coastal waters of the Tulear region are heavily fished, and the fishermen report a progressive decrease in the average size of fish caught over the last 15 years. Management measures are suggested, based on preliminary findings.     

Catch Composition on Red Shrimps’ (Aristaeomorpha foliacea and Aristeus antennatus) Grounds in the Eastern Ionian Sea

In the present study, the catch composition and the catch per unit effort (CPUE) by weight and numbers in red shrimps’ (Aristaeomorpha foliacea and Aristeus antennatus) grounds was examined in the southern part of the eastern Ionian Sea, in order to collect important information for the Greek waters, where no deep-water fishery exists. In the depth stratum 500–700 m, the catch of the commercial species represented a high proportion (>70%) of the total catch. Red shrimps and several other commercial species were found in important quantities. The present results suggest the possibility of developing a deep-water fishery in Greece. In such a case, attention should be paid because of the high vulnerability of A. foliacea – the main deep-water fishing resource in the area – to the fishing pressure.     

河北唐山曹妃甸-乐亭海域自然牡蛎礁分布及生态意义

牡蛎礁是生态系统服务价值最高的海洋生境之一,目前我国自然牡蛎礁分布和生态现状的基础信息仍然较缺乏。于2019年3月对河北唐山曹妃甸-乐亭海域自然牡蛎礁的空间分布、生态环境、牡蛎生物学和礁体动物群落开展了调查,并评估了该牡蛎礁的生态系统服务价值。该海域自然牡蛎礁分布于溯河(SR)、溯河口海域(SRE)和捞鱼尖海域(LYJ),总面积约15 km~2,是目前我国面积最大的自然牡蛎礁。基于96个牡蛎样品的16S rDNA检测,共识别出92个长牡蛎Crassostrea gigas、3个侏儒牡蛎Nanostrea fluctigera和1个巨蛎属未知种Crassostrea sp.。自然牡蛎礁中牡蛎平均密度介于104—3912个/m~2之间,不同礁区间牡蛎平均密度的大小排序为：SRE>SR>LYJ (P<0.05),平均生物量的大小排序为：SR>SRE>LYJ (P<0.05),平均壳高的大小排序为：SR>SRE=LYJ (P<0.05)。在该牡蛎礁内记录到49种礁体动物,其中软体动物16种、节肢动物16种、环节动物8种、棘皮动物5种、腔肠动物2种...     

Energy flow to two abundant consumers in a subtropical oyster reef food web

Oyster reefs are among the most threatened coastal habitat types, but still provide critical habitat and food resources for many estuarine species. The structure of oyster reef food webs is an important framework from which to examine the role of these reefs in supporting high densities of associated fishes. We identified major trophic pathways to two abundant consumers, gray snapper (Lutjanus griseus) and crested goby (Lophogobius cyprinoides), from a subtropical oyster reef using stomach content and stable isotope analysis. The diet of gray snapper was dominated by crabs, with shrimp and fishes also important. Juvenile gray snapper fed almost entirely on oyster reef-associated prey items, while subadults fed on both oyster reef- and mangrove-associated prey. Based on trophic guilds of the gray snapper prey, as well as relative δ¹³C values, microphytobenthos is the most likely basal resource pool supporting gray snapper production on oyster reefs. Crested goby had omnivorous diets dominated by bivalves, small crabs, detritus, and algae, and thus were able to take advantage of prey relying on production from sestonic, as well as microphytobenthos, source pools. In this way, crested goby represent a critical link of sestonic production to higher trophic levels. These results highlight major trophic pathways supporting secondary production in oyster reef habitat, thereby elucidating the feeding relationships that render oyster reef critical habitat for many ecologically and economically important fish species.     

The reproductive biology of two deep-water,reef-building scleractinians from the NE Atlantic Ocean

The reproductive ecology of colonies of Lophelia pertusa (Linné 1758) and Madrepora oculata Linné (1758) from the Porcupine Seabight (Thérèse Mound and South Porcupine Seabight site) and the Darwin Mounds (NE Rockall Trough—L. pertusa only) was investigated using histological techniques. Samples of L. pertusa exhibited seasonal reproduction, whereas the evidence for M. oculata is equivocal but suggests multiple cohorts of gamete production. L. pertusa produces a single cohort of around 3,000 oocytes, whereas M. oculata produces two cohorts, with a total fecundity of around 60 oocytes. The maximum observed oocyte size in L. pertusa was 140 μm and in M. oculata was 405 μm. From these oocyte sizes and the timing of reproduction, a lecithotrophic larva is expected, though not observed. This seasonality of reproduction fits with the phytodetrital food fall occurring around July in the Seabight area. L. pertusa was found to be non-reproductive at the Darwin Mound site. Though unable to be specifically tested, this may suggest that the increased trawling activity in this area might be keeping colonies below sexually viable sizes, as seen in numerous shallow water situations. All areas in the NE Atlantic are coming under threat from increased fishing and commercial exploration practices. This study shows that these highly seasonal reproducers could be sensitive to these fishing operations and care must be taken so as not to repeat the destruction that has occurred on shallower reefs.     

Impact of Bottom Trawling on Deep-Sea Sediment Properties along the Flanks of a Submarine Canyon

The offshore displacement of commercial bottom trawling has raised concerns about the impact of this destructive fishing practice on the deep seafloor, which is in general characterized by lower resilience than shallow water regions. This study focuses on the flanks of La Fonera (or Palamós) submarine canyon in the Northwestern Mediterranean, where an intensive bottom trawl fishery has been active during several decades in the 400–800 m depth range. To explore the degree of alteration of surface sediments (0–50 cm depth) caused by this industrial activity, fishing grounds and control (untrawled) sites were sampled along the canyon flanks with an interface multicorer. Sediment cores were analyzed to obtain vertical profiles of sediment grain-size, dry bulk density, organic carbon content and concentration of the radionuclide ²¹⁰Pb. At control sites, surface sediments presented sedimentological characteristics typical of slope depositional systems, including a topmost unit of unconsolidated and bioturbated material overlying sediments progressively compacted with depth, with consistently high ²¹⁰Pb inventories and exponential decaying profiles of ²¹⁰Pb concentrations. Sediment accumulation rates at these untrawled sites ranged from 0.3 to 1.0 cm y⁻¹. Sediment properties at most trawled sites departed from control sites and the sampled cores were characterized by denser sediments with lower ²¹⁰Pb surface concentrations and inventories that indicate widespread erosion of recent sediments caused by trawling gears. Other alterations of the physical sediment properties, including thorough mixing or grain-size sorting, as well as organic carbon impoverishment, were also visible at trawled sites. This work contributes to the growing realization of the capacity of bottom trawling to alter the physical properties of surface sediments and affect the seafloor integrity over large spatial scales of the deep-sea.