2.4 FAO Statistical Area 34 - West Africa
2.4.1 Survey data
220.127.116.11 West African Slope
Golovan (1978) reviewed the results of several Russian research surveys carried out on the continental slope of West Africa between 32° and 4° N. and at depths between 500 and 1920m. He recorded one species of the family Chlamydoselachidae, six species of the family Scyliorhinidae, 15 species of the family Squalidae and one species each of the families Scymnorhinidae and Echinorhinidae.
Research surveys on the continental slope off West Africa using an epibenthic sledge and a small otter trawl indicated the presence of Apristurus spp., Etmopterus princeps, Etmopterus spinax, Centroscymnus coelolepis and Centroscymnus crepidater (Merrett and Domanski 1985).
18.104.22.168 Canary Islands
Ublein et al. (1998) give details of the depth distribution of Galeorhinus galeus, Squalus megalops, Galeus melastomus, Centrophorus lusitanicus, Centrophorus granulosus, Etmopterus pusillus, and Deania calcea from longline surveys around the Canary Islands. The squalene oil content of the livers of the following deep-water sharks caught in a longline survey of the Canary islands has been determined: Centroscymnus coelolepis, Centroscymnus cryptacanthus, Centrophorus squamosus, Centrophorus lusitanicus, Deania histricosa, Deania profundorum, and Etmopterus princeps (Hernandez-Perez et al. 1997).
2.4.2 The fisheries
Muñoz-Chápuli (1985) has analysed the catches of demersal sharks from trawlers operating between about 27°–37°N off the Moroccan coast and around the Canary Islands. The species present and their abundance in each of three areas are shown in Table 32 Area A is between latitudes 27°– 31°N and includes the Canary Islands, area B is the Moroccan Coast between 31°–34°N and area C is between 34°–37°N). Only Squatina aculeata and Mustelus asterias were not found deeper than 200m. Otherwise there are apparently no published statistics that would enable deep-water demersal shark landings to be identified separately from other coastal and pelagic sharks.
2.5 FAO Statistical Area 47 - Southeast Atlantic
Compagno et al. (1991) have provided a detailed account of the distribution of offshore cartilaginous fishes of the west coast of southern Africa. Depth distributions and an indication of abundance from survey data are given for 32 species, most of which are truly demersal species. Sharks are undoubtedly caught as a bycatch of the hake fisheries and Compagno et al. (1991) have shown that the diversity indices for cartilaginous fishes are much lower at depths and areas where hake dominate the biomass.
2.6 FAO Statistical Areas 48, 58 and 88 (Southern Ocean - CCAMLR areas)
The only elasmobranch fishes included in the CCAMLR reporting system are the rays. There are only three known sharks, Lamna nasus, Somniosus pacificus and Etmopterus sp. (Kock et al. 1985, Stevens Pers. Comm.). These were reported at depths between 100 and 500m off the Kerguelen Islands and elsewhere in the Southern Ocean.
On the slope of the Kerguelen shelf, rays, mainly Bathyraja eatoni and B. irrasa, are a common bycatch from the long-line fishery and sometimes also occur as bycatch in the trawl fishery. Three species of sharks are commonly recorded in the fishery: Lamna nasus in the icefish fishery (shelf depth of 150–250m, Somniosus microcephalus in the Patagonian toothfish fishery, both longline and trawls, from the slope and deepsea, from 300 to 900m and probably deeper. A more rare shark, Etmopterus of granulosus is taken in the deep-sea longline fishery (1000 – 1200m). In the Crozet Island fishery a ray, Raja taaf, is a common bycatch in the longline fishery. The sharks appear to be uncommon but S. microcephalus has been recorded as bycatch from the longline fishery.
2.7 FAO Statistical Area 513 - Western Indian Ocean
Much work on the deepwater elasmobranch resources of the Indian Ocean has been done be research vessels of the Southern Scientific Research Institute of Marine Fisheries, Kerch, Ukraine and not all the information available at that institute in the YugNIRO database has yet been published. As is indicated, the information in this section generally refers to depths greater than 450m.
In the northern Gulf of Aden not less than 10 species of deepwater sharks occur. Among these are:
Heptranchias perlo (Hexanchidae) at depths of 490–530m; a common species
Etmopterus pusillus (Dalatiidae) at depths of 450–600m, rare
Centrophorus granulosus (Centrophoridae) 410–605m, common
Deania profundorum (Centrophoridae) 470–600m, common
Echinorhinus brucus (Echinorhinidae) 405–430m, rare
Pristiophorus sp.n. (Pristiophoridae) 465–600m, rare
Halaelurus hispidus (Scyliorhinidae) 410–605m, very common
Eridacnis radcliffei (Proscylliidae) 410–470m, very common
lago omanensis (Triakidae) 405–605m, common
lago sp.n. 470–485m, rare
In the Sokotra Island region 12 species of deepwater sharks have been recorded:
Etmopterus pusillus (Dalatiidae) at depths of 490–500m, a rare species
Centrophorus granulosus (Centrophoridae) 400–710m, common
Deania profundorum (Centrophoridae) 490–710m, common
Squalus megalops (Squalidae) 400–555m, rare at these depths
Pristiophorus sp.n. (Pristiophoridae) 400–710m, rare
Halaelurus hispidus (Scyliorhinidae) 400–710m, common
Apristurus sp. 700–710m, rare
Cephaloscyllium sp. 405–442m, rare
Eridacnis radcliffei (Proscylliidae) 400m, common
Eridacnis sinuans, 400–555m, common
lago sp.n. (Triakidae) 400–500m, rare
Alopias superciliosus (Alopiidae) 400m, rare at this depth.
3 This section kindly contributed by Dr Sergei I. Usachev, Southern Scientific Research Institute of Marine Fisheries, and Oceanography, Crimea, Ukraine and includes material from Gubanov (1993).
There are no fisheries targeting deepwater sharks in these areas except for Alopis superciliosus. Deepwater sharks are generally taken by bottom trawls as bycatch of shrimp and lobster fisheries. No recent information exists about the fisheries in these areas.
Detailed data on species composition and depth ranges of deepwater sharks of Walters Shoals in the Southwestern Indian Ocean were obtained by the Russian R.V. Vityaz expedition to the western Indian Ocean in 1988–1989 and are available in Parin, Sagaidachny and Shcherbachev (1993). They describe the following elasmobranch fauna:
Centroscymnus coelolepis (Squalidae) captured at depths of 1150–1260m
Centroscymnus crepidater, 1060–1080m
Centrosymnus owstoni, 1175–1760m
Dalatias licha, 775–1260m
Deania calceus, 930–1030m
Deania profundorum, 775–1030m
Deania quadrispinosa, 600–1360m
Etmopterus brachyurus, 650–950m
Etmopterus baxteri, 970–1200m
Etmopterus granulosus, 940–1400m
Etmopterus lucifer, 785–1115m
Etmopterus molleri, 655m
Etmopterus pusillus, 928–940m
Squalus mitsukurii, 250–775m
Squalus cf mitsukurii, 650–660m
Zameus squamulosus, 1100–1230m
Apristurus profundorum (Scyliorhinidae) 1030–1110m
Apristurus cf. Platyrhynchus, 1090–1100m
Apristurus sp.n., 1340–1560m
Halaelurus sp.n., 855–1200m
Halaelurus canescens 830–1120m
Parmaturus sp., 1500–1560m
2.8 FAO Statistical Area 61 - Northwest Pacific
Japan is historically one of the world's most important shark fishing nations, although landings have declined in recent years (Rose 1996). In 1993 the total catch was 25673t, of which 77% was taken by tuna longliners in the distant, offshore and coastal fisheries. In the CITES report, no mention is made of targeted deep-water fisheries or of bycatch of deep-water fisheries. No detailed data on species composition is available from Japanese statistics after 1968 (Bonfil 1994). Bonfil names many of the sharks of importance in Japan, but no deep-water demersal species is included.
Yano and Tanaka (1984) state that deep-water sharks have been exploited in Suruga Bay (Japan) since the 1940s for the squalene in their liver oil. Most of them belong to the genera Centroscymnus, Centrophorus and Deania. Some aspects of the distribution and biology of Centroscymnus coelolepis and Centroscymnus owstoni have been described by Yano and Tanaka (1984, 1988). The vertical distribution and feeding habits of Etmopterus lucifer, Etmopterus unicolor and Centroscyllium ritteri from the continental slope off Chosi (Japan) have been described by Baba et al. (1987).
In the 1960s and 1970s a major international fishery developed on the southern Emperornorthen Hawaiian seamounts for the pelagic armourhead (Pseudopentaceros wheeleri), which undoubtedly affected many other species (Wilson and Seki 1994). Probably one of the most important bycatch species was the demersal shark, Squalus mitsukurii. In a survey of the southeast Hancock Seamount between 1985 and 1988, Wilson and Seki (1994) noted a significant decline in CPUE and estimated that about 80% (±55%) of the initial population had been removed by fishing.
2.9 FAO Statistical Area 81 (part) - New Zealand
2.9.1 The deep-water demersal shark species
Cox and Francis (1997) have described the sharks of New Zealand.
2.9.2 Survey data
In a survey of the deep-water fish resources off the North Island of New Zealand, the following deepwater shark species were recorded: Chlamydoselachus anguineus, Heptranchias perlo, Centrophorus squamosus, Centroscymnus plunketi, Centroscymnus owstoni, Centroscymnus coelolepis, Centroscymnus crepidater, Dalatias licha, Deania calcea, Etmopterus baxteri, Etmopterus lucifer, Etmopterus sp. A, E. cf. pusillus, Oxynotus bruniensis, Odontaspis ferox, Mistukurina owstoni, Apristurus spp., Parmaturus sp., Gollum attenuatus and Pseudotriakis microdons (Clark and King 1989).
In a survey for orange roughy in southern New Zealand waters, the following species were recorded: Centrophorus squamosus, Centroscymnus coelolepis, Centroscymnus crepidater, Centroscymnus owstoni, Centroscymnus plunketi, Centroscymnus Sp. A, Dalatias licha, Deania calcea, Etmopterus baxteri, Etmopterus lucifer, Apristurus spp. A, B, C and D (Clark and Tracey 1992). Four of these were amongst the top ten by weight of all fish caught during the survey: E. baxteri, C. plunketi, C. crepidater and C. owstoni.
2.9.3 The fisheries
Bonfil (1994) reported that between 1989 and 1992, about 4% of the elasmobranchs landed in New Zealand consisted of 13 species of deep-water sharks and at least three species of batoid. Because of the scarcity of information concerning the relevant stocks, effectively it is unknown if New Zealand's deepwater resources of elasmobranchs can withstand the current fishing pressure (M. Francis, NIWA, Wellington, Pers. Comm).
King and Clark (1987) considered that the New Zealand sharks Deania calcea, Etmopterus baxteri, Dalatias licha, Centrophorus squamosus, Centroscymnus plunketi, Centroscymnus owstoni, Centroscymnus coelolepis and Centroscymnus crepidater were of large enough size and sufficiently abundant to be of commercial interest. Deania calcea tended to be the most abundant species in the north, E. baxteri in the south and Centroscymnus in the centre. The livers of all eight species can be used for oil and squalene. The squalene content was highest in D. calceus, C. squamosus and D. licha. A potential problem was that the mercury levels in the small number of samples analysed were above the New Zealand permitted levels.
In a recent paper on New Zealand shark fisheries (Francis in press), Dalatias licha is recorded as an important bycatch that is landed for its flesh. Landings of Deania calcea peaked in the late 1980s and have been low ever since. This may have been caused by changes in the markets for the liver oil, but no data are available for exports except for 1985, when 23t were exported to Japan. Table 33 gives the reported landings data for New Zealand species that may be considered as deepwater, i.e. characteristically taken as depth >200m (Francis in press). A decline had been evident since the peak fishing year of 1994–95, mainly in the landings of ghost sharks (Hydrolagus spp).
Further information on the potential value of shark liver oils is given by Summers (1987) and Summers and Wong (1992).
2.10 FAO Statistical Areas 57, 71 and part 81 (Australian waters)
2.10.1 The deep-water demersal shark species
Australia has a diverse shark fauna, including many deep-water demersal species (Last and Stevens 1994). Many deep-water species, especially of the genera Etmopterus and Apristurus, have not yet been described.
2.10.2 Survey data
Newton and Klaer (1991) provide the following provisional list of sharks from their deepwater survey of the slopes of the Great Australian Bight: Heptranchias perlo, Apristurus spp., Cephaloscyllium nascione, Galeus, boardmani, Centrophorus moluccensis, Centroscymnus owstoni, Centroscymnus coelolepis, Dalatias licha, Deania calcea, Deania quadrispinosa, Deania sp., Etmopterus brachyurus, Etmopterus lucifer, Etmopterus sp., Oxynotus bruniensis, Squalus acanthias and Squalus mitsukurii. In a survey of the upper slope of the southeastern Australian continental slope, Graham et al. (1997) caught 25 species of shark. The most abundant were Squalus megalops, Centrophorus moluccensis, Squalus mitsukurii, Centrophorus harrissoni and C. uyato.
|1986– 87||1987– 88||1988– 89||1989– 90||1990– 91||1991– 92||1992– 93||1993– 94||1994– 95||1995– 96||1996– 97|
2.10.3 The fisheries
The elasmobranch fisheries of Australian waters are small but quite well-documented (Bonfil 1994). Almost all are coastal or offshore pelagic sharks and area 57 accounts for most of the landings. Rose (1996), quoting Bentley (1996), reports that Centrophorus uyato and Squalus mitsukurii and, to a lesser extent Centrophorus moluccensis and C. harrissoni, have recently begun to be exploited for their liver oil off southern Australia.
Deep-water dogfish (mainly Deania calcea, Centroscymnus owstoni, Centroscymnus coelolepis, Centroscymnus crepidater and some Etmopterus and Apristurus spp.) are taken mainly as bycatch in the orange roughy fishery (Stevens Pers. Comm). Little has been published, other than papers on their taxonomy and the squalene oil (Deprez et al. 1990). The flesh of some species is not utilised because of its high mercury content (Last and Stevens 1994). Stevens also notes that there is some targeting of other less deep-living species (Centrophorus spp.) both for meat and liver oil and that these species also figure in bycatch from trawl fisheries. Squalus megalops and Squalus mitsukurii are common on the upper slopes and small quantities are marketed. The latter is sold under the name of 'greeneye dogfish' (Last and Stevens 1994).
3. MANAGEMENT AND CONSERVATION
Deep-water fisheries have developed quite rapidly in recent years, both as a response to improved technology and the need to find new resources to replace over-exploited shallow-water fisheries. These deep-water fisheries can often develop rapidly and can be depleted long before any appropriate management action can be put in place. This situation is exacerbated by a basic lack of knowledge of the fish and their habitat. Only in two areas is our knowledge sufficiently advanced to enable some discussion of the problems as they relate to deep-water sharks. In Australia and New Zealand the surveys associated with the orange roughy fisheries have yielded valuable information on the fish assemblages, including the sharks. There is also information on landings and estimates of discards. In the northeastern Atlantic knowledge of the deep-water ecosystem has advanced considerably in the last few decades. The developing fisheries have been quite well documented by the NATO Advanced Research workshop (Hopper 1995), the ICES Study Group on the Biology and Assessment of Deep-sea Fisheries Resources (Anon. 1995b, 1996b, 1998b) and the unpublished reports of the EC FAIR Deep-fisheries Project (95–655) Developing deep-water fisheries: data for the assessment of their interaction with and impact on a fragile environment. This section will use these investigations to illustrate the problems of management and conservation of deep-water shark resources.
The first major difficulty for most statistical areas is that there is a lack of basic data on landings of deep-water demersal sharks. The term 'sharks various', or similar terms, includes a wide range of shelf and oceanic pelagic and demersal sharks. Separating the various groups from reported landings is difficult. Some estimates can be made using a knowledge of the fisheries and in particular the fishing gear being used. However, if there are several types of fisheries in operation, as off southwest Ireland (see Chapter 1), then it becomes difficult. It is probable that in assessing the relative importance of shark species to fisheries, the deep-water catsharks (Scyliorhinidae) and many small, deep-benthic and oceanic dogfish (Squalidae) fall into the category of 'minimal catch species' (Compagno 1990). However, although the landings may be minimal in terms of all shark species, the discarded catch in the new and developing fisheries on the continental slopes and seamounts is probably quite considerable. Another problem with landings data is that they do not fully take into account the landings of shark products, such as livers or fins (Rose 1996).
If landings data are to have any use for assessment purposes, it is essential that the species are identified correctly. This in turn pre-supposes that all species are adequately described. Unfortunately, this is not the case and even in the northeastern Atlantic, with a history of over 100 years of research, certain genera such as Apristurus and Centrophorus are in urgent need of revision. Even for a fishery biologist, many of the deep-water sharks are difficult to identify. Within the ICES area, 27 different species of demersal shark were recognised by the ICES Study Group on Elasmobranch Fishes (SGEF) (Anon. 1997a). The Study Group also compiled basic biological information on each of the species. It is therefore not surprising that landings dataof deep-water demersal sharks are almost always in grouped categories.
In the longline fisheries of Portugal, some of which target deep-water sharks (see 22.214.171.124), a variety of species are landed. In an effort to check the validity of the official landing statistics for sharks, a market sampling programme was carried out by Institute de Investigaçãdas Pescas e do Mar (IPIMAR) under the auspices of the EC FAIR Deep-fisheries Project. The results showed that the identifications by the fishermen were reasonably accurate. Unidentified sharks only accounted for about 7% and erroneous identification were less than 3% (EC FAIR 95–655, unpublished report).
In the bottom trawl fishery to the west of the British Isles two species, Centroscymnus coelolepis and Centrophorus squamosus, account for most of the landings. The main market is France, where they are collectively known as ‘siki’. The landings of siki have increased considerably during the 1990s, but the information on the relative proportions of the two species is not obtainable from the statistics. Sampling of the landings at the French port of Concameau by the Laboratoire de Biologic Marine du Collège de France has shown that the relative proportions of the two species can vary quite considerably between season and years (EC FAIR 95–655, unpublished report).
ICES ACFM (Advisory Committee on Fisheries Management) has consistently recommended that landings should be recorded at species level (Anon. 1997b). However, one of the problems has been the lack of standard three-digit identifier codes. The ICES SGEF report of 1997 recommended a list of species for changes and additions to the “FAO STATLANT codings (Anon 1997a). These codings are given in full in Anon (1998a), which also recommends changes to the common names used for Galeus spp. and Apristurus spp. Table 34 lists the codings appropriate to the deep-water demersal sharks. While common names have some obvious uses, they can also cause problems. Many of the deep-water species have global distributions and even in the same language there are numerous common names. At the start of the EC FAIR Deep-fisheries Project all participants agreed to use scientific names in all their reports.
|3 - alpha identifier||Species (scientific name)||Species (common name)|
|SHO||Galeus melastomus||Blackmouth catshark|
|SYC||Scyliorhinus canicula||Small-spotted catshark|
|PTM||Pseudotriakis microdon||False catshark|
|SOR||Somniosus rostratus||Little sleeper shark|
|GUP||Centrophorus granulosus||Gulper shark|
|CPU||Centrophorus uyato||Little gulper shark|
|GUQ||Centrophorus squamosus||Leafscale gulper shark|
|CPL||Centrophorus lusitanicus||Lowfin gulper shark|
|ETX||Etmopterus spinax||Velvet belly|
|ETP||Etmopterus princeps||Great lanternshark|
|DNA||Deania spp.||‘Deania’ dogfishes|
|DCA||Deania calcea||Birdbeak dogfish|
|CYO||Centroscymnus coelolepis||Portuguese dogfish|
|CYP||Centroscymnus crepidater||Longnose velvet dogfish|
|CYY||Centroscymnus cryptocanthus||Shortnose velvet dogfish|
|SYO||Scymnodon obscurus||Smallmouth knivetooth dogfish|
|SYR||Scymnodon ringens||Knivetooth dogfish|
|SCK||Dalatias licha||Kitefin shark|
|CFB||Centroscyllium fabricii||Black dogfish|
|OXY||Oxynotus centrica||Angular roughshark|
|OXN||Oxynotus paradoxus||Sailfin roughshark|
The landings of deep-water demersal sharks often account for only a small part of the total shark catch. Those brought to the surface and subsequently discarded are generally so badly damaged and stressed by changes in pressure and temperature that few would survive. The discarding of a significant proportion of the top predators will have important effects on the ecosystem and for biodiversity. In the northeastern Atlantic there is an almost unique situation where there are survey data which pre-date the commercial fishery. There have also been several investigations of discarding in the current fishery. For this reason, it is worthwhile considering this area as an example of some of the problems. The following is a summary of the present status of knowledge.
The continental slope lying to the west of Scotland and Ireland is of considerable interest in terms of deep-water fish and fisheries. Some of the earliest descriptions of deep-water fish were made in this area (Murray and Hjort 1912) and the cruises of the Helga at the end of the 19th Century did much to describe the fauna, including some of the deep-water sharks (Holt and Byrne 1910). There were relatively few surveys in the early part of the 20th century and most simply listed the catch from surveys extending into deeper water for species such as hake (Merluccius merluccius). The situation changed in the 1970s when the loss of fishing opportunities for traditional species resulted in countries such as the United Kingdom and Germany becoming interested in the potential for exploiting deepwater demersal species. In 1973 and 1974 the UK Ministry of Agriculture Fisheries and Food carried out a series of four research vessel and two charter vessel cruises to explore the potential of the deepwater continental slope at depths between about 500 and 1200m. A report of some of the main features of these surveys was published (Bridger 1978). The UK carried out a further survey in 1978. Recognising the importance of these surveys as a baseline for the fisheries that have subsequently developed in these areas, a proposal to work up and archive the logbook data from these surveys was funded by the European Commission (Gordon and Swan 1997). At about the same time, the Institut fur Seefischerei (ISH) in Hamburg also began a series of TIFI cruises between 1974 and 1986. Although some of the data on the teleost species was published (Ehrich 1983), the sharks were not considered to be of commercial interest and were excluded. However, some data on the distribution, abundance and biomass of the sharks can be obtained from an analysis of the data from eight of the cruises (Ratz 1984). It has also been recognised that these surveys represent a valuable resource on the status of the fish assemblages prior to the current exploitation. These data are presently being compiled and analysed as a contribution by ISH to the EC-funded Fair Deep-fisheries Project.
In addition to these surveys which covered a wide geographical area, there have been others which have concentrated on seasonal sampling of more discrete areas. Between 1975 and 1992 the Scottish Association for Marine Science (SAMS) investigated the deep water demersal fish of Rockall Trough between about 55° and 57°N (Gordon and Bergstad 1992; Gordon and Duncan 1985; Gordon and Swan 1993) as part of a more detailed study of the area (Mauchline 1986). The depth range covered by these surveys was from 245 to 2996 m, which made it possible to define the total depth range of all the shark species. Another similar survey was carried out jointly by SAMS and the Institute of Oceanographic Sciences (IOS) in the Porcupine Seabight between 1979 and 1983. General descriptions of the fish populations are given by Merrett et al. (1991a,b). Further information on the deep-sea benthic programme is provided by Jackson et al. (1991) and Rice et al. (1991).
The data on the depth distribution, abundance and biomass by area and depth from these surveys have been summarised by Gordon and Swan (1997b). These data, to some extent, represent the status of the stocks prior to the current exploitation of the deep-water fish populations.
The recent growth of deep-water fishing in the Atlantic began in the 1960s when the USSR and other eastern European countries began to exploit species such as roundnose grenadier (Coryphaenoides rupestris) off Canada at depths down to about 900m. The fishery developed rapidly, with landings peaking at almost 84 OOOt in 1971. The fishery also extended to the eastern North Atlantic (Reykjanes Ridge and Hatton Bank, east of the Rockall Plateau) and the peak catch of over 30 000t of roundnose grenadier was landed in 1975. These fisheries have declined to a much lower level, mainly for political and economic reasons. In the Rockall Trough deep-water fishing began in the early 1970s when first German and then French vessels began to exploit blue ling (Molva dypterygia) at depths down to about 800m. Landings of other deep-water species did not begin until about 1989 when the French fishing industry began to develop markets for species such as roundnose grenadier, black scabbardfish (Aphanopus carbo) and squalid sharks. It is probable that the blue ling fishery began as a targeted fishery on spawning aggregations and the bycatch of other species might have been minimal. However, as the fishery developed, it changed to a more multi-species fishery and it is likely that all the species that are now landed were discarded. As the market for these species and especially roundnose grenadier developed, the fishery moved into deeper waters (to about 1200m). In 1992 stocks of orange roughy (Hoplostethus atlanticus) were found and exploited, thus extending the fishery into even deeper waters (down to 1900m) (Charuau et al. 1995). Scottish trawlers also fish in the deep water, although most target monkfish (Lophius piscatorius) at depths down to about 800 m. In all these fisheries different species of demersal sharks are caught, depending on depth and area. They are either discarded whole, or discarded after removing the liver, or landed. Only two species, Centroscymnus coelolepis (Portuguese dogfish) and Centrophorus squamosus (leafscale gulper shark) are generally landed for human consumption under the name of ‘siki’ (see 3.2).
In addition to the bottom trawl fisheries, there also exist longline fisheries. Norwegian longliners traditionally fish for ling (Molva molva) and tusk (Brosme brosme) along the outer shelf and upper continental slope to the west of the British Isles. Experimental longline surveys in deep water have also been carried out by Norway (Stene and Buner 1991; Olsen 1995). The shark information from Stene and Buner (1991) has been summarised by Gordon and Swan (1997b) (Table 5).
Spain has a longline fishery for hake along the edge of the continental slope and the livers of deep-water sharks are retained. In some cases the carcass is also landed. Recently, there has been a targeted fishery for deep-water sharks in this area (Piñeiro et al. 1998).
The first study of discarding in the Rockall Trough resulted from Irish longline and trawl surveys (Connolly et al. 1996). The longline survey took place in November to December 1995 and the results are based on 17 valid sets between depths of 540 and 1224m. The trawl survey was carried out using a commercial trawler in November 1995. The trawl used was a commercial rockhopper trawl, with a 4.5m headline height and a wing spread of about 24m. The 100mm mesh codend was lined with a 10mm liner. In the trawl surveys the weight of the total catch was estimated from the average weight of a box and the species composition was recorded. The landings were recorded from the haul sheet as the number of boxes. The number of boxes of discards was also recorded and representative boxes were analysed. The weight of discards of each species was then expressed as the weight per tonne of roundnose grenadier landed.
The landings from the trawl were dominated by roundnose grenadier and Centroscymnus coelolepis. There was a wide range of discard species but the deep-water sharks, Deania calcea, Centrophorus squamosus, and Centroscymnus crepidater were among the dominant species. Table 35 shows the weight in kg of the sharks discarded per tonne of roundnose grenadier landed. The total discards of all species amounts to about 7530t per 8000t of roundnose grenadier landed. Shark discards comprise about 52% of the discards of all species.
|Shark species||Kg discarded per tonne roundnose grenadier landed||Tonnes discarded in 1995||Numbers discarded in 1995|
Only 12 species were caught by the longline and of these, six were sharks. The retained sharks were Centrophorus squamosus and Centroscymnus coelolepis which comprised, on average, 75% and 9% of the catch, respectively. The discarded species were Centroscymnus crepidater, Deania calcea, Etmopterus spinax and Galeus melastomus and they comprised about 4% of the total catch and 90% of the discards.
Investigations of discarding by the commercial French and Scottish trawler fleets have been carried out by the Scottish Office Marine Laboratory as a contribution to the EC FAIR Deep-fisheries Project (Blasdale and Newton 1998). Both fleets fish at depths down to about 900 m and only two species, Deania calcea and Centroscymnus crepidater, make a small contribution to the total discards. The mean discard rate for Deania calcea for both fleets was about 2kg/hr.
Also, as a contribution the EC Fair Deep-fisheries Project, IFREMER have carried out discard studies on French trawlers fishing in the Rockall Trough. The main emphasis has been on the roundnose grenadier but some information on other species, including the sharks, has been published (Dupouy et al. 1998). The shark data in terms of numbers of individuals landed and discarded in 1996 is shown in Table 36. The shark species most frequently discarded is Deania calcea.
Deep-water sharks can form a significant component of the discards in both the bottom trawl and longline fisheries. There are quite noticeable differences between the different studies, but this is to be expected as a result of the different areas and depths fished and the gear type used.
|Species||March 96||May/June 96||Sept. 96||Dec. 96|
|3 (0.0)||2(1.2)||4 (2.7)||8 (7.6)|
|1||7(4.9)||3 (2.1)||8 (6.6)|
Centroscymnus coelolepis +
|Centrophorus squamosus||49(93.7)||2(2.1)||8(13.2)||5 (4.6)|
In the commercial bottom trawl fishery, the codend mesh is generally 100mm stretched mesh. However, Kelly et al. (1998) have observed that there was little difference in the length frequency composition of roundnose grenadier obtained using a commercial codend and a research codend of 10mm, suggesting that the meshes are almost closed under the tensions on the trawl in fishing at great depth. Deep-water fish tend to have fragile skins and are not well endowed with mucus. It is probable that most fish, including sharks, which enter the trawl and subsequently escape suffer a high mortality. These escapees have sometimes been called ‘no catch discards’. Thus the total catch of fine-meshed research bottom trawls is probably a good indication of the fish assemblage that is removed by commercial trawling. However, there is a further complication caused by the selectivity of the trawl itself.
In research surveys in the Rockall Trough and Porcupine Seabight, particular emphasis was placed on comparing the catch composition obtained by using different bottom trawls (Gordon and Duncan 1985: Gordon and Bergstad 1992; Gordon et al. 1996; Merrett et al. 1991a). The small shrimp trawls fished on a single warp, which are often used in deep-water surveys, were particularly inefficient at capturing fast swimming species such as the sharks. The converging bridles ahead of the trawl probably serve to deflect these species out of the path of the net. In the commercial fishery only paired warp trawls are used. Little is known of the catching efficiency of these trawls, although it is likely that headline height may influence catches. Some evidence from studies of the diet suggest that Etmopterus spinax, Etmopterus princeps, Centroscymnus crepidater and Apristurus spp. may feed higher in the water column than Centroscymnus coelolepis, Centrophorus squamosus, Deania calcea and Centroscyllium fabricii (Mauchline and Gordon 1983). The French industry tend to use lighter trawls with high headline heights, whereas the Scottish vessels use so-called ‘scraper trawls’ of low headline height, specifically to target Lophius spp. Some significant differences in discards of teleost species were found between the two fleets (Blasdale and Newton 1998), but the discards of Deania calcea were not significantly different between the two fleets. However, these results may reflect the differences in the depths fished by the two fleets rather than in the design of the trawl. The recent trend towards the use of twin-rig trawling in deep water could cause changes in the shark bycatch. The disturbance in front of the trawl caused by the third towing warp could deflect sharks and other fast swimming species out of the path of the nets in the same way as a single warp trawl.
The depth fished is likely to have the greatest influence on the catch composition. In a multivariate analysis of all the trawl catch data in the Rockall Trough, Gordon and Bergstad (1992) showed that depth accounted for most of the variance, with gear type only second in importance. Table 1 shows quite clearly how the catch composition, abundance and biomass changes with depth in the Rockall Trough. This information is summarised diagrammatically in Figure 4. Similar results were obtained in a comparison of the above SAMS data and data from IFREMER surveys (Lorance, Pers. Comm.) To estimate the discarding of shark species, a knowledge of the depth of the target fishery is essential.
Depth distributions of deep-water sharks in the Rockall Trough.
The deep-water sharks are top predators and therefore readily take baited hooks. Longlining is therefore relatively unselective and can result in high discard rates (Connolly and Kelly 1996). If deepwater longlining were to increase in the northeastern Atlantic, the landings and discards might significantly increase, especially in some areas (Table 5).
3.4 Fisheries effects
Although there have been many surveys in the Rockall Trough, direct comparisons between the fish assemblages before and after the fishery are difficult because different gears have been used. In the commercial fishery trends in CPUE are also difficult to interpret because, as in any new and developing fishery, efficiency improves with experience. To overcome this problem, Lorance and Dupouy (1998) analysed the fleet in terms of vessel size, power and technological development. From this, they identified a fleet that had changed little as the fishery developed and used this to compare trends in CPUE for the target species. The results were used to calculate a CPUE index of abundance for the squalid sharks, Centroscymnus coelolepis and Centrophorus squamosus combined, which shows a reduction of approximately 50% between 1991 and 1996 (Table 16). It should be noted that although 1991 was taken as the starting point, these species were being caught and discarded in earlier fisheries for species such as blue ling. There is no information on the possible impacts of the fishery on the stocks of the discarded bycatch species.
The dramatic decline in the landings of Dalatias licha from the Azores (Anon 1998a) may be attributed to a high level of exploitation, but fluctuations in the price of liver oils may also contribute (Anon 1995a).
The Spanish longline fisheries which either target sharks or have sharks as a bycatch show considerable annual and seasonal fluctuations in the landings (Piñeiro et al. 1998). It is probable that many of these fluctuations are caused by changing conditions such as weather, price of liver oil, relative value of other species etc. It is therefore difficult assess the possible impact of the fishery on the stocks.
In New Zealand a study of the population changes associated with the orange roughy fishery on the Challenger Plateau by dark and Tracey (1994) found an estimated reduction in the biomass to about 20% of the virgin level. However, the biomass indices of the other bycatch species did not show the same trends between 1987 and 1989. The biomass of Centrophorus squamosus and Centroscymnus owstoni showed little change. A declining abundance was suggested for Deania calcea and Dalatias licha. There appeared to be no compensatory species replacement for orange roughy.
Graham et al. (1997) have compared the catches of their upper slope bottom trawl survey of southeastern Australia in 1996/97 with an earlier survey in 1976/77. Although natural variability cannot be ruled out, there was a consistent trend indicating that elasmobranchs had been fished down to a low level. The exception to this was the spiky dogshark (Squalus megalops), which showed an increase over the 20 year period. Some of the deeper-living dogsharks were virtually absent from some areas. It appears that the shark bycatch of the multi-species teleost trawl fishery and the growing importance of sharks for both meat and oil has severely depleted the shark stocks.
The discovery of large stocks of the teleost Pseudopentaceros wheeleri on North Pacific seamounts led to an intensive trawl fishery. Following a dramatic decline in catches, the fishery was closed, although surveys continued. The greatest bycatch in this fishery was the demersal shark Squalus mitsukurii. A detailed study on one seamount using longlines showed a decline in CPUE of more than 50% over two years. Further modeling suggested that the initial population had been reduced by 80% as a result of fishing. The decline in the population was not accompanied by a decline in mean length, perhaps indicating that there was little or no recruitment.
3.5 Vulnerability to exploitation
The biological characteristics of elasmobranch fishes apply equally well to the deep-water species and have been listed as follows by the ICES Study group on the Elasmobranch Fishes (Anon 1997a):
However, the Study Group also notes a number of other features that may increase the vulnerability of deep-water sharks to fishing impacts. The survival rate of discards and even escapees is either nil or low. Many species have a wide geographical range, probably involving long migrations. Life history stages can be separated, as for example in the squalid sharks, where for most species the juveniles are not found on the European continental slope. Because deep-water fisheries are comparatively recent, there has been a lack of monitoring or sampling of the stocks and hence a scarcity of biological and life history information. Methods of age estimation are unresolved and validation will be difficult.
The problems associated with obtaining catch statistics for sharks have been outlined by Compagno (1990). In assessing the relative importance of shark species to fisheries, Compagno placed the deep-water catsharks (Scyliorhinidae) and many small, deep-benthic and oceanic dogfish (Squalidae) in the category of minimal catch species.
The main problems associated with deep-water shark management can be summarised as follows.
With few exceptions, the landings of deep-water demersal sharks are of minor importance compared to the fisheries for coastal or oceanic pelagic sharks.
Most landings are a bycatch of other fisheries and the landings tend to be a mixture of several species.
The deep-water demersal sharks are less well known than the coastal sharks (eg Squalus acanthias) or the oceanic pelagics (e.g Prionace glauca) and are usually landed in grouped categories such as “sharks various”.
Often only parts of the shark, e.g. liver or fins, are retained.
Sharks of small adult size or the juveniles of marketable species are discarded. No records are kept of discards and there have been few scientific studies of discarding.
Sharks brought up from deep water in bottom trawls and subsequently discarded will suffer a high mortality.
Almost all sharks will take bait and therefore longlining is not a selective fishery and discarding rates can be high.
Little is known of the distribution, stock structure and migrations of the deep-water demersal sharks.
There are few age estimates of deep-water demersal sharks and none have been validated.
Little is known of the reproductive biology of the deep-water sharks. The squalid sharks, which are the most important in the fisheries, are ovoviviparous and the gestation period could be two to three years.
Because many of the fisheries where deep-water sharks are taken as a bycatch are in the process of developing, CPUE data are unreliable.
Many of the fisheries fall into the category of ‘highly migratory species and straddling stocks’, which is likely to result in uncontrolled fishing in international waters.
Bonfil (1994) commenting on the large bycatch of sharks, most of which are discarded, in the New Zealand deep-water orange roughy fishery stated the following. “The impact of this level of bycatch on the local stocks (of) sharks is unknown but it must be highly damaging and likely to lead to unsustainable exploitation. But this is difficult to verify as little information exists about the biology and population biology of these species. More research is needed on the levels of by catch, survival of discards and the deep sea shark populations themselves”. This statement is still pertinent and is equally appropriate to the ever increasing expansion of deep-water fishing.
The following people are acknowledged for contributing material to sections relating to their area of specialization: B. Seret, Muséum National d'Histoire Naturelle, Laboratoire d'Ichtyologie, France; G. Duhamel, Musée National d'Histoire Naturelle, Paris - Southern Ocean - CCAMLR area; J. Stevens, CSIRO, Australia and Southern Oceans; M. Francis, NIWA, New Zealand; José Miguel Casas, Instituto Español de Oceanografia, Cabo Estay - Canido, Spain
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