Many who would recognize the absurdity of a plan to sustain large and growing numbers of people by hunting buffalo, deer, wild birds, rabbits and squirrels, seem to disengage their power of reason when it comes to the sea. Apparently believing, somehow, that ocean systems are fundamentally different from those on the land - that they can year after year yield huge, commercially viable takes of wild-caught organisms and rebound indefinitely (Earle 1996).

Compared to the continental shelf, the living resources of the deep sea are very different and are particularly sensitive to any fishing activities, whether as a result of direct or indirect exploitation. The poor knowledge of deep-sea fish populations, especially their life history biology; low density of most species; worries about the sustainability of stocks so remote from their sources of food, and the inevitable difficulties faced with fishing in this environment, are all factors which make the removal of fish from this environment likely to put these populations in grave danger.

4.1 Poor knowledge

In the cold realm of the world’s deep oceans, the life processes of many species appear to be very slow, and life spans are long (Turekian et al. 1975). The biology and physiology of long-lived, low fecundity species, characteristic of the deep ocean environment mean that these species often have to grow almost to full adult size, which can take several decades, before they begin to breed. (Vincent 1995; pers. comm. Nigel Merrett).

Most deepwater species have delayed maturity, low growth, low mortality, long life span and low fecundity (Stearns 1976; Adams 1980). These are the traits that allow such populations to persist, so anything that shortens life and the number of times an organism reproduces endangers the population (Norse 1993). Such characteristics make deep-sea biota especially vulnerable to exploitation and slow to recover from disturbances (FAO 1993; Norse 1993).

The orange roughy can take several decades to reach sexual maturity (FAO 1993). The roughy is thought to start reproduction at 20-35 years and produce between 30-50,000 eggs. To put this into perspective, this is about 100 times less than the Atlantic cod (Bell et al. 1992; Clark et al. 1994; Koslow et al. 1995). Thus there will always be high catches of older fish when the fishery begins; but with continuing exploitation the catches will decline (Gordon and Hunter 1993) and any removal by a fishery will take many years to recover (Haedrich 1996a), if indeed it is able to.

Deepwater species generally have very low natural mortality (e.g. FAO 1993), which means few are eaten as prey by other fish, marine mammals or other predators. The orange roughy has nearly the lowest potential fecundity of all exploited fish, combined with the lowest growth rate and greatest longevity, and does not have the potential to recover as fast as species like cod (Angel 1997). It has been suggested that most sexually mature adults may naturally die of old age in the absence of any fishery (pers. comm. Nigel Merrett), although orange roughy have been found in the stomachs of sperm whale feeding in the deep ocean of New Zealand. This is how orange roughy were discovered in New Zealand.

The ‘great whales’ are also known to be long-lived species. Their fate following exploitation by the whaling industry is well documented. The blue whales which were once exploited off the coast of Scotland were almost driven to extinction by whaling. They are known to live up to 80-90 years (IUCN 1991), a similar age to some of the fish which have been exploited off Scotland in recent years.

In contrast, species with early maturity, rapid growth rates, high rates of mortality, small body size and short life span generally show high fecundity, i.e. they produce large numbers of young (Stearns 1976; Adams 1980). This fecundity tends to be maximised in conditions of variable food resources where small, rapidly hatching larvae are potentially vulnerable to starvation but able to capitalize on increases in the availability of food (Millar 1984; Mann et al. 1984). Such circumstances are found in shallower more productive waters. Shallow water fish populations are thus likely to be far more robust than any deepwater fish populations and have been shown to be able to recover following an intense fishery.

Our knowledge of the deep-sea and the biology of the species is incredibly poor. We do not know how quickly a particular species can decline, whether it is targeted by a fishery or caught as bycatch. Some of the species which may be caught by a fishery in the Atlantic Frontier are considered ‘rare’. How can we monitor the impact of a fishery when we have no reliable estimates for the biomass of a species?

4.2 Low density

Fishermen believe that the deep-sea has great abundance of ‘untapped resources’ that could lead to expanded food production. This is unlikely to be true (Norse 1993; Scottish Fishing Monthly 1996b).

The main limiting factor is that at a depth of 800 metres fish abundance decreases to about 10 per cent of that at the shelf edge at a depth of 200 metres. At 1,000 metres it is only 1 per cent as abundant (Gordon 1992b). The world’s most productive fishing grounds are found in the shallower waters, such as the North Sea or off the west coast of South America, not in deeper waters. It is no accident that people capture sardines from more productive shallow waters than the impoverished deep ocean environment (Norse 1993).

The world’s most numerous and widely distributed fish are bristlemouths, which occur in the open ocean at depths from 500 to 2,000 metres. But these fish are unlikely to become a food source as they have a very high water content, they are relatively slow growing, and their density in the sea is low. Their numbers are only huge because the total area of the deep-sea where they live is so great (Norse 1993).

Many deepwater species targeted by fishermen are concentrated into narrow strips on the steep continental slopes or sea-mounts, which give, much higher ratios of catch rates to biomass than shallower waters. Fishermen often confuse high biomass with high sustainable yields. Unfortunately, fishermen tend to think that high catch rates indicate high abundance, which can lead to problems in depleting a stock (Kenchington 1996). In other words they think because there are a lot of fish present there will continue to be a lot of fish - which is wrong.

4.3 Sustainable stocks?

Possibly the most important limiting factor in deep-sea ecology is food availability. All food production in the deep sea, except at hydrothermal vents and seeps, is fuelled, either directly or indirectly, by the import of organic matter from the productive surface waters (Gage and Tyler 1991; Gordon 1993a).

Because deepwater species are food-limited (Gordon 1993a), reproduction is related to food availability. Studies off Australia have suggested that in years when food is reduced female orange roughy do not spawn, instead conserving energy for their own survival (Bell et al. 1992; Gordon and Hunter 1994a). The study showed that some 45 per cent of the sexually mature females of the orange roughy did not reproduce in one particular year (Gordon et al. 1995). No comparison studies of the orange roughy in Scottish waters has been conducted, however it is known that the deepwaters here are far less productive than the Southern Ocean (pers. comm. Nigel Merrett).

“It is a curious twist of human nature that makes us apparently unable to raise the possibility that there may be no such thing as a ‘surplus’ just waiting to be taken.”
Dr Sylvia Earle.

There is some evidence to suggest that some lower slope and abyssal species may only spawn once in a lifetime, perhaps in response to some favourable condition such as a large food-fall. One such species might be the abyssal armed grenadier (Nematonurus armatus) which is thought to reproduce only once at the end of its supposedly long life. This large grenadier only occurs in any significant numbers at depths greater than 3,000 metres. This species is almost certainly slow growing (Gordon and Hunter 1994a; Stein 1985) and it has recently come to light that this grenadier might undertake long-distance seasonal migrations across the sea floor of the north-east Atlantic (Rice 1995).

Whilst fisheries scientists and politicians have traditionally referred to ‘sustainable’ fishing at levels close to maximum sustainable yield, it seems not to have occurred to those people charged to managing fisheries, that the right number for us to remove to ensure the sustained health of a wild population of deepwater fish might be zero (Earle 1996).

4.4 Natural dangers

The ocean environment imposes considerable demands on the design of vessels and their equipment (Tumilty 1995). Natural dangers of the deep ocean include sandbanks on the seabed which can collapse, engulfing the trawl, and huge amounts of deep sea sponges which clog the trawl, making it difficult to haul onboard (Knox 1996). In rough seas what would often be considered a nuisance can become a real danger.

There are fairly heavy losses of trawl gear in these fisheries (Gordon 1996b). The French deep-water trawlers from Lorient which fish off Scotland are fitted with a device which automatically cuts the nets free from the vessel if it becomes snagged and the winch gets jammed. This emergency safety system prevents their trawlers being dragged down if the vessel looses stability (pers. comm. French fishermen, Lochinver 1997; pers. comm. Dr John Wilson).

Deepwater corals such as Lophelia pertusa are often caught in trawl nets fishing close to the seabed. These have to be broken up with hammers back onboard the trawler. Nets which trawl through these corals wear out very quickly (France-Eco-Peche 1992). New Scottish vessel owners are having to learn how fish these new grounds through trial and error. Some have “torn a lot of gear and wasted a lot of hauls fishing areas that are not viable”. (Mitchell 1995). Around the north of rock outcrop of Rockall, beyond the 200 metres depth mark, fishermen are well aware of these corals which can have “devastating results on gear.” (Fishing News 1997b).

The suitability of deepwater grounds north-west of the Butt of Lewis have been the subject of discussion among fishermen, as it is known to be the site of a scuttled ship. Scottish Fishermen’s Federation Chief Executive Bob Allan said: “In these times when the protection of the marine environment has got a very high focus indeed, I would have thought it incumbent on one or another of the agencies to come forward and tell fishermen what the risks are.” (Allan 1995). Perhaps these same government ‘agencies’ should also be telling fisherman and fishermen’s leaders about the risks to deepwater fish populations. One thing that would seem to be clear from these ‘new’ deepwater fisheries is that neither the fisheries managers, nor the fisherman, have consulted each other about exploiting these deepwater populations - ie the Government authority has no control of these fishermen.

4.5 Blind destruction

Trawling is likely to be the principal method of fishing in deepwater for the foreseeable future and can be highly destructive to both target and non-target species, as mixed catches are inevitable and the level of discards is high (Tumilty 1995). Other methods, such as longlining and gillnets, also blindly catch unwanted species and can lead to high levels of mortality to these non-target species.

Trash species
Inevitably as fishermen move into deeper water to exploit species for which there is already a market, other species, such as deepwater sharks, rabbit fish and the smoothhead, will be caught and, all or most will be discarded (Gordon and Hunter 1994).

The 1970s Ministry of Agriculture, Fisheries and Food (MAFF) survey referred to them as “trash” species, condemned as being unpalatable, or because they were “simply too small or too rare, or both, to be worth processing”. The survey identified some 40 different species, some of them previously regarded as extreme rarities. Others has to be sent to specialists for identification (Bridger 1978).

All deep-sea fish will die once brought to the surface, even if returned to the water immediately. In most cases their stomach inverts through their mouth as they are brought to the surface. Large scale capture and discarding of immature fish will have serious consequences for the stocks (Gordon and Hunter 1994a). There are no figures for discards of small fish or non commercial species available but it is very likely that they represent a considerably higher tonnage than those fish that are landed (Gordon 1996b).

In terms of weight, the MAFF survey identified the dominant “trash” species to be smoothheads and various rabbitfish (Bridger 1978). Almost 60 per cent of the catch from depths of 1,000-1,250 metres, consists of the smoothhead (Alepocephalus bairdii) which is always discarded because of its watery flesh. The smoothhead is often the dominant species at depths below 1,000 metres to the west of the British Isles (Gordon and Hunter 1994a) and is considered a “nuisance fish” by fishermen (Gordon and Swan 1997b).

The different body shape of many of these deepwater species will mean that many of them are caught as immature juveniles by trawls. For example, the large head of the roundnose grenadier results in immature fish being retained by the trawl, and one Irish survey estimated that this could amount to between 5 and 10 per cent of the weight of the catch. There is also anecdotal evidence that immature orange roughy are discarded at sea and not recorded (Gordon and Hunter 1994a).

The Scottish deepwater boats are currently exploiting monkfish in the zone from 200-800 metres, for which there is a high market value. This deepwater fishery probably requires considerable trawling effort to achieve viable catches and as a consequence, results in the discarding of other deepwater species and even some traditional species that are managed by quota (Gordon and Hunter 1994a).

‘Invisible’ discards
Very few deep sea fish have a slimy skin and very often the scales tend to be large and easily lost. Grenadiers generally have few scales left after coming out of the net. The black scabbard fish is generally white, having lost its fragile skin in the net (Gordon and Hunter 1993). Others have a thin fragile skin which is easily damaged by the trawl. The fragility of deep-sea species may result in a high mortality of fish which enter the mouth of the trawl and subsequently escape through the meshes of the net causing “invisible” discards (Gordon and Hunter 1994a).

Many species of fish which have a small adult size may suffer considerable damage in escaping from the trawl as ‘invisible discards’. This has important implications in terms of the loss of the food source of the larger fish, being an integral part of the deep-sea food chain. In addition immature commercial fish may suffer high levels of mortality with repercussion in the size of the adult population (Gordon and Hunter 1993; Gordon and Hunter 1994a).

It is possible that trawling at about 1,000 metres and deeper in the Rockall Trough, where good catches of adults are made, could result in a significant mortality of juvenile roundnose grenadier through ‘invisible’ discarding, with a consequent reduction in future recruitment to the adult stock (Gordon and Hunter 1994a).

Gear trouble
Static fishing gear such as longlines, bottom gillnets and traps are much more likely to be used at greater depths where the fish tend to have a well developed sense of smell and are attracted by bait (Gage and Gordon 1995a). Research has shown that fish are attracted to bait over a very long distance, which is another indication of the extreme low density and the vulnerability of deepwater species to fishing.

One of the problems of longlining is that there is always some loss of catch caused by the drag through the water as the line approaches the surface and also when the fish leaves the water and weight increases. Longlining also has the disadvantage of catching various deepwater sharks, which are highly vulnerable to fishing (Gordon 1992a).

Gillnets and fish traps which have been lost can continue to catch fish in what is known as ‘ghost fishing’. This will have a detrimental and unquantifiable effect on fish stocks. It is often argued that in shallow water, lost gillnets soon become fouled and/or bundled up by currents and so become ineffective. However, in deepwater fouling is less likely and the currents are lower strength. It is thought that gillnets will remain effective for much longer periods in deepwater (Gordon 1994a; Gordon 1996b) and will go on killing fish for years (Gordon 1992b; Gordon 1996b).