Scottish Marine and Freshwater Science Volume 3 Number 3: Clyde Ecosystem Review

A review of previously published information and knowledge concerning the Firth of Clyde ecosystem and suggestions for further studies in the Clyde required in order to support sustainable use of this active Scottish marine ecosystem


6.1 Introduction

The fishing industry has had a major impact on the image and character of the Firth of Clyde and many of its coastal towns. It is a complex, traditional industry and holds great economic importance for many rural communities around Scotland's coast, including the Clyde, as well as political influence (SSMEI, 2010). In the Clyde, fishing occurs throughout the area and the main landing ports are Campbelltown, Tarbert, Ayr and Troon and some smaller fishing ports, Greenock, Largs and Rothesay. The Clyde had once been a productive fishery for demersal, pelagic and shellfish but is now primarily a shellfish fishery. The locations of the main fishing ports in the region where landings have been recorded are shown in Figure 6.1.

Figure 6.1 The locations of the main Clyde fishing ports where landings have been recorded. Also shown are 1 nm (orange) and 3 nm (red) limits from the coast.

Figure 6.1 The locations of the main Clyde fishing ports where landings have been recorded. Also shown are 1 nm (orange) and 3 nm (red) limits from the coast.

6.2 Legislation

In Scottish Inshore waters the principle legislation for fisheries management is the Inshore Fishing (Scotland) Act 1984. The principal fisheries management legislation Acts that are applicable to the Clyde are included in Table 6.1. The list should not be considered a comprehensive list and does not include the statutory instruments implemented under the different acts throughout the years.

Table 6.1 The principal fisheries management legislation Acts that are applicable to the Clyde. The list should not be considered a comprehensive list and does not include the statutory instruments implemented under the different acts throughout the years.

Year Title
1885 Sea Fisheries Scotland Act
1891 Fisheries Act
1893 North Sea Fisheries Act
1915 Fishery Harbours Act
1934 Illegal Trawling Act
1955 Fisheries Act
1962 Sea Fish Industry Act
1964 Fishery Limits Act
1966 Sea Fisheries Regulation Act
1967 Sea Fisheries (Shellfish) Act
1967 Sea Fish (Conservation) Act
1968 Sea Fisheries Act
1970 Sea Fish Industry Act
1973 Sea Fisheries (Shellfish) Act
1976 Fishery Limits Act
1984 Inshore Fishing (Scotland) Act
1992 Sea Fisheries (Wildlife Conservation) Act
1992 Sea Fish (Conservation) Act
1994 Inshore Fishing (Scotland) Act
2000 Sea Fisheries (Shellfish) Amendment (Scotland) Act
2005 The Registration of Fish Sellers and Buyers
2009 Marine and Coastal Act
2010 Marine Scotland Act

Some of the legislation changes have had a profound effect on the behaviour of the fisheries such as following the introduction of the Buyers and Sellers registration in 2005 which resulted in an almost complete stop to black landings around Scotland. However, since the Buyers and Sellers regulation there has also been a dramatic increase in the levels of discarding of finfish across all age groups suggesting that the legislation has controlled landings rather than catch.

A ban on trawling in the Clyde was originally introduced in the late nineteenth century after fishery scientists at the time suggested that the Firth of Clyde fisheries were becoming depleted due to excessive trawling (Thurstan and Roberts, 2010). The Act was then amended in 1934 to include beam and otter trawling under the Illegal Trawling (Scotland) Act 1934. The closure remained in place until 1962 when the Sea Fisheries (Scotland) Byelaw (No.65) came into effect which permitted summer (1 May to 30 September) otter trawling within the Firth of Clyde except within a 3 nautical mile limit (Figure 6.1). Further legislation in 1968 (Sea Fisheries (Scotland) Byelaws no.'s 80 and 83) enabled fishing to take place throughout the year. The 3 nautical mile limit remained until 1984 (Inshore Fishing (Scotland) Act) when it was repealed under pressure from the industry during a time when demersal finfish landings were in decline.

Recently there has been a call to re-instate the 3 n.mile limit, most strongly from the Scottish Sea Angling Conservation Network (SSACN). Sea angling was at one time a major sport and tourist attraction in the Clyde, which began in the 1960s. However, with the decline in catches sea angling in the Clyde is not what it used to be. SSACN blame poor fisheries policies and practices combined with political ineptitude, resulting in the removal of many gear and access restrictions in the 1980's, to be the reason that the quality of sea angling has drastically declined. This suggestion was further supported by Thurstan and Roberts (2010) which linked the collapse of the demersal fisheries in the Clyde with the removal of the 3 nautical mile closure.

With regards to legislation applicable to the herring fishery in the Clyde, it was also prohibited to pelagic trawling for herring until 1962. In the same year a Byelaw was passed which prohibited fishing for herring by any method between midnight on Fridays and midnight on Sundays. The spawning ground for herring located on the Ballantrae Bank had originally been closed in 1860 but this was repealed in 1867. The bank stayed open to fishing until 1972 when a seasonal closure was introduced from 15 February until the 30 April each year. There was an exemption for anchored drift nets but they became included in the seasonal closure in 1977 (Bailey et al., 1986).

The measures which remain in force in order to protect the spring-spawning herring are as follows;

  • a complete ban on herring fishing from 1 January to 30 April
  • a complete ban on all forms of active fishing from 1 February to 30 April on the Ballantrae Bank spawning grounds
  • a ban on herring fishing between 00,00 Saturday morning and 24,00 Sunday night.

6.3 Stock Assessment

ICES provides scientific advice on the management of the important commercial species of fin fish and some shellfish stocks in all areas of the north-east Atlantic. For the purpose of assessment, ICES divide sea areas into sub areas and divisions. Annual Total Allowable Catch (TAC) quotas are agreed for each of the fisheries in each division. Ideally these are based on scientific advice which is given in accordance with the precautionary approach and aimed at keeping stocks above the reference point of Bpa (if this is defined for the stock). This work is summarised in the annual ICES advice. ICES is responsible for providing scientific advice on TACs and other conservation measures to the international fisheries commissions, including the EU. In addition, since 2010 ICES has given advice in relation to Maximum Sustainable Yield (MSY).

The Firth of Clyde falls into Division VIa which stretches from the North Channel of the Irish Sea to 60ºN and west to 12ºW including the coast of Scotland as far east as 4ºW including the Clyde. For the main demersal species stock assessments are carried out at the level of VIa. Thus assessment and setting of Total Allowable Catches is carried out at the level of the 'West of Scotland' rather than the Clyde. The main exceptions to this are Clyde herring, Nephrops and scallops, which are assessed locally.

6.4 Pelagic Fisheries

6.4.1 Herring

The Firth of Clyde was once the centre of a major herring fishery which had been prosecuted in Scottish inshore waters since the 15th century (Rorke, 2005) although it has been subject to major fluctuations (Bailey et al., 1986). The biology of the Clyde herring stock has been studied since the late 1800's following the opening of a temporary laboratory at Tarbert, Loch Fyne (Gordon and De Silva, 1980). In the past the fishery was almost entirely dependent on spring-spawning herring which spawn locally on Ballantrae Bank in the Firth of Clyde until about 1969 when a new component of autumn-spawning herring gradually increased in importance in landings and was predominant by 1972 (HAWG, 1978). Although autumn-spawned herring formed a greater part of the Clyde population, there was no evidence of autumn spawning occurring within the Firth (Wood, 1960; Saville, 1962).

Although there had been no significant change in the abundance of the total herring population, the spring-spawners had decreased in the early 1970s which was accompanied by an increase in the absolute abundance of autumn-spawners. In 1979 TAC regulations were imposed on the stock in addition to a closure of the fishery from October through till May in response to the clear evidence of a decrease in the abundance of spring-spawning herring (Bailey et al., 1986). The cause of this decline may be natural as it can be attributed with the reduced recruitment of the local spring-spawning stock, but the effects of intensive exploitation cannot be ruled out (Bailey et al., 1986).

In Scotland, records of herring landings were kept even before the Fishery Board for Scotland was formed in 1882. The fishery was prosecuted by anchored drift net and ring-netting until pair trawling arrived in 1968 and became the predominant method by 1973 (Bailey et al., 1986). Catches increased to a peak between the late 1950s and mid 1960s averaging around 14,000 t per annum after which the stock began to collapse (Bailey, 1986) (see Figure 6.2).

The Firth of Clyde is part of ICES Division VIa and in the 1970s all the herring stocks in the adjacent areas to the Clyde were at a low level. This led to Division VIa herring fishery being closed in 1978 but because of the complex origin of the Clyde herring population, ICES advised that the area be treated as a separated management unit. A small TAC of 2000t was agreed in 1979 for the area (Bailey et al., 1986) and the first analytical assessment was carried out in 1982 (Hatfield et al., 2007). Management of the Clyde has remained separate since 1979.

Landings of herring from the Firth of Clyde fluctuated about a long-term average of 14,200t in the period 1893-1960, from 1960-1978 the average landings were down to 8300t and only 2400t from 1978-1984 when landings became regulated. Since 1991 catches in the Clyde have exceeded 1,000 t only once (see Figure 6.3). The reasons for the collapse of the Clyde herring are not clear cut however declining spawning stock biomass, high fishing mortality and lack of recruitment to the local spring-spawning stock may well have been causative (Bailey et al., 1986).

Figure 6.2 Herring landings in the Firth of Clyde (1000 tonnes) 1890-1984 (Source, Bailey et al., 1986).

Figure 6.2 Herring landings in the Firth of Clyde (1000 tonnes) 1890-1984 (Source, Bailey et al., 1986).

Figure 6.3. Herring landings (1000 tonnes), 1955 to 2009. Spring and autumn spawning herring combined. Agreed TAC 1984 - 2009.

Figure 6.3. Herring landings (1000 tonnes), 1955 to 2009. Spring and autumn spawning herring combined. Agreed TAC 1984 - 2009.

Today herring in the Clyde are exploited by a small number of local trawlers and by pair trawlers from Northern Ireland. There is not much information on the current status of the stock as catch and sampling data availability has been minimal since the early 2000s. Thus, the precise status of the stock is uncertain; the stock is low but there is currently no evident downward trend. The Clyde herring stock is recognised as a separate stock but is not currently assessed analytically.

In 2011 under the provisions of the TAC and Quota Regulations (57/2011), the European Commission has delegated the function of setting the TAC for certain stocks which are only fished by one Member State, to that Member State - setting out a mechanism for how the TAC should be determined. This provision currently applies only to the Clyde Herring stock. However, as mentioned, Clyde Herring is a very data poor stock which makes this task more difficult. It has been proposed, in the absence of a detailed assessment, to set the TAC for 2011 based on the recent stability of the fishery at an average of the TAC set for the last 3 years.

Despite the management measures imposed on the Clyde herring fishery such as the closure of the spawning fishery, there is so far no evidence of recovery of the recruitment to this stock.

6.5 Demersal Fisheries

In the Clyde yields of demersal fish increased rapidly after the repeal, in 1962, of the long standing ban on trawling in the area up to 3 nautical miles from the coast. The demersal fish landings continued to increase up until 1973 when landings reached a maximum before starting to decline (Hislop, 1986). In 1984 the 3 nautical mile ban was also lifted in order to try and maintain catch levels. This proved ineffective and demersal fin fish landings continued to decline until the early 2000's when the directed fishery effectively ceased (Heath and Speirs, 2010). In addition, the total demersal fishing effort in the Clyde had risen since the 1960's almost entirely due to an increase in trawling for Nephrops from which there was a large by-catch of whitefish. According to the Scottish Sea Fisheries Statistical Tables (SSFST), which were published annually by DAFS, the annual weights of all demersal species landed at Clyde ports, in the early 1980's, ranged from 4,000 to 10,000 tonnes, valued at £2-5 million. In terms of value, cod and hake were the most important species in the Clyde fishery; these species composition of landings in the Clyde are unusual for a Scottish inshore fishery in that the proportion of haddock is rather small, while that of hake is relatively large.

The demersal fishery was seasonal on the Clyde grounds and most of the fishing effort took place during the winter and spring, partly because catch rates were relatively high at this time of year and partly because the area provides shelter from the winter weather. The Clyde fishery depends to a large extent on young fish and the relative importance of the principle species fluctuates from year to year in response to variations in the strength of the recruiting year class. Although there was no conclusive evidence that the Clyde populations were self-contained, all the major species are known to spawn within or close to the area and tagging experiments had shown that there was not much mixing between Clyde fish and those from surrounding areas. Historically, the most important single species in the Clyde demersal fishery in terms of landings has been whiting (Gambell, 1965).

In 1986, the demersal fish stocks in the Clyde were considered to be exploited intensively with the fishery there depending to a large extent on the younger age groups and thus the fishery was highly sensitive to variations in the strength of the recruiting year class (Hislop, 1986).

Demersal fishing in the Clyde Sea has changed dramatically since the 1986 review by Hislop. Since the mid 1980's, as mentioned previously, there has been a collapse in the stocks of the main commercial demersal species, cod, haddock and whiting and a transition to a Nephrops dominated fishery in the Clyde (see Figure 6.4.). Although the target fishery is Nephrops the demersal fisheries in the Clyde Sea area are a mixed fishery and as a result of this all groundfish stocks are exploited. Cod, haddock and whiting are the predominant roundfish caught in this mixed fishery with highest catches in the winter, although there can also be important by-catches of other species such as hake and saithe (WGNSDS, 2008). A mixed fishery by its nature is difficult to determine appropriate management strategies for and in recognition of this, ICES has given the following advice for fisheries in the west of Scotland, they should fish,

  • without catch or discard of cod and whiting in Division VIa
  • with minimal catch of common skate and undulate ray
  • within the biological exploitation limits of all other stocks

Thurstan and Roberts (2010) used historical records and anecdotal information to describe changes in the fisheries of the Clyde. It has been asserted by this study that the record of demersal fish landings demonstrates that the Clyde is an ecosystem in 'ecological meltdown'. However, fisheries landings data alone are insufficient to support such a strong claim. For example, discard data can form an important component of the total species catch in demersal fisheries and often render landings data inadequate for monitoring trends and assessing the state of an ecosystem (Stratoudakis et al., 1999).

For the purpose of this review it is useful in the first instance to reproduce the trends in landings for the fisheries in the Clyde using historical landings data. Figure 6.4 shows the trends in landings (taken from the Fisheries Management Database at MSS) for the three main demersal fish species cod, haddock and whiting and for the Nephrops fishery. The arguments made by Thurstan and Roberts (2010) is evident from the plot; the decline in demersal fin fish landings corresponds with the increase in Nephrops landings and the transition to a Nephrops dominated fishery in the Clyde.

Figure 6.4 Landings from 1960-2010 for cod, haddock, whiting and Nephrops in the Clyde (ICES Stat Rectangles 39E4 + 39E5 + 40E4 + 40E5. Source, MSS).

Figure 6.4 Landings from 1960-2010 for cod, haddock, whiting and Nephrops in the Clyde (ICES Stat Rectangles 39E4 + 39E5 + 40E4 + 40E5. Source, MSS).

6.5.1 Cod

The Atlantic cod ( Gadus morhua) is found in all Scottish waters from shallow coastal areas to waters deeper than 200m. It is a key species as it is an important marine predator and a highly valued commercial resource providing one of the major fishery resources in the North Atlantic dating back centuries (Hughes and Nickell, 2009; Hutchinson et al., 2001). They are fished throughout the North Sea and West Coast of Scotland However, high exploitation rates in the 1980s and 1990s have depleted the cod stocks off the west coast of Scotland, like many other Atlantic cod stocks, to an all time low and these stocks are now showing some signs of recovery (Wright et al., 2006).

In response to the depleted state of the cod stocks the emphasis of recent management has been to help their recovery with the implementation of the Cod Recovery Plan in 2001 ( ICES, 2001). Under the recovery plan the immediate requirement was to allow as many cod as possible to spawn before the end of April when the spawning season finishes. The seasonal closure remains in the Clyde from February 14th - April 30th when all bottom trawling is prohibited in the outer closed area shown in Figure 6.5 and only trawling for Nephrops is allowed in the inner closed area. ICES have previously advised a zero catch of cod which would provide the highest probability of stock recovery in the context of the precautionary approach. However, this would effectively mean a closure of all the mixed demersal and Nephrops fisheries on the West Coast and, due to social and economic issues, fisheries managers have never fully implemented the recommendation of 'zero catch'.

Figure 6.5 Clyde closed areas to fishing for cod.

Figure 6.5 Clyde closed areas to fishing for cod.

Understanding the Clyde cod structure and migration habits is important for the local management of the stock. Cod in the west of Scotland have shown a high degree of fidelity to their spawning grounds with recent studies estimating that between 67-97% of inshore cod remains within 100 km of their spawning ground (Wright et al., 2006). The scale of juvenile and adult movements indicated by recent studies suggests that there are sub-stocks within ICES division VIa which may be reproductively isolated due to limited movements. Therefore, recovery within the Clyde is likely to be dependent on self-recruitment in this area and movements into the region from the nearby Inner Hebrides. Although the closed area contains an important spawning aggregation, the spatial scale and seasonal limit is too small to protect this local sub-stock (Wright, 2010).

6.5.2 Hake

The deep waters of the Firth of Clyde are known to have provided excellent fishing grounds for hake (Hislop, 1986). ICES provides advice for the Northern hake stock as a widely distributed migratory stock which can be found in the Kattegat and Skagerrak straits adjoining Norway, Sweden and Denmark, in the North Sea, the Channel and also in the waters to the west of Scotland. Currently the state of the stock is unknown but trends based assessment indicates an increase in SSB for the Northern hake stock ( ICES, 2011). Information specific to hake in the Clyde is limited but discarding rates of juvenile hake is substantial in the area (Stratoudakis et al., 2001). Figure 6.6 shows the landings of hake in the Clyde from 1960-2010, which show a significant decline in landings since the early 1990s to virtually zero.

Figure 6.6 Landings of hake in the Clyde from 1960-2010 ( ICES Stat Rectangles 39E4 + 39E5 + 40E4 + 40E5. Source, MSS).

Figure 6.6 Landings of hake in the Clyde from 1960-2010 (ICES Stat Rectangles 39E4 + 39E5 + 40E4 + 40E5. Source, MSS).

6.5.3 Saithe

Saithe in the Clyde has been described as a boom-and-bust fishery (Thurstan and Roberts, 2010) which was discovered in the late 1960s and peaked in 1973 with landings of over 7000 tonnes; however the catch quickly collapsed to very little by the 1990s (see Figure 6.7).

Figure 6.7 Landings of saithe in the Clyde from 1960-2010 (ICES Stat Rectangles 39E4 + 39E5 + 40E4 + 40E5. Source, MSS).

Figure 6.7 Landings of saithe in the Clyde from 1960-2010 (ICES Stat Rectangles 39E4 + 39E5 + 40E4 + 40E5. Source, MSS).

6.6 Shellfish fisheries

6.6.1 Nephrops

The Langoustine, Norway Lobster, Dublin Bay Prawn, or Nephrops norvegicus (L) is the most commercial shellfish resource in UK waters and is extensively exploited throughout Europe. Up until the early 1950's Nephrops attracted little commercial attention in Scotland and was rarely on sale in the region. However, in the last 50 years an important fishery for Nephrops has developed in Scotland and is now the largest in Europe. These developments were delayed in the Clyde by the Herring Fishery (Scotland) Act 1889, which prohibited otter and beam trawling in the Clyde until 1962 when otter trawling for Nephrops in the Firth of Clyde was permitted in the summer (Bailey et al. 1986). Shellfish such as Nephrops, scallops, lobster, edible and velvet crab are now the most important resource to the inshore fleet of the region. The majority of the inshore trawling fleet are dependent on Nephrops which are landed all year.

During the summer Nephrops are exploited by the Clyde fleet to the west of the Kintyre peninsula and in the more sheltered and accessible waters of the Firth of Clyde in the winter months. Nephrops are targeted by small demersal trawlers of no more than 30 m and are allowed to use a mesh size of 70mm, prior to 2009, and 80mm from 2009 onwards in the cod-end provided provided that Nephrops constitute a certain minimum proportion by weight of the total landings (Tuck et al., 1997; ICES, 2011).

In more detail, the changes in technical measure regulations applying on the West of Scotland ( ICES VIa) were designed to improve the selectivity and reduce unwanted catches of small fish. The minimum mesh size in the Nephrops fishery was increased from 70mm to 80mm as part of the 2009 EU emergency measures to deal with reduced gadoid populations. A second measure was the requirement for Nephrops vessels operating on the west coast to use a square mesh panel with a mesh size of 120mm as part of the Scottish Conservation Credits scheme (in place since 2008).

Owing to its burrowing behaviour, the distribution of Nephrops is restricted to areas of a particular sediment type; mud, sandy mud and muddy sand. Burrow density and animal size are related to sediment characteristics and hydrography (Chapman and Bailey, 1987). Around Scotland it has been found that areas of fine sediments are characterised by large Nephrops occurring in low densities and areas of coarser sediments are characterised by smaller Nephrops at higher densities (Tully and Hillis, 1995). In the Clyde, burrow densities are higher in the south and the average size of Nephrops is smaller. This is particularly evident from the differences in discard composition between the north and the south of the Clyde. Catches from the south have a higher proportion of small Nephrops resulting in large proportions of undersized Nephrops and Nephrops heads (Bergmann et al., 2002). Trawls in the south also generate larger quantities of roundfish discards (35%), including the important commercial species whiting, cod and haddock (Bergmann et al., 2002)

Nephrops stocks have been identified on the basis of population distributions which are defined as separate functional units ( FU) and are the level at which ICES Working Groups collects fishery data and performs assessments. The Clyde Sea area ( FU13) is one of three distinct functional units within ICES area VIa where Scotland's Nephrops stocks are assessed, although the stocks are considered to be functionally separate they are currently managed under the West of Scotland single stock definition.

Survey assessments of the stock are carried out annually by an underwater TV survey ( UWTV) which determines Nephrops burrow density on the seabed. The information gathered provides an index of stock abundance which is independent of the fishery and together provides information on the status and predicted stock level.

The Nephrops fishery has also developed in recent years due to an increase in catches by static gear such as creeling. Almost all of the Clyde including the sealochs is targeted by bottom trawlers fishing for Nephrops; VMS data linked to landings shows the areas which are targeted by boats greater than 12m in length ( Figure 6.8). This figure suggests that the area covered by the UWTV survey is slightly smaller than the area covered by fishing activity ( WGCSE, 2010). The perception of the state of the stock in the Firth of Clyde is that it is stable and with the recent high catches by trawlers it is perceived to be a sustainable fishery. However, a reduction in abundance in 2007 coupled with the increase in landings has led ICES to advise a more precautionary approach in recent years ( Figure 6.9).

Figure 6.8 The distribution of VMS pings (shown in red) recorded from Nephrops trawlers >12 m length in 2010.

Figure 6.8 The distribution of VMS pings (shown in red) recorded from Nephrops trawlers >12 m length in 2010.

Figure 6.9 Nephrops, Clyde ( FU13), Firth of Clyde subarea. Time-series of revised TV survey abundance estimates (not adjusted for bias), with 95% confidence intervals. (Source, ICES, 2011).

Figure 6.9 Nephrops, Clyde (FU13), Firth of Clyde subarea. Time-series of revised TV survey abundance estimates (not adjusted for bias), with 95% confidence intervals. (Source, ICES, 2011).

For most analytical models used for assessment purposes, it is assumed that the population is homogenous in terms of its biological characteristics. The Nephrops stock in the Firth of Clyde has been shown to consist of a mixture of sub-unit populations differing from each other in growth, size composition and density which invalidates the assumption of homogeneity.

The north-east Atlantic Nephrops trawling fisheries have been ranked as having the fifth highest discard ratio in the world (number of by-catch to number of target species) (Catchpole et al. 2005). Unsurprisingly the discarding rates are also high in the Clyde Nephrops fishery with the proportion of the catch discarded being estimated between 66 and 80% in the Clyde Sea (Bergmann et al., 2002; Stratoudakis et al., 2001). The discards mainly consist of small demersal fish particularly young whiting ( Merlangius merlangus) (Stratoudakis et al., 2001 and see Section 7). The use of more selective gear can help reduce by-catch levels such as the use of the Swedish selection grid (Nordmore grid) which may allow the Nephrops fishery to be decoupled from cod and other demersal fish species (Valentinsson and Ulmestrand, 2008). The Swedish grid has been adopted by the Swedish Nephrops fishery and has recently been trialled for the west coast of Scotland Nephrops fishery ( MSS, 2011). The trials resulted in a significant loss in the smaller Nephrops (< 41-45mm carapace length) attributed to the selection of the 80mm cod-end and no significant loss of larger Nephrops ( MSS, 2011). There has, however, been a reluctance to adopt this method in the Clyde Nephrops fishery due to handling difficulties and the decrease in retention of the larger size classes of Nephrops.

6.6.2 Scallops

The most important commercial molluscan species in the Clyde Sea are the scallop ( Pecten maximus) and the related but smaller queen scallop ( Chlamys opercularis). Both species occur wherever the sea bed consists of sand, fine gravel or sandy gravel. The distribution of suitable sediments results in the species being found in a narrow strip of sea bed on each side of the Clyde and around Arran in depths no greater than 40m (Mason and Fraser, 1986). Both the scallop and the queen are filter feeders, the scallop lies recessed in the sediment, the queen, however, do not recess into the sediment and will swim sporadically for a short time.

A dredge fishery for the scallop started in the Clyde Sea in the 1930's but remained small during the 1940s and 1950s with landings averaging 112t (Mason, 1983) but the scallop fishery provided a useful income for a few boats between other fisheries. In the 1960s there was a radical change in the fishery; a large demand for scallops had developed in continental Europe resulting in a huge expansion of the fishery. The advent of processing factories in Scotland also allowed fishing for scallops to occur throughout the year (Mason and Fraser, 1986). Scallops are now exploited in the Clyde by a fleet of dredging boats which developed in the area in the late 1960's. The Ayr and Campbeltown Fishery District, includes the Clyde Sea Area and has been the major locus of the fishery in Scotland since the fishery began.

The traditional method of catching scallops is by dredge which basically consists of a rigid metal frame and toothed bar that digs into the sea bed at an angle of 60-70º; this is towed along the sea bed and the scallops are scraped into a netted bag (Mason, 1983). In the early days of the Scottish scallop fishery. A small percentage of the catch is also taken by hand by SCUBA divers along the coast. Figure 6.10 shows the distribution of Vessel Monitoring System ( VMS) pings from scallop boats greater than 15m, this plot indicates that scallop dredging by larger vessels occurs mostly along the coast of the Clyde.

Figure 6.10 The distribution of VMS pings (shown in blue) recorded from scallop vessels >12 m length in 2010.

Figure 6.10 The distribution of VMS pings (shown in blue) recorded from scallop vessels >12 m length in 2010.

Many fisheries are controlled by one or more of a number of types of measures such as quota restrictions, mesh regulations, minimum landing size, closed areas and seasonal closures. Not all of these measures are suitable for scallops due to the fragmented nature of the fisheries which is prosecuted in remote areas and landed in many isolated harbours. The most applicable measure for conserving scallop stocks is the minimum landing size and is the most commonly practised (Mason, 1983). In September 2003 The Prohibition of Fishing for Scallops (Scotland) Order 2003 was implemented. This put in place new measures for scallop conservation that limited the number of dredges a scallop vessel can tow to a maximum of 8 per side for Scottish Inshore waters ( FRS, 2006). Currently there is no catch limit on UK scallop fisheries. They are mostly managed through minimum sizes, restriction on dredge numbers and seasonal closures in some areas. Scallop stocks are highly susceptible to overfishing due to their limited mobility and confinement to a particular type of sea bed. However, scallop stocks have largely held up well on all grounds, even in the Clyde where fishing has gone on the longest.

The historical landings series shows marked fluctuations resulting from a combination of variable recruitment and changes in fishing effort ( ICES, 2006). After the rapid development of the fishery in the late 1960s, landings declined sharply in the early 1970s, only to increase again reaching a peak in the late 1970s. Following the high reported landings of 1984, landings declined to a low point in 1988 after which they increased fairly steadily until 1998 and have fluctuated since then, probably influenced by the effect of area closures ( ICES, 2006). The historical landings show that until recently annual landings have been, typically, between 200 and 300 tonnes. However, since 1999, there has been an upward trend with the 2003 landings exceeding 500 tonnes for the first time since the mid 1970's (see Figure 6.11). The most recent assessment for scallops was done in 2006 which is mainly due to a lack of data and the fact that sampling of Clyde scallop landings is quite limited and irregular. Because of limited port sampling, the age composition data are insufficient to carry out a VPA assessment for the Clyde and no survey data is available ( ICES, 2006). Catches are reported by individual ICES rectangles and in recent years rectangle 40E4 has accounted for almost all of scallop Clyde landings. All recorded landings are taken by dredge and the main landing ports are in Campbeltown, Troon and Ayr.

Figure 6.11 Landings of scallops in the Clyde from 1967-2010 (Source, MSS).

Figure 6.11 Landings of scallops in the Clyde from 1967-2010 (Source, MSS).

6.6.3 Other Shellfish

There are small creel fisheries for crabs, lobsters and shrimp mostly on a part-time basis. Gathering mussels was a flourishing industry in Scotland at the end of the nineteenth century and accounted for 29% of the total shellfish landings, but now only a few are taken, mainly in the lochs and mainly for human consumption. Beds of cockles are found in several parts of the Clyde, including the Isle of Bute, the Ayrshire coast and at Stranraer (Mason and Fraser, 1986). The native oyster was once abundant and widespread in Scottish waters and now it is scarce with the only commercial fishery remaining in Scotland is in Loch Ryan ( SNH, 2011; Mason and Fraser, 1986)

Around the Arran coast electric fishing for razor fish is known to occur. Fishing for razors using electricity is reported to be a very effective method. However, there is limited knowledge of the effect of the electric field on other species (Breen et al., 2011). The legality of using electricity as a fishing method is an issue of concern.

6.7 Sea Angling

Sea angling is a long established leisure activity in the Clyde and the area once had a reputation of being one of the best sea angling destinations in the UK. The Upper Clyde has experienced a significant decline in sea angling. Large number of anglers were attracted and fished from both boat and the shore and there is evidence that some travelled on a fairly regular basis from as far afield as the English Midlands.

The governing body is the Scottish Federation of Sea Anglers, which has approximately 64 affiliated clubs (Murison and Robson, 1997). Sea Angling has three main forms, angling from the shore; inshore fishing; and deep sea fishing. Sea angling occurs throughout the Clyde and the area once held major competitions including the Scottish Open Shore, the European Boat Cod Festival and the Saltcoats Sea Angling Competition. In their prime (1970s), these competitions attracted up to 1300 entrants and visiting anglers enjoyed unparalleled sport (Donnelly et al., 2010; SSACN, 2010).

In response to sharp falls in angler number, and the poor catches in the 1988 White Horse Whisky competition, the Scottish Tourist Board commissioned the Clyde Sea Angling Study ( CSAS). After reviewing the available evidence, the CSAS study concluded that the Clyde fish stocks declined as a result of the increased commercial fishing effort, much of which resulted directly from the Inshore Fishing (Scotland) Act 1984 which opened up the zero to three mile zone to all mobile gear fishing activity). There was, however, no policy response and a great many sea angling dependent jobs were subsequently lost, as CSAS had predicted. The angling charter fleet has now gone and angler numbers are much reduced (Marine Scotland, 2009).

6.8 Environmental Effects of Fishing

It is clear that the primary objective of all fishery management strategies must be to determine fishing pressure on fish stocks in order to manage this pressure and maintain a sustainable industry. However, additional consideration should be given to the conservation of habitats and the protection of biodiversity which includes monitoring the damage caused by the impacts of fishing to target species, discards, benthic infauna and sediments (Hauton et al., 2003).

6.8.1 Trawling

The effects of towed fishing gear on the marine environment has attracted considerable interest recently with many studies examining this issue using various approaches (Currie and Parry, 1996; Tuck et al., 1998; Thrush et al., 1998;). The use of such gear affects much more than the species being caught. As the gear is dragged along the sea bed it disturbs both the sediments and the organisms and plants that live there. Many of the species affected by the fishing gear have no commercial value but they do play important ecological roles (Coggan et al., 2001). Direct changes can result from the crushing of individuals while the partial excavation and damage of near surface dwelling organisms can attract mobile predators/scavengers. In the majority of cases, the immediate impacts on benthic organisms are quantitative in nature such that changes in the abundance of various organisms occur rather than their elimination. Trawling can also reduce habitat complexity by damaging the physical and biological structures of the sea floor ( NRC, 2002).

The obvious impacts of trawling are the physical effects of the doors and that trawling results in a reduction of the abundance of large, fragile organisms and an increase in the abundance of opportunistic species (Tuck et al., 1998; Coggan et al., 2001). Trawl damaged fauna provide an artificial food subsidy to an area which, over time, generally seems to shift the species composition in favour of scavenger feeders (Ramsay et al., 1997). A study on the effects of trawl disturbance was carried out in an un-fished sea loch of the Clyde, Loch Gareloch, which found that measures of diversity decreased while the number of individuals increased suggesting a disproportionate increase in a few dominant species (Tuck et al., 1998). The main species that showed a consistent increase in abundance in association with disturbance was the cirratulid family which are considered to be opportunistic in nature (Tuck et al., 1998).

The principle fishery today in the Clyde is a Nephrops fishery and as shown from the VMS data (see figure 6.8) nearly the entire Clyde is targeted by Nephrops trawlers. In terms of the direct effects on Nephrops, the passage of ground gear over the sea bed may close over the entrance to burrows of Nephrops and other species. However, the animals have been observed to be able to open the burrows again easily thus the effect on uncaught Nephrops is minimal (Coggan et al., 2001).

6.8.2 Dredging

The impacts of trawls and dredges are often thought to be similar as they are both towed along surface sediments where they are likely to damage organisms. However, the toothed scallop dredge, designed to dig into the sediment may be the most damaging of all demersal fishing gears to benthic communities (Jenkins et al. 2001; Collie et al., 2000). Scallop dredging has been found to markedly change the topography of the sediment by flattening mounds and filling depressions in soft sediments directly after trawling (Currie and Parry, 1996). However the long-term effects of trawling and dredging differ greatly with dredging affecting the most vulnerable species which are slow-growing and long-lived such as sponges and coral.

Although many studies have been unable to detect the long term effects of gear impact on sedimentary habitats (Hall-Spencer and Moore, 2000), an ongoing scallop dredging study over 5 years has been shown to alter benthic communities, driving them from one state to another often causing the community to become more heterogeneous (Bradshaw et al., 2001). Hall-Spencer and Moore (2000) showed that scallop dredging could kill and bury up to 70% of living maerl (a habitat of special conservation interest in EU waters) in the path of the dredge. Atlantic maerl beds are usually characterised by coarse sediment, clean water and significant bottom currents, thus tend to provide good scallop fishing grounds (Macdonald et al., 1996). The difference between un-fished and fished areas confirms that maerl beds are a particularly fragile habitat. Most of the Clyde maerl grounds have been extensively modified by scallop dredge.

A hydraulic dredge fishery for razor clams (Ensis sp.) exists on a small scale in Scotland although increased demand from Europe and the Far East has led to a growth of interest in the fishery (Hauton et al., 2003). Hydraulic dredging is another severely damaging fishing activity which has the potential to expose deep-burrowing long-lived bivalves, such as the otter shell (Lutraria sp.) which would otherwise rarely find itself exposed on the sediment surface (Hauton et al. 2003). The capacity of these deep-burrowing species to rebury is low compared to bivalves that live closer to the surface of the sediment as they bury into the sediment as juveniles.

6.8.3 Discards

Another significant effect of fishing results from the discards which are produced by commercial fishing worldwide. Discards are by-catch organisms which are returned to sea as they are considered undesirable. This can be for a variety of reasons such as that they have no commercial value, are below minimum landing size or are surplus to quota (Bergmann et al. 2002; Catchpole et al., 2005). Discarding behaviour can be highly variable and depends on current legislation and market conditions (Rochet et al., 2002). The rate of discarding in the Clyde Sea Nephrops fishery is very high and the proportion discarded has been estimated between 66-80%. Wieczorek et al. (2001) estimated that approximately 80 tonnes of biomass are discarded every working day by Nephrops trawlers in the Clyde Sea and that every year a minimum of 25,000 tonnes are discarded in the area.

Beam trawling results in two forms of potential biomass for scavengers, discarded by-catch and disturbed, damaged and dead benthic infauna (Groenewold and Fonds, 2000). Most of the discards from beam trawlers end up on the sea floor, 80% of flatfish, 90% of invertebrates, and 20% of roundfish. The majority of the discarded material in the Clyde Sea reaches the sea bed within a few minutes where it becomes available to benthic scavengers. Invertebrate discards fall to the sea bed rapidly where they are readily consumed by megafaunal scavengers and amphipods (Bergmann et al. 2002). The overall impact of this is that discarding subsidises benthic populations and the food web in general, by making available food resources that would otherwise be unattainable for organisms at a particular trophic level (Kaiser and Haddink, 2007). This carrion could therefore also be subsidising the Nephrops population. However, the extent to which this occurs in the Clyde is unknown as there is a lack of information in the ecological energetics of the species involved locally.

Although the majority of material discarded sinks to the sea bed discards are also exploited at the sea surface by seabirds. Many seabirds especially gulls associate with fishing boats all around the British Isles and discards and offal provide a large quantity of food. This availability of large amount of discards is one of the factors which may have contributed to the rapid growth of some seabird populations (Garthe et al. 1996; Dunnet et al., 1990). Recently there has been a concerted change in discarding practice around Scotland which could have a significant impact on the breeding populations along the coast.

6.9 Suggestion for further work

While the Nephrops stock in the Clyde is regularly surveyed, and there is some survey data describing the demersal stocks (see next section), there is no up to date survey of the herring stock in the Clyde.

In order to estimate the current abundance and distribution of herring and sprat in the Clyde Sea, a fisheries acoustic survey should be conducted, using a multi-frequency scientific echosounder and a pelagic trawl for validation. The survey should include all areas of the Clyde Sea, as close inshore as is possible to navigate the survey vessel, and include the major loch areas (Kyles of Bute, Loch Fyne, Loch Striven, Loch Ryan) and the Clyde Estuary (to Port Glasgow). This survey should result in estimates of abundance at age for herring and sprat, and maps of their distribution.


Back to top