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Home , Carradale, Dunoon, Firth of Clyde, Kyles of Bute, Loch Fyne, Loch Long

Trawling and Dredging in the Clyde Sea Area: History, Impacts and Prospects for Recovery

A report to the Sustainable Inshore Fisheries Trust

M. R. Ryan & D. M. Bailey University of

July 2012

Table of Contents

Table of Contents...... 2

Executive Summary ...... 3

Glossary of Terms...... 5

1. INTRODUCTION...... 6

2. BACKGROUND INFORMATION...... 6 2.1 Clyde Sea Area ...... 6 2.2 Background – Habitats ...... 9 2.3 Background – Important marine species of the Clyde Sea Area ...... 13 2.3.1 Atlantic , Gadus morhua ...... 14 2.3.2 Haddock, Melanogrammus aeglefinus ...... 15 2.3.3 Herring, Clupea harengus ...... 15 2.3.4 Whiting, Merlangius merlangius ...... 17 2.3.5 Plaice, Pleuronectes platessa ...... 18 2.3.6 Norway lobster, Nephrops norvegicus ...... 18 2.3.7 Scallops ...... 19 2.4 Background – Fishing Gear ...... 19 2.4.1 Otter Trawls ...... 19 2.4.2 Beam Trawls ...... 20 2.4.3 Creels and Pots ...... 21 2.4.4 Scallop Dredges ...... 22 2.5 Background – History of Fishing Regulation ...... 23 2.6 Background – History of Fishing Activity ...... 25

3. TRAWLING AND DREDGING – EXTENT AND INTENSITY ...... 27

4. IMPACTS OF TRAWLING AND DREDGING...... 29 4.1 Impacts – Habitat and Community Composition ...... 29 4.2 Impacts – Bycatch ...... 36

5. POTENTIAL FOR RECOVERY ...... 40 5.1 Potential for Recovery – Habitats ...... 40 5.2 Potential for recovery – Fish stocks ...... 41

6. REFERENCES...... 45

2 Executive Summary The Clyde Sea Area has long been an important region within , providing economic, cultural, and environmental benefits for hundreds of years. The area is rich in diverse marine habitats, and contains eelgrass, maerl, gravel, sand, and mud habitats used by a wide variety of commercially valuable fish and invertebrate species. The fishery in this area has long been one of Scotland’s most important, with high-quality herring forming the basis for the early fishery, and Nephrops shifting to prominent since the 1970s.

Changes to fishery regulations have changed both the nature of the fishery itself and the economic value of the catch. A trawling ban within the Clyde Sea Area was implemented in 1889, repealed outwith 3 nautical miles of the coast in 1962, and repealed within 3 nautical miles of the coast in 1984. This opening of the Clyde to towed gears was accompanied by gear improvements that opened previously unfished areas to trawling and dredging . Approximately 83% of the Clyde is now subject to fishing pressure, with many heavily fished areas being trawled multiple times in a given year.

Trawling and dredging alter the seafloor by smoothing mounds, filling depressions, destroying emergent epifauna, overturning rocks, removing or burying plants, and causing high mortality to maerl beds. The Clyde Sea Area is a nursery area for cod, herring, whiting, and scallops, and these species feed back into resident or semi- resident adult populations. Changes caused by trawling and dredging may alter the suitability of the habitat for juvenile fish and crustaceans, potentially altering the Clyde’s role as a nursery for these species.

Changes to benthic communities caused by long-term trawling pressure are characterized by a shift from slow-growing, sessile, or fragile megafauna to highly mobile, smaller, and more physically resistant species. Additionally, non-target animals are captured as bycatch, suffering high mortality rates. Bycatch can be juvenile fish species, invertebrates, and large, non-target fish species, and is estimated to be as high

3 as 25000 tonnes each year for the Clyde Nephrops fleet. Mortality caused by bycatch likely has negative impacts on local fish populations.

In the event of a ban on trawling and dredging, the potential for recovery is very difficult to predict. Maerl recovery from existing damage may not be possible, but halting further damage is critical for this habitat and its ecological role. It is possible the conditions in the Clyde represent a new stable state for the ecosystem and even complete cessation of trawling and dredging might not result in change towards a more natural state. However, available data suggests that the characteristics of the Clyde Sea Area ecosystem (populations of fish with limited external exchange, continued presence of juvenile fish, presence of suitable juvenile fish habitat) make recovery plausible, and potentially more likely to occur here than in other heavily fished ecosystems.

4 Glossary of Terms

Clyde – Unless otherwise specified, refers to the entire Clyde Sea Area

Bycatch – All non-target organisms captured during fishing operations, includes both discards and landed bycatch

Discards – Non-target organisms discarded by fishing vessels while at sea, may include whole organisms or partial organisms (e.g. Nephrops ‘heads’)

Epifauna – Benthic organisms that live on the surface layer of bottom substrates

Maerl – Refers to several species of calcareous red or pink coralline

Eelgrass – Ribbon-like seagrass that grows in muddy substrate and requires high light levels

Granulometric – Refers to the size distribution of particles in mud, sand, gravel, or rock

Otter trawl – Towed fishing gear that utilizes otterboards to keep the mouth of the net open during fishing

Beam trawl – Towed fishing gear that utilizes beams (usually steel) to keep the mouth of the net open during fishing

Rock-hopper trawl – Towed fishing gear with large rubber bobbins attached to the footrope to minimize snagging on rocks and protruding substrata

Creel – Static fishing gear that is made of a wooden or metal frame covered in mesh, with a hard (plastic) or soft (mesh) opening to allow crustaceans to enter but not to escape

Pot – Static fishing gear that is made of a wooden or metal frame covered in mesh, with a bucket-shaped opening in the top to allow crustaceans to enter, but not to escape

Demersal – Refers to the lower layer of the water column, close to and highly influenced by the seafloor

Pelagic – Refers to open water, generally the middle or upper layers of the water column

Benthic – Refers to being on or in the substrates of the seafloor

Infauna – Benthic organisms living beneath the surface of bottom substrates

5 1. INTRODUCTION

Fishing has been a source food and employment for people for thousands of years. Technological developments over the past 150 years have led to increased efficiency in fishing capability, and expanded the range of habitats and locations accessible to fishing fleets. As a consequence, there is increased potential for fishing activities to have an impact on fish populations and habitats. These impacts are direct and indirect, and the extent of their capacity to create change in marine ecosystems has only recently begun to be understood in detail. This understanding is hampered by a lack of sufficiently long-term data and a lack of information about ecosystems’ ‘pre-fished’ states (Greenstreet and Rogers 2000). Nonetheless, it is known that direct impacts from fishing can occur via intentional and accidental capture of animals, and by incidental damage to other animals and habitats by the physical movement of fishing gear. Indirect effects of fishing include changes in nutrient cycling, carbon transfer from the marine to the terrestrial system, changes to food availability for scavengers, and changes in the food chain due to removal of animals of certain species or sizes. This report will summarize the known literature regarding the fishing activities of bottom trawling and dredging as they pertain specifically to the Clyde Sea Area on the west coast of Scotland. Additionally, background information on commercially important fish species, fishing gear, fishing regulations, and the habitats of the Clyde Sea Area is provided.

2. BACKGROUND INFORMATION

2.1 Clyde Sea Area The Clyde Sea Area is located on the west coast of Scotland, at latitude c. 56° N, and is comprised of the Clyde , several sea lochs, and a wide basin enclosing several islands (Mill 1887). The main freshwater inlet is the Clyde river, which flows northwest through Glasgow and outlying communities, then widens into a at (Mill 1886). The Clyde river has been extensively modified by man in the last 200 years, including changes to the structure of the banks and the depth of the river (Boyd 1986).

6 Pollution has also impacted the environment of the Clyde, and high sulphur content from chemical works has been found in shallow and deepwater mud of the Clyde Sea Area (Chumley 1918).

The marine environment becomes predominant downstream of , but when the are extreme and the water levels are low, salt water is able to penetrate into the river to the tidal weir at (Tivy 1986). At Greenock, the firth bends west and three sea lochs, the Gareloch, Long, and , branch north (Mill 1886). The main firth opens to the south, forming the basin that makes up a large portion of the Clyde Sea Area (Halliday 1969). The , , and lie at the northern end of this main basin, and the sea lochs of , and Loch Riddon extend north from the basin’s northernmost tip (Halliday 1969). Loch Fyne is the longest sea loch in Scotland, and it contains the deepest parts of the Clyde Sea Area (Ross, Thompson and Donnelly 2009). The lies almost in the centre of the basin, and extends south from the southernmost part of the main basin (Halliday 1969). The Clyde is connected to the via the gap between the Mull of and Corsewall Point (Halliday 1969). The exact delineation of the Clyde Sea Area has not remained consistent over the years. While the has always been the northern point on the line of demarcation, the southern point has variously been identified as , the Rhins of north of Loch Ryan, and Corsewall Point (Mill 1887, Chumley 1918, Halliday 1969). As such, estimates of the total area of the Clyde Sea Area range from 3004 km 2 to 3650 km 2 (Chumley 1918, Donnelly, Thompson and Ross 2010). For the purposes of this report, the Clyde Sea Area shall refer to the area demarcated by a line running from the Mull of Kintyre to Corsewall Point, enclosing a sea area of 3650 km 2. A watershed 8676 km 2 in size drains into the Clyde Sea Area, which is bordered by approximately 700 km of coastline (Chumley 1918, Ross et al. 2009).

7 The Clyde contains several deep basins, the largest of which is the Arran Basin, a deep area extending into the southern portion of Loch Fyne, and down either side of the north end of Arran (Hoyle 1889). The Basin is located north of and extends north towards the Clyde estuary (Hoyle 1889). The Great Plateau stretches between the east coast of Arran and the mainland, and extends southwest around the south coast of Arran (Matthews et al. 1999). The water depth of the Great Plateau is generally less than 50m (Matthews et al. 1999).

8 2.2 Background – Habitats The diverse habitats within the Clyde support a wide variety of marine life. The coastline is characterized by mainly rocky shores, but sandy beaches, mud flats and gravelly areas also present (Mason and Fraser 1986, Chumley 1918). Away from the coast, mud is the dominant habitat, interspersed with sandy mud and muddy sand (Mason and Fraser 1986).

9 Information on the distribution of biogenic habitats within the Clyde is sparse, but the information available suggests these habitats are widespread and that they are utilized by several commercially and ecologically important species.

Eelgrass ( Zostera sp.) is a seagrass that grows in sandy or muddy habitat, and requires a lot of light to photosynthesize, limiting growth in the UK to depths <15m (Davison and Hughes 1998). Seagrass beds have been found around Arran, in shallow (<10m) depths in Lamlash Bay, forming large meadows in Whiting Bay, and in at least three locations along the west coast of the island (Axelsson et al. 2009, COAST 2005, Davison and Hughes 1998). Seagrass is also present in at least 3 areas of Loch Ryan (MESH 2012). Seagrass beds are preferred habitat for several species of fish and crustaceans, and can reduce predation rates for young fish (Linehan, Gregory and Schneider 2001, Laurel, Gregory and Brown 2003, Lazzari, Sherman and Kanwit 2003).

10 The gaping file shell Limaria hians is a species of bivalve that forms reefs which can serve as habitat for at least 265 species of invertebrates (Hall-Spencer and Moore 2000b). This species is found on substrates ranging from coarse sand to bedrock at depths of up to 98 m (Hall-Spencer and Moore 2000b). In the Clyde, a bed of Limaria can be found in Loch Fyne, although as of 2010, this was comprised of mainly dead shells (Hall-Spencer and Moore 2000b, Duncan 2010).

Maerl is a term that refers to several species of calcareous red or pink that grow very slowly (typically 1 mm per year) into complex beds (Hall-Spencer 2001, OSPAR Commission 2010, Hauton, Hall-Spencer and Moore 2002). The two main maerl-forming species in Scotland are Phymatolithon calcareum and Lithothamnion glaciale (Hall-Spencer 2001). Maerl grows where sufficient light is available (generally up to depths of 40m, depending on ; in the Clyde most maerl is limited to depths up to 24 m) and where currents provide water exchange (Hall-Spencer 2001, OSPAR Commission 2010). Maerl beds generally grow on clean gravel, sand or muddy mixed substrates and may be comprised of living or dead maerl, or some combination of the two (OSPAR Commission 2010). Little is known about the maerl reproduction and its capacity to colonize new areas, but the growth rates suggest establishment of new beds is a very slow process (OSPAR Commission 2010). Maerl beds form complex habitat that are known to support up to 66 macroalgal species and almost 500 species of benthic fauna in Scottish waters (Barbera et al. 2003). Maerl beds generally support more diverse communities of organisms than surrounding habitats (OSPAR Commission 2010). This is also true for damaged or dead maerl beds, although they do not support communities as diverse as beds containing living maerl (OSPAR Commission 2010). Maerl beds in the Clyde serve as nursery habitat for juvenile fish, including cod Gadus morhua , saithe Pollachius virens , and pollack Pollachius pollachius (Kamenos, Moore and Hall-Spencer 2004c). Juvenile scallops Pecten maximus have increased survivorship in maerl, highlighting the nursery role this habitat fulfills for scallops in the Clyde (Kamenos, Moore and Hall-Spencer 2004a, Kamenos, Moore and Hall-Spencer 2004b). Maerl also serves as protective habitat for other invertebrates, including clams Mya arenaria , sea urchins Psammechinus miliaris and Echinus

11 esculentus , and starfish Asterias rubens (Kamenos et al. 2004b). In the Clyde, maerl beds have been identified between the Cumbrae islands, off the coast of , off the south coast of Cumbrae, at Otter Spit and off Tarbert in Loch Fyne, Loch Goil, Bay on Arran, in the , southeast of Holy Island, and off the north tip and the southwest of the Isle of Bute, (Hall-Spencer 2001, Axelsson et al. 2009, MESH 2012, Langmead et al. 2008).

12 2.3 Background – Important marine species of the Clyde Sea Area

The west coast of Scotland has traditionally been viewed as richer in marine flora and fauna than the east coast, and the Clyde Sea Area received particular attention (Chumley 1918). Hake Merluccius merluccius , whiting Merlangius merlangius , cod, saithe, and plaice Pleuronectes platessa are demersal species that have historically spawned in or near the Clyde (Hislop 1986). There appears to have been limited exchange between populations of demersal species inside and outside of the Clyde Sea Area, suggesting that the Clyde supported semi- or fully resident fish populations (Hislop 1986). Furthermore, the Clyde has been identified as a nursery area for cod, haddock, and whiting, particularly shallow areas with high complexity, large particle size, and high levels of algal coverage (Ware 2009). Nursery areas provide increased recruitment to adult populations compared to non-nursery habitat via any combination of increased a) density b) growth c) juvenile survival rates and d) migration to adult habitats (Beck et al. 2001). Alternatively, “Effective Juvenile Habitat” (EJH), contributes more to the adult population than other habitats (Dahlgren et al. 2006). It has been determined that destruction of complex habitat used by commercially important juvenile fishes in the Clyde has the potential to reduce year class strength (Ware 2009).

Several fish species feed on zooplankton as larvae and juveniles, therefore changes to the abundance of phytoplankton and zooplankton have the potential to impact commercial fish populations (Helle and Pennington 1999, Hop, Gjosaeter and Danielssen 1994, Steingrund and Gaard 2005). In the Clyde, seasonal changes occur in the phytoplankton (diatoms dominant in winter/spring, dinoflagellates increase in the early summer and peak in August), and populations of zooplankton in the surface and subsurface waters feed on the diatom blooms (Chumley 1918, Adams 1986, Boney 1986). Long-term data sets suggest zooplankton abundance on the west coast of Scotland is declining (specifically Calanus finmarchicus and Para-Pseudocalanus ), but not to the extent seen in the , and abundance is still high during April and May, when they are crucial prey for larval gadoids (Bailey et al. 2011). It should be

13 noted, however, that a shift in the species composition of zooplankton communities has occurred, with fewer large copepod species present (Bailey et al. 2011).

2.3.1 , Gadus morhua Atlantic cod are distributed throughout the north Atlantic Ocean, with populations of resident coastal cod and populations of wide-ranging migratory fish (Scott and Scott 1988, Beacham et al. 2002, Nielsen et al. 2009). Cod have been found throughout the Clyde, and appeared to be locally abundant in some locations (Bagenal 1965). According to otolith and tagging data, Clyde cod are a resident fish population, mixing little with cod from other populations (Galley, Wright and Gibb 2006, Wright et al. 2006b, Wright et al. 2006a). Cod aggregate to spawn in large groups (Gibb et al. 2004), and cod spawning occurs near the entrance of the Clyde Sea Area (Wright et al. 2006a). Cod have a pelagic egg and larval stage, followed by settlement onto banks and into nearshore areas (Scott and Scott 1988). Juvenile cod around Scotland are found predominantly within 60 km of the coast, and particularly in sheltered coastal areas (Gibb, Gibb and Wright 2007). As settled age-0 fish, cod off the west coast of Scotland feed on a wide variety of crustaceans, polychaete worms, and small fish such as flatfish and sandeels (Ellis and Gibson 1995, Brown et al. 1989, Gibson and Ezzi 1987, Ware 2009). The Clyde supports higher densities of age-0 fish than the surrounding areas and the highest densities of juvenile cod are found at depths of less than 20m in complex habitat (Gibb et al. 2007, Ware 2009). This agrees with research from Canada and Norway demonstrating the importance of shallow, complex habitat for juvenile cod (Gregory, Anderson and Dalley 1997, Gotceitas, Fraser and Brown 1997, Pihl et al. 2006). Juvenile cod are present in the Clyde all year , but are more abundant in shallow nearshore habitats (<7m depth) from September to November (Kamenos et al. 2004c). Juvenile abundance decreases as water depth increases, and young fish move into deeper water as they age (Ware 2009). Juvenile cod remain close to their settlement area, and little mixing occurs between different cod stocks during the juvenile stage (Gibb et al. 2007). One of the key factors in cod recruitment success is the carrying capacity of juvenile habitat (Heath et al. 2008). Detailed surveys are required to

14 determine the exact timing, magnitude, and locations of juvenile cod settlement in the Clyde Sea Area.

2.3.2 Haddock, Melanogrammus aeglefinus Haddock aggregate to spawn, and will spawn repeatedly from February to mid-March (Gibb et al. 2004). Haddock may be reproductively active from age 2, but older fish produce more eggs for a given size (Gibb et al. 2004, 2012). Haddock have pelagic eggs and these have been found in icthyoplankton surveys in the Clyde Sea Area, but the origin of these eggs is unknown (Gibb et al. 2004). Adult haddock off the west coast are known to have lived in the inshore as juveniles, but there appears to be little population structuring of haddock on the west coast of Scotland (Wright et al. 2010). Adult haddock are piscivorous, but feed mainly on polychaetes, crustaceans, and echinoderms (Greenstreet, McMillan and Armstrong 1998). Haddock appear only sporadically in the shallow nearshore areas of the Clyde and may not have specific nursery areas (Hislop 1996). However, use of complex habitat in shallow areas of the Clyde Sea Area by juvenile haddock has been reported (Howarth 2012, Ware 2009), so more research on this topic is needed to determine the extent to which juvenile haddock utilize complex nearshore habitat in the Clyde.

2.3.3 Herring, Clupea harengus Herring in the Clyde have historically been divided into two groups, the spring spawning and autumn spawning populations. These two groups are morphologically distinct (different numbers of vertebrae, fin rays, and gill rakers), and behavioural differences (time of spawning) indicate they were probably reproductively isolated from one another (Wheeler 1969). Herring spawn in shallow water, leaving beds of sticky eggs adhered to the substrate (Wheeler 1969). Spawning areas for herring in the Clyde Sea area are located on Ballantrae Bank and the Iron Rock Ledges off the south coast of Arran (Marshall, Nicholls and Orr 1937, Parrish et al. 1959). Spawning can occur in waves, and older, more mature individuals may contribute disproportionately to spawning, potentially making recruitment success at least partly dependent on the age structure of the population (Lambert 1987). Some herring may be able to spawn when they are as

15 young as two, but the majority of the population spawns by age four (Marine Scotland 2012). Larval herring feed on diatoms and other unicellular organisms, then shift to copepod nauplii and small copepods as they grow (Marshall et al. 1937). Juvenile herring are opportunistic foragers, feeding mainly on copepods, but supplementing their diet with whatever is abundant in the water column and integrating fish into their diet as they grow (Marshall, Nicholls and Orr 1939, Wheeler 1969, Blaxter 1965, de Silva 1973b). Juvenile fish may remain in the nearshore for some time before moving offshore to join the main adult population (Marine Scotland 2012, Parrish et al. 1959, de Silva 1973a). Young herring often stay close to the spawning banks for the first few years of life (Wheeler 1969), so the spawning grounds for spring-spawning herring in the Clyde may also have served as nursery grounds for young fish. It has been suggested that the entire life cycle of spring-spawned herring up until the point of spawning took place within the Clyde Sea Area, after which there may have been some exchange with herring stocks of the Outer and the northwest coast of Ireland (Bailey et al. 1986b).

Spring-spawning herring were abundant in the Clyde until the 1960s when a shift to autumn-spawning fish in catches indicated the decline of spring-spawned herring (Bailey et al. 1986b). However, as early as 1930, the catches were largely dependant on young fish, and were highly susceptible to fluctuations in year-class strength (Thurstan 2007). As of 1986, the stock was at a very low level, but it was still possible to catch herring in spawning condition on the Ballantrae Bank in early spring (Bailey et al. 1986b). Unlike the spring-spawning fish, autumn-spawned herring in the Clyde appear to come from several spawning areas outside the Clyde, including the Irish Sea, Minch, and Isle of Man (Bailey et al. 1986b). The grouping of herring eggs makes them susceptible to large-scale mortality from storm-damage, predation, and toxic algae (Morrison et al. 1990, Aneer 1987, Tibbo, Scarratt and McMullon 1963). In 1990, a mass mortality (98%) of spring spawned herring eggs off the south coast of Arran was caused by a sedimenting diatom bloom (Morrison, Napier and Gamble 1991). Such mass mortality events may have significant consequences for already depleted herring populations in the Clyde (Morrison et al. 1991). Water currents may also disperse

16 pelagic larvae away from potential nursery areas, and even out of the Clyde entirely, particularly those herring spawned at Ballantrae Bank (Parrish et al. 1959, Rankine, Cargill and Morrison 1990).

2.3.4 Whiting, Merlangius merlangius Whiting spawn in shallow waters throughout the spring, with peak spawning in April (Bowers 1954, Gibb et al. 2004). Females can reach sexually maturity by age 2 and most are sexually mature at age 3 (Bowers 1954). Males may begin spawning as age 1 fish, but most reach sexual maturity at age 2 (Bowers 1954). Young whiting feed on fish, crustaceans and polychaetes, with size and seasonal abundance of food largely determining the exact composition of their diets (Gordon 1977b). Adult whiting are largely piscivorous, but also feed on crustaceans (Greenstreet et al. 1998).

Whiting were historically abundant in the Clyde and generally distributed throughout the area (Bagenal 1965). Young whiting inhabit shallow areas of complex habitat, moving into deeper water as they grow (Bowers 1954, Ware 2009). Early tagging studies suggested that whiting emigrated from the Clyde after reaching sexual maturity (Garrod and Gambell 1965, Hislop 1986). However, additional recaptures over time suggested that emigration of adult whiting from the Clyde were less than originally thought, with 315 out of 329 recaptured fish caught within the Clyde (Hislop 1986). In the 1970’s, whiting found in shallow coastal waters on the west coast of Scotland were predominantly young (0-3 years) and high of young whiting appear to be due to an active inshore migration, then a subsequent migration to deeper water as they age (Gordon 1977a). Currently, the majority of whiting in the Clyde are age-1 fish, and they are the dominant demersal species present in the ecosystem (Heath and Speirs 2012). Otolith data suggests that whiting currently within the Clyde are likely to have drifted in as larvae from spawning grounds outside the Clyde, probably the Irish Sea (Ware 2004). It is unknown whether larval drift from external spawning grounds has always been the source, or partially the source, of Clyde Sea Area whiting, or if this is a new phenomenon.

17 2.3.5 Plaice, Pleuronectes platessa Plaice spawn in the Clyde Sea Area from February to May, with maximum spawning occurring in March (Poxton 1986). Following a planktonic larval stage, plaice are carried into nearshore by currents and settle into coastal nursery areas (Poxton 1986, Gibson and Ezzi 1987). The coast, particularly Irvine and Bays have been identified as nursery areas for plaice in the Clyde, although eggs and larvae are generally widespread within the Clyde Sea Area from February to April (Poxton, Eleftheriou and McIntyre 1982, Poxton 1986). Young plaice feed on Tellina siphons, spionid palps, crustaceans and polychaetes (Poxton, Eleftheriou and McIntyre 1983). Year class strength is largely dependent on food availability in any given year (Poxton et al. 1983). Tagging studies indicate that there is some mixing of plaice from the Clyde Sea Area with Irish Sea plaice, although this mixing appears to be limited (Hislop 1986, Poxton 1976).

2.3.6 Norway lobster, Nephrops norvegicus Nephrops are burrowing crustaceans that live in muddy sediments with a silt/clay fraction above 20% by (Atkinson 1986). Nephrops is common throughout the Clyde, and is found at depths ranging from 14 to 230m (Allen 1967), however this ubiquity within the Clyde may not have always been the case. Loch Fyne was reported as having no Nephrops in 1918, despite containing apparently suitable habitat in the form of sandy mud in the shallows and fine silty mud in the deeper portions (Chumley 1918). Nephrops become sexually mature at approximately 3 years of age (minimum carapace length of sexually mature females ranges from 20mm to 33mm), spawn from July to September, and hatch eggs in late spring of the following year (Allen 1967) summarized in (Bailey, Howard and Chapman 1986a). The larvae remain in the water column for six to eight weeks and then the juvenile settle into benthic habitat (Marine Scotland 2012). The Clyde population appears to have a wider size range of animals and lower abundance than populations elsewhere (Bailey et al. 1986a). Nephrops in the north area of the Clyde are larger and live at lower densities than Nephrops in the southern part of the Clyde (Combes and Lart 2007, Bergmann et al. 2002a). Nephrops

18 burrows can be up to 30cm deep and the burrowing activity has the capacity to alter the substrate characteristics of the seafloor (Atkinson 1986).

2.3.7 Scallops Scallops Pecten maximus (and queens Aquipecten opercularis ) inhabit areas of sand, gravel, sandy gravel and sometimes muddy gravel (Mason and Fraser 1986). Scallops are filter feeders that live partially buried in the bottom substrate. They are widespread around the edges of the Clyde and the coast of Arran at depths of approximately 10-40 m (Mason and Fraser 1986). Scallop fishing in the Clyde began to increase in 1961, when the year-round fishery for this species developed, encouraged by improvement in processing technologies (Mason and Fraser 1986, Thurstan 2007). As of the mid 1980s, the Clyde supported a small and fluctuating fishery for scallops (Mason and Fraser 1986).

2.4 Background – Fishing Gear Several different types of gear are used for capturing demersal fish species, of which otter trawls (towed by single or paired vessels) and beam trawls interact directly with bottom substrates (Galbraith, Rice and Strange 2004). Nephrops fishing can be conducted with creels, pots, and otter trawls (Galbraith et al. 2004). Scallop fishing employs the use of toothed dredges that ‘comb’ the seafloor to a depth of approximately 10 cm and scrape the target organisms into a collecting bag (Bradshaw et al. 2000, Galbraith et al. 2004).

2.4.1 Otter Trawls Nephrops trawlers use modified otter trawls for fishing. Otter trawls towed by a single boat are kept open horizontally by otter boards attached to the ropes ahead of the net itself (Galbraith et al. 2004). Nets pulled by pairs of trawlers do not require otter boards, as the net is kept open by the separation of the two boats, each pulling one side of the net (Galbraith et al. 2004). Multiple nets may also be pulled by a single boat, particularly Nephrops trawlers (Galbraith et al. 2004). Trawl nets are usually funnel shaped, with different numbers of panels comprising the body of the net (Galbraith et al. 2004). The

19 top panel is held up by floats and keeps fish from escaping over the top of the net when used for demersal fish trawling (Galbraith et al. 2004). Otter trawls used for Nephrops have a square mesh panel inserted into the top panel to allow fish to escape (Bergmann et al. 2002a, McIntyre, Fernandes and Turrell 2012). The bottom rope is weighted and often equipped with heavy rubber discs or bobbins (rock-hoppers) to keep it on the seafloor while minimizing the likelihood of snagging (Galbraith et al. 2004).

2.4.2 Beam Trawls Beam trawls are kept open by a single heavy steel tube (the beam) that holds open the mouth of the net (Galbraith et al. 2004). They can range from 4m to 12m in width (Kaiser et al. 1996). Two beam trawls are often pulled in concert, one from each side of the vessel (Galbraith et al. 2004). Beam trawls often utilize ‘tickler’ chains, chains that scrape across the bottom ahead of the net, flushing bottom-dwelling fish off the bottom

20 so they can be captured by the trawl net (Jones 1992). Tickler chains are used mostly in flatfish fisheries (Jones 1992, Kaiser and Spencer 1995).

2.4.3 Creels and Pots Creels are small traps that sit on the seafloor and are baited with salted or fresh fish, and can be used singly, or in sets connected by a central line (Galbraith et al. 2004). Creels are generally D-shaped with either a ‘hard’ (plastic) or ‘soft’ (netting) eye that allows crustaceans to enter (Galbraith et al. 2004). Pots are similar to creels, but are shaped like an inkwell, with an entry bucket fixed to the top through which crab can enter (Galbraith et al. 2004).

21

2.4.4 Scallop Dredges Newhaven scallop dredges are characterized by 77cm wide mouths with 10cm spring- loaded teeth spaced 8cm apart (Hall-Spencer and Moore 2000a). It is common to pull multiple (limited by law to 8 on each side, 16 in total) dredges of this size behind a boat to maximize extraction from undulating seafloors (Kaiser et al. 1996, Boulcott and Howell 2011). Such ‘gangs’ of dredges fish approximately 6.6 km 2 for every 100 hours of fishing effort (Kaiser et al. 1996, Beukers-Stewart and Beukers-Stewart 2009).

22 2.5 Background – History of Fishing Regulation Regulation of the fishery in the Clyde Sea Area can be traced back to the Herring Fishery (Scotland) Act 1889, which prohibited trawling in the Clyde Sea Area to trawlers larger than 8 tonnes (in addition to the Firth and the ) (Thurstan and Roberts 2010). This restriction largely limited fishing in the Clyde Sea Area to seine netting and drift netting, the predominant means of fishing in the area until the 1960s.

In 1962, otter trawling during the summer (May 1 to September 30) for Nephrops was permitted outwith 3 nautical miles of the coast, but only from midnight Sunday to midnight Friday (Sea Fisheries (Scotland) Byelaw (No. 65))(Bailey et al. 1986a). Additional regulations brought in following the introduction of otter trawling to the Clyde included limitations on otterboards (must be wooden and not exceed 15 square feet), bycatch (must not exceed 25% of the total catch by weight), and vessel size (no longer than 70 ft; 21 m)(Bailey et al. 1986a). In 1968, regulations were modified to allow trawling throughout the year and the requirement for wooden otterboards was removed (Sea Fisheries (Scotland) Byelaws no.’s 80 and 83)(Bailey et al. 1986a). During the 1960’s, demersal trawling became the predominant means of fishing by the Clyde fleet (Thurstan 2007). Improvements in engine power and rock hopper gear lead to expansion of the fishery into areas where the rough bottom substrates would have previously prevented access (Thurstan 2007). This resulted in increased catches of demersal fish until 1973, when landings peaked (Hislop 1986).

Herring fishing on the Ballantrae Banks was permitted until 1972, when seasonal closures (no fishing from January to March, boats using anchored drift nets exempt until 1976) were implemented (Bailey et al. 1986b). In 1979, the European Economic Community (EEC) implemented Total Allowable Catch (TAC) restrictions, limiting the amount of herring that could be legally landed. Current restrictions on herring fishing are the ban on weekend fishing between midnight Friday and midnight Sunday, and a complete ban on all fishing activity on the Ballantrae Banks from Feb 1 to Apr 1, and a complete ban on all herring fishing from Jan 1 to Apr 30 (McIntyre et al. 2012).

23 In 1984, new regulations allowed for trawling within 3 nautical miles of the coast, effectively opening up the entire Clyde to fishers (Heath and Speirs 2012). In the 1980s, the fleet ranged in size from 80 to 100 vessels, 10-21 m in length, operating out of ports throughout the Clyde (Bailey et al. 1986a). Beginning in 1990, multi-rig trawls became more common due to their increased area coverage without a significant increase in the engine power required (IMBC, UMBSM and IRPEM 1994). Multi-rig trawls employ two or three trawls pulled parallel to each other (IMBC et al. 1994). Beginning in 2001, temporary closed areas were implemented within the Clyde and are intended to protect spawning cod and herring (Donnelly et al. 2010). In a strip running from the Mull of Kintyre to the , fishing is prohibited from February 14 to April 30, with the exception of scallop dredging and creel fishing (Donnelly et al. 2010). The area between Loch Ryan, the south coast of Arran and the Mull of Kintyre is also closed from February 14 to April 30 to fishing with the exception of scallop dredging, Nephrops trawling and creel fishing (Donnelly et al. 2010). Mobile gear is prohibited at all times within the Gareloch in addition to restrictions and activities in other areas associated with the (Donnelly et al. 2010). Through the 1990s, landings of demersal fish continued to drop (Thurstan 2007). There is a currently a mixed demersal fishery operating in the Clyde regulated by Total Allowable Catch restrictions, but targeted demersal fishing in the Clyde effectively ceased by 2005 (McIntyre et al. 2012, Heath and Speirs 2012).

Additional regulations limit the proportion of small Nephrops ‘tails’ that may be landed. ‘Tailed’ Nephrops are generally smaller animals that have the cephalothorax removed at sea. A maximum of 290 tails per kg may be legally landed (Immature Nephrops Order 1979 and Nephrops tails (Restriction on Landing) Order 1979). Additionally, legally landed Nephrops must have a carapace size of at least 20mm and constitute 30% of the catch (Wieczorek et al. 1999, IMBC et al. 1994). Protected bycatch must be minimum landing size and constitute no more than 60% of the total catch by weight in order to be landed legally (Wieczorek et al. 1999, IMBC et al. 1994, Bergmann et al. 2002a). Nephrops nets prior to 2009 could use mesh 70mm in size with a panel of 80mm mesh inserted into the top sheet to allow escape of juvenile fish, but in 2009 this

24 changed to 80mm mesh with a 120mm mesh insert (McIntyre et al. 2012, Bergmann et al. 2002a).

The scallop fishery has no limits on catch, but is governed by minimum size regulations, seasonal closures, and the limit of 16 dredges per boat (McIntyre et al. 2012). Minimum landing size for great scallops is 100mm (SL) throughout UK waters, except for the Irish Sea and English Channel, where the minimum landing size is 110mm (SL) (Howell et al. 2006, Beukers-Stewart and Beukers-Stewart 2009). Queen scallops must have a minimum shell height of 40 mm (Beukers-Stewart and Beukers-Stewart 2009).

2.6 Background – History of Fishing Activity Fishing ports within the Clyde Sea Area are , Tarbert, , , Girvan, Greenock, , and (Donnelly et al. 2010).

The herring fishery in the Clyde was conducted with anchored drift nets and ring-nets until 1967, after which pair-trawlers were introduced and by 1973 had become the predominant fishing method (Strange 1977, Bailey et al. 1986b). During the seasonal closures on the Ballantrae Bank, herring continued to be caught as bycatch in other areas by demersal trawling fisheries (Bailey et al. 1986b). The TAC resulted in fewer total days at sea by pair-trawlers, but extensive discarding of herring was known to occur as late as 1984 (Bailey et al. 1986b).

The fishery for Nephrops in the Clyde began in the early 1950s and expanded into a fishery worth £3.75 million in 1983 (Bailey et al. 1986a). In 1997, 21000 tons of Nephrops were landed in Scotland, approximately 3000-4000 tons from the Clyde (Wieczorek et al. 1999). Nephrops are fished throughout the Clyde, including the inner sea lochs at depths ranging from 14m to 200m (Wieczorek et al. 1999). Trawling is permitted throughout the year, but not on weekends (from 1986 and applicable to all mobile gear), and fishing success is cyclical, with the greatest landings per unit effort (LPUE) occurring from July to December, a trend thought to be related to and burrowing behaviour (Bailey et al. 1986a, Bergmann et al. 2002a).

25

The fishery for scallops and queens in the Clyde and elsewhere expanded in the 1960s, and included both dedicated scallop fishers and herring fishers during the herring off- season (Mason and Fraser 1986, Allen 1962). Scallop fisheries appear to have had little impact on the health of scallop stocks, however there are concerns that changes to age and size distributions have occurred (Mason and Fraser 1986, McIntyre et al. 2012, Beukers-Stewart and Beukers-Stewart 2009).

The demersal fisheries in the Clyde have been somewhat dependent on younger fish since at least 1965, leaving the fishery highly susceptible to fluctuations in year class strength (Hislop 1986). Cod landings decreased in the early 1960s, then increased again as fishing effort increased (Thurstan 2007). In the early 1980s, cod landings again declined, continuing to do so until negligible landings were made by 2005 (Thurstan 2007). Whiting landings began to significantly decline in the early 1970’s, as did haddock landings, albeit with more fluctuations (Thurstan 2007). The changes in the composition of the demersal fish community that resulted from these population decreases in the Clyde Sea Area have been examined in depth (Thurstan and Roberts 2010, Heath and Speirs 2012). Prior to 1960, 13 taxa comprised 95% of fish captured in survey trawls but by 1995, 80% of survey trawl catches were whiting (Heath and Speirs 2012). Furthermore, a decrease in the average maximum attainable size (an estimate of the maximum fish length for a given species in the Clyde based on the largest fish caught in trawl surveys) after 1990 demonstrated the absence of large fish species and large individuals (Heath and Speirs 2012). This is corroborated by a significant decrease in the mean length of demersal fish from 60cm in the 1920s to less than 20cm in the 1990s (Heath and Speirs 2012). Comparisons to the Hebrides show that the Clyde Sea Area underwent a much more drastic reduction in diversity and mean fish size than other areas of the west coast (Heath and Speirs 2012). The changes in the fish community of the Clyde is hypothesized to be a result of opening the Clyde to trawling in 1962 (Heath and Speirs 2012). This shift from larger animals to smaller ones over during the 20 th century has also shown to have occurred in the Irish Sea and the North Sea off south east coast of England (Rogers and Ellis 2000).

26 3. TRAWLING AND DREDGING – EXTENT AND INTENSITY Including boats from , approximately 164 vessels participated in the Clyde Nephrops fishery as of 2012 (Sinclair 2012). Some of these vessels also participate in scallop dredging (Combes et al. 2007). The soft, muddy fishing grounds around are predominantly fished by larger trawlers (Combes et al. 2007). Arran and the upper sea lochs, where the substrates are coarser, are fished by boats with heavier, “hoppier” gear (Combes et al. 2007). In addition to trawling activity, these areas are often targeted by creel fishers on weekends when trawling is forbidden (Combes et al. 2007).

Tracking of 18 vessels in the Clyde Nephrops fleet chosen to be representative of the fleet in terms of engine power, gear, and area fished reveals some trends regarding fishing intensity. The study employed GPS-linked recording devices to track vessels through “pixels” of space 2500m 2 in size (Marrs et al. 2002). Nephrops trawlers exploit all areas of continuous mud within the Clyde, with the only exceptions being unsuitable substrate, debris that could be a to nets, and restricted areas such as locations used for explosive disposal (Marrs et al. 2002). At the time of the study approximately 40-80 vessels operated in the Nephrops fishery, and the18 boats tracked trawled almost 70% of the Clyde Sea Area at least once during the 12 month study period (Marrs et al. 2002), suggesting the intensity exerted by the entire fleet is much higher. The maximum number of times a pixel was fished in a given month ranged from 13 to 35 (Marrs et al. 2002). Some areas were fished up to 250 times in a year (Marrs et al. 2002). Overall, of the 3650 km 2 area of the Clyde, approximately 3000km 2 are fished by trawlers (Wieczorek et al. 1999). Kilbrannan Sound, Lower Loch Fyne, east of Arran, and west of the Cumbraes were areas that consistently received high fishing pressure (Marrs et al. 2002). This pattern continued at least until 2007-2006 when the most intensively fished areas were identified as west of Ailsa Craig and east of Arran (Combes et al. 2007).

27

28

4. IMPACTS OF TRAWLING AND DREDGING

4.1 Impacts – Habitat and Community Composition Several studies from around the world quantify the impacts of trawling activities on habitats and organisms. Trawling reduces the structural complexity of the seafloor by smoothing out mounds and filling in troughs or burrows (Coggan et al. 2001). Trawling can also displace boulders, thereby reducing structural complexity (Freese et al. 1999).

Much of the Clyde is characterized by sandy or muddy sediments (Mill 1894, Tivy 1986). One study suggests that these habitats experience little long-term visible physical damage from trawling, with resuspension of sediments being one major effect,

29 and visible marks from trawling apparent for only 52 hours post-trawl (Fonteyne 2000). In contrast, physical changes caused by 16 months of trawling (10 trawls of 45 min duration completed on 1 day each month) and monitored by sidescan and RoxAnn in the deep-water mud of the Gareloch regained similarity to a control site 18 months after cessation of trawling (Tuck et al. 1998).

In addition to physical disturbance, trawling alters the benthic community structure in ways that may not be immediately apparent. In the same trawling experiment described above, differences in community structure between trawled and control sites in the Gareloch were identified only after 5 months of trawling disturbance, and remained for at least 18 months after trawling ceased. (Tuck et al. 1998). The differences included more species and organisms at the trawl site, but lower evenness, a result caused by an increase in a few dominant organisms following trawling (Tuck et al. 1998). In general, soft sediment biota take years to recover from trawling, and slow growing sponges or coral may take as long as 8 years to recover (Kaiser et al. 2006). Trawling can damage or destroy epifaunal invertebrates and fragile sessile organisms (Freese et al. 1999). Intensively fished areas around the Isle of Man have shifted from complex, communities dominated by emergent, mostly sessile, high biomass megafauna to communities dominated by small, hard-shelled, infaunal organisms (Kaiser et al. 2000a). A separate report linked intensive trawling to a shift from predation/scavenging communities towards communities of filter feeders (Coggan et al. 2001). Another study of seabed communities found that heavily disturbed areas are dominated by harder- shelled molluscs and scavengers, while undisturbed areas are dominated by complex epifauna (Collie, Escanero and Valentine 1997). Repeated beam trawling can lead to a reduction in benthic infaunal species number and abundance depending on the habitat characteristics (Kaiser and Spencer 1996). In stable sediment communities, this change is greater that in more naturally disturbed habitats and is due to reduction or removal of less common species (Kaiser and Spencer 1996). In the Clyde, trawling has been shown to increase the similarity between different locations, reduce overall habitat and community diversity, suggesting the trawling has already reduced habitat complexity in the Clyde Sea Area (Coggan et al. 2001). Structurally complex megafauna serve as

30 habitat for shrimp, polychaetes, brittle stars and small fish (Collie, Escanero and Valentine 2000a). Many of these organisms are food for demersal fish, suggesting the reduction in benthic fauna that accompanies trawling and dredging activity may reduce food availability for commercially important fish species (Collie et al. 1997). Such shifts in community structure may be temporary, but might also represent an alternate stable state for highly disturbed benthic communities (Kaiser et al. 2000a).

Trawling is linked to negative long-term trends in benthic productivity (Ball, Munday and Tuck 2000). While trawling activity may temporarily increase productivity (animal biomass) due to increased scavengers and accompanying predators, this effect is temporary (days) and a long-term decrease is apparent, due to habitat damage, trawling mortality, and removal of fauna (Ball et al. 2000, Coggan et al. 2001). This response has also been shown in the Mediterranean, where the increase in predator and scavenger behaviour present immediately following trawling lasts several days (Demestre, Sanchez and Kaiser 2000). In the long term post-trawling, benthic communities typically exhibit one or more of the following: reduced abundance, diversity, biomass, and/or size of individual organisms (Ball et al. 2000, Demestre et al. 2000, Collie et al. 1997, Collie et al. 2000a). Additionally, trawling reduces benthic species richness, production, and biomass (Hiddink et al. 2006). Observations on areas with variable bottom-trawling history in the North Sea show that long-term trawling can shift benthic ecosystems on the sea-basin level scale (Tillin et al. 2006). In the North Sea, mortality of invertebrate species due to physical damage or resulting disturbance ranged from 5% to 40% following a single trawl passage over sandy sediment (Bergman and van Santbrink 2000). Trawling has been identified as the likely reason for the disappearance of several benthic invertebrate species from some areas of the southern North Sea (Bergman and van Santbrink 2000).

Trawling intensity can also affect the degree of disturbance to the benthos. At more intensively fished sites in the Clyde, 100% of Neptunea antiqua and an increased proportion of Asterias rubens exhibited trawl damage (Coggan et al. 2001). Work on the Dutch continental shelf indicates that increased trawling intensity alters the species

31 composition more drastically than less intensive trawling activity, although this result is also likely to have been affected by environmental gradients over the areas surveyed (Craeymeersch et al. 2000). Estimates of annual mortality for megafaunal invertebrates in the North Sea range from 5% to 39%, with population recovery of affected species likely dependent on species life history traits (Bergman and van Santbrink 2000). Habitats that are exposed to high levels of natural disturbance, or have been subjected to fishing pressure for long periods of time may not exhibit community changes after beam trawling (Kaiser et al. 1996). This may be due to the community already being adapted to disturbance, or that changes due to fishing pressure have already occurred (Kaiser et al. 1996).

Scallop dredges are considered to be the most damaging types of towed fishing gear with respect to habitats (Collie et al. 2000b). Dredging can have variable impacts on habitat and species composition depending on the substrate and intensity of the dredging effort (Bradshaw et al. 2000, Boulcott and Howell 2011). Dredging can change the granulometric properties of the substrate, potentially altering its suitability for settlement for larval animals (Hauton et al. 2002, Hall-Spencer and Moore 2000a, Hall- Spencer et al. 2001). Dredging may also affect non-target substrates. Rocky substrates are often avoided by fishers because of their poor suitability for scallop and the potential for damage to dredges. However, when suitable scallop habitat is nearby, rocky reefs may be subject to inadvertent dredging (Boulcott and Howell 2011). A study conducted in Kilbrannan Sound in the Clyde Sea Area revealed damage to vulnerable organisms such as sea sponges (Boulcott and Howell 2011). However, the impact of trawling activity on the biota of rocky substrates appears less than that for animals in softer sediments (Boulcott and Howell 2011). Animals in previously stable mud, coarse gravel, or biogenic habitats are the most negatively impacted by dredging (Collie et al. 2000b).

Similarly to trawling, dredging reduces benthic community complexity and reduces heterogeneity between previously different habitats (Bradshaw et al. 2001, Bradshaw et al. 2000). Such reduction in heterogeneity reduces the complexity of habitats (Bradshaw et al. 2001), potentially altering the suitability of the habitat for mobile species and

32 reducing the suitability of habitat for larval scallop settlement (Auster and Langton 1999, Auster et al. 1996, Bradshaw, Collins and Brand 2003). Long-term data from the Irish Sea suggests that 60 years of dredging activity has contributed to a shift from slow moving and sessile taxa to robust, fast-moving and scavenging taxa (Bradshaw, Veale and Brand 2002). Some studies suggest the impacts of dredging are less severe and that recovery may be possible. Experiments conducted in shallow (10m) sandy substrates in Loch Ewe show the destruction of sessile, fragile, and large epibenthic fauna, but noted no differences in substrate or small organisms after 9 days of trawling with varied intensity (Eleftheriou and Robertson 1982). Comparison of two muddy-maerl sites of the south-west cost of Ireland, one with ongoing dredging activity, one where dredging ceased six month prior to sampling, show a shift in the recovering site from a scavenger-dominated community to one dominated by filter-feeding bivalves (De Grave and Whitaker 1999).

Slow growing biogenic habitats like maerl or Limaria hians are highly susceptible to the impacts of dredging. A meta-analysis of bottom fishing gear reveals that the most severe damage to benthic habitat occurs when biogenic habitats such as maerl are fished by scallop dredgers (Kaiser et al. 2006). Scallop dredging in the Clyde has likely been responsible for the reduction in L. hians , a formerly widespread -building species (Hall-Spencer and Moore 2000b). Dredging can break and bury live maerl thalli, and silt can bury maerl adjacent to the dredge track, resulting in long-term mortality (Hauton et al. 2002, Hall-Spencer and Moore 2000c, OSPAR Commission 2010, Barbera et al. 2003). These plumes of sediment can cover maerl up to 8m on either side of the dredge track (Hauton, Hall-Spencer and Moore 2003b).

Experiments conducted at two sites in the Clyde (one with no previous history of dredging and one with extensive dredging history) show that a single sweep with a Newhaven scallop trawl can kill up to 70% of living maerl in the dredge path, with no signs of regrowth after 4 years (Hall-Spencer and Moore 2000a). Hydraulic bivalve dredges can bury up to 5.2 kg of maerl per metre exclusive of that which may later be covered by sediment (Hauton et al. 2003b). Flora and fauna on the surface, and buried

33 up to 10cm deep are affected directly by dredging, but some deeper burrowing species are more resistant to dredging effects (Hall-Spencer and Moore 2000a). Dredge tracks (troughs and ridges) in maerl are visible for up to 2.5 years, representing a major shift in the physical structure of the environment (Hall-Spencer and Moore 2000a).

Benthic communities in maerl beds that have had previous dredging but had recovered to a modified condition take a shorter amount of time to return to the modified community structure (1-2 years) following a second round of dredging (Hall-Spencer and Moore 2000a). The recovery of the community structure does not, however, reflect the impact on the maerl itself, which may take hundreds or thousands of year to re-grow (Hall-Spencer and Moore 2000c). Dredging alters the granulometric properties of the substrate, potentially altering the suitability of the site for maerl re-colonization and the attractiveness of the site for maerl-inhabiting organisms during their settlement phase (Hauton et al. 2002, Hall-Spencer et al. 2001). Dredging reduces the habitat complexity of maerl, and dredge-damaged maerl is similar to that of a gravel substrate, with smaller thalli and less structural heterogeneity (Kamenos, Moore and Hall-Spencer 2003).

Few maerl beds in the Clyde Sea Area have been unaffected by dredging activity, with the single unaffected bed found by (Hall-Spencer and Moore 2000a) located near a communication cable that prohibited trawling/dredging. Detailed surveys of Lamlash Bay on Arran showed that most maerl beds have sustained extensive damage and the healthiest maerl bed recorded contained 5-10% living maerl (Axelsson et al. 2009). However, there is one area containing patches of 90% living maerl in Lamlash Bay (COAST 2005, Kamenos, Moore and Stevenson 2004d). Comparison of samples from the Tan Buoy maerl bed south of Cumbrae taken from 1885-1891 and 1995-1997 showed a decrease in size of maerl and the amount of living maerl, a result attributed to scallop dredging (Hughes and Nickell 2009).

An examination of the impacts of fishing gear of habitat and the best ways to minimize negative impacts has identified spatial closures as the best available method to reduce damage to maerl beds (ICES 2005). An assessment of the evidence for maerl decline to

34 due fishing activity concluded that widespread declines were occurring and there was significant evidence to implicate the role of fishing activities in this decline, particularly dredging (ICES 2005).

A meta-analysis of impacts and recovery from trawling suggests dredges have roughly the same negative impact on the suspension- and deposit- feeders in sand, mud and gravel substrates (Kaiser et al. 2006). Modifications to dredges can mitigate the negative impacts on the benthos. New dredges with rubber lips instead of the traditional teeth have lower bycatch and higher scallop catches than traditional dredges, however damage and injury to benthic megafauna is comparable (Hinz et al. 1973).

35 4.2 Impacts – Bycatch

The small mesh used by Nephrops trawls ( ≥70mm for single-rig trawls, ≥80mm for multi- rig trawls) results in significant mortality to non-target species, both as discards (fish smaller than legal landing size or non-marketable species) and landed bycatch (Stratoudakis et al. 2001, Bergmann et al. 2002a). Fishing activity has both long term and short term impacts on target and non-target species. One immediate result of fishing activity is a decrease in density and diversity of bycatch species that is variable depending on the species and habitats in question (Auster and Langton 1999). Another immediate impact of fishing is an increase in scavenger activity (Bergmann et al. 2002b, Caddy 1973). Increases in scavenger activity may have the potential to affect disease transmission in the trawling area, but further work on this needs to be completed (Wieczorek et al. 1999).

The total weight of discards produced in the Clyde Sea Area by the Nephrops fleet in 1997 was estimated to be at least 25000 tons (Wieczorek et al. 1999). Based on Nephrops catches of between 3000 and 4000 tons in 1997, this means for every ton of Nephrops landed, at least 6 tons of discards were produced. The ratio of overall bycatch to Nephrops is even higher, with 9 kg of bycatch for every 1 kg of Nephrops landed (Bergmann et al. 2002a). Bycatch ranges from 77 to 86% of total catch by volume and is predominantly non-target invertebrates by volume (Bergmann et al. 2002a). In 2006- 2007, 38% of the total catch (by weight) was discarded, a total of 3531 tons from the Nephrops fleet (Combes and Lart 2007). The discrepancy between this study and (Bergmann et al. 2002a, Wieczorek et al. 1999, Stratoudakis et al. 2001) may be due to a reduction in the abundance of bycatch species, changes in gear selectivity, or increases in Nephrops abundance in the Clyde (Combes and Lart 2007). Overall, fish mortality is high and bycatch is suspected to have a negative effect on commercially important fish species at the population level (Wieczorek et al. 1999).

The composition of non-target invertebrate bycatch can vary enormously between trawls. In some trawls, echinoderms comprise up to 80% of invertebrate discards from

36 Nephrops trawlers, predominantly starfish A. rubens and brittlestar Ophiura ophiura (Bergmann and Moore 2001a). Conversely, discards from another trawl may be comprised of up to 89% decapod crustaceans, including harbour crab Liocarinus depurator , squat lobster Munida rugosa , and hermit crab Pagurus bernhardus (Bergmann and Moore 2001b). Mortality for bycatch species may be immediate and due to damage caused by the trawling and sorting process, such as removal of limbs, crushing injuries, and exposure to air (Bergmann and Moore 2001b, Bergmann and Moore 2001a). Alternatively, mortality for bycatch species may occur over longer time scales due to pathogens, stress, or inability to recover from injury (Bergmann and Moore 2001b, Bergmann and Moore 2001a). There is also the potential for increased susceptibility to predation (Bergmann and Moore 2001a, Bergmann and Moore 2001b). In the echinoderms, short-term mortality ranged from 0% for A. rubens to 31% for O. ophiura and long-term mortality ranged from 11% for A. rubens to 100% for O. ophiura (Bergmann and Moore 2001a). In the decapod crustaceans, overall mortality estimates ranged from 68-84% for M. rugosa and from 51-72% for L. depurator (Bergmann and Moore 2001b). Echinoderms may not be affected at the population level by these mortality levels due to the resilience of A. rubens to trawling and the high reproductive capacity of O. ophiura , combined with a reduction in the number of natural predators of both species (Bergmann and Moore 2001a). The potential impact on decapod crustacean populations was not evaluated.

Fish discards from the Nephrops fishery are predominantly young whiting (less than 19cm), but poor cod, long rough dab, hake, and Norway pout are also common (Stratoudakis et al. 2001). During the period of 1982-1998, bycatch biomass by unit effort decreased, but the proportion of total catch comprised of bycatch increased (Stratoudakis et al. 2001). By 1998, 70% of fish bycatch by weight was discarded and it was estimated that 4500 tonnes of fish was discarded annually by the Clyde Sea Area Nephrops fleet (Stratoudakis et al. 2001). A study during the 2005-2006 fishing season estimated the amount of discards to be much lower, calculating that the Nephrops trawling fleet was responsible for 1631 tonnes of fish discards (Combes et al. 2007).

37 Use of larger mesh sizes increases the average size of fish bycatch, possibly indicating a reduction in the mortality of young fish (Stratoudakis et al. 2001).

Based on Nephrops catches, the Clyde Sea Area can be divided into ‘north’ and ‘south’ areas (divided by a line running from Troon to the northern tip of Holy Island) apparently determined by hydrographical, geological, sedimentogical and biological differences (Wieczorek et al. 1999). In the south, total catch biomass is comprised of approximately 30% Nephrops , 55% fish discards, and 15% invertebrate discards while in the north, catches are approximately 4% Nephrops , 36% fish discards and 60% invertebrate discards (Bergmann et al. 2002a). The northern area tends to land larger individuals and more whole individuals, while the south is characterized by smaller Nephrops and a large proportion of the landings are Nephrops tails from smaller individuals (Wieczorek et al. 1999). The southern area discards contain more undersized commercial fish (55% by weight) than the north (32 % by weight) and more roundfish in the discards (25-35% of total discard biomass in the south compared to 1-5% in the north; (Wieczorek et al. 1999). There are also differences between the two areas in the amount of bycatch that is discarded. Approximately 80% of the total catch biomass in the north ends up being discarded, compared to 65% of the catch in the south ends up being discarded (Wieczorek et al. 1999). In the north, discarded fish are predominantly young whiting, haddock and Atlantic cod, while in the south, discarded fish are young whiting, haddock and herring (Wieczorek et al. 1999).

(Combes et al. 2007) have suggested that the “north-south” divide of the Clyde is an over- simplification, and that gear type also plays a role in the nature of Nephrops catch and bycatch. According to comprehensive vessels and catch surveys, “clean gear” (gear with no rock hoppers, used to fish in muddy grounds) catch greater proportions of smaller Nephrops , especially during the summer (Combes et al. 2007). Conversely, heavier, rock-hopper gear that is used for rockier, harder substrates target larger Nephrops (Combes et al. 2007). This study suggests that it is the substrate type (and subsequently the gear used to target it), that influences catch composition and that the

38 “north-south” divide can be explained by the distribution of substrates within the Clyde (Combes et al. 2007).

Trawling can cause disproportionately higher mortality in some bycatch species compared to others. Fragile species, particularly fish, swimming crabs and urchins, experience higher mortality from beam trawls than other, more robust species (Kaiser and Spencer 1995). More resilient species, such as hermit crabs, starfish and molluscs, had survivorship of at least 60% six days following capture in a beam trawl (Kaiser and Spencer 1995). Damage inflicted on fish that may affect their long-term mortality included scale loss, bruising or cuts caused by contact with other organisms such as urchins, and stress from air exposure (Kaiser and Spencer 1995).

Beam trawling captures a greater number of species than scallop dredges, with faster species being more likely to escape the slower scallop dredges (Kaiser et al. 1996). Scallop dredges are more efficient at capturing their target species, while beam trawls are more indiscriminate fishers (Kaiser et al. 1996).

Fishing in maerl beds can result in larger amounts of bycatch compared to other substrate types, with 8-15 kg of other organisms (not including maerl) for every 1 kg of scallop caught (Hall-Spencer and Moore 2000a). In terms of area, dredging maerl beds using hydraulic bivalve dredges results in 529g of discards per m 2 of dredging activity (Hauton, Atkinson and Moore 2003a). Dredging also results in additional death and injury to animals on the seafloor that may not be readily observable, suggesting bycatch may not be the only non-target mortality caused by dredging activity (Jenkins, Beukers- Stewart and Brand 2001).

39

5. POTENTIAL FOR RECOVERY

5.1 Potential for Recovery – Habitats Recovery of habitat from trawling and dredging activity has been the subject of numerous studies and meta-analyses that demonstrate the difficulty of predicting recovery potential damaged habitat, particularly slow-growing habitat.

A meta-analysis of the recovery potential for habitats subjected to bottom trawling reveals sponges and soft coral may take 8 years (Kaiser et al. 2006). However the authors of this meta-analysis noted that the lack of experiments conducted in pristine biogenic habitats may have biased the results (Kaiser et al. 2006). In contrast, another meta-analysis suggest slow-growing biogenic habitats such as maerl and sponges may take up to 15 years to recover from fishing activity (Collie et al. 2000b). This issue is further complicated by language. “Recovery” may refer to absence of physical evidence of trawling, partial regrowth of biogenic habitat, or full recovery to a pristine state. While examination of a previously pristine maerl bed four years after a single experimental trawl showed no signs of maerl regrowth, physical markings from the dredge were no longer detectable after 18 months (Hall-Spencer and Moore 2000c). Maerl is an

40 extremely slow-growing organism (approximately 1 mm per year), and its full recovery to a pristine state following trawling damage is unlikely to occur on time scales relevant to humans (Airoldi and Beck 2007, Hall-Spencer et al. 2003). OSPAR has characterized maerl habitats as having “poor” recovery potential, meaning only partial recovery can be expected in 10 years and full recovery in at least 25 years (IMPACT 1998). Essentially, maerl is a “non-renewable” resource, and prevention of further damage is critical (Barbera et al. 2003). Recommendations of the BIOMAERL team include provisions regarding prohibiting use of heavy towed gears over selected maerl beds, including maerl in the evaluation of MPA designation, and implementing no-take zones in areas with maerl (Barbera et al. 2003).

In contrast to slow-growing biogenic habitats such as maerl, short-lived organisms, such as polychaete worms can recover from trawling on time scales of less than 1 year (Kaiser et al. 2006). A meta-analysis of fishing activity concluded that communities in sandy habitats largely recovered from disturbance after approximately 100 days, but certain species (the clam M. truncata ) took up to two years to recover, and this recovery time does not apply for repeatedly disturbed fishing grounds (Collie et al. 2000b). Experiments conducted in England confirmed the faster rate of recovery following disturbance to sandy habitats compared to muddy habitats (Dernie, Kaiser and Warwick 2003). There is an extreme paucity of data surrounding the recovery of intensively fished areas, making conclusions and predictions about the recovery potential of such areas difficult, if not impossible (Collie et al. 2000b).

5.2 Potential for recovery – Fish stocks Recovery of fish stocks is hampered by the degree to which fish have been depleted. Fish stocks that do not have large enough populations to successfully reproduce are less likely to recover following fishing closures (Gell and Roberts 2002). Recovery is also hampered if important habitat is not protected or has already been destroyed (Gell and Roberts 2002). It is also necessary to consider that ‘recovery’ may not be possible. A review of studies of the North Sea suggests that fishing pressure has shifted the North Sea ecosystem from one of large, slowly reproducing animals to one of smaller,

41 faster reproducing animals (Frid and Clark 2000). It is possible this represents a non- reversible shift in the nature of the ecosystem.

Bailey et al. (1986) suggested that the population of spring-spawned herring in the Clyde was unlikely to increase without an increase in recruitment to the stock. It is unknown if such an increase in recruitment would be likely to occur as a result of trawling restrictions, as a decrease in spring spawning herring stocks throughout the northeast Atlantic occurred at the same time as the shift in Clyde spring-spawning herring, suggesting a possible natural change unrelated to fishing pressure (Bailey et al. 1986b). Analysis of long-term data suggests that climate and fishing have contributed to shifts in stocks of cod and herring, indicating cessation of fishing pressure may not be enough to allow full recovery of fish stocks (Brunel and Boucher 2007). For herring, the possibility that age-structure contributes to spawning success (Lambert 1987) may mean that recovery is unlikely until there is a significant increase in older fish.

The relative isolation of the Clyde from surrounding fish populations suggests that local changes are likely to have positive remedial effects on fish populations. It is important to note that any recovery will lag behind changes to fishing policy, taking up to 20 years before changes in the fish community are detectable (Heath and Speirs 2012). Because of its isolation from other populations, cod recovery in the Clyde is likely to be largely influenced by the rate of intrinsic population increase (Wright et al. 2006b). The current low population levels suggest recovery could be slow (Wright et al. 2006b). Habitat recovery has been shown to lead to increased densities of the species that utilize the habitat (Warren et al. 2010), suggesting habitat recovery in the Clyde could result in increased juvenile fish abundance. This result is confirmed by analyses of changes in the distribution of eelgrass in the Norwegian Skagerrak that correspond with changes in juvenile fish recruitment (Fromentin et al. 1998).

Haddock on the west coast of Scotland do not exhibit population structuring in the form of resident populations (Wright et al. 2010), suggesting a closed area in the Clyde is unlikely to affect this particular species. Modelling work suggests that removal of

42 mortality by the Nephrops fleet would have no impact in whiting or haddock stocks, and only a limited impacts on cod stocks of the west coast of Scotland (Bailey et al. 2011). It is unknown if this extrapolation would be valid if applied solely to stocks at the level of the Clyde Sea Area.

Closed areas show potential for benthic community and fish stock recovery. Life history affects recovery of fish stocks, with longer lived species taking longer to recovery following closure (Gell and Roberts 2002). There is evidence that in areas protected from fishing, fish populations shift a more robust age distribution, with animals living longer and growing bigger than they would when fishing pressure is present (Gell and Roberts 2002). Voluntary agreements made in 1978 between fishing groups (potters/creelers and trawlers) off the coast of Devon to limit certain types of fishing activity in certain areas and at certain times of the year has resulted in increased diversity and biomass in the areas where towed gear is prohibited (Kaiser, Spence and Hart 2000b). The authors noted that benefits to mobile gear fishermen (i.e. spillover) were unknown at the time of the study (Kaiser et al. 2000b). Examination of a closed area of the Isle of Man suggests benthic community heterogeneity increases after closure of fishing, and this change is reversed if fishing is re-started (Bradshaw et al. 2000). Additional work in the Baltic Sea shows that macrobenthic communities take as long as 5 years to recover following dredging activity in sandy substrate (Bonsdorff 1983). In gravelly habitats off the Isle of Man, some species increased in numbers in the 10 years after cessation of trawling (scallops and the seven-armed starfish Luidia ciliaris , some crabs and brittle stars), while another (starfish A. rubens ) seems to have decreased (Bradshaw et al. 2000).

Sampling from within and outwith areas closed to trawling and dredging show that scallops within closed areas attain greater size and are present in greater abundance than scallops in locations subject to dredging (Kaiser et al. 2007, Beukers-Stewart et al. 2005, Bradshaw et al. 2001). Scallops within a closed area exhibited increased reproductive output compared with scallops of the same size from outside the closed area (Kaiser et al. 2007). Scallops within a closed area also exhibit higher growth rates

43 and grow larger than those outside the closed area (Beukers-Stewart et al. 2005, Bradshaw et al. 2001). This result has also been found in the Clyde, where the Lamlash Bay No-Take Zone (NTZ) contained larger and older king and queen scallops than surrounding areas 3 years after implementing the no-take policy (Howarth 2012).

Studies from Australia indicate that trawling reduces habitat heterogeneity, thereby reducing the abundance of commercially valuable fish that associate with complex habitat (Auster and Langton 1999). Cessation of trawling led to an increase in the abundance of commercially valuable fish (Auster and Langton 1999). Experiments in the Irish Sea comparing closed areas with fished locations show that closed areas have increased benthic community heterogeneity (Bradshaw et al. 2001). This result is also seen in the Clyde, where 3 years after the implementation of a NTZ in Lamlash Bay, the biodiversity, number of species, and number of individuals was higher inside the no-take zone than outside (Howarth 2012). Furthermore, cod and haddock were more abundant within the NTZ than outside it (Howarth 2012). Changes in the age, biomass, and size of scallops inside the NTZ compared to outside were apparent as early as 2 years following the closure (Howarth 2010, Howarth et al. 2011). Recovery of commercially valuable fish has been demonstrated to occur when marine reserves are put in place in South Africa, where fish populations important for angling increased following the establishment of a marine reserve (Bennett and Attwood 1991). Overall, these results suggest recovery of heavily fished areas is possible following cessation of fishing activity, however the nature and timescale of recovery is unknown.

Temporary area closures may lead to a decrease in heterogeneity due to displaced fishing effort, and permanent closure should be considered for long-term mitigation of fishing impacts (Dinmore et al. 2003). Additionally, closure of some fishing activity may be less effective than cessation of all fishing activity, as continuing with some activity can still result in bycatch or habitat disturbance for the species of interest (Gell and Roberts 2002).

44 6. REFERENCES

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