Energy subsidies for marine benthic communities

16.4.4 Energy subsidies for marine benthic communities

Food-falls of carrion can have a profound effect on local diversity and production processes of deep- sea benthic communities (Dayton and Hessler 1972). Although fishing activities occur in oceanic waters, the majority of fishing activities are con-

fined to more shallow coastal waters that are less (b) than 100 m deep. Hence the energy subsidies to

deep-sea communities from current levels of fish- ing activities are likely to be less significant than for shallow waters.

The greatest energy subsidies to benthic com- munities are generated by towed bottom fishing gears. These trawls and dredges are specifically de- signed to remain in close contact with the seabed and thereby maximize catches of bottom-dwelling species such as flatfishes and scallops (Misund et al., Chapter 2, this volume). Not surprisingly, these gears are particularly heavy and robust; a typ- Fig. 16.4 Towed bottom fishing gears are designed to ical beam trawl fished in the North Sea will weigh remain in close contact with the seabed: (a) 12-m wide

up to 10 tonnes in air. Most bottom fishing gears commercial Dutch beam trawls can be fitted with up to are fitted with chains or toothed bars across the 27 tickler chains that are attached between the two

beam shoes, (b) scallop dredges are fitted with toothed mouth of the net or dredge (Fig. 16.4; see also bars that dig scallops out of the seabed.

Misund et al. Chapter 2, this volume). These modi- fications are designed to dig out or disturb the tar- get species from the substratum whereupon they areas of trawl disturbance often exceeds their are caught in the following net. Although these initial density before trawling started by between fishing gears are very effective at catching the tar-

4 and 10 times (Ramsay et al. 1996; Fonds and get species (Cruetzberg et al. 1987), they also catch, Groenwold 2000). Examination of the gut contents damage or kill in situ a large number of non-target of scavenging fish reveals that they eat a wider fish and invertebrates (Bergman and Van Santbrink variety and larger rations of prey in areas of trawl 2000). The odours emitted from this carrion rap- disturbance (Kaiser and Spencer 1994; Kaiser and idly attract mobile scavenging species into areas Ramsay 1997). Furthermore, they are able to con- of the seabed that have been trawled (Kaiser and sume prey such as burrowing sea urchins and large Spencer 1994; Ramsay et al. 1996, 1997). Fishers bivalve molluscs that normally would be unavail- have known of this phenomenon for many years able to them (Kaiser & Spencer 1994). In addition and it is not uncommon to see trawlers fishing one to scavenging fishes, a wide variety of invertebrate after the other along the same navigational plot. scavengers are attracted into areas of trawl distur- The density of scavenging fishes attracted into the bance. The most commonly observed taxa that

353 feed upon fisheries carrion are starfishes (Aster- reliable and constant source of high-quality food

Ecosystem Effects of Fishing

oidea), hermit crabs (Paguridae), crabs (Decapoda) (Camphuysen et al. 1993). Furthermore, we now and gastropods (Gastropoda) (Hill and Wassenberg know that sea birds will often choose not to feed on 1990; Kaiser and Spencer 1996a; Ramsay et al. fisheries discards especially when rearing chicks 1997). When hermit crabs are found in high back- (Camphuysen and Garthe 2000). In contrast, ben- ground densities, they can locate carrion within thic invertebrates are unable to actively search minutes of its arrival on the seabed and can reach for sources of fisheries-generated carrion and rely densities in excess of 300 m -2 (Kaiser and Spencer on its chance occurrence on the seabed. Thus, 1996a; Ramsay et al. 1997). It is not surprising at present, this source of carrion may be too unpre- therefore that dominance hierarchies occur among dictable to give significant benefit to most inver- the scrum of scavenging invertebrates such that tebrate scavenger populations. In line with our smaller conspecifics are excluded from the food rationale for the relative lack of cascade effects resource as are other less competitive species observed in marine systems as a result of fish- (Ramsay et al. 1997; Kaiser et al. 1998). The ing activities, it is perhaps not surprising that we amount of carrion generated on the seabed by bot- only see population effects of subsidies at higher tom fishing is at least equivalent to the amount of trophic levels (sea birds) whereas they are not ap- material discarded at the surface of the sea parent, or are less obvious, at lower trophic levels. (Bergman and Van Santbrink 2000). However, this carrion is calculated to supply only 7% of the max- imum annual energy requirements of invertebrate

16.5 EFFECTS OF

scavenging megafauna in the southern North Sea

REMOVING PREDATORS

(Fonds and Groenwold 2000). While the responses of populations of scaveng-

16.5.1 The ‘krill surplus’ hypothesis

ing sea birds have been correlated in part with Commercial whaling dramatically reduced the subsidies from discarding practices, the responses abundance of baleen whales in the Antarctic of fish and invertebrate scavenger populations to Ocean, and these whales were the main consumers fisheries subsidies are far less clear (Ramsay et al. of krill (mostly Enphausia superba). The reduced 2000). Several studies indicate that populations predation on krill was expected to release krill pro- of flatfish species that are known to be facultative duction for species that formerly competed with scavengers have increased in abundance or in- whales. This was known as the ‘krill surplus hy- creased their growth rate in response to increased pothesis’. Predicted krill production in the south- fishing effort. For example, the growth rates of west Atlantic is 28.6 to 97.6 million tonnes yr -1 , plaice (Pleuronectes platessa) and sole (Solea while sea bird, seal, whale and fish consumption in solea ) have increased in relation to the increase in the same area was estimated as 32.6 million tonnes bottom fishing effort and hence disturbance of the yr -1 (Trathan et al. 1995). If the distributions and seabed (Millner and Whiting 1996; Rijnsdorp and feeding behaviour of krill feeders overlapped in Leeuwen 1996). However, there are many alterna- space and time then competition could reason- tive explanations for the increased growth of plaice ably be expected. However, a recent analysis by and sole, which include the reduction of the popu- Murphy (1995) suggests that many krill predators lations of their main predators and the enhance- cannot compete because they feed in different ment of polychaete communities due to the effects areas at different times. As a result, the effects of of eutrophication.

competitive interactions between krill predators Why should there be such large differences in are minor in comparison with the oceanographic the responses of scavenging sea birds and fishes and factors that determine krill abundance on feeding invertebrates to energy subsidies? Sea birds are able grounds. In general, temporal and spatial variation to actively search for fishing vessels over large areas in krill abundance appears to drive the dynamics of and hence discards from trawlers are a relatively krill consumers. The variation is due to recruit-

Chapter 16

ment fluctuations and the differential transport of were calculated for the outcomes of interest, such krill by ocean currents. In the waters around South as changes in yields of different species following a Georgia, for example, krill abundance fluctuates seal cull. Contrary to the prediction of the simplis- by a factor of twenty between years, and in years of tic models, Yodzis showed that a cull of seals was low abundance krill predators suffer catastrophic more likely to be detrimental to total yields than breeding failures (Croxall et al. 1988; Brierley beneficial. et al. 1997; Murphy et al. 1998). The early models that predicted the existence of a ‘krill surplus’ were based on a homogeneous view of the food chain and assumed that krill were equally vulnerable

16.6 EFFECTS ON

to predators when in fact they were not (Murphy

BENTHIC COMMUNITIES

AND HABITATS

16.5.2 Culling mammals to

All continental shelf areas are subject to fishing

boost fishery yields using towed bottom-fishing gear. Fishing in the

marine environment is perhaps the best example Fishers see marine mammals eating fish and often of a method of harvesting animals that can lead treat them as competitors. Indeed, there are many to habitat change. In a recent article, Watling and examples of fishers culling marine mammals Norse (1998) compared the effects of fishing in an attempt to boost fishery yields (Earle 1996; with towed bottom-fishing gears to clear-cutting Hutchinson 1996). There are similar traditions in of forests in terrestrial systems. This is perhaps freshwater systems, where by anglers sometimes understandable, given the manner in which towed remove predatory birds and fishes in the hope that bottom fishing gears operate (see section 16.4 and this will increase their catches (Cowx, Chapter 17, Misund et al., Chapter 2, this volume). Watling this volume).

and Norse’s (1998) assertion was that the inciden-

Traditionally, the simple logic that marine tal or even deliberate removal of topographically mammals and fishers competed directly was complex seabed habitats would have detrimental followed, and the debate focused on how much effects on the associated species assemblage as in the mammals ate and whether this consumption terrestrial systems. This is perhaps not surprising was significant in relation to catch (Beddington and there is good evidence to demonstrate these et al. 1985; Crawford et al. 1992). This argument effects in some marine systems. For example, assumes a simple food web as perceived by many Sainsbury (1987; 1998) reported that as sponges and fishers, in which fishers compete with a marine soft-coral communities were removed as bycatch mammal for a prey. However, such systems are in trawls off the northwestern shelf of Australia, embedded in a much more complex system in the associated fish species were greatly reduced in which the interactions between fishers and marine abundance. Only after these areas were protected mammals do not follow the rules predicted by from bottom fishing did they observe slow regen- simple models (Beverton 1985; Yodzis 1998; Pauly eration of the habitat as sponges and corals began and Christensen, Chapter 10, this volume).

to recolonize and grow. Once this process had

A recent analysis by Yodzis (1998) considered begun, then the populations of associated fish the potential effects of a fur seal cull on fishery species began to increase once again. In temperate yields from the Benguela upwelling ecosystem off estuarine systems, oysters are important reef- West Africa. Here, the simple model, where seals forming organisms that add structural complexity and fishers compete for fish that feed on every- to the seabed and increase species and habitat thing else, was discarded, and Yodzis described the diversity. Oysters also improve water quality system using a food web containing 29 species or through their filtration activities as they bind species groups (Fig. 16.1). Probability distributions particulate organic matter and remove pollutants A recent analysis by Yodzis (1998) considered begun, then the populations of associated fish the potential effects of a fur seal cull on fishery species began to increase once again. In temperate yields from the Benguela upwelling ecosystem off estuarine systems, oysters are important reef- West Africa. Here, the simple model, where seals forming organisms that add structural complexity and fishers compete for fish that feed on every- to the seabed and increase species and habitat thing else, was discarded, and Yodzis described the diversity. Oysters also improve water quality system using a food web containing 29 species or through their filtration activities as they bind species groups (Fig. 16.1). Probability distributions particulate organic matter and remove pollutants

Many fisheries that use towed bottom gears occur over coarse sediments in relatively shallow waters. The species that live in these habitats are adapted to physical disturbance as they are regu- larly subjected to wave action, strong currents and tidal scouring (Kaiser 1998). Hence, it might be expected that the effects of fishing disturbance in such habitats would be relatively minor in com- parison with other more stable environments. The ecological significance of fishing disturbance will vary according to the intensity, frequency and history of disturbance that has occurred over a particular area of the seabed. This implies that in some habitats a certain level of physical distur- bance may be sustainable, whereas in others it may have long-lasting ecological effects. This is proba- bly the most relevant question for current research as the implicit assumptions would form the basis for predicting the likely outcome of bottom- fishing practices in different habitats.

There now exists an extensive literature on manipulative experiments designed to ascertain the immediate effects of fishing disturbance on benthic community structure. The majority of thesestudies demonstrate significant short-term reductions in species number and counts of individual organisms as a result of fishing disturbance (Jennings and Kaiser 1998; Auster and Langton 1999). While these studies have been important for understand- ing the mechanism of community change and as- certaining those species that are most vulnerable to disturbance, they are unable to replicate the in- tensity, frequency or long-term disturbance histo- ry associated with the fishing activities of an entire fleet of fishing vessels. A better approach to study- ing the long-term effects of fishing disturbance at the scale of the fishing fleet is to compare areas sub- jected to different known intensities of fishing dis- turbance. A number of studies have employed this approach and have independently shown similar findings (Collie et al. 1997; Thrush et al. 1998; Bradshaw et al. 2000; Kaiser et al. 2000a,b).

Using a combination of fishing effort data and direct observations from side-scan sonar surveys, Collie et al. (1997) were able to identify compar- able substrata that experienced different inten- sities of scallop dredging on the Georges Bank, northwest Atlantic. Areas that were less frequent- ly fished were characterized by abundant bryo- zoans, hydroids and worm tubes which increased the three-dimensional complexity of the habitat. Furthermore, examination of evenness within the community suggested dominance by these structural organisms, which indicated that this environment was relatively undisturbed. In con- trast, the more intensively dredged areas had lower species diversity, lower biomass of fauna, and were dominated by hard-shelled bivalves (e.g. Astarte spp.), echinoderms and scavenging decapods. The higher diversity indices observed at the less intensively dredged sites were attributable to the large number of organisms, such as polychaetes, shrimp, brittle stars, mussels and small fishes, that were associated with the emergent sessile fauna (Collie et al. 1997). Many of these associated species were also important prey for commercially exploited fishes such as cod (Bowman and Michaels 1984). This study emphasizes how fish- ing activities can degrade habitat characteristics on which commercial fish species depend for suc- cessful growth and survival (see Benaka 1999).

Bradshaw et al. (2000) were able to take advan- tage of the creation of an area closed to fishing to monitor subsequent changes in benthic commu- nity structure. The closed area was located off the south of the Isle of Man, Irish Sea, in an intensively fished scallop ground. After several years surveyed plots within the closed area had become signifi- cantly different to similar plots of seabed in the adjacent seabed that remained open to fishing. Bradshaw et al. (2000) then tested the hypothesis that these changes had occurred as a result of the lack of fishing disturbance. Within the area closed to fishing, experimental plots were fished with the same scallop dredges that are used by the commer- cial fleet. As a result of their experimental fishing, Bradshaw et al. (2000) altered the benthic commu- nity in the experimental plots within the closed area such that it became similar to that in the adja-

Ecosystem Effects of Fishing

355

356

Chapter 16

cent commercial fishing ground. This manipula- bottom disturbances for 20 years. Standard benthic tion convincingly demonstrated that the seabed surveys of the infauna and epifauna within the surrounding the closed area was maintained in different areas revealed that the areas that had

a permanently altered state by bottom-fishing been closed to towed bottom-fishing gears were disturbance.

dominated by emergent sessile epifauna that had In a similar study, Kaiser et al. (2000a) investi-

a relatively high biomass. In contrast, the areas of gated differences in benthic community structure seabed subjected to either seasonal or continuous and habitat complexity in areas exposed to differ- towed bottom-fishing activities were dominated ing levels of bottom-fishing activity according to by small-bodied opportunistic species and mobile the restrictions imposed by a voluntary manage- scavenging fauna (Fig. 16.5). ment agreement. This agreement exists between

The resolution of fishing-effort data sets re- fixed-gear and towed bottom-gear fishers that op- mains a major obstacle to comparative studies of erate off the south Devon coast in England. The the long-term effects of repeated bottom-fishing agreement was instigated in 1978. All fishing by disturbance. The simplest solution is to install scallop dredgers and trawlers is prohibited in areas tracking devices to fishing boats that would record reserved for the setting of pots or fixed bottom gear. their precise position. These devices are now fitted Some areas are open to towed bottom gears for to a proportion of certain fleets for the purpose of limited periods of the year. The closed areas extend monitoring compliance with fisheries regulations out to 6 nautical miles from the shore, beyond (Rijnsdorp et al. 1998). However, these schemes which there are no operational restrictions on are in their infancy and do not yet provide a long- bottom-fishing activities. This management sys- term historical record of seabed disturbance. tem permitted Kaiser et al. (2000a) to compare Kaiser et al. (2000b) were able to partly overcome areas of the seabed that had been exposed to differ- some of these problems to compare areas of the ing levels of fishing disturbance over a prolonged seabed off the Isle of Man that are subjected to period. The areas open to pot fishing only were either high or low levels of scallop-dredging considered to have experienced only infrequent disturbance. They chose this fishery as the subject

(c) High disturbance 100

(a) Low disturbance

(b) Medium disturbance

100

100

90 90 90 80 B 80 80 70 70 70 60 A 60 60 50 50 50 40 40 40 30 30 30 Abundance/biomass (%) 20 20 20 10 10 10

1 10 100

1 10 1 10 100

Species rank

Fig. 16.5 Abundance/biomass curves of samples collected from (a) areas protected from towed bottom-fishing gear (low disturbance), (b) areas open seasonally to towed bottom-fishing gear and those areas (c) that are fished all year with towed bottom-fishing gear (high disturbance). As the level of bottom-fishing disturbance increases the biomass curve (B) converges with the abundance curve (A), which is a typical response in stressed communities. (Source: adapted from Kaiser et al. 2000a.)

of their investigation because fishing effort has been recorded at a relatively small spatial scale since the early 1980s (5 ¥ 5 nautical miles). Never- theless, even this relatively fine scale of fishing- effort data presents problems of sampling at the appropriate scale when one considers that the sam- pling devices used collect organisms from areas

of 0.1 m 2 (grabs) to approximately 1 m 2 (dredges).

Kaiser et al. (2000b) were able to corroborate the disturbance history of the specific areas of seabed sampled from the disturbance scars that occurred in the shells of a long-lived bivalve, Glycymeris glycymeris. Bivalves lay down annual growth rings, hence it was also possible to determine the year in which the disturbance occurred. Fishing gears that come into contact with bivalves leave distinctive scars in the shell matrix when the ani- mal’s mantle withdraws in response to physical in- jury (Fig. 16.6). The occurrence of these scars in the shells of G. glycymeris confirmed that the chosen heavily fished areas had experienced significantly higher levels of fishing disturbance consistently over many years compared with low mean levels of disturbance experienced in the chosen areas of low fishing effort. An examination of the benthic com- munities in these areas showed that large biomass emergent epifauna were more prevalent in the areas of seabed that had been fished sporadically over many years, whereas those areas that had

experienced constantly high levels of fishing dis- turbance were dominated by more opportunistic and scavenging species.