Michel J. Kaiser

The effects of fishing on marine ecosystems

ABSTRACT We review the effects of fishing on benthic fauna, habitat, diversity, community structure and trophic interactions in tropical, temperate and polar marine environments and consider whether it is possible to predict or manage fishing-induced changes in marine ecosystems. Such considerations are timely given the disillusionment with some fishery management strategies and that policy makers need a scientific basis for deciding whether they should respond to social, economic and political demands for instituting or preventing ecosystem-based management. Fishing has significant direct and indirect effects on habitat, and on the diversity, structure and productivity of benthic communities. These effects are most readily identified and last longest in those areas that experience infrequent natural disturbance. The initiation of fishing in an unfished system leads to dramatic changes in fish community structure. As fishing intensity increases the additional effects are more difficult to detect. Fishing has accelerated and magnified natural declines in the abundance of many forage fishes and this has lead to reduced reproductive success and abundance in birds and marine mammals. However, such donor-controlled dynamics are less apparent in food webs where fishes are the top predators since their feeding strategies are rather more plastic than those of most birds and mammals. Fishers tend to target species in sequence as a fishery develops and this leads to changes in the composition of the fished communities with time. The dramatic and apparently compensatory shifts in the biomass of different species in many fished ecosystems have often been driven by environmental change rather than the indirect effects of fishing. Indeed, in most pelagic systems, species replacements would have occurred, albeit less rapidly, in the absence of fishing pressure. In those cases when predator or prey species fill a key role, fishing can have dramatic indirect effects on community structure. Thus fishing has shifted some coral reef ecosystems to alternate stable states because there is tight predator–prey coupling between invertebrate feeding fishes and sea urchins. Fishing has reduced, and locally extirpated, populations of predatory fishes. These reductions do not have a consistent effect on the abundance and diversity of their prey: environmental processes control prey populations in some systems, whereas top-down processes are more important in others. By-catch which is discarded during fishing activities may sustain populations of scavenging species, particularly seabirds. We conclude by identifying the circumstances in which new research is needed to guide managers and stress the importance of unfished control sites for studies of fishing effects. We discuss the advantages and disadvantages of closed area management (marine reserves) and the conditions under which such management is likely to provide benefits for the fishery or ecosystem.

A quantitative analysis of fishing impacts on shelf-sea benthos

1. The effects of towed bottom-fishing gear on benthic communities is the subject of heated debate, but the generality of trawl effects with respect to gear and habitat types is poorly understood. To address this deficiency we undertook a meta-analysis of 39 published fishing impact studies. 2. Our analysis shows that inter-tidal dredging and scallop dredging have the greatest initial effects on benthic biota, while trawling has less effect. Fauna in stable gravel, mud and biogenic habitats are more adversely affected than those in less consolidated coarse sediments. 3. Recovery rate appears most rapid in these less physically stable habitats, which are generally inhabited by more opportunistic species. However, defined areas that are fished in excess of three times per year (as occurs in parts of the North Sea and Georges Bank) are likely to be maintained in a permanently altered state. 4. We conclude that intuition about how fishing ought to affect benthic communities is generally supported, but that there are substantial gaps in the available data, which urgently need to be filled. In particular, data on impacts and recovery of epifaunal structure-forming benthic communities are badly needed.

The effects of beam-trawl disturbance on infaunal communities in different habitats

Beam-trawling is a source of physical disturbance to marine sedimentary communities in areas less than 50m deep, on the western European continental shelf. Chains attached between the beam-trawl shoes are designed to penetrate the upper few cm of the sediment, which leads to the damage or removal of some infaunal and epifaunal species. In some areas, beam-trawling may be frequent and intense, leading to speculation that it may generate long-term changes in the local benthic fauna. As part of a larger MAFF study examining the ecological effects of beam-trawling, we investigated its local impact on an infaunal community in the north-eastern Irish Sea. Studies of this type are complicated by the heterogeneity of the environment, hence we adopted a replicated, paired control and treatment design to maximize the chances of detecting any effects due to trawling. A side-scan sonar survey revealed that the experimental area was characterized by mobile megaripples in the south-eastern sector of the experimental area and stable sediments with uniform topography in the north-western sector. Multivariate analysis of the species abundances from the control areas separated the fauna into two distinct communities which corresponded to the different substratum characteristics. Data from the two regions were therefore treated separately when testing for the effects of trawling. In the north-western sector, trawling led to 58% decrease in the mean abundance of some taxa and a 50% reduction in the mean number of species per sample. Multivariate analysis revealed that differences between control and fished sites were largely due to the reduction or removal of less common species. These effects were less apparent in the mobile sediments of the south-eastern sector, which had a naturally impoverished fauna and high level of heterogeneity. Univariate variables, such as abundance and the total number of species per sample, indicated that the variation between replicate samples increased as a result of trawling disturbance. However, examination of the community data using an index of multivariate dispersion revealed no difference between fished and unfished areas. This suggests that the effects of fishing disturbance are consistent between replicate samples. Fishing with demersal gears modifies communities in relatively stable sediments. Frequent and repeated physical disturbance by fishing gears may lead to long-term changes in the benthic community structure of these habitats.

Coexistence of genetically modified (GM) and non-GM crops in the European Union. A review

The adoption of genetically modified (GM) crops in the European Union (EU) raises questions on the feasibility of coexistence between GM and non-GM crops. Regulations to ensure that different cropping systems can develop side-by-side without excluding any agricultural option are currently implemented or developed by member states. The aim of this review is to explore whether nationally or regionally proposed coexistence strategies comply with the general principles established by the European Commission that ask for science-based and proportionate coexistence measures. In the first part, existing legal requirements and potential sources of adventitious mixing are reviewed. It is discussed what type of coexistence measures might be necessary to keep GM inputs below the legal tolerance threshold of 0.9%. Concentrating on cross-fertilisation as the major biological source of adventitious mixing in maize, it is then assessed to which extent available scientific data on cross-fertilisation can explain the diversity of currently proposed isolation distances by several member states. In the second part, it is analysed whether currently proposed isolation distances reflect contending policy objectives towards GM crops that largely exceed the economic scope of coexistence. It is investigated how coexistence is intersecting with a wider debate about the role of GM crops in agriculture. Based on the analysis of existing cross-fertilisation data, it is concluded that some of the currently proposed isolation distances are not in line with the coexistence principles laid down by the European Commission: they are (i) excessive from a scientific point of view; (ii) difficult to implement in practice; (iii) rarely proportional to the regional heterogeneity in the agricultural landscape; and (iv) not proportional to the farmers’ basic economic incentives for coexistence. Hence, the range of proposed isolation distances cannot simply be explained by different interpretations of available scientific data, possible error intervals and remaining uncertainties inherent in the scientific process. It is argued that other than scientific issues must be at play. One might thus claim that coexistence has become an arena of contending values and visions on the future of agriculture and on the role GM crops might play therein.

Trawl disturbance on benthic communities: chronic effects and experimental predictions

Bottom trawling has widespread impacts on benthic communities and habitats. While the direct impacts of trawl disturbances on benthic communities have been extensively studied, the consequences from long-term chronic disturbances are less well understood. The response of benthic macrofauna to chronic otter-trawl disturbance from a Nephrops norvegicus (Norway lobster) fishery was investigated along a gradient of fishing intensity over a muddy fishing ground in the northeastern Irish Sea. Chronic otter trawling had a significant, negative effect on benthic infauna abundance, biomass, and species richness. Benthic epifauna abundance and species richness also showed a significant, negative response, while no such effect was evident for epibenthic biomass. Furthermore, chronic trawl disturbance led to clear changes in community composition of benthic infauna and epifauna. The results presented indicate that otter-trawl impacts are cumulative and can lead to profound changes in benthic communities, which may have far-reaching implications for the integrity of marine food webs. Studies investigating the short-term effects of fishing manipulations previously concluded that otter trawling on muddy substrates had only modest effects on the benthic biota. Hence, the results presented by this study highlight that data from experimental studies can not be readily extrapolated to an ecosystem level and that subtle cumulative effects may only become apparent when fishing disturbances are examined over larger spatial and temporal scales. Furthermore, this study shows that data on chronic effects of bottom trawling on the benthos will be vital in informing the recently advocated move toward an ecosystem approach in fisheries management. As bottom-trawl fisheries are expanding into ever deeper muddy habitats, the results presented here are an important step toward understanding the global ecosystem effects of bottom trawling.

Co-management Policy Can Reduce Resilience in Traditionally Managed Marine Ecosystems

Best-practice environmental policy often suggests co-management of marine resources as a means of achieving sustainable development. Here we consider the impacts of superimposing co-management policy, in the form of territorial user rights for fishers over an existing traditional community-based natural-resource management system in Chile. We consider a broad definition of co-management that includes a spectrum of arrangements between governments and user groups described by different levels of devolution of power. We used participatory rural appraisal techniques and questionnaires to understand the mechanisms that underpin the traditional management system for the bull-kelp “cochayuyo” (Durvillaea antarctica). Traditional management was based on the allocation of informal access rights through a lottery system. This system was controlled by a complex web of traditional institutions that were shown to be successful in terms of equity and resilience. Using a similar approach, we analyzed the effects of superimposing a government-led co-management policy into this traditional system. Two major effects of the new policy were encountered. First, traditional institutions were weakened, which had negative effects on the levels of trust within the community and intensified conflict among users. Second, the management system’s adaptive capacity was reduced, thereby jeopardizing the ecosystem’s resilience. Our results suggest that the devolution of power to this kind of fisher community still has not reached the level required for fishers to legally address the local deficiencies of the Chilean co-management policy. Additionally, legal adjustments must be made to accommodate traditionally managed ecosystems that offer benefits comparable to those mandated under the formal policy. A fuller understanding of the interactions between co-management and traditional institutions can help us to identify ways to promote resilience and facilitate equal access by mitigating the potential negative effects of co-management policy and informing its future implementation.

Responses of benthic scavengers to fishing disturbance by towed gears in different habitats

The aggregation and feeding behaviour of invertebrate scavengers in areas disturbed by trawling was investigated at three different localities. At each site a fishing disturbance was created using a commercial 4 m beam trawl and scavenger density was quantified using a light beam trawl. At one site two diver surveys were also carried out; along a line fished with a scallop dredge or a beam trawl on two separate occasions. For all experiments the fished and adjacent unfished control areas were sampled before, and at intervals after, the initial fishing disturbance. Sampling with the light beam trawl revealed that hermit crabs Pagurus bernhardus moved into areas which had been fished with a 4 m beam trawl at an experimental site near Anglesey. The density of these hermit crabs increased significantly in the fished area after fishing had taken place, but no change in density occurred in the adjacent control (unfished) area. At two other sites (Red Wharf Bay, Anglesey and a site offshore from Walney Island) there were no detectable increases in scavenger numbers in the fished areas. Furthermore, at the site near Walney Island, numbers of hermit crabs P. bernhardus, swimming crabs Liocarcinus depurator and starfish Asterias rubens actually decreased after fishing. Thus the responses of scavengers to towed fishing gears varied considerably between different communities. At Red Wharf Bay, divers observed similar responses of scavengers to both beam trawl and scallop dredge disturbance. Four predatory species were observed feeding in the fished area; starfish A. rubens, hermit crabs P. bernhardus, brittlestars Ophiura ophiura and whelks Buccinum undatum. These predators fed on damaged bivalves, echinoderms, crustaceans, whelks and polychaetes. The proportion of starfish feeding in the fished area was significantly higher after fishing had taken place. Demersal fishing activities provide food for scavengers in the form of damaged animals which are left in the tracks of the trawl or dredge. The responses of scavengers to fishing disturbance are not always manifested as a large increase in their abundance. It is clear that the magnitude of response varies between species and between habitat types.

Recovery of soft sediment communities and habitats following physical disturbance

Physical disturbance in soft sediment habitats disrupts the sediment structure and can lead to the death or emigration of resident biota. Current methods used to quantify the response of benthic assemblages to physical disturbance are time consuming and expensive, requiring the analysis of a time series of samples to ascertain the time taken for a disturbed area to converge on a condition similar to that found in adjacent control areas (the recovery time). We designed an experiment that studied the effects of two intensities of physical disturbance on both the habitat and fauna of a sheltered sand flat to ascertain whether the recovery of the biota could be predicted from physical attributes of the habitat. Benthic community recovery from the lower intensity disturbance occurred within 64 days of the disturbance, whereas recovery after higher intensity disturbance did not occur until 208 days post-disturbance at this site. Sediment granulometry and percentage organic content did not alter as a result of the disturbance in either treatment. However, the depth of water that remained in the disturbed pits decreased with time, and correlated with the temporal changes in community structure. Thus although this was the most crude physical parameter measured it best described the recovery process of the fauna and may encapsulate the entire suite of other more subtle habitat changes that occur at the same time. By quantifying the persistence of physical features in different soft sediment habitats it might be possible to develop a more amenable and rapid framework for assessing the longevity of the effects of physical disturbance.

Engagement in co-management of marine benthic resources influences environmental perceptions of artisanal fishers

SUMMARY The perceptions of resource users towards any conservation policy can be a major determinant of its success. While co-management policy can enhance the management of fisheries, to date there have been few reports concerning how engagement in co-management regimes may affect fishers’ perceptions. This paper assesses the determinants of fishers’ environmental perceptions within a comanagement regime in Chile. Group meetings and structured questionnaires showed fishers’ environmental perceptions were composed of four domains of concern, termed ‘water pollution’, ‘stock conservation’, ‘conservation/profit trade-offs’ and ‘charismatic species’. Fishers’ dependence on diving and/or the length of time fishers had engaged with the policy affected their perceptions of each of these domains. Perceptions of the ‘water pollution’ domain were related to length of time fishers had participated in co-management, probably because fishers gained an increased understanding of the market and its need for unpolluted produce. Attitudes towards ‘conservation/profit trade-offs’ tended to become linked to an increasingly conservation-oriented ethic with increasing time of engagement with the policy. Given that the length of time fishers engaged with co-management differentially affected fishers’ perceptions of environmental domains of concern, public officials and funding agencies should be careful not to judge co-management prematurely. It takes time to educate participants and overcome distrust. Through co-management processes in Chile, fishers’ environmental awareness increased, and sustainable behaviours may consequently increase in the future.

Chelal morphometry, prey-size selection and aggressive competition in green and red forms of Carcinus maenas (L.)

Abstract Carcinus maenas exhibits a range of adult colour forms, from green to red. Coloration is at least partially correlated with the length of intermoult. There are no external morphometric differences between colour forms, but red crabs have a greater chelal dry mass per unit chelal size, shown particularly in the chelal integument, apodemes, and to a lesser extent the closer muscle. Sarcomere length of the closer muscle of the crusher chela is not significantly different in the two colour forms. Strain gauges mounted in a model mussel showed that red crabs, of ≈21 mm chelal height, exerted a mean force of 21.5 N and green crabs, of similar size, 16.7 N. Red crabs exerted a greater proportion of slow chelal contractions and maintained them for longer than green crabs. Red crabs preferred larger mussels and usually dominated green crabs in aggressive disputes over prey. The wider range of potential prey and aggressive dominance of the red crabs probably places them at a competitive advantage over the green forms, which find refuge in marginal habitats, such as high shore-levels and estuaries.