Stephen J. Hall

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.

Conserving livestock breed biodiversity.

Growing awareness of the importance of conserving the biodiversity of livestock breeds is paralleled by genetic advances that will help objective planning of conservation. Inventories of breeds, long advocated, are now being established and concepts originally formulated for the quantification of species diversity are being applied. The breeds thus conserved will provide valuable resources for the future of agriculture, especially in the developing world.

Assessing the Relative Importance of Trophic Links in Food Webs

In recent years much effort has gone into the analysis of food web graphs in a search for recurrent properties—the so-called web statistics (see recent reviews by Pimm et al. (1991) and Hall and Raffaelli (1993)). The data requirements for these analyses are relatively undemanding, requiring no more than a matrix describing what eats what, but while binary representations of species interactions have proven useful for addressing some aspects of community organization, such as connectance (see Bengtsson and Martinez (this volume)), it has become increasingly clear that links vary in their dynamic importance (e.g. Paine (1980, 1992)).

Compartments and predation in an estuarine food web

The proposition that strong functional predator-prey links are associated with compartments in food webs (Paine 1980) is examined for a large, well-documented web, the Ythan estuary. A simple method for the detection and description of infrastructure within binary food webs is described and this is applied to 12 webs previously analysed by Pimm & Lawton (1980) and to the Ythan web. There was evidence for significant compartmentation in several real webs, all of which are reasonably well documented, aquatic and benthic. However, there was no evidence of compartments in the Ythan web

Disturbance of intertidal soft-sediment benthic communities by cockle hand raking

Recent awareness of the ecosystem effects of fishing activities on the marine environment means that there is a pressing need to evaluate the direct and indirect effects of those activities that may have negative effects on non-target species and habitats. The cockle, Cerastoderma edule (L.) is the target of a commercial and artisanal fishery that occurs in intertidal and estuarine habitats across Northern Europe. Cockles are harvested either mechanically using tractor dredges or suction dredges or by large numbers of individual fishers using hand rakes. This study examined the effects of hand raking on the non-target species and under-sized cockles associated with intertidal cockle beds and the effects of size of the patch of sediment disturbed on subsequent recolonisation. Hand raking led to an initial three-fold increase in the damage rate of under-sized cockles compared with control plots. The communities in both small and large raked plots showed community changes relative to control plots 14 days after the initial disturbance. The small raked plots had recovered 56 days after the initial disturbance whereas the large raked plots remained in an altered state. Samples collected over a year later indicated that small-scale variations in habitat heterogeneity had been altered and suggest that while effects of hand raking may be significant within a year, they are unlikely to persist beyond this time-scale unless there are larger long-lived species present within the community.

Impacts of fishing gear on marine benthic habitats

Fishing affects seabed habitats worldwide. However, these impacts are not uniform and are affected by the spatial and temporal distribution of fishing effort, and vary with the habitat type and environment in which they occur. Different fishing methodologies vary in the degree to which they affect the seabed. Towed bottom fishing gears and hydraulic harvesting devices re-suspend the upper layers of the sedimentary habitat and hence re-mobilize contaminants and fine particulate matter into the water column. The ecological significance of these fishing effects has not yet been determined. Structurally complex habitats (e.g. seagrass meadows, biogenic reefs) and those that are relatively undisturbed by natural perturbations (e.g. deep-water mud substrata) are more adversely affected by fishing than unconsolidated sediment habitats that occur in shallow coastal waters. Structurally complex and stable habitats also have the longest recovery trajectories in terms of the re-colonization of the habitat by the associated fauna. Comparative studies of areas of the sea bed that have experienced different levels of fishing activity demonstrate that chronic fishing disturbance leads to the removal of high-biomass species that are composed mostly of emergent seabed organisms. These organisms increase the topographic complexity of the seabed and have been shown to provide shelter for juvenile fishes, reducing their vulnerability to predation. Conversely, small-bodied organisms, such as polychaete worms and scavengers, dominate heavily fished areas. Such a change in habitat may lead to changes in the composition of the resident fish fauna. Fishing also has indirect effects on habitat through the removal of predators that control bio-engineering organisms such as algal-grazing urchins on coral reefs. However, such effects are only manifested in those systems in which the linkages between the main trophic levels are confined to less than ten species. Management regimes that aim to incorporate both fisheries and habitat conservation objectives can be achieved through the appropriate use of a number of approaches, including total and partial exclusion of towed bottom fishing gears, and seasonal and rotational closure techniques. Different management regimes can only be formulated and tested once objectives and criteria for seabed habitats have been defined.

Long-Term Habitat Changes and their Implications for Future Fisheries Management

Recovery rate estimates for a variety of habitats would suggest that some areas of the seabed will continue to be held in a permanently altered state by the physical disturbance associated with fishing activities. What is clear from the studies undertaken to date is that there exist communities and habitats that are so sensitive to physical disturbance that all forms of bottom-fishing with towed gear should be excluded from these areas forthwith. While it would appear that gear restriction management regimes have the added benefit of conserving habitats, target species and benthic fauna within the management area, at present it is not possible to determine whether there are any wider benefits for the fishery that exploits target species outside the management area.