Mortality. Population mortality occurs either as part of the catch (landings plus discards) or incidentally either by killing benthic and demersal species or making them more vulnerable to scavengers and other predators.
Increased food availability. Discarded fish, fish offal, and dead benthic organisms become food for scavenging species.
Loss of habitat. Some fishing gears cause the disturbance or destruction of seafloor habitat.
Indirect effects are the downstream consequences of a direct effect. Reductions in the total biomass of target fish, along with the direct effects noted above, could be expected to affect predators, prey, competitors of a target species, and overall seafloor community structure. Indirect effects also encompass potential changes in the flow of materials and energy through ecosystems and shifts in the balance among the processes of primary production, primary consumption, and secondary production.
Human activities such as trawling can be considered a disturbance to environments, and their effects are often compared with natural disturbances that occur in the same ecosystems. It is important to ask whether human disturbances represent selective pressures at novel spatial or temporal scales or are just slight changes in the scale of existing natural disturbance. Natural disturbances can occur with different periodicities, spatial effects, and patterns of recovery (e.g., Lake, 1990; Pickett and White, 1995). Periodic disturbances can be considered pulse events, and a population or community assemblage might respond in several ways. If the disturbance is not too intense, and if the interval between disturbances is long relative to the attributes of the community, or if the system is resilient, the community could return to its previous state. Ecological disturbance theory also suggests that, even if each individual pulse disturbance does not have a large acute effect, there could be a threshold of intensity or a cumulative level beyond which persistent changes in the ecosystem occur. Resilience is the degree to which an ecosystem’s long-standing composition, structure, and function can recover from disturbance (Holling, 1973). The disturbance paradigm predicts that short-lived, highly motile or dispersing species with high reproduction rates will recover from disturbance faster than will long-lived, sessile, low-dispersing species (Pickett and White, 1995).
The following sections are based on the results of previous studies and reviews. They summarize commonly observed effects of fishing on the seafloor with respect to gear type, the nature of the seafloor habitat, frequency of disturbance (natural and from fishing gear), and rates of recovery to the pretrawling or predredging state.
Studies of the effects of mobile fishing gear on benthic habitat have used observation and experiment. Observational studies compare the benthic habitat in trawled areas with the habitat in lightly trawled or untrawled places. One difficulty with this approach is finding habitats that are similar in all respects other than the degree of fishing. In any given region, benthic areas inhabited by commercial concentrations of fish and shellfish, not closed by regulation, will be trawled or dredged at some frequency. Quantifying how much trawling has occurred in lightly trawled areas can be impractical given the limited scale of benthic studies. It is difficult to assess how much trawling actually occurs in an area solely from effort data collected in most fisheries. A full evaluation of the effects of trawling and dredging on habitat will require higher resolution effort data to translate the results of smallscale experimental studies to effects at the ecosystem level.
Experimental studies generally use the before/after control/impact design. In this approach, an experimental area is trawled and compared before and after trawling (before/after comparison) and with a site that has not been trawled recently (control/impact comparison). This design often involves direct sampling of fauna, video observations, and sonar scans of the control and disturbed sites. The primary limitation of this design is that it is based on the assumption that the control and experimental sites are equivalent. A study by Lindegarth et al. (2000) suggests multiple evaluation sites are needed to assess the effect of trawling on benthic habitat. The authors showed that the interpretation of experimental studies varies depending on the control and treatment sites compared. Although the need for multiple control sites and replicate trawling is acknowledged within the scientific community, application is limited by ship time, funding constraints, and existing and shifting management regimes.