The impact of habitat disturbance by scallop dredging on marine benthic communities:what can be predicted from the results of experiments?

Abstract

Field experiments were conducted on 2 subtidal sandflats to identify the short-term impacts of commercial scallop dredging on macrobenthic communities. The 2 sites (1400 m2) were situated 14 km apart, both at about 24 m depth, with similar exposure aspects and were characterised by infaunal communities dominated by small and short-lived species. Prior to dredging, preliminary sampling failed to reveal significant differences in the density of common macrofauna within each site, although community composition was distinctly different between sites The experiment was initiated by using a commercial scallop dredge to dredge half of each study site. Macrofauna samples were collected in both the dredged and adjacent reference plot at each site irnmed~ately after dredging and again 3 mo later. The density of common macrofaunal populations at each site decreased as a result of dredging, with some populations still significantly different from the adjacent reference plot after 3 mo. Significant compositional differences in the assemblage structure between dredged and reference plots were also recorded at each site over the course of the experiment. The f~n d~n g s of this experiment are considered a conservative assessment of bottom disturbance by fishing because of the area of seabed used, the types of community present and the intensity of disturbance used in the experiment. The findings of this and similar short-term experiments are discussed in light of the need to predict and assess possible large-scale changes to benthic communities as a result of habitat disturbance by fishing.

The primacy of top-down effects in shallow benthic ecosystems

“…Before proceeding, we should make it clear that we share the opinions o~ ~ Dayton et al (1995), Jackson et al (9001), and Steneck and Sala (2005) who have argued that virtually all of the published predation studies have been carried out under conditions that are fundamentally difterent tkom those that existed when most predator-prey relationships evolved. This is because humans have removed extraordinary quantities of large consumers from the world's coastal oceans, so that many are ecologically extinct, and some have been so tor as long as several hundred years (see examples in Dayton et al 1995Dayton et al , 1998Jackson et al 2001;Myers and Worm 2003;Steneck and Sala 2005).…”

“…This is because humans have removed extraordinary quantities of large consumers from the world's coastal oceans, so that many are ecologically extinct, and some have been so tor as long as several hundred years (see examples in Dayton et al 1995Dayton et al , 1998Jackson et al 2001;Myers and Worm 2003;Steneck and Sala 2005). Because we believe that most coastal ecosystems are now devoid, or nearly devoid, of apex predators, as well as many mid and lower order consumers (Panly et al 1998), it is unlikely that any of the published studies were conducted in ecosystems whose species composition was simile" to that prior to human intervention.…”

“…2000). Most studies of nutrient pollution have been conducted long afier the numbers and diversity of larger consumers were dramatically reduced by centuries of intense harvesting (Dayton et al 1995(Dayton et al , 1998Pinnegar et al 2000;Myers and Worm 2003;Pandolti et al 2005). We now know that the harvesting of large consumers, in many cases to fhncfional extinction, can trigger indirect ettects that result in altered species composition and abundance at several trophie levels, and can be *Corresponding author; tele: 251/861-7533; fax: 251/861-7540; e-mail: kheck@dist org very similar to those reported to result t}om eutrophication (Heck et at.…”

Clam dredging effects and subsequent recovery of benthic communities at different depth ranges

“…For example, Pranovi et al (2004) in a multidisciplinary study of the immediate effects of mechanical clam harvesting in the Venice lagoon detected a significant decrease in the total number of individuals after the haul. Thrush et al (1995) observed a decrease in macrofaunal taxa due to scallop dredging and Kaiser and Spencer (1996) reported a general decrease in biodiversity after beam trawling.…”

“…Biodiversity loss may persist over long periods of time, namely due to the reduction of habitat complexity, resulting from the removal of tubicolous organisms and epibenthic species (Dayton et al, 1995;Kaiser et al, 2000). These organisms are particularly important as they provide protection for commercial species, and their removal will potentially affect fisheries .…”