Influence of mesh size and tooth spacing on the proportion of damaged organisms in the catches of the Portuguese clam dredge fishery

Abstract

Experiments to assess the effect of mesh size and tooth spacing on the catch of Spisula solida were undertaken with the aim of determining an optimal combination of these two characteristics to minimize the dredging impact on by-catch species. However, our data showed that tooth spacing, mesh size and the interactions between these two factors did not affect the number of damaged macrofaunal individual's caught. This may be because infauna entered the dredge without passing through the space between the teeth and the mesh of the net bag closed as it was stretched by the weight of the contents, preventing the escape of the caught individuals. Thus, independently of mesh size, when the dredge is towed over the sediment, the retained individuals were injured due to abrasion between animals and/or between animals and debris. The severity of injuries inflicted by dredging on different macrobenthic species is related to their morphology and fragility.

Manila Clam (Tapes philippinarum Adams & Reeve, 1852) in the Lagoon of Marano and Grado (Northern Adriatic Sea, Italy): Socio-Economic and Environmental Pathway of a Shell Farm

“…The alteration of sediment features may influence the colonization and presence of benthic species, including target ones, leading to long-term changes in community structure (Pranovi & Giovanardi, 1994;Pranovi et al, 1998). In this way, there has been increasing recent interest in evaluating the collateral effects of commercial cultivation of bivalves on the marine environment (Sorokin et al, 1999;Bartoli et al, 2001;Jie et al, 2001;Dame et al, 2002), especially on macrobenthic communities (Kaiser et al, 1996;Drake & Arias, 1997;Spencer et al, 1997;Gaspar et al, 2002;. The main effect on non-target species is a reduction in species richness and abundance (Commito, 1987;Dittman, 1990;Guenther, 1996;Ragnarsson & Raffaelli, 1999;Beadman et al, 2004;Pranovi et al, 2004), although in some cases the opposite effect has been recorded (Mantovani et al, 2006).…”

Population structure and species dynamics of Spisula solida, Diogenes pugilator and Branchiostoma lanceolatum along a temporal–spatial gradient in the south coast of Portugal

“…Spisula solida is heavily fished in the south coast of Portugal Gaspar et al, 2002;Chícharo et al, 2002a,b). The present study raises questions of proper fisheries management.…”

“…In marine soft sediments, physical disturbance seems to be a key factor controlling the spatial and temporal compositions of the populations inhabiting those areas (Dernie et al, 2003). This disturbance may be naturally induced, such as waves and tide impacts (Snelgrove et al, 1998;Hewitt et al, 2003), but may also be due to anthropogenic perturbations, such as fishing (Chícharo et al, 2002a,b;Gaspar et al, 2002;Dernie et al, 2003) and dredging activities (Sardá et al, 2000). At a smaller scale, complex biological interactions, which in turn control sediment properties, also play an important role in regulating patterns of the biota (Snelgrove et al, 1998;Reise, 2002;Hewitt et al, 2003).…”

“…The dredges are designed to dig clams out of the sediment, disturbing the seabed both in terms of physical structure and biological communities (Gaspar et al, 2003). Besides the impact on the S. solida populations, these fisheries may also have impacts on other non-target species, such as the hermit crabs, which frequently constitute the by-catch of these fisheries (Gaspar et al, 2002(Gaspar et al, , 2003Palma et al, 2003). Hermit crabs do not seem to be strongly affected by injuries caused by the dredges (Gaspar et al, 2002).…”

“…Besides the impact on the S. solida populations, these fisheries may also have impacts on other non-target species, such as the hermit crabs, which frequently constitute the by-catch of these fisheries (Gaspar et al, 2002(Gaspar et al, , 2003Palma et al, 2003). Hermit crabs do not seem to be strongly affected by injuries caused by the dredges (Gaspar et al, 2002). The by-catch discards of the bivalve dredge fishery may also constitute an important source of food for benthic scavengers, such as D. pugilator (Chícharo et al, 2002b;Palma et al, 2003), therefore influencing their population dynamics.…”