Substratum heterogeneity of dredged vs un-dredged maerl grounds

Maerl grounds are formed from loose-lying, normally non-geniculated, coralline red algae (Corallinaceae). Such grounds are found in areas characterized primarily by high water movements in the photic zone. Maerl grounds form shallow marine biotopes of high biodiversity, analogous to kelp forests and sea grass beds, all of which are habitats created by marine phototrophs (Birkett et al., 1998; BIOMAERL team, 1999). Maerl grounds can be productive areas for the collection of adults and juveniles of commercially important species such as queen scallops (Aequipecten opercularis) (Hall-Spencer, 1998). Gadoids (N.A. Kamenos, personal observation) too, may be attracted by substratum heterogeneity. High habitat heterogeneity is known to equate to high biodiversity in many marine and terrestrial systems (Ormond et al., 1997). Towed demersal ¢shing gears, especially scallop dredges, cause major physical disturbance to the seabed. The destructive impact of such gears is particularly marked on biogenic substrata, such as sea grass and maerl grounds (Hall, 1999; Kaiser & de Groot, 2000). Hall-Spencer & Moore (2000) have shown that a single passage of towed demersal gear can kill maerl thalli due to burial, but there are no data on the mechanical damage su¡ered by maerl thalli due to gear impaction.