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The dependence of wild seed to replenish mussel culture plots is a limiting factor in areas where spat recruitment is poor. One way to maximize recruitment of mussel spat is to optimize the artificial substrates used as collectors. This study compares the performance of two 5 m long rope collectors with different diameters (14 and 32 mm) for recruiting planktonic spat in Baie de M’diq (Tétouan), an embayed area of high potential for marine aquaculture in the Alboran Sea. Multiple collectors of both diameters were suspended from a long-line system settled in this bay during the main spawning season. Each metre of the 5 m collectors was treated as a different depth grouping and the settled mussels were counted with a microscope. The mean settlement was greater on the thick collector (179.7 ± 62.5 spat m−1) than on the thin collector (34.7 ± 11.6 spat m−1). Significant differences were found in spat abundance between depth samples on thick collectors but not so on thin collectors. Spat abundance on the deepest metre of the thick collector (259.3 ± 14.9) was twice that on the shallowest metre (107.7 ± 11.30). These results are interpreted in terms of aggregation behaviour of mussels that seems to be influenced by length and surface of collectors placed well above the thermocline. We provide recommendations on how to maximize the artificial collection of juveniles in turbulent areas of high mussel spawning but low recruitment. [ABSTRACT FROM PUBLISHER]