The role of fish predation on recruitment of Mytilus galloprovincialis on different artificial mussel collectors

Fish predation interferes with mussel seed population dynamic and is an important limiting factor on seed supply in several areas of mussel farming production. In the present study we assessed the impact of fish predation in a mussel farm sited in the Ría de Ares-Betanzos (Galicia, NW Spain). To assess fish impact on recruitment, we have quantified mussel recruitment densities with or without excluding fish predation. The experiment was carried out using four different collector rope designs that could contribute to decrease the fish predation impact on the amount of mussel seed collected for cultivation.The unprotected long-line (fish exposed treatment) showed lower recruitment densities than the protected one (fish exclusion treatment) for every collector design tested (between 38 and 58%; ANOVA, p < 0.001) with the exception of non-filamentous loop complement ropes (NF-L), which showed similar density values (9104 ± 316 and 7855 ± 375 indiv/m in the protected and unprotected long-line, respectively; Tukey p > 0.05). In addition, in the protected long-line recruitment densities were homogeneous between collector designs (8820 ± 635 indiv/m; ANOVA p > 0.05) whereas in the unprotected one statistically significant differences between collectors were observed. These results pointed out the influence of fish predation in the amount of mussel seed collected and its different effect between collector designs. The homogeneous density recorded in the protected long-line between collector designs would suggest the presence of another regulation factor of population size when fish predation is excluded. This regulation factor could be the intra-specific competition derived by space and food limitations of the studied area. With regard to the collector design, differences in recruitment density in the unprotected long-line would suggest different degree of protection from predators depending on collector texture and lacing complexity which could enhance the strength of seed attachment and create space refuges from fish.