Using stereoscopic video cameras to evaluate seagrass meadows nursery function in the Mediterranean

Seagrasses provide an important ecological value as nursery habitats, hosting higher juvenile densities than their surrounding less-structured habitats by offering shelter and food to early stages of fish. However, the quantitative evaluations of this nursery function remain elusive. Surveys assessing abundances of juvenile fish over seagrass meadows have been largely conducted by diving visual censuses, where typically only a shallow depth range is adequately covered. Within this study, we developed a low-cost stereo-baited video camera (SBRUV) with high precision and accuracy able to deliver length frequency distributions, including the smallest juvenile fraction. The capacity of the SBRUV system increased the synopticity with respect to typical surveys. We tested the system over seagrass meadows of Posidonia oceanica in a Mediterranean bay, investigating the relationship of juvenile abundance and size of the sparid Diplodus annularis with depth, time of day and protection status (i.e. inside and outside of a Marine Protected Area, MPA). We found significant effects of the depth on the length of the older size classes fraction (larger sizes at deeper stations) and an effect of time of the day on the abundance (less abundant during the evenings), and an opposite pattern was observed for early juvenile’s abundances. The MPA protection had no effect in D. annularis population structure. Interestingly, the relative abundance of D. annularis early juveniles was comparable at all depths, from 2 to 20 m, which suggests a potentially higher nursery value of P. oceanica meadows than earlier thought.