Designing a coastal monitoring marine biodiversity survey, using trammel nets and gillnets in Portugal

Although coastal areas are of great importance, they often lack long term monitoring surveys, which are essential for effective management, fisheries sustainability, and marine spatial planning. To address this, we conducted two experimental surveys aimed at assessing communities and their biodiversity, with the objective of designing a monitoring program for marine coastal biodiversity, based on gear type, depth, sampling arrangement, total net length, and panel position within the haul. Depth was found to be the most important factor shaping biodiversity, while gear type had the greatest impact on assessing species richness. Overall, both number of species and diversity increased with depth, though significant differences were only found between hauls deployed at 10 m and deeper (30–50 m). Trammel nets caught a larger number of species compared to gillnets, and there was evidence of an interaction between depth and gear, in terms of abundance and at the community level; samples taken deeper than 10 m showed more gear-related differences. We found that neither the panel position within the haul nor the number of nets had any impact on species diversity or abundance. Similarly, there was no evidence for an effect related to the arrangement of nets, as results were consistent whether using separated or continuous net panels. However, the number of nets used as a sampling unit significantly influenced the results, as the variation in species abundance and diversity with depth and gear type was similar, when 20 or 30 nets were used, but became more variable when only 10 nets were used, even with increased replication. We concluded that both trammel nets and gillnets should be included in a coastal biodiversity monitoring program, as each catches a different set of species, including both demersal and pelagic taxa. Further, we recommend using a minimum of 20 nets (ideally 30+) per station, with replication. The evaluated monitoring system has a minimal impact on the ecosystem, can be easily deployed using commercial vessels and effectively captures a large number of species, being thus, highly recommended to be used in coastal monitoring surveys.