Epsilon Archive for Student Projects

Abstract

Mysids (Opossum shrimp) play an important role in shaping aquatic food webs by competing for food resources with fish while at the same time acting as prey for fish. Despite their important role, they remain neglected in broader ecological assessments because the standard monitoring methods fail to capture mysids on a quantitative level. A sampling method that has shown promise in capturing mysids are light traps, a passive tool that captures phototactic species using an artificial light source. The light traps are considered a suitable method for sampling in fragile ecosystems and areas with complex structure, as a light trap causes minimal damage to the environment. This thesis aims to describe and evaluate the usefulness of light traps for the monitoring of littoral mysids (Neomysis integer and Praunus flexuosus) in a complex coastal environment and to investigate what potential biotic and abiotic factors might affect the catch of mysids. The results showed that quatrefoil light traps modified with a net to keep large predators away captured more mysids than unmodified traps. The presence of three-spined sticklebacks (Gasterosteus aculeatus) was associated with lower catches of mysids, and night duration seems to be an important abiotic driver leading to higher catches during longer nights. The effect of night duration also affected catches of sticklebacks in a similar manner. In contrast, catches of sticklebacks increased over the sampling period, rising temperatures and declining night duration in benthic trap nets (without artificial lights). The positive effect of temperature on stickleback catches is expected as the catchability of fish normally increases in traps and gillnets due to increased activity levels. The light traps captured all sizes of mysids, from juveniles to adults, which is relevant for monitoring. In conclusion, light traps modified to keep predators away may be a promising monitoring method for littoral mysid species in shallow and structured environments that prevent standard methods from being used. Further statistical analyses are, however, needed in order to account for the herein identified sources of variability and to establish a standardized catch-per-unit index for broader application.