Stage dependent habitat use under conflicting predation pressure: An experimental test with larval and juvenile two-spotted gobies, Gobiusculus flavescens Fabricius

Abstract

The effect of size, predator types and presence of multiple predators on the microhabitat use of larvae and juveniles of a sublittoral, semipelagically schooling fish, the two-spotted goby ( Gobiusculus flavescens), was tested in two experiments. Larvae (15 and 25 days old, Experiment I) and juveniles (mean ± 1 S.E.: small, 15.9 mm ± 1.28; medium, 19.2 mm ± 1.43; and large, 23.4 mm ± 1.67, Experiment II) were allowed to choose between two sections of the tanks; an upper, representing a water column habitat, and a lower, artificially vegetated, representing the hyperbenthic habitat. Position of larvae or juveniles and the activity level of juveniles were recorded. Predator treatments were: (I) no predators (control), (II) a pelagic predator, the jellyfish Aurelia aurita L., (III) a hyperbenthic predator, the mysid Praunus flexuosus O.F. Müller or (VI) both predator types simultaneously. In Experiment I predators were restricted to the habitat which they were chosen to represent, while goby larvae could move freely. In Experiment II both predators and juvenile gobies were allowed to move freely between compartments. Increasing age caused larval gobies, but not juveniles to shift downwards. Only 25-day-old larvae and small juveniles avoided the mysid by shifting upwards. Larval response to A. aurita was also size dependant: 25-day-old larvae avoided medusae by shifting downwards, while 15 day olds did not. Emergent multiple predator effects were found for the vertical distribution of 15-day-old larvae and small juveniles. Larger juveniles were more active than smaller, both in the upper and the lower sections of tanks. P. flexuosus caused juvenile gobies in their vicinity (i.e. in the lower section) to increase their activity level, while small juveniles (but not medium-sized or large) increased their activity level even when further away (i.e. in the upper section). The presence of A. aurita led to a reduction in activity of small juveniles in its vicinity (i.e. in the upper section), while no response was observed among older juveniles or juveniles further away from the predator (i.e. in the lower section). Emergent multiple predator effects on the activity level of juveniles were not observed.