The effects of barriers on the climbing ability of two amphidromous gobiids

Background/Question/Methods

Many members of the family gobiidae exhibit amphidromous life histories on tropical islands, drifting from freshwater breeding areas to the ocean as larvae and actively migrating back into freshwater as juveniles. Some species are known for their exceptional climbing ability, scaling large waterfalls to reach upstream habitats. While previous work has focused on the climbing ability of juvenile gobies, retaining climbing ability could also be advantageous for adult gobiids, who can be subject to downstream displacement during floods.  On the island of Moorea in French Polynesia, amphidromous gobiids comprise a large part of the freshwater vertebrate fauna. We hypothesized that the climbing ability of these fishes would be negatively impacted by high water flow and steep stream gradients, as well as the presence of man-made stream barriers.  In this study, Sicyopterus spp. and Stiphodon elegans adults were challenged to climb under simulated waterfall conditions in an experimental arena, where ramp angle and flow rate were manipulated. We analyzed interactions between challenge variables and climbing ability using t-tests and ANOVA.  Additionally, we surveyed sites above and below man-made barriers on the island and conducted visual surveys of presence/absence for the two focal species.  

Results/Conclusions

Climbing trials demonstrated clear species-specific differences in climbing performance.  While increases in waterfall angle negatively affected the climbing success of Stiphodon elegans, Sicyopterus spp. maintained high climbing success even under steeper slopes.  Completion time increased for Sicyopterus spp. during the steeper climbing challenge.  When considering the different water flow rates, Stiphodon elegans was more successful and slower at lower flow rates. Our ongoing analysis of climbing performance is focusing on quantifying the ability for fishes to pause or rest during their ascent, as well as the effect of morphological variance on climbing performance.  Stream surveys suggested that the construction of barriers plays a key roll in migration success among Sicyopterus species, which could influence their distributions in island streams.  Understanding limits to the climbing ability of amphidromous gobiids thus has importance for both understanding species distributions and for their conservation in island streams.