Effects of sea cucumber fishing on tropical seagrass productivity

Fishing can drive major ecological change in coastal ecosystems and is typically examined through top-down trophic impacts. However, the massive removal of biomass can also disrupt key ecological bottom-up processes, though how these effects shape ecosystems is poorly understood. Here, we examined the ecological roles of two species of commercially exploited sea cucumbers thought to promote primary productivity in nutrient-poor environments through nutrient provisioning and sediment processing. Using a large-scale field experiment, we tested whether simulated sea cucumber fishing affected seagrass productivity in a natural system comprising reef and seagrass patches that varied in abundance of vertebrate nutrient providers (i.e., fishes). Our findings were scale- and metric-specific: while we could not detect a change in patch-level seagrass productivity in response to simulated sea cucumber fishing, individual leaf extension rates were similar to 15% lower at sites where all sea cucumbers were removed, relative to the highest density, unfished sites. Interestingly, there was no concomitant effect of nutrients from the more abundant reef-associated fishes, which contribute far more nutrients overall than sea cucumbers. This suggests that sea cucumbers are likely mediating seagrass growth through mechanisms other than direct nutrient provisioning, perhaps through processes associated with sediment processing. Our study demonstrates the potential consequences of under-regulated and unmonitored sea cucumber fishing on foundation species like seagrasses, while highlighting the importance of taking a community-based approach to these types of field experiments.