Non-trophic interactions amplify kelp harvest-induced biomass oscillations and biomass changes in a kelp forest ecological network model

Abstract

Kelp forests are important marine ecosystems providing habitat for numerous species. Despite over 50 yr of mechanical harvesting in the Northeast Atlantic, the indirect impacts of kelp harvesting and associated habitat loss on faunal species within kelp forests remain poorly understood. We investigated the consequences of kelp harvesting by developing an allometric trophic network model for a subtidal Northeast Atlantic kelp forest (dominated by Laminaria hyperborea). Additionally, we designed a novel mechanistic model to explore the non-trophic interactions between kelp and age-class 0 Atlantic cod Gadus morhua and between kelp and European lobster Homarus gammarus, specifically focusing on the increased survival benefits provided by the kelp habitat. Simulations were conducted over a 50 yr period, incorporating harvesting cycles of 2, 5, and 9 yr, as well as low and high harvesting intensities. Our findings reveal the complex dynamics resulting from kelp harvesting. The recovery of kelp biomass was observed with 5 and 9 yr harvesting cycles, whereas a decline was observed with a 2 yr cycle. Furthermore, the non-trophic interactions facilitated a higher pre-harvest biomass for both European lobster and Atlantic cod compared to scenarios without this interaction. These results highlight the multitrophic effects of kelp harvesting and emphasize that the recovery of kelp-associated species may not necessarily align with kelp recovery, depending on harvesting intensity and recovery periods. Importantly, our study contributes to a better understanding of the ecological consequences of kelp harvesting and underscores the need for sustainable management practices to mitigate habitat loss in kelp ecosystems.