Linking social and ecological networks in coastal fisheries in North Carolina
Joseph Luczkovich, Rebecca Deehr, Jeffrey Johnson
Humans have caused significant impacts to ecological networks because of their fishing in coastal food webs – they are “keystone” species.Intensive fishing by humans often causes a trophic cascade (indirect effect), disproportionately affecting other species not the target of the fishery.In addition, humans are connected through a network of behavioral interactions (multiple individual fishers engaged in and switching between fishing gear “fleets” that capture particular species), which may also exhibit properties of indirect network effects.To examine the resilience of the social and ecological networks, and the potential for such indirect effects in both ecosystems and social systems, we prepared social and ecological networks of fishing activities in coastal fisheries in NC (USA).We compared two analytical methods for assessing network robustness to perturbation:the “Key Player” index from social network analysis and “Keystoneness” indices from ecological network analysis.We built two species-node ecological networks (65 species nodes and 5 fishing “fleets”) in adjacent bays where the use of trawling and other commercial fishing gears has been intensive or restricted for 30 years.We also created a fisherman (actor) by gear-species (event) affiliation network, which allowed us to examine the ways fishermen might switch among fisheries.Fisherman networks with high average node density were relatively stable; removing any one individual using “key player” analyses did not fragment the network.However, in one case, even though the average density of the affiliation network was high, there was an uneven distribution of links between two key nodes, so that removal of one of these nodes fragmented the networks.Using this same approach on the ecological network models, we observed several species that are also likely “key players”.In areas open to intensive shrimp trawlingthe top five “key players” that would maximally fragment the network if removed were white shrimp, striped mullet, detritus, blue crabs and southern flounders.In the areas closed to shrimp trawling, the top five “key players” were zooplankton, blue crabs and southern flounders, ctenophores and southern kingfish.Species with the greatest relative total impact (“keystoneness” outputs form the ecological network models) were, spot, zooplankton, weakfish, phytoplankton, and brown pelicans. It is interesting to note that these different approaches to examining resilience produced “key” species are often the target of commercial fisheries (shrimp, blue crabs and flounders are the most highly valued fishery species in NC).Such species are subjected to many management actions, with regulations on season, method of capture, quotas, and closed areas.Restriction of fishing with gill nets, for example, will increase trawling and clamming activities, increasing the ecosystem impacts associated with those gears.Conversely, reducing shrimp trawling will cause fisherman to shift to gill netting for declining species (like southern flounder and red drum).Because single-species management plans have indirect impacts both on ecosystems and the social systems, both social and ecological network models are useful tools for fishery management.