Grouped Licenses Network Analysis
Based on Stoll et al. 2016 license categories
| License Type | Target Species | Stoll 2016 Grouping |
|---|---|---|
| Aquaculture license | Aquaculture | Aquaculture |
| Commercial fishing crew | Finfish | Commercial Fishing |
| Commercial fishing single | Finfish | Commercial Fishing |
| Commercial pelagic and anadromous crew | Pelagic/Anadromous | Pelagic & Anadromous |
| Commercial pelagic and anadromous single | Pelagic/Anadromous | Pelagic & Anadromous |
| Commercial shellfish | Shellfish | Shellfish |
| Commercial shrimp crew | Shrimp | Northern Shrimp |
| Commercial shellfish + 70 | Shellfish | Shellfish |
| Commercial shrimp single | Shrimp | Northern Shrimp |
| Commercial shellfish under 18 | Shellfish | Shellfish |
| Elver dip net | Elver | Elver |
| Elver dip net crew | Elver | Elver |
| Elver 1 fyke net | Elver | Elver |
| Elver 1 fyke net crew | Elver | Elver |
| Elver 2 fyke net | Elver | Elver |
| Elver 2 fyke net with crew | Elver | Elver |
| Elver dip net 1 fyke net | Elver | Elver |
| Elver dip net 1 fyke net crew | Elver | Elver |
| Eel pot | Eel | Eel |
| Green crab | Green crab | Green crab |
| Lobster crab apprentice | Lobster | Lobster Apprentice & Student |
| Lobster crab apprentice + 70 | Lobster | Lobster Apprentice & Student |
| Lobster crab apprentace under 18 | Lobster | Lobster Apprentice & Student |
| Lobster crab class i | Lobster | Lobster Class I |
| Lobster crab class ii | Lobster | Lobster Class II |
| Lobster crab class ii + 70 | Lobster | Lobster Class II |
| Lobster crab class iii | Lobster | Lobster Class III |
| Lobster crab class iii + 70 | Lobster | Lobster Class III |
| Lobster crab + 70 | Lobster | Lobster Apprentice & Student |
| Lobster crab student | Lobster | Lobster Apprentice & Student |
| Lobster crab under 18 | Lobster | Lobster Apprentice & Student |
| Mussel dragger | Mussel | Mussel |
| Menhaden commercial | Menhaden | Menhaden |
| Menhaden non commercial | Menhaden | Menhaden |
| Mussel hand | Mussel | Mussel |
| Marine worm digging | Marine worm | Marine worm |
| Quahog (mahogany) | Quahog clam | Mahogany Clam |
| Sea cucumber drag | Sea cucumber | Sea Cucumber |
| Scallop dragger | Scallop | Scallop (Dragger) |
| Scallop diver | Scallop | Scallop (Hand) |
| Scallop with tender | Scallop | Scallop (Tender) |
| Spat | Spat | Spat |
| Sea urchin dragger | Sea urchin | Sea Urchin (Dragger) |
| Sea urchin diver | Sea urchin | Sea Urchin (Hand) |
| Sea urchin raking | Sea urchin | Sea Urchin (Hand) |
| Surf clam boat | Surf clam | Surf Clam |
| Sea urchin with tender | Sea urchin | Sea Urchin (Tender) |
| Seaweed | Seaweed | Seaweed |
| Seaweed supplemental | Seaweed | Seaweed |
Unique license divisions
Time Series
Total number of a particular license group issued per year.
Annual Species Metrics
Degree centrality
“The degree of a node is the number of other nodes that single node is connected to. Important nodes tend to have more connections to other nodes. Highly connected nodes are interpreted to have high degree centrality.”
Commercial fishing and lobster licenses (Class I-III) are the most central license types in the network, with their degree centrality increasing over time. Marine worm, scallop dragger, and seaweed license have also become of increasing centrality over time.
Betweeness
“[Betweenness] ranks the nodes based on the flow of connections through the network. Importance is demonstrated through high frequency of connection with multiple other nodes. Nodes with high levels of betweenness tend to serve as a bridge for multiple sets of other important nodes.” Source
License types with high degrees of betweenness are license types that might not be particularly central to the network but are influential to the flow of the system around them. As in, disruption to license types with high levels of betweenness may impact the overall structure of the network significantly.
Initially, we had interpreted license types high betweenness and relatively low centrality to be important “fringe” fisheries that may not be a harvesters focal fishery, but one with low cost of entry that may serve as a supplement or back-up options when landings are low. It is unclear at this time whether that is an appropriate interpretation of this metric.
Annual Network Plots
Groups, or modules, are denoted by color. Community structure is determined by a multi-level optimization of modularity, known as Louvain clustering. > It is based on the modularity measure and a hierarchical approach. Initially, each vertex is assigned to a community on its own. In every step, vertices are re-assigned to communities in a local, greedy way: each vertex is moved to the community with which it achieves the highest contribution to modularity. When no vertices can be reassigned, each community is considered a vertex on its own, and the process starts again with the merged communities. The process stops when there is only a single vertex left or when the modularity cannot be increased any more in a step. Since igraph 1.3, vertices are processed in a random order.
Edge Density and Modularity
Below is the time series of network edge density. The density of a graph is the ratio of the actual number of edges and the largest possible number of edges in the graph, assuming that no multi-edges are present. A network with higher density suggests more connections within the network. In the case of license holdings, a region with a denser network means that fishers have a more diversified permit portfolio and have flexibility to participate in multiple activities.
High modularity and low density have different subgroups of the target species that alternate over the time period, while high density implies species are being caught simultaneously (Nomura et. al, 2021)
Chord Diagrams
Clustering
Heatmap
