Species Networks
Species Grouping
To correlate species distribution changes with changes in license holdings in Maine, license types have been grouped by target species as described by Maine State Legislation.
| License Type | Target Species |
|---|---|
| Aquaculture license | Aquaculture |
| Commercial fishing crew | Finfish |
| Commercial fishing single | Finfish |
| Commercial pelagic and anadromous crew | Pelagic/Anadromous |
| Commercial pelagic and anadromous single | Pelagic/Anadromous |
| Commercial shellfish | Shellfish |
| Commercial shrimp crew | Shrimp |
| Commercial shellfish + 70 | Shellfish |
| Commercial shrimp single | Shrimp |
| Commercial shellfish under 18 | Shellfish |
| Elver dip net | Elver |
| Elver dip net crew | Elver |
| Elver 1 fyke net | Elver |
| Elver 1 fyke net crew | Elver |
| Elver 2 fyke net | Elver |
| Elver 2 fyke net with crew | Elver |
| Elver dip net 1 fyke net | Elver |
| Elver dip net 1 fyke net crew | Elver |
| Eel pot | Eel |
| Green crab | Green crab |
| Lobster crab apprentice | Lobster |
| Lobster crab apprentice + 70 | Lobster |
| Lobster crab apprentace under 18 | Lobster |
| Lobster crab class i | Lobster |
| Lobster crab class ii | Lobster |
| Lobster crab class ii + 70 | Lobster |
| Lobster crab class iii | Lobster |
| Lobster crab class iii + 70 | Lobster |
| Lobster crab + 70 | Lobster |
| Lobster crab student | Lobster |
| Lobster crab under 18 | Lobster |
| Mussel dragger | Mussel |
| Menhaden commercial | Menhaden |
| Menhaden non commercial | Menhaden |
| Mussel hand | Mussel |
| Marine worm digging | Marine worm |
| Quahog (mahogany) | Quahog clam |
| Sea cucumber drag | Sea cucumber |
| Scallop dragger | Scallop |
| Scallop diver | Scallop |
| Scallop with tender | Scallop |
| Spat | Spat |
| Sea urchin dragger | Sea urchin |
| Sea urchin diver | Sea urchin |
| Sea urchin raking | Sea urchin |
| Surf clam boat | Surf clam |
| Sea urchin with tender | Sea urchin |
| Seaweed | Seaweed |
| Seaweed supplemental | Seaweed |
License Portfolio by Species
Using Theresa Burnham’s code for building the individual license portfolio, license types were grouped by target species. A binary matrix was created, whereas 1 denotes an individual license holding for a corresponding year and 0 denotes no license held. With this co-occurrence matrix, we can quantify which license types were held together over the past 30 years and how those holdings have changed.
Targeted Species
Time Series of Targeted Species
The plots below show the total number of licenses issued per year for a particular target species.
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.”
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
Similar to plotting the license holdings as a network over time, we plot the target species pertaining to these permits over time. This makes for a more easily interpretable graph, as well as increased potential for correlating license changes to species distribution changes.
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
Limited Entry Fisheries
According to Maine State Legislation, there are 6 limited entry species including lobster, which is limited by harvest zone. Most species became limited entry in response to a fishery collapse, while some aim to avoid collapse. The most recent limited entry fishery is menhaden, which was passed by the legislature in 2023.