Why is there a difference?
Within the Global Fishing Watch AIS processing, every AIS position is estimated as potential ‘fishing’ or not. Fishing effort is intended to act as a high level summary of the summed hours of potential fishing for a given vessel or area over time. However, when looking at the individual voyage of a given vessel it is not always realistic to see a single coordinate marked as potentially fishing. Therefore, GFW developed fishing events to estimate when a series of positions indicate a possible fishing ‘event’.
Filters are applied to group vessel positions into fishing events, but apparent fishing effort includes all positions identified as fishing for a given vessel. In general, fishing events and apparent fishing effort should reflect similar values, but there are a few exceptional cases which may lead to observable discrepancies – namely, in the case where a vessel has numerous fishing positions which appear consecutively, but are 10 km apart or more than 2 hours apart; these positions may get excluded from fishing events, but be included in apparent fishing effort calculations. The result would be a greater number of apparent fishing hours than what is reflected in hours a vessel is detected in fishing events.
Apparent fishing effort
Global Fishing Watch analyzes AIS data collected from vessels that our research has identified as known or possible commercial fishing vessels, and applies a fishing detection algorithm to determine “apparent fishing activity” based on changes in vessel speed and direction. The algorithm classifies each AIS broadcast data point for these vessels as either apparently fishing or not fishing and shows the fishing on the Global Fishing Watch apparent fishing effort heat map.
Fishing events
Fishing events use those data points as input and summarize them into one event for easier analysis. Fishing events are defined using the following restrictions:
- Consecutive positions identified as fishing are grouped together
- Fishing positions which appear consecutively, but are 10 km apart or more than 2 hours apart are separated.
- Fishing events within 1 hour and 2 km of another fishing event but that may have intermittent transit points are grouped together.
Finally, the dataset is restricted by removing fishing events that are brief and fast, and thus possibly less likely to indicate a realistic fishing event. The following short fishing events are removed:
- Events less than 20 minutes;
- Events comprised of 5 or fewer positions;
- Events that cover distance of less than 0.5 km (for all gears except estimated squid gear);
- Events that cover distance of less than 50m (for estimated squid gear) in distance; and
- Abnormally fast moving vessel events with an average vessel speed of 10 knots per hour or greater.
Ongoing review
An internal review is underway to evaluate options for closer alignment of apparent fishing effort and fishing events.