The map is mostly based on AIS data; AIS transponders are only required on vessels larger than 100 tons, thus excluding smaller coastal fishing vessels. EDIT A

The map is mostly based on AIS data; AIS transponders are only required on vessels larger than 100 tons, thus excluding smaller coastal fishing vessels. EDIT According to wikipedia, I'm wrong (https://en.wikipedia.org/wiki/Automatic_identification_system https://en.wikipedia.org/wiki/Automatic_identification_syste... ). The international limit (IMO SOLAS) is apparently 300 gross tons, though local jurisdictions can have stricter requirements, e.g. it mentions that the EU requires all fishing vessels longer than 16m to be equipped. EDIT2 wikipedia is wrong, the EU limit for fishing vessels is 15m as of 2014-05-31 (http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32009R1224&from=EN http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELE...).