Do big boats tow big nets?

Fishing vessel capacity for trawlers is generally expressed in terms of length, tonnage, and engine power, assuming that a larger vessel has a greater fishing power. Management uses effort-control measures such as kW-day limits based on this assumption. Many studies have shown a weak and noisy relationship between effort and modelled catches, and explanatory models often require the inclusion of a skipper or vessel effect to explain the variance. A key element in this effect is the choice of gear size. Relationships are investigated between metrics of the vessel (length, tonnage, and power) and the gear towed (length of groundgear, or circumference of the net opening) in Scottish and Irish whitefish, Nephrops, and pelagic otter trawlers. Often, the vessel size did not correlate with that of the gear, or did so only for smaller vessels (<1000 hp). The key implication is that effort management based on vessel metrics alone is not appropriate, because it is a poor predictor for gear size, and hence for fishing power. Effort restrictions may actually encourage the adoption of larger gears for a given vessel, to maximize the value of a limited-time resource.