Pervasive declines in monkfish (Lophius americanus) size structure throughout the northwest Atlantic

Shifts in size structure have been documented for many commercially exploited marine fish stocks, thought to be attributed to size-selective harvesting practices coupled with changing oceanic conditions. Northwest Atlantic monkfish (Lophius americanus) is a commercially valued species in the United States that is commonly caught as bycatch in Canadian scallop and groundfish fisheries. This uniquely positioned stock is bisected by the Canadian–American jurisdictional boundary, with considerable differences in exploitation and management across its range. The status of this species was assessed two decades ago (2000) in the Maritimes region of Canada and more recently (2016) in the United States, with both countries reporting considerable decreases in the average and maximum size attained by fish. A comprehensive understanding of the magnitude of this size decrease and the drivers underlying these changes across this species’ range is lacking. Here, using 55 years of fishery-independent survey data, we apply a linear mixed effects model to quantify temporal changes in monkfish length frequencies. Widespread decreases were evident in both countries, with a 48% reduction in historic body length, indicating that the large fish once prevalent in the 1970s have become uncommon. Length varied significantly as a function of seasonality and bottom temperature. Discrepancies exist in the average observed size between countries when controlling for year, season, and area; with monkfish observed by the United States trawl survey 6 cm larger on average than those observed by the Canadian trawl survey. The implications of this difference should be considered when combining fish length data from multiple surveys. Irrespective of survey differences, the truncation of size structure in Northwest Atlantic monkfish is significant and understanding how these drivers, among others hypothesized to affect size structure (e.g. fishing mortality, density dependence), are associated with population viability will be necessary to ensure its long-term sustainability.