Relative importance of population size, fishing pressure and temperature on the spatial distribution of nine Northwest Atlantic groundfish stocks

Abstract

The spatial distribution of nine Northwest Atlantic groundfish stocks was documented using spatial indicators based on Northeast Fisheries Science Center spring and fall bottom trawl survey data, 1963-2016. We then evaluated the relative importance of population size, fishing pressure and bottom temperature on spatial distribution with an information theoretic approach. Northward movement in the spring was generally consistent with prior analyses, whereas changes in depth distribution and area occupancy were not. Only two stocks exhibited the same changes in spatiotemporal distribution in the fall as compared with the spring. Fishing pressure was the most important predictor of the center of gravity (i.e., bivariate mean location of the population) for the majority of stocks in the spring, whereas in the fall this was restricted to the east-west component. Fishing pressure was also the most important predictor of the dispersion around the center of gravity in both spring and fall. In contrast, biomass was the most important predictor of area occupancy for the majority of stocks in both seasons. The relative importance of bottom temperature was ranked highest in the fewest number of cases. This study shows that fishing pressure, in addition to the previously established role of climate, influences the spatial distribution of groundfish in the Northwest Atlantic. More broadly, this study is one of a small but growing body of literature to demonstrate that fishing pressure has an effect on the spatial distribution of marine resources. Future work must consider both fishing pressure and climate when examining mechanisms underlying fish distribution shifts.

Fig 1. Northeast Fisheries Science Center bottom trawl survey offshore strata.

Stock strata definitions are shown for Gulf of Maine and Georges Bank cod (upper left), the northern and southern stocks of red hake and silver hake (upper right), Georges Bank and (spring only) Southern New England-Mid Atlantic yellowtail flounder (lower left). Ecoregions used in previous analyses are also shown (lower right). Further details, as well as the stock strata definition for white hake, are provided in S1 Supporting Information.

Fig 2. Proportion of stocks for which a variable was the most important predictor, by spatial indicator.

Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km²), depth (m) and positive area (PA; km²). Predictor variables are: Northeast Fisheries Science Center (NEFSC) bottom trawl survey stratified mean kg per tow (biomass; kg), catch/NEFSC stratified mean kg per tow (relative F) and NEFSC stratified mean bottom temperature (°C). Proportions may not sum to one in the case of ties.

Fig 3. Maps for the three cases in this study where a significant change in the center of gravity was accompanied by a significant decrease in inertia.

Centers of gravity (left) and inertia (right) for Georges Bank yellowtail flounder at five year intervals in the spring, 1970–2015 (upper), and in the fall, 1965–2015 (middle); and Southern New England-Mid Atlantic yellowtail flounder in the spring, 1970–2015 (lower). Closed Area II and the Hague Line are also shown in the upper and middle panels. Due to the tight grouping of the centers of gravity in the lower left panel the year labels are shown in the inset plot. Year labels for both stocks are truncated where necessary for clarity.