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

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.


Time-series analysis
Estimate, standard error, t-value and p-value for linear regressions of Atlantic cod (Georges Bank) center of gravity (XCG, YCG), inertia, depth and positive area (PA) as a function of year. P-values < 0.05 are in bold. Corresponding time series plots are shown below, with significant linear fits indicated by a solid line. Sample sizes: spring (n = 47); fall (n = 54).

Predictor variables
Time-series of predictor variables for Atlantic cod (Georges Bank). Predictors and associated units are: Northeast Fisheries Science Center (NEFSC) bottom trawl survey stratified mean kg per tow (biomass), catch/NEFSC stratified mean kg per tow (relative F) and NEFSC stratified mean bottom temperature (°C). Data for all three predictors were set to NA for years in which there were insufficient bottom temperature recordings to calculate a stratified mean. Sample sizes: spring (n = 37); fall (n = 41).

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for Atlantic cod (Georges Bank). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ). 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). Predictor variables with the highest summed Akaike weights for each indicator are in bold.

Model-averaged parameter estimates
Model-averaged predictor estimates by spatial indicator for Atlantic cod (Georges Bank). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ). Catch data in the most recent assessment (NEFSC, 2013) begins in 1982, so data for all three predictors were set to NA for years prior. Additionally, data for all three predictors were set to NA for years in which there were insufficient bottom temperature recordings to calculate a stratified mean. Sample sizes: spring (n = 31); fall (n = 29).

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for Atlantic cod (Gulf of Maine). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ).

Time-series analysis
Estimate, standard error, t-value and p-value for linear regressions of red hake (northern) center of gravity (XCG, YCG), inertia, depth and positive area (PA) as a function of year. P-values < 0.05 are in bold. Corresponding time series plots are shown below, with significant linear fits indicated by a solid line. Sample sizes: spring (n = 48); fall (n = 54).

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for red hake (northern). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ). 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). Predictor variables with the highest summed Akaike weights for each indicator are in bold.

Model-averaged parameter estimates
Model-averaged predictor estimates by spatial indicator for red hake (northern). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for red hake (southern). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2

Model-averaged parameter estimates
Model-averaged predictor estimates by spatial indicator for silver hake (northern). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for silver hake (southern). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ).

Model-averaged parameter estimates
Model-averaged predictor estimates by spatial indicator for silver hake (southern). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ). 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).

Predictor variables
Time-series of predictor variables for white hake (unit). Predictors and associated units are: Northeast Fisheries Science Center (NEFSC) bottom trawl survey stratified mean kg per tow (biomass), catch/NEFSC stratified mean kg per tow (relative F) and NEFSC stratified mean bottom temperature (°C). Data for all three predictors were set to NA for years in which there were insufficient bottom temperature recordings to calculate a stratified mean. Sample sizes: spring (n = 41); fall (n = 46).

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for white hake (unit). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2

Model-averaged parameter estimates
Model-averaged predictor estimates by spatial indicator for white hake (unit). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for yellowtail flounder (Georges Bank). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ).

Relative importance of predictors on spatial distribution
Relative importance of predictor variables on spatial indicators for yellowtail flounder (Southern New England-Mid Atlantic). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ). 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). Predictor variables with the highest summed Akaike weights for each indicator are in bold.

Model-averaged parameter estimates
Model-averaged predictor estimates by spatial indicator for yellowtail flounder (Southern New England-Mid Atlantic). Spatial indicators and associated units are: geographically referenced longitude and latitude of the center of gravity (XCG and YCG, respectively; km), inertia (km 2 ), depth (m) and positive area (PA; km 2 ). 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).