Northeast Fisheries Science Center Reference Document 22-04

Update on the Spatial Distribution 

of Butterfish, 1982-2019

March 2022
Northeast Fisheries Science Center Reference Document 22-04

Update on the Spatial Distribution 

of Butterfish, 1982-2019

by Charles F. Adams1

1 NOAA Fisheries, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts 02543, USA

US DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Northeast Fisheries Science Center

Woods Hole, Massachusetts

March 2022
Northeast Fisheries Science Center (NEFSC) 

Reference Documents 

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Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, 

the NEFSC completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office. 

This document may be cited as: 

Adams CF. 2022. Update on the spatial distribution of butterfish, 1982-2019. US Dept Commer 

Northeast Fish Sci Cent Ref Doc. 22-04; 12 p.
TABLE OF CONTENTS

Abstract.......................................................................................................................................1

Introduction.................................................................................................................................2

Methods.......................................................................................................................................2

Results.........................................................................................................................................3

Discussion...................................................................................................................................3

References Cited .........................................................................................................................4

Tables..........................................................................................................................................5

Figures.........................................................................................................................................6
1

ABSTRACT

This document updates a prior analysis of the spatial distribution of butterfish (Peprilus 

triacanthus) in the Northwest Atlantic Ocean with 6 additional years of Northeast Fisheries 

Science Center (NEFSC) spring and fall bottom trawl survey data. The primary findings are that 

there was a significant increase in area occupancy for all ages of butterfish over the spring time 

series, as well as a significant increase in area occupancy for age 0 butterfish in the fall, due in part 

to a range expansion into the Gulf of Maine. It is recommended that inclusion of the NEFSC spring 

bottom trawl survey data in the assessment model should be considered in the upcoming research 

track, as well as Gulf of Maine and outer Georges Bank strata.
2

INTRODUCTION

The spatial distribution of butterfish (Peprilus triacanthus) in the Northwest Atlantic 

Ocean from 1982-2013 has been previously characterized (Adams 2017) using several spatial 

indicators based on Northeast Fisheries Science Center (NEFSC) spring and fall bottom trawl 

survey data. This analysis found no significant northward movement of butterfish in spring or fall 

over the course of either time series. However, there was a significant increase in the area occupied 

by ages 1-3 in the spring that was correlated with surface temperature. It was also concluded that 

the spatial distribution of age 0 recruits is driven by environmental conditions.

A research track assessment for butterfish is scheduled for fall 2021. In preparation for this, 

I update the prior spatial distribution analysis with 6 additional years of data (2014-2019) in this 

document. Given the cancellation of the 2020 NEFSC spring and fall bottom trawl surveys due to 

the COVID-19 pandemic, this document will provide the most up-to-date information on the 

spatial distribution of butterfish for the upcoming research track. 

METHODS

Survey Data

Methods and equations for the present analysis are identical to those used previously 

(Adams 2017) and are only briefly summarized here. As noted above, the analysis was based on 

NEFSC spring and fall bottom trawl survey data. Strata used were a combination of those referred 

to as offshore in the most recent benchmark assessment (Adams et al. 2015), as well as Gulf of 

Maine and outer Georges Bank strata (Figure 1).

Age-length keys from each cruise were used to transform the length frequencies observed 

at each trawl station into age frequencies. Butterfish abundance at each station was disaggregated 

into age 0 to 4+ for the fall and age 1 to 4+ for the spring. Data for 2009-2019 were converted to 

FRV Albatross IV units using the length-based calibration in Miller (2013).

Spatial Analysis

Distances between points (trawl stations) were computed in a Euclidean reference system. 

This was done by setting the minimum longitude and latitude of the strata for each stock as (0, 0) 

and converting all coordinates to kilometers. The cosine of the midpoint latitude (39° 49′ N) was 

used to convert longitude. This process is also known as geographical referencing (Rivoirard et al. 

2000).

Four spatial indices were calculated: the center of gravity (CG), which is the bivariate mean 

location of the population (hereafter referred to as the X- and Y- components of the CG); the 

inertia, which is the variance of the CG; abundance weighted depth; and the positive area (PA), 

which is the area (km2) occupied by fish abundances greater than 0. The PA is analogous to the 

proportion of positive tows, albeit weighted with the areas of influence.

Changes in the spatial distribution of butterfish over time were evaluated with a linear 

regression (i.e., a generalized linear model with Gaussian distribution and identity link function) 

that modeled the relationship between each spatial indicator and year. This was done for each age 

class within a season. Models that exhibited serial correlation were corrected with a first order 

autoregressive fit. Spring 2014 and fall 2017 were omitted from this analysis due to limited or no 

sampling of Southern New England and Mid-Atlantic strata in those surveys.
3

RESULTS

Spatial indicators showed interannual variation in the spring for all age classes (Figure 2). 

There was a significant increase in area occupancy for all ages of butterfish over the course of the 

time series. However, there was no change in the CG, inertia, or depth for any age class in the 

spring.

Spatial indicators also showed interannual variation in the fall for all age classes (Figure 

3). There was a significant eastward movement of the XCG for age 0 butterfish, but no change in 

the YCG. This was primarily due to 2016 and 2019, which had the 2 easternmost XCGs in the 

time series. There was no change in inertia for any age class over the course of the time series. 

Age 1 butterfish exhibited a significant decrease in depth. Finally, age 0 butterfish have shown a 

significant increase in area occupancy over the course of the fall time series. A post hoc

summarization (Table 1, Figure 4) reveals that this is due in part to an increase of age 0 butterfish 

in the Gulf of Maine over the past decade.

DISCUSSION

The objective of this study was to update a prior analysis of butterfish spatial distribution 

(Adams 2017) with 6 more years of data in preparation for the upcoming research track assessment. 

Some trends continued, new ones emerged, and others were no longer significant.

There has been a significant increase in area occupancy for all age classes in the spring. In 

the earlier analysis, these trends were only observed for age 1-3 butterfish; they have continued 

and now include age 4+. This adds further support for my previous suggestion that the spring index 

should be considered for inclusion in the assessment model (Adams 2017). It should be noted that 

butterfish distribution over the shelf is still not on the scale as in the fall (which can be seen by 

comparing the PA plots in Figures 2 and 3), but the trajectory appears to be that this difference 

will shrink as shelf warming continues.

Several significant trends were observed in the fall. The significant eastward trend in the 

XCG was due primarily to 2 new years of data. Previously, there was a significant increase in the 

inertia for ages 1-2, indicating that these age classes were becoming more dispersed around their 

respective CGs (Adams 2017); however, the additional years of data show that this is no longer 

the case. There was also a significant decrease in depth for ages 1-2, which now only applies to 

age 1 butterfish. Finally, there is now an increase in area occupancy for age 0 butterfish, which is 

due in part to a range expansion into the Gulf of Maine.

In conclusion, this update of butterfish spatial distribution provides additional evidence 

that inclusion of the NEFSC spring bottom trawl survey data in the assessment model should be 

considered in the upcoming research track, as well as Gulf of Maine and outer Georges Bank strata.
4

TABLES

Table 1. Northeast Fisheries Science Center fall bottom trawl survey number of positive tows for 

age 0 butterfish in Gulf of Maine strata (24, 26-30, and 36-40), 1982-2019. 

Year Positive

2016 59

2015 43

2014 35

2009 34

2012 31

2019 30

2010 27

2013 27

2000 26

2011 24

2018 19

1998 18

1994 16

1995 15

2002 14

1990 13

1997 13

1999 12

2006 8

1985 7

1982 6

1984 6

1986 6

1993 6

2004 6

2007 6

1996 5

2001 5

2008 5

1983 4

1989 4

1987 3

1991 3

1992 3

2003 3

1988 2

2005 2
5

FIGURES

Figure 1. Northeast Fisheries Science Center bottom trawl survey strata used in this study include 

the butterfish stock assessment offshore strata (cross hatch), as well as Gulf of Maine and outer 

Georges Bank strata (white).
6

Figure 2. Spring time series of spatial indicators for butterfish ages 1 to 4+. Solid lines indicate a significant linear fit at the level of α = 

0.05. Note that linear models were tested on XCG and YCG, not the back-transformed longitude and latitude, which are shown here to aid 

interpretation.
7

Figure 3. Fall time series of spatial indicators for butterfish ages 0 to 4+. Solid lines indicate a significant linear fit at the level of α = 0.05. 

Note that linear models were tested on XCG and YCG, not the back-transformed longitude and latitude, which are shown here to aid 

interpretation.
8

Figure 4. Positive area maps for age 0 butterfish in fall 1988 and fall 2016, when the number of 

positive tows in Gulf of Maine strata (24, 26-30, and 36-40) were 2 and 59, respectively. Black 

polygons indicate positive tows.
9

REFERENCES CITED

Adams CF. 2017. Age-specific differences in the seasonal spatial distribution of butterfish 

(Peprilus triacanthus). ICES J Mar Sci 74(1):170-179. Accessible at: 

https://doi.org/10.1093/icesjms/fsw128

Adams CF, Miller TJ, Manderson JP, Richardson DE, Smith BE. 2015. Butterfish 2014 stock 

assessment. US Dept Commer Northeast Fish Sci Cent Ref Doc 15-06; 110 p. Accessible 

at: https://repository.library.noaa.gov/view/noaa/5022

Miller TJ. 2013. A comparison of hierarchical models for relative catch efficiency based on pairedgear data for US Northwest Atlantic fish stocks. Can J Fish Aquat Sci 70(9):1306-1316.

Accessible at: https://doi.org/10.1139/cjfas-2013-0136

Rivoirard J, Simmonds J, Foote KG, Fernandes P, Bez N. 2000. Geostatistics for estimating fish 

abundance. Oxford: Blackwell Science
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