Sagarese et al.: Spatiotemporal overlap of Squalus acanthias and commercial fisheries within the northeast U.S. marine system 
103 
ing spiny dogfish during period 2 because the entire 
quota was filled during period 1 off New England. 
Catchability can be affected by changes in the spa- 
tial distribution of a species on fishing grounds, either 
by increasing or decreasing their availability or vul- 
nerability to the fishery (Freon et al., 1993; Smith and 
Page, 1996; Godo et al., 1999). Off the coast of Peru, 
the catchability of Peruvian anchoveta ( Engraulis rin- 
gens) changed as vulnerability increased because of 
high densities coupled with improved detection ability 
through sonar and radar (Bertrand et al., 2004). Com- 
mercial CPUE can be artificially high if fishing occurs 
in high-density areas because of hyper aggregation, the 
aggregation of fish at low abundances (Rose and Kul- 
ka, 1999). The risk of hyperaggregation is related to 
temporal and spatial behavior of a species and varies 
among species (Frisk et al., 2011). 
Hyperstability can occur if CPUE remains high 
while stock abundance declines (Hilborn and Walters, 
1992). Atlantic cod was a prime example of this con- 
cept in the late 1980s and early 1990s, when commer- 
cial CPUE remained relatively stable near the south- 
ern extent of their range while the size of the fishery 
footprint decreased (Hutchings, 1996; Rose and Kulka, 
1999; Salthaug and Aanes, 2003). At the same time, 
a southward shift in distribution during the 1990s in- 
creased the vulnerability of Atlantic cod to domestic 
fleets within the Canadian Exclusive Economic Zone 
and to foreign fishing fleets outside the Canadian Ex- 
clusive Economic Zone, thereby altering its catchability 
(Rose et al., 1994; Rose and Kulka, 1999). For spiny 
dogfish, inshore shifts in population centroids during 
spring (males: -50 km; females: -20 km) (NEFSC 3 ) 
indicate that spiny dogfish may be more available to 
commercial fisheries that operate inshore. 
Frisk et al. (2011) ranked spiny dogfish as a species 
of greatest concern to undergo overexploitation because 
of its potential for nonlinearity in survey catchability 
combined with a slow life history (Musick, 1999). Large- 
scale seasonal movements and resulting distributional 
changes, both spatially and temporally, likely modify 
the availability of spiny dogfish to both the NEFSC 
bottom trawl survey and commercial fisheries. Chang- 
ing availability to the bottom trawl survey may ex- 
plain the large fluctuations in interannual abundance 
observed in survey estimates (Rago and Sosebee 1 ). For 
commercial fisheries, altered availability of spiny dog- 
fish can lead to increased bycatch and mortality if it 
becomes more coincident with fishing grounds. 
The study described here does not attempt to ad- 
dress apparent increases in abundances estimated by 
bottom trawl surveys. Rather, we examined whether in- 
creased spatial overlap between spiny dogfish and com- 
mercial fisheries has manifested as increases in local 
abundance on fishing grounds — a hypothesis that has 
motivated a number of studies where bycatch reduc- 
tion of spiny dogfish has been examined (Tallack and 
Mandelman, 2009; Chosid et al., 2012; O’Connell et al., 
2012). The objectives of this study were 1) to describe 
the behavior of major commercial fisheries that catch 
spiny dogfish (either directly or indirectly as bycatch) 
between 1989 and 2009, 2) to quantify spatial overlap 
and investigate spatiotemporal interactions between 
spiny dogfish distribution (derived from the NEFSC 
bottom trawl survey) and commercial fisheries (i.e., ef- 
fort and catch) during this time period, and 3) to ex- 
plore and relate changes in availability of spiny dogfish 
to catchability on commercial fishing grounds. 
Materials and methods 
Data sources 
Fishery-dependent data Data collected by the NEFSC’s 
large-scale Northeast Fisheries Observer Program (NE- 
FOP) were assumed representative of commercial fish- 
ery distribution and behavior between 1989 and 2009. 
Within NEFOP data, observed trips were selected care- 
fully to ensure representation of fleet performance by 
season, area, and other factors (Murawski et al., 1995). 
At-sea sampling provided catch (total, retained, and 
discarded), effort, location, and associated biological 
and fishery data (e.g., gear) on a tow-by-tow basis with 
a high spatial resolution (Murawski et al., 1995). 
Fishery-independent data Spiny dogfish distribution 
and abundance were derived from annual NEFSC bot- 
tom trawl surveys conducted with an OT during au- 
tumn and spring. These stratified random sampling 
surveys sample groundfish including spiny dogfish from 
the NES LME from Cape Hatteras, North Carolina, 
north to the U.S. boundary located within the Gulf of 
Maine (GM). Four regions were surveyed, including the 
GM, Georges Bank (GB), southern New England (SNE), 
and the Mid-Atlantic Bight. Offshore strata have been 
sampled during autumn since 1963 and inshore strata 
have been sampled during autumn since 1972, where- 
as the spring survey has sampled groundfish includ- 
ing spiny dogfish from these regions since 1968 and 
1973, respectively. The number of stations sampled per 
stratum was proportional to its area; however, inshore 
strata were sampled at approximately 3 times the rate 
at which offshore strata were sampled. Survey design 
and data collection methods have remained relatively 
consistent throughout the time series and are detailed 
in Azarovitz (1981) and Azarovitz et al. 4 Correction fac- 
tors based on field experiments were applied for chang- 
es in vessels, gear, and doors when necessary. A transi- 
tion in 2009 from the NOAA Ship Albatross IV to the 
NOAA Ship Henry B. Bigelow brought about changes to 
the trawling gear and survey protocol (Brown et al. 5 ) 
4 Azarovitz, T., S. Clark, L. Despres, and C. J. Byrne. 1997. 
The Northeast Fisheries Science Center bottom trawl sur- 
vey programme. ICES Committee Meeting (C.M.) document 
1997/Y:33, 21 p. 
5 Brown, R. W., M. Fogarty, C. Legault, T. Miller, V. Nordahl, R 
Politis, and P. Rago. 2007. Survey transition and calibra- 
tion of bottom trawl surveys along the Northeastern Conti- 
