McBride et al.: Expansion of spawning and nursery grounds of Centropristis striata into a warming Gulf of Maine 
333 
fidelity of age-0 black sea bass, at least until they mi¬ 
grate offshore as winter approaches; therefore local 
conditions are relevant to juvenile growth and survival 
(Miller et al., 2016). Although more experimental work 
would be useful to define a full response to tempera¬ 
ture and salinity combinations, first-year growth con¬ 
ditions off southern New England do not appear to be 
suboptimal in relation to the central portions of the 
middle Atlantic seaboard. 
Spawning and nursery habitats 
Black sea bass spawning during June was historical¬ 
ly documented in Nantucket Sound by Wilson (1889), 
and later, Kolek (1990) identified spawning black sea 
bass in shallow waters, <6 m, along the south shore of 
Cape Cod during May-July. In our analysis of spawn¬ 
ing fish from trawl samples, ripe females were deeper 
and more variable in deeper water (mean: 18.7 m [SD 
21.1], range: 6-146 m, n=341) than that reported for 
running ripe females (mean: 12.2 m [SD 5.3], range: 
5-35 m, n=83; MADMF and NEFSC spring surveys, 
1984-2017). These characterizations of spawning depth 
are similar to, or even shallower than, Drohan et al.’s 
(2007) characterization of black sea bass spawning: at 
20-50 m depth. Recaptures of spawning fish in subse¬ 
quent years, on the spawning grounds where they were 
tagged, indicate homing behavior to Nantucket Sound 
(Kolek, 1990). 
We assume limited movements by spawning fish 
between locations of capture and spawning, which ap¬ 
pears reasonable on the basis of field observations of 
tagged fish (Fabrizio et al., 2013). Our conclusion that 
spawning is expanding into the Gulf of Maine is de¬ 
pendent on whether the developing females observed 
there since 2000 do spawn there. Our prediction is that 
actively spawning females (i.e., ovulating) will eventu¬ 
ally be documented in the Gulf of Maine, or that with 
gonad histological methods, partially spent females 
(i.e., with fresh postovulatory follicles) between batch 
spawning events will be identified there. 
The NMFS-NEFSC trawling data may reflect a bias 
caused by a switch to a larger trawl and research ves¬ 
sel in 2008, which increased catches (NEFSC 1 ). How¬ 
ever, expansion of mature adults into the southern 
Gulf of Maine was also evident in the MA-DMF sur¬ 
vey, which has not switched gear or ships during this 
period (Suppl. Material). Another concern is to explain 
why some age-0 fish were present in the southern Gulf 
of Maine in the 1980s, before spawning adults were 
documented. Some researchers have suggested early 
life stages of bass are transported through the Cape 
Cod Canal, but no ichthyoplankton data exist to con¬ 
firm such transport. 
These first approximations of areal extent and dy¬ 
namics of spawning suggest additional avenues for re¬ 
search. Tagging returns from previous years by Kolek 
(1990) and Moser and Shepherd (2009) indicate that 
a combined use of acoustic tags may not only be suc¬ 
cessful for documenting philopatry, but may also be 
successful for contrasting spawning activity in differ¬ 
ent habitats and elucidating details of black sea bass 
mating systems, as has occurred recently for Atlantic 
cod (Gadus morhua) (e.g., Dean et al., 2014; Zemeckis 
et al., 2014). 
Further focus on age-0 fish appears warranted. For 
example, the success of newly settled fish in habitats 
in the Gulf of Maine depends on the prey base there. 
Are there aspects of habitat at time of settlement that 
are being preconditioned by warming or is there already 
suitable habitat there now, but the area lacked compe¬ 
tent larvae (i.e., larvae capable of undergoing metamor¬ 
phosis) previously. Black sea bass do not use estuarine 
habitats extensively as nurseries, but age-0 juveniles 
do use polyhaline, subtidal habitats in estuaries (this 
study; Able and Hales, 1997; Peters and Chigbu, 2017), 
and larvae and juveniles are tolerant of a wide range 
of salinities (10-32; Berlinsky et al., 2000). The expan¬ 
sion of early life stages, those of both larvae and settled 
juveniles, on Georges Bank off the coast of New Eng¬ 
land (Suppl. Material) is quite recent and warrants con¬ 
tinued scrutiny, as well. Given this breath of possible 
settlement habitats, examining the microchemistry and 
isotopic signatures of otolith cores may be fruitful to 
test the hypothesis that fish settling at more northern 
latitudes have been responsible for the recent trend in 
increasing biomass of the northern stock. 
This study details black sea bass spawning and 
nursery habitats in a poorly studied region, noting that 
habitat use has been dynamic over the last 4 decades. 
Spawning adults and juveniles have extended north¬ 
ward as coastal waters warm, and possibly to the east, 
on Georges Bank (Suppl. Material). Nye et al. (2009) 
did not include black sea bass in their review of chang¬ 
ing fish distributions in continental shelf waters of the 
U.S. Northeast, but more recent examinations of the 
same data set show that the distribution range of this 
species is moving northward (Bell et al., 2015; Kleis- 
ner et al., 2016; Miller et al., 2016). Such analyses 
have been conducted at a coarse level, i.e., by examin¬ 
ing changes in geographic centroids, without consider¬ 
ation of life stage. The most recent stock assessment 
(NEFSC 1 ) showed that the northern (north of Hudson 
Canyon off the New Jersey coast) but not the southern 
contingents of black sea bass have been increasing in 
abundance, which could give the impression that the 
fish are moving north, when what is changing is the 
productivity rates of each set of contingents. Also, the 
geographic distribution of this species may appear to 
be moving ‘north’ in spring because warmer tempera¬ 
tures initiate an earlier migration to inshore waters 
off Long Island, New York, as well as north of Cape 
Cod (Suppl. Material). Warm-temperate species, like 
black sea bass, are preconditioned to wide seasonal 
fluctuations in temperature, and thus may be better 
able to expand into new habitats caused by warming 
seas at high latitudes. Still, just because this stock mi¬ 
grates seasonally, we should not assume that aggregate 
changes in the distribution of this population are solely 
the result of individuals per se moving farther north. 
