Lazzari: Dynamics of larval fish abundance in Penobscot Bay, Maine 
91 
The dominant fishes in Penobscot Bay lay demersal and 
adhesive eggs (Bigelow and Schroeder, 1953). Fourteen of 
16 species (87%) of larvae collected in Penobscot Bay in 
1998 and and 17 of 22 (78%) of the larvae collected in 1997 
hatched from demersal eggs. Twenty-two species of larvae 
were taken in the central coastal area of Maine and a com- 
paratively large number of pelagic-egg species (41%) com- 
pared with demersal-egg species, as might be expected in 
an area strongly influenced by coastal water (Chenoweth, 
1973). In upper Sheepscot Bay, 42 species of fish larvae 
were taken (Shaw, 1981), but this relatively high number 
reflects the long time series (nine years) of this survey and 
the inclusion of several coastal species that were rare in 
the total catch. Even so, these catches were dominated by 
demersal egg-laying species (76%). In the Damariscotta 
estuary and Sullivan Harbor, only one of 22 species (<5%) 
taken as larvae came from pelagic eggs (Townsend, 1981; 
1984). The ichthyoplankton of the St. Lawrence estuary 
were almost exclusively forms from demersal eggs (Able, 
1978) and only seven of 25 (28%) larval forms collected 
came from pelagic eggs. However, pelagic eggs did occur in 
the St. Lawrence estuary, but these were usually nonresi- 
dent species, such as S. scombrus and G. morhua, whose 
larvae were virtually absent from the upper bay. Gadus 
morhua were very rare in Penobscot Bay and the Sheep- 
scot estuary (Chenoweth, 1973; Shaw, 1981) and were ab- 
sent from the Damariscotta estuary and Sullivan Harbor 
(Townsend, 1984). Typically, the abundance of larvae from 
demersal eggs peaked in the winter-spring period whereas 
larvae from pelagic eggs peaked in the summer (Che- 
noweth, 1973). 
Egg type has major implications on larval fish ecology. 
Coastal shelf spawners, such as G. morhua , E. cimbrius, 
H. plattesoides , and T. adspersus lay pelagic eggs that 
may be dispersed over a wide area by the counterclock- 
wise current flow in the Gulf of Maine (Sherman et ah, 
1984). Nearshore spawners, such as P americanus, P. gun- 
nellus , Myoxocephalus spp., U. subbifurcata , and C. haren- 
gus lay demersal eggs and depend on protected areas of 
the coastline for nursery areas. Larvae that hatch from de- 
mersal eggs are less likely to be transported out of their 
nearshore nursery grounds, rather they are entrained in 
the landward moving bottom layer (Pearcy and Richards, 
1962; Norcross and Shaw, 1984). 
Variation in distribution and abundance 
Larval fish assemblage composition in Penobscot Bay 
varied considerably over time but not among sampling 
locations. Interannual and monthly differences in both spe- 
cies composition and abundance of ichthyoplankton in the 
bay appeared to be associated with differences in environ- 
mental conditions, particularly with temperature. Tempo- 
ral patterns of larval fish abundance corresponded with 
seasonality of reproduction (Bigelow and Schroeder, 1953). 
Larvae that hatched from demersal eggs from late winter 
through early spring, such as Myoxocephalus spp.,Ammo- 
dytes sp., and Pholis guunellus, were abundant in Penob- 
scot Bay in March and April and gradually their numbers 
in plankton samples declined. Such a spawning pattern 
suggests that adults use the bay and nearshore areas for an 
extended period of time. Larvae of taxa that hatch later in 
spring such as P. americanus, L. atlanticus and U. subbifur- 
cata, were abundant in Penobscot Bay in May and June. 
These results suggest that temperature is the principal 
factor controlling the temporal occurrence of fish larvae 
in Penobscot Bay. Temperature is a primary factor asso- 
ciated with the occurrence and distribution of young stag- 
es of fish inhabiting temperate and cold oceans (Oben- 
chain, 1981; Frank and Leggett, 1982; Laprise and Pepin, 
1995). This association may be critical in ecosystems char- 
acterized by a short growing season (Conover, 1992). How- 
ever, the influence of temperature on larval occurrence no 
doubt resulted from species-specific responses of spawn- 
ing adults. The late winter-early spring species, such as 
Myoxocephalus spp., Ammodytes sp., and Pholis gunnel- 
lus, had a negative association with temperature, whereas 
late-spring spawners, such as P. americanus , L. atlanti- 
cus, and U. subbifurcata, had a positive association. The 
synchronous emergence of the larvae of several species of 
fishes in the planktonic environment may be a strategy 
to further reduce predation (Frank and Leggett, 1983) re- 
gardless of temperature. The association observed in this 
study between salinity and ichthyoplankton may be arti- 
factual. Although the influences of temperature and sa- 
linity could not be separated, it seems unlikely that the 
minute differences in salinity observed between samples 
would greatly affect marine animals. 
Penobscot Bay plays an important role in the early life 
history of fish inhabiting the central coast of Maine by 
offering favorable habitat for ichthyoplankton. A hydro- 
graphic front off of Penobscot Bay separates the coastline 
into two different hydrographic regimes (Bigelow, 1927). 
West of Penobscot Bay, a combination of increased runoff 
and reduced tidal mixing, favors more rapid development 
of vertical stratification in spring and summer. East of Pe- 
nobscot Bay, tidal mixing is enhanced and the develop- 
ment of vertical stratification is much reduced t hroughout 
the summer months. Vertical stability of the water column 
is controlled more by salinity stratification than by tem- 
perature, and peaks of stability are the result of influxes of 
low-salinity surface waters (Townsend, 1984). Therefore, 
the peak in abundance of both phytoplankton and zoo- 
plankton occurs earlier in the western Gulf of Maine and 
gradually spreads to the east with the onset of vertical 
stratification during spring and summer (Bigelow, 1927). 
Marine fishes may be categorized as those that use the 
estuaries, such as Penobscot Bay, as primary spawning 
and nursery areas and those that do not. Winter-early- 
spring spawning species ( P. gunnellus, Ammodytes sp., H. 
americanus, Myoxocephalus spp. ) belong to the first group 
of resident demersal fishes whose larvae hatch from de- 
mersal eggs and use the bays and estuaries as nursery ar- 
eas. Typically, these larvae disappear from collections in 
April and May. Two species that spawn in late spring, P. 
americanus and U. subbifurcata, also use Penobscot Bay 
as a spawning and nursery habitat and have areas of 
greatest larval abundance in the midbay and lower bay, re- 
spectively. The larvae of E. cimbrius, G. morhua, T. adsper- 
sus, S. scombrus and H. plattessoides, although present, 
