193 
Seasonal variability 
in ichthyoplankton abundance 
and assemblage composition 
in the northern Gulf of Mexico off Alabama 
Email address for contact author: fhernandez@disl.org 
1 Dauphin Island Sea Laboratory 
101 Bienville Boulevard 
Dauphin Island, Alabama 36528 
2 Department of Marine Sciences 
University of South Alabama 
307 University Boulevard, LSCB Rm 25 
Mobile, Alabama 36688 
Abstract — Multiyear ichthyoplankton 
surveys used to monitor larval fish 
seasonality, abundance, and assem- 
blage structure can provide early indi- 
cators of regional ecosystem changes. 
Numerous ichthyoplankton surveys 
have been conducted in the north- 
ern Gulf of Mexico, but few have had 
high levels of temporal resolution and 
sample replication. In this study, ich- 
thyoplankton samples were collected 
monthly (October 2004-October 2006) 
at a single station off the coast of 
Alabama as part of a long-term bio- 
logical survey. Four seasonal periods 
were identified from observed and 
historic water temperatures, includ- 
ing a relatively long ( June-October) 
“summer” period (water tempera- 
ture >26°C). Fish egg abundance, 
total larval abundance, and larval 
taxonomic diversity were significantly 
related to water temperature (but not 
salinity), with peaks in the spring, 
spring-summer, and summer periods, 
respectively. Larvae collected during 
the survey represented 58 different 
families, of which engraulids, sciae- 
nids, carangids, and clupeids were 
the most prominent. The most abun- 
dant taxa collected were unidenti- 
fied engraulids (50%), sand seatrout 
(Cynoscion arenarius, 7.5%), Atlantic 
bumper ( Chloroscombrus chrysurus, 
5.4%), Atlantic croaker ( Micropogo - 
nias undulatus, 4.4%), Gulf menha- 
den ( Brevoortia patronus, 3.8%), and 
unidentified gobiids (3.6%). Larval 
concentrations for dominant taxa were 
highly variable between years, but 
the timing of seasonal occurrence for 
these taxa was relatively consistent. 
Documented increases in sea surface 
temperature on the Alabama shelf 
may have various implications for 
larval fish dynamics, as indicated by 
the presence of tropical larval forms 
(e.g., fistularids, labrids, scarids, and 
acanthurids) in our ichthyoplankton 
collections and in recent juvenile sur- 
veys of Alabama and northern Gulf 
of Mexico seagrass habitats. 
Manuscript submitted 8 October 2009. 
Manuscript accepted 8 January 2010. 
Fish. Bull. 108:193-207 (2010). 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Frank J. Hernandez Jr. (contact author)’ 
Sean P. Powers' 2 
William M. Graham 1 
Ichthyoplankton surveys provide fish- 
eries-independent information that is 
inherently “ecosystem-based”; entire 
larval fish assemblages are collected 
(i.e., early stages of both exploited and 
unexploited finfish species) along with 
zooplankton predators and prey, and 
often with a suite of environmental 
observations (e.g., salinity, tempera- 
ture). At the ecosystem level, infor- 
mation on larval assemblages can be 
used to detect changes in marine fish 
community composition and abun- 
dances over time (Sherman et ah, 
1984). Previous studies have indicated 
that larval assemblages are the result 
of convergent spawning strategies by 
multiple species taking advantage of 
favorable environmental conditions for 
larval fish survival (Doyle et ah, 1993; 
Sherman et al., 1984). The composi- 
tion of larval fish assemblages varies 
spatially and temporally because of 
the behaviors of the larvae (Gray and 
Miskiewicz, 2000; Hare and Govoni, 
2005) and the spawning adults (Sher- 
man et al., 1984; Hernandez-Miranda 
et al., 2003), as well as oceanographic 
transport and mixing processes (Auth, 
2008; Muhling et al., 2008). Variabil- 
ity in any of these factors, therefore, 
may result in a different structure 
of larval fish assemblages. Because 
larval fish survival is closely tied with 
primary and secondary productivity 
in coastal oceans, changes in larval 
fish assemblage structure (over larger 
time scales) can be an early indica- 
tor of climate-related environmental 
shifts (Auth, 2008; Brodeur et al., 
2008). 
Despite the importance of the re- 
gion to fisheries, seasonal variabil- 
ity in larval fish assemblages in the 
northern Gulf of Mexico has been 
examined in relatively few studies. 
Much of the previous ichthyoplankton 
research has focused on estuarine as- 
semblages (Raynie and Shaw, 1994; 
Tolan et al., 1997) or on relatively 
short-term interactions between as- 
semblages and specific oceanograph- 
ic features, such as the Mississippi 
River plume (Sogard et al., 1987; Go- 
voni et al., 1989) or the Loop Current 
(Richards et al., 1993). Other studies 
have used ichthyoplankton survey 
data from the National Marine Fish- 
eries Service’s (NMFS’s) gulf-wide 
Southeast Monitoring and Assess- 
ment Program (SEAMAP), but these 
studies are typically focused on a sin- 
gle species (Scott et al., 1993; Lycz- 
kowski-Shultz and Ingram, 2003; Ly- 
czkowski-Shultz and Hanisko, 2007). 
Ditty et al. (1988) summarized the 
available ichthyoplankton literature 
at the time to provide information on 
larval fish seasonality for the entire 
northern Gulf of Mexico, and more 
