D’Alessandro, E.K.

Sponaugle, S.

Aquatic Commons

Patterns and Processes of Larval Fish Supply  
to the Reefs of the Upper Florida Keys 
 
EVAN K. D’ALESSANDRO and SU SPONAUGLE  
Rosenstiel School of Marine and Atmospheric Science 
University of Miami 
4600 Rickenbacker Causeway 
Miami, Florida 33149 USA 
 
 
ABSTRACT 
To examine the role of cyclic environmental parameters and stochastic 
hydrodynamic events on the delivery of fish larvae to shallow coral reefs along 
a major western boundary current, a series of light traps was deployed over 
shallow coral reefs along the upper Florida Keys every other night for six 
months from May-October of 2002 and 2003.  Wind speed, current velocity, 
and water temperature were measured concurrently.  In total, 27,649 larval 
fishes from 55 families, encompassing pelagic, mesopelagic, seagrass and reef 
species, were collected during the two year time series.  Besides small silvery 
fishes in the families Clupeidae, Engraulidae, and Atherinidae, the collections 
were dominated by the families Pomacentridae, Chaenopsidae, Tripterygiidae, 
Gerreidae, Labrisomidae, Opistognathidae, Scaridae, Labridae, Scorpaenidae, 
Lutjanidae, Monocanthidae, and Sphyraenidae.  The temporal nature of larval 
delivery in the study area appears both cyclic and episodic.  Physical data 
indicated that 2 - 3 mesoscale eddies associated with the Tortugas Gyre passed 
through the study area each year, each with a residence time of several days.  
Such eddies caused a weakening or reversal of the mean northeast flow.  The 
relative influence of these mesoscale features was taxon specific.  Results from 
this study will improve our understanding of the dynamics of larval fish supply 
and how it is affected by physical environmental factors.   
 
KEY WORDS: Larval fish, recruitment, Florida Keys 
 
Patrones y procesos de la oferta de larvas de peces en los 
arrecifes de los cayos superiores de la Florida 
 
Para examinar el papel de parámetros medioambientales cíclicos y de 
eventos hidrodinámicos estocásticos en la distribución de larvas de peces en 
arrecifes coralinos someros en cercanías a una corriente importante, se 
instalaron una serie de trampas de luz a lo largo de los cayos superiores de la 
Florida cada noche de por medio por un período de seis meses, de mayo a 
octubre del 2002 y del 2003. Al mismo tiempo se midieron la velocidad del 
viento y la temperatura.  Durante el período de muestreo se recogieron un total 
de 27 649 larvas de peces pertenecientes a 55 familias, representando especies 
pelágicas, mesopelágicas, de pastos marinos, y arrecifales.  Además de 
sardinas de las familias Clupeideae, Engraulidae, y Atherinidae, las coleccio-
nes estuvieron dominadas por las familias Pomacentridae, Chaenopsidae, 
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Tripterygiidae, Gerreidae, Labrisomidae, Opistognathidae, Scaridae, Labri-
dae, Scorpaenidae, Lutjanidae, Monocanthidae, y  Sphyraenidae.  El carácter 
temporal de la entrega de larvas en el área de estudio parece ser tanto cíclica 
como episódica.  La información física indica que entre 2–3 eddies asociados 
con el ‘Tortugas Gyre’ pasan por el área de estudio anualmente, y se quedan en 
la zona por varios días.  Estos eddies ocasionaron una disminución, o una 
inversión de la corriente de flujo del noreste.  Se encontró que la influencia 
relativa de estos parámetros de mesoescala fue específica para cada taxa.  Se 
espera que los resultados de este estudio mejoren nuestro entendimiento de la 
dinámica que controla la oferta de larvas de peces de arrecife y cómo está 
afectada por los parámetros medioambientales. 
 
PALABRAS CLAVES: Peces en arrecifes coralinos, larvas, cayos superiores 
de la Florida 
 
 
INTRODUCTION 
The coral reefs of the Florida Keys are located on the western edge of the 
Florida Current (FC), a major western boundary current with predominantly 
northeasterly flow.  Replenishment processes to the fish populations on these 
reefs are important due to high natural mortality and recreational fishing 
pressure.  Although the supply of settlement stage fish larvae is a prerequisite 
of replenishment, very little is known about temporal patterns of larval supply.  
The supply of larvae to coral reefs is variable over time and strongly 
influenced by the physical environment.  Cyclic processes such as lunar phase 
and tidal amplitude may act as cues for settlement while less predictable 
hydrodynamic variability may affect the delivery of larvae to suitable habitat 
(Dufour 1991, Thorrold et al. 1994, Sponaugle and Cowen 1996).   
Although the FC flows consistently to the northeast, there is considerable 
variability in flow along its northwestern front where the coral reefs of the 
Florida Keys are situated.  Mesoscale (> 50 km) recirculation features 
(Tortugas eddies), originating as meanders of the eastern leg of the Loop 
Current, propagate along this front causing a weakening or reversal of the 
northeast flow along the shelf.  It has been hypothesized that these features 
enhance delivery of fish larvae to the Florida Keys (Lee and Williams 1999).  
These eddies are considerably elongated and sheared apart by the time they 
reach the eastern Straits of Florida and the reefs of the upper Florida Keys.  
Smaller, faster moving, submesoscale (< 50 km) frontal eddies cause similar 
current reversals on shorter time scales.   
The focus of this study was to measure the effect of cyclic environmental 
processes and episodic current reversals associated with the Tortugas gyre on 
the temporal patterns of larval fish supply to the reefs of the upper Florida 
Keys.  The null hypothesis was that larval fish supply to the reefs of the upper 
Florida Keys is temporally random.  
 
 
  
 
  D’Alessandro, E.K. and S. Sponauagle  GCFI:57   (2006) Page 251  
 
 
MATERIALS AND METHODS 
Light traps modified from Sponaugle and Cowen (1996) were used to 
measure the temporal pattern of larval fish supply.  The design consisted of a 
cylinder of Nitex netting with three funnel-shaped openings surrounding a 
submersible fluorescent light.  Weather permitting, three replicate traps were 
deployed on French Reef, Key Largo every other night for two six-month 
periods (May-October, 2002 and 2003).  Traps were attached to Florida Keys 
National Marine Sanctuary mooring buoys at sunset and retrieved at dawn.  
Samples were preserved in 95% ethanol for later sorting.  Fish larvae were 
removed and identified to the lowest taxonomic level possible.   
Wind and tide data were obtained from the NOAA C-MAN Molasses Reef 
tower located approximately two km southwest of the study site.  Current flow 
was monitored by two acoustic Doppler current meters fixed to subsurface 
mooring at 21 and 4 meters depth approximately 4 km northeast of the study 
site (T. Lee, RSMAS, University of Miami).  Prior to analysis, the raw current 
meter data were broken into V-alongshore and U-cross-shelf components, 
rotated to standard oceanographic convention, filtered with a low pass 40 hour 
filter to remove high frequency variability, and regressed against wind data to 
remove wind induced water flow.  Ocean color satellite imagery was provided 
by the Institute for Marine Remote Sensing at the University of South Florida.   
In order to examine possible cycling in the biological data, larvae of all 
fish taxa (excluding silvery bait fishes of the families Clupeidae, Atherinidae, 
and Engraulidae) were grouped together and the mean number of fish larvae 
per trap per night sampled was calculated.  The Autocorrelation Function 
(ACF), was applied to these data to determine the lag at which the data cycled 
in time.   The entire twelve month time series was then collapsed into a single 
30 day lunar cycle with day 1 corresponding to the new moon and day 15 
corresponding to the full moon.  Raleigh circular statistics were then used to 
determine if separate taxa were randomly distributed over the lunar cycle.  If 
this test indicated that a taxon was non-randomly distributed over the lunar 
cycle, the mean day about which the data were centered was calculated.  Only 
taxa in which more than 100 individuals were caught were analyzed in this 
manner.  These were Stegastes partitus (Pomacentridae), Pomacentridae 
(excluding S. partitus), Chaenopsidae, Tripterygiidae, Gerreidae, Labrisomi-
dae, Opistognathidae, Sparisoma spp. (Scaridae), Labridae, Scorpaenidae, 
Lutjanidae, Monocanthidae, and Sphyraenidae.  Because these thirteen taxa 
comprised almost three quarters of the total number of fish larvae captured 
(excluding silvery bait fishes), they were considered to be representative of the 
entire data set.   Bray-Curtis cluster analyses were also carried out with these 
twelve taxa.  Data were separated by year and transformed into proportions of 
the total number of each taxon caught in order to prevent clustering due to 
overall abundance.  Any cycling was then removed from the data set by fitting 
a sin wave (corresponding to phase of the moon) to the data and obtaining the 
residuals.  The residuals reflected episodic variability that could then be 
compared to episodic physical features such as passage of mesoscale eddies. 
Current meter data as well as ocean color satellite imagery were carefully 
reviewed to determine periods when mesoscale eddies passed by the study site.   
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These periods were then compared with the biological data to examine the 
effects of mesoscale eddies on the supply of fish larvae to the study site.   
 
 
RESULTS AND DISCUSSION 
In total, 154 nights were sampled over the two six-month periods.  496 
samples were collected on French Reef yielding 7,791 larval fishes from 55 
different families (silvery bait fishes excluded). 
The ACF revealed that larval supply of all fish taxa collected was cycling 
at a lag of approximately 30 days and in synchrony with the lunar/tidal 
amplitude cycle.  Raleigh tests determined that all taxa combined as well as the 
twelve most abundant taxa separately were non-randomly distributed over the 
lunar cycle at the α = .05 level.  All fish combined were centered on a mean 
lunar day of 27.1 and the mean lunar days about which separate taxa were 
centered ranged from 24.1 and 1.1, between the third quarter and new moons.  
The multi-species cyclic nature of these data suggests that the lunar/tidal 
amplitude cycle is a common cue for a majority of fish larvae captured in the 
traps.  The number of dark hours (hours when the moon is visible over the 
horizon), as well as the percentage of the moon illuminated, are decreasing 
during this period and reach a minimum at the new moon.  Inversely, the tidal 
amplitude is increasing to a peak at the new moon during this time.  The tidal 
amplitude and lunar cycle are in phase in the study area and teasing apart the 
influence of these two cycles is difficult.  Because of the large range of mean 
lunar days for separate taxa, it is likely that both cycles are important and the 
degree to which each influences larval supply varies among different families.   
Examination of current meter data as well as ocean color imagery revealed 
that three mesoscale eddies passed by the study site during each six month 
sampling period.  Comparison of the residuals (lunar/tidal amplitude cycling 
removed) of all fish larvae combined and periods where mesoscale eddies 
passed by revealed that the eddies increased variability in the data when 
present or up to a week after their passage.  The first eddy in 2002 and the first 
two in 2003 enhanced larval supply to the study site.  The remaining three 
eddies appeared to disrupt larval supply.  Whether eddy passage increased or 
decreased larval supply may depend on when during the lunar cycle eddies 
were present.  Full moon periods consistently had very low larval supply 
whether or not an eddy passed.  Larval supply increased when eddies were 
present during the period encompassing the third quarter and new moons. 
Cluster analyses indicated that the where the influence of the mesoscale 
eddies was positive, it was also taxon-specific.  Because all twelve of the most 
abundant taxa shared a similar cyclic pattern, the cluster analysis created 
groups largely due to episodic variability in the data.  Three different groups of 
larvae clustered according to their presence during the three eddies that 
enhanced larval supply.  There was no obvious clustering related to reproduc-
tive strategy.  The fact that different families were affected differentially by 
each eddy may simply reflect differential presence in nearshore waters during 
the eddy’s propagation along the reefs. 
In conclusion, the supply of fish larvae to the reefs of the upper Florida 
Keys follows a predictable cyclic temporal pattern which is closely linked to 
  D’Alessandro, E.K. and S. Sponauagle  GCFI:57   (2006) Page 253  
 
variation in the lunar/tidal amplitude cycles.  However, this cycle may be either 
disrupted or enhanced by the episodic passage of mesoscale frontal eddies. 
 
 
LITERATURE CITED 
Dufour, V. 1991.  Variations of fish larvae abundance in reefs: effect of light 
on the colonization of the reefs by fish larvae. C.R. Academy of Science 
313(4):187-194 
Lee, T.N. and E. Williams. 1999.  Mean distribution and seasonal variability of 
coastal currents and temperature in the Florida Keys with implications for 
larval recruitment. Bulletin of Marine Science 64(1):35-56 
Sponaugle, S. and R.K. Cowen. 1996.  Nearshore patterns of coral reef fish 
larval supply to Barbados, West Indies. Marine Ecology Progress Series 
133(1-3):13-28 
Thorrold, S.R., J.M. Shenker, E.D. Maddox, R. Mojica, and E. Wishinski. 
1994.  Larval supply of shorefishes to nursery habitats around Lee 
Stocking Island, Bahamas. II. Lunar and oceanographic influences. 
Marine Biology 118:567-578 
 
 
 
 
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Patterns and processes of larval fish supply in the reefs of the upper Florida Keys

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Patterns and processes of larval fish supply in the reefs of the upper Florida Keys

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