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Recruitment of Estuarine Fishes - Long-term
Research and Monitoring
This program is designed to answer the following questions.
What is the timing of larval recruitment to temperate estuaries?
How does this vary between resident and transient species?
Between facultative and obligate estuarine users? Between
closely related species with similar life histories? What,
if any, is the relationship between larval recruitment and
settlement? Is there a relationship between larval supply
(abundance) and juvenile settlement/abundance?
In an attempt to answer some of these questions, we collected
weekly, quantitative ichthyoplankton samples over 6 years
(1989-1994, 1309 samples) to identify temporal scales of variability
in the abundance and occurrence of larval fish assemblages
inside Little Egg inlet (see map) (Witting et al. 1999). We
collected species that spawn in the estuary (30%), both the
estuary and the continental shelf (35%), continental shelf
(25%), and the Sargasso Sea (10%). The following analyses
suggest an annually repeated seasonal progression of species
assemblages: 1) the rank abundance of the 20 dominant species
did not change significantly from year to year, 2) variation
in the density of the dominant species was primarily explained
by intraannual rather than interannual variation, and 3) multivariate
analysis of the assemblage matrix identified five seasonal
assemblages that occurred during all six years. We found that
the timing and duration of each of these seasonal groups were
correlated with two characteristics of the annual temperature
cycle, magnitude (higher or lower temperature) and trajectory
(increasing vs. decreasing temperature). As a result of this
study, we suggest that the repeated occurrence of larval fish
assemblages in temperate estuaries along the U.S. coast may,
in part, be driven by local environmental processes. This
weekly sampling has continued through 2002 and includes over
2400 samples and over 120,000 fish.
In recent years the emphasis on fish recruitment has focused
more on survival of postsettlement young-of-the-year (YOY).
In an effort to evaluate the effectiveness of this approach
we have attempted to determine the relationship between larval
abundance for resident and transient fishes (18 species) based
on the above ichthyoplankton sampling program inside Little
Egg Harbor, and subsequent abundance in sampling gears (traps,
trawls; see below) designed to capture YOY fishes. This analysis
is based on relatively long (4-12 years) sampling programs
in the JCNERR in southern New Jersey. Several significant
relations between larval and YOY abundance were found between
these varied species, sampling gear, and the manner in which
abundance was estimated. As a result of this analysis we are
continuing sampling and analyses with a focus on wire mesh
traps (killitraps) and otter trawls as the most effective
manner in which to monitor these relationships between larval
and juvenile fish abundance in the most cost-effective manner.
In a developing attempt to determine annual variation in
fish habitat utilization we conducted a three-year study to
provide a large-scale perspective of fish assemblage structure
across an ocean-estuarine ecotone (salinity 0.1-32 ppt) based
on sampling at 12 stations along 40 km from the Mullica River
(river), Great Bay (bay), and the adjacent inner continental
shelf (ocean) in southern New Jersey. Otter trawl collections
were dominated by YOY of most of the species encountered.
Species richness and abundance appeared greatest in the ocean,
decreased (with an increase in inter-station variability)
in the bay, and appeared to increase again towards the uppermost
river stations. The same areas contained three non-discrete,
but identifiable, fish assemblages. Members of the searobin
(Triglidae) and butterfish (Stromateidae) families characterized
the ocean and bay, whereas representatives of the temperate
bass (Percichthyidae) and bullhead catfish (Ictaluridae) families
characterized the river. Several species, including bay anchovy
(Anchoa mitchilli) and weakfish
(Cynoscion regalis), exhibited
a ubiquitous distribution across the sampling area. Further
analyses identified salinity and geographic distance among
the variables examined, as the most important determinants
in shaping the assemblages. Other contributors included habitat
heterogeneity and water depth. These observations, to date,
indicate that large-scale patterns in the structure of this
estuarine fish assemblage are primarily a result of individual
species' responses to dominate environmental gradients, as
well as ontogenetic migrations, whereas smaller-scale patterns
appear to be the result of habitat associations that are most
likely driven by foraging, competition, and or predator avoidance
strategies. This sampling program and analyses is continuing
with the addition of several stations in each area.
Specific Research
Projects
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