SOUTH ATLANTIC BIGHT
RECRUITMENT EXPERIMENT:
EGG SURVEYS AND INLET STUDIES
Principal Investigator: Peter B.
Ortner
Collaborating scientist(s): David
Checkley (Scripps Institution of Oceanography)
Objective:
The objectives of the egg survey work are:
1) to complete validation of the continuous assessment methodology through
intercomparison of net and pump data collected during the year 4 field season;
and,
2) to describe and understand observed variability in spawning distribution
and timing
observed during the years 3, 4, and 5 field seasons.
3) to assess the degree to which improved knowledge of spawning may be used
to
better understand interannual variability in recruitment.
Our specific hypotheses include:
that storm passage stimulates spawning;
that frontal upwelling areas are preferred spawning sites;
that particular geographic sites are preferred;
that larvae originating from particular patches are more likely
to be successfully transported across the shelf )
The objective of the inlet experiment is:
1) to determine the distribution of particles of various sizes (and motile
abilities) in relation to tidally driven water exchange across the Beaufort inlet mouth.
Rationale:
A basic premise of SABRE is that by studying the birthdate history
of survivors (larvae, late larvae, and juveniles) one can determine which
life history phase or passage (spawning, transport across the shelf, inlet
ingress, estuarine development, inlet egress) regulates recruitment
variability in annual cohorts of transgressive species like Atlantic
menhaden (figure 1). A first step in the process
of estimating survivorship is to determine the time-zero distribution.
This has been estimated in other fisheries, with limited success, from
spawning biomass. That approach is unfeasible in this fishery.
Prior to SABRE menhaden eggs had occasionally been caught in
icthyoplankton net tows taken in the South Atlantic Bight but never with
the frequency and abundance expected based upon the abundance of adults.
We argued that the basic method used, net tows at discrete stations were
unsuited to the sampling requirements since the postulated spawning area
was extensive and extended for hundreds of kilometers over a comparatively
wide shelf and spawning events were discrete and of relatively short
duration. In short it would be easy to miss the highly contagious
distribution of newly spawned eggs and after only a few days these eggs
would be so dispersed they would become comparatively rare. Moreover with
net collection methods realtime feedback is limited and adapative sampling
strategies (modifying search patterns or resampling hot spots) is
difficult or impossible. Prior to SABRE one continous method had been
used, the in-situ optical particle counter (OPC) developed by A. Herman of BIO,
but with limited success. Unfortunately while particles of the actual
diameter of menhaden eggs (ca. 1.6mm) are comparatively rare, the eggs are
largely transparent with more than half of their cross-sectional area
consisting of virtually transparent perivitelline space. The OPC which is
actually measures occluded light is sensitive not only to cross- sectional
area but also to opacity and determines the area of newly hatched eggs to
be less than 0.8mm which as luck would have it are the approximate size of
the dominant extremely abundant copepods of the SAB. In addition it
proved difficult to tow the OPC at full ship speed during the severe sea
states regularly encountered during the midwinter spawning period. SABRE
needed a system that furnished realtime feedback continously under any sea
conditions at maximal ship speeds. In the original SABRE five-year
implementation plan a ship-mounted pump based survey system was postulated
and that is exactly what was eventually developed and deployed.
Method:
A system was assembled that consists of a submersible pump sampler, a
concentrator, a realtime detection device and a mechanical sample collector.
The first was designed by D. Checkley (Scripps Institute of
Oceanography), the second was adapted by AOML from the design of A. Herman
(Bedford Institute of Oceanography), the third was developed by AOML in
conjunction with A. Herman and the last was the creation of D. Checkley.
Together these constitute the Continuous
Underway Fish Egg Sampler (CUFES).
In brief, a high volume pump sample delivers ca. 600l per minute from an inlet
port mounted on a pipe attached shipside at a depth of ca. 3m. The
concentrating unit, retains particles greater than 1mm in a reduced flow
of ca. 20l/min discarding the remaining 580l overboard. This step markedly
increases the system signal to noise ratio by discarding most of the
confounding small copepods. The flow then passes
through an OPC/video detector which is continously monitored by AOML or
NMFS personnel and thence to into a collection cup with 1mm Nitex mesh.
At intervals the cups are collected and either preliminarily examined
under the microscope or immediately preserved for subsequent microscopic
examination. The flow passes in turn the OPC and then a silhouette video
camera system. When a particle is detected that might be an egg the
individual video frame is grabbed and frozen briefly upon a large display
monitor. All particles detected are associated with individual video
frames and can be readily indentified. The latest version of the system
saves in digital form the specific region-of-interest of frames
containing particles which results in improved image quality and
considerably reduced digital storage requirements. The system is integrated
with a Global Positioning System (GPS) so that particle distributions can be
accurately mapped in time and space [see Checkley et al, 1997] .
Accomplishments:
Using CUFES SABRE was able to determine the preferred spawning locale for
Atlantic menhaden off the Carolina coast. Previous studies (confirmed by
SABRE work) suggested that spawning occured within waters of a comparatively
narrow temperature range (figure 3)
between the very warm waters of the Gulf Stream and
the very cool waters of the inner shelf. The black bars indicate the
number of observations of eggs at the indicated temperature, while the
white bars indicate the number of observations overall at
that temperature during the survey.
Uniquely we determined that there was a geographic preference as well.
Egg patches mapped by CUFES along with concommitent hydrographic data
(temperature, salinity and chlorophyll fluorescence)
clearly showed that spawning occured in shallow waters located
just south of Cape Lookout shoals in waters of very different origin
(figure 4).
This was an unexpected result for a pelagic spawning clupeid and unique in
the literature.
As discussed above CUFES incorporates an OPC and video system. From
our laboaratory measurements of the optical particle size of menhaden eggs
we were able to define their optical size over a comparatively narrow range.
Differencing the signal as we crossed the egg patch (visible in the video
in real time], that is subtracting the observations in an adjacent control
region from those within the patch, we were able use the OPC to automatically
enumerate the number of eggs encountered (figure 5)]. These estimates were within 1% of
those made by counting all the eggs caught in the mechanical collector
and imply we will eventually be able to totally automate the procedure.
CUFES has already been officially adopted by the CALCOFI program to
define spawning by egg production of sardine and anchovy populations off
California and by the South Africans to define populations in the Agulhas
Current. The technology is generally applicable to pelagic spawners with
buoyant eggs. Last, we were even able to age the eggs encountered using
the in situ video. Digitized images of individual eggs are sufficient to
determine their developmental age (figure
6). These data are used with physical models to
calculate advection trajectories and dispersion coefficients.
Key Reference:
Checkley, D.C., P.B. Ortner, L.R. Settle and S.R. Cummings. A continuous underway fish egg sampler. Fisheries Oceanog. 6:2, 58-73 (1997).
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