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).

Click here to return to the AOML project overview page.
Click here to return to the Build Sustainable Fisheries page.

Page last modified: .
Please direct any questions or comments to the OCD Webmaster.