R/V SEWARD JOHNSON Cruise 9503
to Georges Bank
Department of State Cruise N0. 95-007
13-24 March 1995
We gratefully acknowledge the very able assistance provided by the officers and crew of the R/V SEWARD JOHNSON and the Marine Technology Group of the University of Miami, Florida.
This report was prepared by Greg Lough, Jim Manning, Elaine Caldarone, Betsy Broughton, Marie Kiladis, Larry Madin, Eric Horgan, Grace-Klein MacPhee, and Donna VanKueren. This cruise was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration.
Cruise Narrative 1
Individual Reports 3
Icthyo-Zooplankton Studies 4
MOCNESS Sampling 5
URI Predation Studies 9
Appendix 2. Eventlog 15
Table 1. Larval fish and zooplankton 6
Table 2. Immunological Work 9
Table 3. 1-m MOCNESS Samples 12
In order to sample the same cohort of larvae, ARGOS/GPS/VHF drifting buoys with drogues were used to tag a parcel of water for the conduct of sampling operations.
Scuba diving operations were conducted to collect gelatinous organisms and record their distribution and behavior.
Loading of the R/V SEWARD JOHNSON began on 12 March 1995. A new spool of 0.688" wire had to be put on for the 10-m MOCNESS, wires terminated and the MOCNESS units assembled and tested before we could leave port. The R/V SEWARD JOHNSON left Woods Hole, Massachusetts at 1600 h on 14 March 1995 to begin a bongo-net survey of the eastern part of Georges Bank. Along the way, the cruise track was altered to search for a missing Longterm Mooring (LT1:Brink,Irish). Recent ARGOS fixes from the Longterm Drifter program (Limeburner, Beardsley) indicated residual flow the previous week having an on-bank component that would keep the drifting mooring just downstream from the Stratification Mooring (ST1: Weller, Beardsley). After several legs of a zig-zag pattern, there was no sign of the missing mooring. We stopped at mooring site ST1 at 0930 h 15 March 1995 to make meteorological observations for calibration and mooring ST2 (NMFS) for a visual check. We also stopped at mooring site LT1 (Irish, Brink) to interrogate the bottom mounted ADCP at 1200 h 15 March 1995 and confirmed that it was still in operation.
Our first bongo station began at 1000 15 March 1995 at 40o54.0', 67o30.0'. At the end of the third bongo station we were informed from Woods Hole that a fisherman had spotted the missing buoy near Hydrographer Canyon. We ceased operations and steamed to the last reported site. The buoy was sighted at 0305 h 16 March 1995 and brought on board at 0358 h. The mooring had evidently escaped any recirculation tendency indicated by the ARGOS drifters which were located further on the bank. The mooring had drifted approximately 110 miles to the southwest only a few miles away from where a particle track simulation preformed on the March flow field case (MARNODENa) of the GLOBEC finite element model (Lynch, Loder) had estimated.
The northeastern part of Georges Bank was surveyed during 16-17 March 1995. Cod larvae were collected on most stations (1-10 per haul); haddock were fewer in number. Gadid eggs were abundant on four stations. At the end of bongo station 18, 17 March 1995, Larry Madin and Erich Horgan conducted scuba diving observations 1020-1130 h. During this time the first MarkIII CTD cast was made (1118-1130 h). Weather forecast indicated a major front passing through the area in the next 12 hours (50 knot winds and 14 foot seas), so at 1130 h the vessel steamed for the lee of Provincetown.
Arrived in Provincetown at 0145 h 18 March 1995 and stayed at anchor until departure 1610 h 19 March 1995. After pulling anchor, a drifter test deployment was conducted off Wood End Cape Cod from 1726-1826 h. We then steamed directly to the Northeast Peak to resume sampling operations. Arrived on station 19 "stratified site" (41o50.0', 66o37.0') at 0912 h 20 March 1995, deployed a drifter at 0952 h, and began MOCNESS and CTD operations following the drifter. The weather became increasingly bad so that after MOCNESS 014 operations ceased at 1756 h 21 March 1995. The drifter was retrieved at 1925 h.
On 22 March 1995 at 0900 h the sea state at station 19 was too rough for sampling so we steamed to a shoal site, station 20, about 50 m depth (41o35.0', 67o09.0'). Upon arrival 1215 h 22 March 1995, conditions were better so we began MOCNESS and CTD operations. A drifter was not deployed at this site because conditions were marginal. The weather improved so we were able to continue operations until 1800 h 23 March 1995 when we left for Woods Hole. Arrived in Woods Hole 0930 h 24 March 1995.
Physical Oceanography (Jim Manning)
A new set of four GPS/ARGOS/VHF drifters were brought on board for this cruise. These instruments were custom designed by Brightwaters Instrumentation according to our specifications for following mid-depth water parcels on Georges Bank. The electronics
are housed in a cylindrical canister (30cm x 60cm) with an external temperature probe. The three antennae are mounted above the canister, a holey sock drogue (1.6m x 6m) is tethered 10m below, and a Minilog temperature probe is attached to the top of the holey sock.
The first test deployment was conducted 1 mile offshore in Cape Cod Bay for one hour. The instrument worked as expected except for a strobe light failure (blown fuse). The following day however, the instrument failed to transmit. As determined back at the lab a week later, a leak evidently occurred in the new style GPS antennae mount that slowly seeped into the electronics.
The second deployment was conducted on the Northeast Peak of Georges Bank for 36 hours. The drifter was successfully tracked via VHF during most of the deployment however the range was limited to less than 1.5 miles probably due to the combination of heavy seas (3m) and shadowing of the radio signal by the ships stack.
In both of these two deployments and previous test deployments, the ARGOS signal was intermittent or non-existent. It was discovered several days after returning home that there was a programming problem at ARGOS in Landover Maryland that contaminated the fixes and the instruments themselves were working properly.
Despite these problems, the 36 hour deployment provided a detail track of the water parcels at one minute intervals over three tidal cycles. As depicted on Figure 2, the sub-tidal drift was towards the east during the first cycle and, as the wind increased towards the south during the later part of March 21st (see Figure 3a ), the eastward drift arrested. The final tidal excursion resulted in no residual. The thermal stratification as measured by the two sensors at the surface and at 10 m never exceeded the resolution of the Minilog probe, 0.2oC.
The R/V Seward Johnson is well equipped with the University of Miami hardware and software for acquisition of shipboard sensor data. Five minute interpolated values were loaded into a MATLAB routine for range and delta checks and then plotted in Figure 3a and 3b.
Some data loss occurred for several hours late in the day on the March 19th. Most of the signal in the SAIL system (seasurface temperature, salinity, and fluorometry) is due to the two excursions during the cruise; one into the warm salty slope water to find the missing mooring and the other to the cold fresh waters of Provincetown harbor. The fluorometry sensor indicates the most variability on Georges Bank especially on March 21-22. The meteorological data was quite variable with wind ranging between near zero to over 25 knots. Local maximum in wind occurred on March 16, 17, 18, 19, and then the largest about midnight on 21st, primarily out of the north with a corresponding drop in barometric pressure. Cycles in the air temperature correspond to those of wind speed. Solar radiation peaks occurred mid-day during the later half of the cruise with the most persistent sun appearing on March 20th which resulted in a corresponding local maximum in seasurface temperature.
Two RDI Acoustic Doppler Current Profilers were on board and operating at 150 and 600 Khz, respectively. Post-processing of these records has not begun.
A total of 20 Seabird (Model 19) cast were made during the bongo survey at stations 1-18 in depths ranging from 52 m to 81 m. While the data from these casts are not reported here, very little structure and variability was seen at real time processing steps. The water column was very well mixed at all stations and differed little between stations. While satellite images indicated the presence of Scotian Shelf Water intrusions, these cast were apparently just north and west of that anomalous lens. Surface temperatures ranged from 4.3oto 5.4oC. Bottom temperatures ranged from 4.6oto 5.1oC. Water bottle samples were taken at stations 2 and 6.
One General Oceanics (GO MarkIII) cast was made at station 18 (64m), eight at station 19 (80m), and nine at station 20 (40m). All cast included water bottle samples typically at the bottom and near the surface during the upcast for salinity calibration purposes. All cast included measurements from a fluorometer, transmissometer, and light sensor in addition to standard CTD variables. These casts were post-processed at sea using the GO CTDPOST routines to generate ascii output. The results are plotted with a MATLAB routine for rough comparisons of four of the variables in Figure 4a and 4b. Again, these preliminary results indicate a very well mixed water column in all respects. Note that the largest change that occurred between cast 1 and cast 2 is due to both the time and space between casts. Cast 1 was conducted prior to going to Provincetown and was actually at station 18. All cast will be reprocessed to pressure averaged and pressure centered intervals. Particular attention will be made to a) near surface stratification relative to shipboard meteorological data and b) light intensity in the water column relative to shipboard radiation sensor.
Bongo-net Survey (G. Lough, E. Broughton, M. Kiladis)
An initial bongo-net survey was made on eastern Georges Bank to locate cod and haddock eggs/larvae for more intensive site studies. Bongo tows were made with a 61-cm frame fitted with 0.333-mm and 0.505-mm mesh nets using standard MARMAP procedures, i.e., double-oblique from surface to within 5 m of the bottom. A SeaBird CTD (Model 19) was attached to the towing wire above the bongo to monitor sampling depth in real time and to record temperature and salinity. Both sides of the bongo were sorted at sea to provide a count on the number of cod and haddock eggs/larvae; these larvae were frozen for biochemical analyses. Bongo stations began 15 March 1995 on the southeastern part of Georges Bank, about midflank and proceeded north along the 70-m isobath to the northeast peak (Figure 5). Stations were nominally 10 miles apart. On the northeast peak, east-west transects were made to about the 40-m isobath. Bongo operations ceased with station 18 on 17 March 1995, having identified an aggregation of cod-haddock eggs/larvae on historic spawning grounds. A range of 1-10 cod-haddock larvae per bongo tow was observed on eastern Georges Bank. Cod larvae were more common than haddock (Figure 5). The smallest and most recently hatched larvae occurred on the northeast peak in the vicinity of stations 8 and 16 , where there also were high numbers of gadid eggs. Larger and older cod-haddock occurred at shoaler stations and to the southeast, consistent with the expected residual drift.
MOCNESS Sampling (G. Lough, E. Broughton, M. Kiladis)
Station 19 (41o50.0', 66o37.0') was our initial deep-water "stratified" site, which was occupied from 0912 h 20 March 1995 to 1925 h 21 March 1995. Bottom depth at this site ranged from 70-90 m. MOCNESS and CTD operations followed a drifter as the station marker (Figure 2). The 1-m MOCNESS with nine 0.333-mm mesh nets was used to sample larval fish and larger zooplankton. The 1/4-m MOCNESS is equipped with nine 0.064-mm mesh nets, designed to sample the smaller plankton such as copepod nauplii. The tow profile for these two nets was nominally 10-m strata within 5 m of the bottom; extra nets were used for special collections. The 1-m MOCNESS nets typically sampled for 5 minutes (ca. 250 m3) and the 1/4-m MOCNESS nets for 2-3 minutes (ca. 30 m3). The MOCNESS and CTD operations generally were alternated. The 10-m MOCNESS equipped with five 3.0-mm mesh nets, only was used twice per 24 h, once at midnight and again at midday. Larger and rarer nekton were collected by the 10-m MOCNESS. Sampling intervals were broaden to 20-m strata, keeping the same alternate depth horizons as the 1-m MOCNESS (0-20, 20-40, 40-60, and 60-80 m), each net sampling about 10 minutes (ca. 5000 m3).
At the deep site station 19, a total of 14 MOCNESS tows were made: eight 1-m MOCNESS, three 1/4-m MOCNESS, and three 10-m MOCNESS tows. MOCNESS-1 m 14 was sorted ashore for cod and haddock eggs and larvae. The vertical distribution of gadid eggs in Figure 6 shows them to have been at very high density, 400-500 per m3, throughout the 90-m water column. Approximately 40% of the eggs were in the late stage of development, tail bud free to before hatching. The duration of cod egg period is about 19 days at 5C, therefore the late stage (III) eggs on Georges Bank were about 12-19 days old. Cod and haddock eggs only can be separated in the late stage III so that the earlier stages have to be partitioned based on the relative proportion of the late stage. The gadid eggs on Georges Bank at the deep site were about 50:50 cod and haddock. Haddock larvae (Figure 6) were uniformly low, 1-4 per 100 m3, throughout the water column. Cod larvae were more abundant than haddock in the upper 40-m of the water column, ranging 10-18 per 100 m3. Both cod and haddock larvae at this site were recently hatched, 2-4 mm, with yolksac present.
A shoal site station 20 (41o35.0', 67o09.0') was occupied from 1215 h 22 March 1995 to 1800 h 23 March 1995. A drifter was not deployed as a station follower because weather was marginal. Since the bottom depth was shoaler, 40-60 m, a double-oblique tow profile could be made with the 1-m MOCNESS (40-30, 30-20, 20-10, 10-0, 0-10, 10-20, 20-30, and 30-40 m), allowing nets 1-4 to be preserved in formaldehyde seawater and net 5-8 to be picked for special preservation (liquid nitrogen frozen). The same double-oblique profile was used for the 1/4-m MOCNESS; however, the 10-m MOCNESS sampled at 10m intervals for a single profile.
At the shoal site station 20, a total of 12 MOCNESS tows were made: six 1-m MOCNESS, three 1/4-m MOCNESS, and three 10-m MOCNESS tows. MOCNESS 1-m 26 also was sorted ashore for a vertical profile of larval cod and haddock (Figure 7 ) . Both eggs and larvae were distributed broadly through the water column at densities ranging from 2-7 per 100m3. Approximately 30% of the eggs were in late stage, last third of development, and about 40% were identified as cod and 60% as haddock. Only one recently hatched haddock larvae was collected at 20-10m depth. Recently-hatched cod larvae, 4-7 mm, were more abundant, ranging 2-6 per 100 m3. The highest density of cod larvae was in the surface 10-0 m.
Biochemistry Studies (E. Caldarone, E. Broughton)
Samples for biochemical analyses were taken from 18 61-cm bongo samples, 15 1-m MOCNESS tows, six 1/4-m MOCNESS tows, and five 10-m MOCNESS tows. All samples were rinsed from the nets on deck using minimal water pressure and transferred to buckets containing ice packs. Samples which were not sorted were preserved immediately in 4% buffered formaldehyde. Samples sorted for fish or invertebrates were picked in seawater filled sorting trays on ice covered light tables. Every effort was made to keep samples cold during processing. Plankton remaining after removal of samples was preserved in 4% buffered formaldehyde.
Table 1. Larval fish and zooplankton removed from the following samplers:
Investigator Bongo.333 Bongo.505 MOC1 MOC1/4
Buckley-Cod 14 16 127
Buckley-Haddock 4 2 6
Clarke-Cod 3 5 71
Clarke-Calanus finmarchis 110
WHOI-Cod 7 4 77
WHOI-Calanus finmarchis 200
Buckley samples were video taped then frozen in liquid nitrogen. Clarke fish samples were video taped then cut in half. The head was preserved in EtOH and the tail was dipped in sucrose and frozen in liquid nitrogen. Clarke copepod samples were video-taped, dipped in sucrose, and frozen in liquid nitrogen. WHOI fish samples were measured (SL) using a Wild M5 steriomicroscope equipped with a micrometer then frozen. WHOI copepod samples were frozen. WHOI phytoplankton samples were sieved into 3 size categories (>.505 mm,<.505 mm->.333 mm,and <.333 mm->.064 mm).
Larval cod and haddock were collected for the future biochemical analysis from bongo tows and seven 1-m mocness tows. A total of 157 cod and 12 haddock were videotaped on-board for length measurements and stored in liquid nitrogen. The larvae will be analyzed for their RNA, DNA, and protein content and length. The data will be used to determine the nutritional condition and growth rate of the individual fish. A comparison of growth rates will be made between fish collected at the shoal site (station 20) and the deeper site (station 19) and also among discrete depths at each site.
WHOI Predation Studies (L. Madin, E. Horgan. M. Butler)
Sampling by the GLOBEC Predation Group on this cruise included:
A total of 76 individual large predators were removed from the MOC-1 and MOC-10 hauls and preserved for antibody analysis for the presence of Calanusin the gut contents. These predators included Themistoguadichaudi, two species of gammarid amphipods (Lyssianassidae, Oediceratidae), Cirolana polita, Crangon septemspinosa, a pandalid shrimp, and Pleurobrachia pileus.
Other live predators from tows or dive collections (Themisto, Pleurobrachia , and Crangon ) were used in laboratory experiments to try to determine gut passage times. They were fed Calanusadults dyed with fluorescent vital stain ( Lyssamine Rhodamine X ) . The Crangonate 12 copepods in about 5.5 hrs. Gut passage times in Crangon fed Calanuswere about 3 and 3.25 hrs (n=2). One pandalid shrimp fed Calanuscleared its gut in 1.5 to 3.75 hrs. Two isopods, Cirolina polita, cleared their guts of juvenile fish in 2 and 3.25 hrs. (n=2) Pleurobrachia digested copepods in about 3 to 7 hrs.
Forty individuals of Pleurobrachiapileuswere removed from 10-m MOCNESS nets and used for a length-weight regression. We measured oral-aboral length, tentacular width and displacement volume. The samples were frozen for subsequent measurement of carbon and nitrogen weights.
On the first dive (station 18) we observed high densities of a large physonect siphonophore, probably a species of Agalma. Colonies were up to 1.51 m in length, and we saw one large cluster of over 100 of them drifting by. Colony fragments of Clytiawere also present, but not as abundant as in May 1994. On the second dive at night the water column was filled with marine snow but few animals. One Nanomia carawas collected and several more seen, along with a few sand lance and 1 Bolinopsis.
Table 2. Immunological Work
STATION# NET# TAXA # individuals
MOC10-007 0 isopods 4
MOC10-007 0 hyperids 2
MOC10-007 2 hyperids 2
MOC10-007 4 gammarids 4
MOC10-007 4 Pleurobrachia 4
MOC1-008 0 chaetognaths 7
MOC1-008 0 Crangon 1
MOC1-008 0 gammarids 5
MOC1-008 0 gammarids 1
MOC1-009 8 hyperids 10
MOC1-009 8 gammarids 3
MOC10-016 0 mysids 3
MOC10-016 0 hyperids 3
MOC10-016 1 Crangon 2
MOC10-016 1 gammarids 2
MOC10-020 0 Crangon 5
MOC10-020 0 Cirolina polita6
MOC10-020 0 gammarids 2
MOC10-020 1 pandalid 1
MOC10-020 0 hyperids 5
MOC10-020 2 euphausids 3
MOC10-025 0 Cirolina polita1
URI Predation Studies (G. Klein-MacPhee, D. VanKeuran)
Introduction: The role of our complement of the predation group in the Georges Bank GLOBEC program is to identify potential gelatinous predators on the target species (cod, haddock, Calanusand Pseudocalanus); to determine their biomass on Georges Bank coincident with the target species; to determine their potential impact on survival of cod and haddock, either directly as predators on eggs and larvae, or indirectly as competitors for their food, Calanus and Pseudocalanus; and to determine the effects of increased temperature on predator impact.
In the pilot field study conducted in April of 1994, we identified two potentially important groups of predators, drifting colonial hydroids and chaetognaths which were collected in large numbers. We conducted several laboratory experiments which showed that the hydroids and hydromedusae were capable of capturing and ingesting Calanuseggs, and cod eggs and larvae and conducted some experiments on gut passage times. Although these hydroids were described by Bigelow in 1924, they have not been mentioned in recent surveys and we were not sure whether their presence was a transient phenomenon produced by storm conditions or whether they were a regular component of the plankton.
Objectives: In the March 13-24 cruise our goals were:
To describe potential gelatinous predators on the
To estimate predation rates of selected predators on target species from feeding rates obtained from gut analysis of preserved specimens together with gut passage times.
To look for the floating hydroids which were present in 1994, quantify their abundance, and determine if their distribution occurred coincidentally with early life stages of cod and haddock.
To perform shipboard experiments on digestion rates of hydroids on Calanus eggs.
To perform shipboard experiments on freshly collected gelatinous predators using target species (Calanusor Pseudocalanus) labeled with fluorescent dye as prey to determine gut passage time at a controlled temperature.
Methods: Plankton collections were made using a 1-m MOCNESS with nine nets (0.333 mm mesh) at shallow and deep stations in the day and at night, where cod larvae and Calanuswere determined to be present. Nets were fished at eight depths in the deep stations and five depths in the shallow stations. The nets were rinsed with sea water and the contents preserved immediately in phosphate buffered formalin.
Additional collections of gelatinous predators were made by divers, and delivered live in plastic bags filled with sea water. These organisms were placed in 4 quart plastic containers in filtered seawater and kept in a temperature control room at 9oC.
The most abundant gelatinous predators encountered at the dive stations were the ctenophore, Pleurobrachia pileusand the colonial hydroids. The ctenophores were used for gut passage experiments. Two series of experiments were conducted. The ctenophores were starved for 12 hours and then transferred by dipper to quart glass jars containing clean sea water, one individual per jar. They were allowed to acclimate for 1 hour then presented with 4 dyed Calanus adults. Calanus were stained with Tetramethyl rhodamine dye, 100µl in 100 ml filtered sea water (concentration of 1:1000) (Mari Butler personal communication). The Calanus were live and fluoresced when viewed under black light. Fifteen to 20 copepods were eventually offered to the ctenophores. The ctenophores were observed every half hour under black light to determine when ingestion occurred and gut passage times. Complete digestion was determined to have occurred when the ctenophores gut no longer fluoresced and dyed feces were observed in the jar.
In the second set of experiments the animals were placed in quart jars containing 20µ filtered sea water, 10 dyed Calanuswere placed in each jar, the ctenophores were allowed to feed for one half hour, then transferred to clean filtered water. The water from the jars they were removed from was filtered and the remaining Calanuscounted so we would know how many animals were ingested. The ctenophores were then observed every half hour. When the animals appeared to have completely digested the food, they were removed from the jars, measured and returned to their holding containers. The jars were filtered through a 20µ screen and the filtrate examined for remains of digested copepods.
Colonial hydroids were used in two digestion experiments. In the first, hydroids collected from a 30 minute Bongo tow (0.505-mm mesh)were given the opportunity to ingest dyed Calanuscopepods and copepodites (method described previously). Since the hydroids were freshly collected, some did have undyed copepodites in their guts at the time of collection. None of the animals appeared to have ingested any food other than copepodites. Hydroids were maintained in the dark at 9oC and observed hourly, with ingestion of dyed food and digestion of both dyed and undyed food noted.
Two days later the same group of hydroids was used in a second digestion experiment. These hydroids were fed Calanuseggs produced by animals collected at the same time as the hydroids in the previously mentioned Bongo tow. Hydroids were starved for over 24 hours, fed Calanuseggs, and maintained in the dark at 9oC. Observations were made hourly for the first 14 hours, and as often as possible after that time. The experiment was discontinued after 60 hours. Eggs were judged to be starting to digest when the membrane of the egg was no longer visible, and totally digested when the hydroid gut was no longer expanded.
Results: We collected eight 1-m MOCNESS sets of samples, preserved them and stored them for sorting (Table 1).
Table 3. 1-m MOCNESS Samples
Date Time Tow Number Description of Tow
3/20/95 1020 Moc 1-001 Day Deep (70m)
3/20/95 1254 Moc 1-002 Day Deep (70m)
3/20/95 1953 Moc 1-005 Night Deep (70m)
3/21/95 1659 Moc 1-014 Night Deep (80m)
3/22/95 1834 Moc 1-018 Night Shallow (40m)
3/22/95 2234 Moc 1-019 Night Shallow (40m)
3/23/95 1012 Moc 1-023 Day Shallow (40m)
3/23/95 1705 Moc 1-026 Evening Shallow (40m)
Experiment 1 -When offered 4 copepods, the ctenophores did not ingest any prey after 2 hours so we increased the number of prey to about 15 Calanusper ctenophore. The ctenophores then ingested prey. Animal 1-1 ingested I-3 copepods which were visible in the gut as a pink mass. Time to digest the prey was 6.5 hours. Animal 1-2 took 3 hours to digest 2 copepods. Animal 1-3 took 7 hours to digest 2-4 copepods (Figure 8). Because the excess copepods were left in the jar for over an hour with the predators I could not be sure they had not ingested prey later; therefore Experiment 2 was conducted where the number of copepods actually ingested was monitored carefully.
Experiment 2- The three ctenophores collected in the 10-m MOCNESS net did not ingest any prey; one did not extend its tentacles, one extended a tentacle and it broke off, the third extended its tentacles, captured a copepod, but did not ingest it. I felt that the animals had been injured in the net and were not behaving normally, so I did not use any net-collected ones for the experiments. One of the diver collected ctenophores also did not feed, but two others did. Ctenophore 2-2 ingested three copepods, ctenophore 3-3 also ingested three. Ctenophore 2-2 took 7 hours to digest three copepods at 9oC, ctenophore 2-3 took 8 hours (Figure 8).
In the first experiment using colonial hydroids the undyed animals present in the gut at the start of the feeding observations were totally digested within 12 hours at 9oC. The dyed Calanusadults were too large to be ingested by the hydroids. In two cases Calanusstage CIII (Mari Butler, personal communication) were partially ingested by hydroids and held for over six hours while the caudal rami were digested. The rest of the animal remained in the dish in both cases, although some unsuccessful attempts were made to ingest them.
During the second digestion experiment 24 Calanuseggs were ingested by colonial hydroids within a half hour period. The eggs were readily eaten, with none rejected. Five animals ingested one egg each, eight animals ingested two, and one individual ingested three eggs. The first digestion began with the hydroid which had ingested three eggs, with membrane integrity lost after 10 hours and gut empty by 17 hours. There was no other digestion noted until after 25 hours had elapsed. The majority of the Calanuseggs were digested between 30 and 34 hours, and at the end of 60 hours four eggs remained undigested although the hydroids were still alive (Figure 9). Although the colonial hydroids readily ingest Calanuseggs and do eventually digest them, the rate is very slow at 9oC.
We collected replicate day and night discrete depth samples at a shallow and deep station. Predator abundance, distribution and gut content analysis for predators will be determined from these samples.
We collected floating hydroids at both of the stations and will examine the preserved samples to determine abundance, distribution and gut contents.
We determined digestion times of hydroids on dyed and undyed Calanuseggs at 9oC in shipboard experiments.
We collected an additional potential predator, the ctenophore Pleurobrachia pileus, which was not present in large numbers during the April 1994 cruise; and determined gut passage times using dyed Calanusadults as prey at 9oC. Pleurobrachia pileus was described by Bigelow in 1924 as one of the most important pelagic coelenterates in the Gulf of Maine from an economic standpoint because it was locally very abundant, present throughout the year, and was a destroyer of smaller planktonic animals in particular copepods (Bigelow 1927). Bigelow also believed it to be an important predator on cod and haddock eggs although he did not offer any direct evidence of this. Gut contents of preserved specimens will be used to determine prey selection and feeding rates.
We collected live hydroids from net tows and diver collections and brought them back to the laboratory as culture stock. We will perform additional laboratory experiments using these animals.
Name Title Organization
SEWARD JOHNSON Officers and Crew
Appendix 2. Eventlog
Figure 1. Area of operations on Georges Bank for the R/V Seward Johnson Cruise 9503, 13-24 March 1995, showing the deep and shallow sites and recovery of the LT1 mooring.
Figure 2. Sampling locations at site 19 relative to ARGOS/GPS/VHF drifter elliptic path. Straight lines indicate start and stop path of ship during net hauls. Numbers indicate cast number and start position. MOCNESS haul #1, for example, was taken on a north-northwestern tack. The drifter path indicates a residual flow towards the east.
Figure 3a. Examples of shipboard meteorologic sensor data interpolated to five minute intervals (see text) .
Figure 3b. Examples of shipboard hull-mounted sensor data (seasurface temperature, salinity, and fluorescence) interpolated to five minute intervals (see text) .
Figure 4a. MarkIII CTD profiles at station 18 (cast 1) and station 19 (cast 2-9) as retrieved by CTDPOST ascii output routine prior to post-processing. Note ranges of the four variables are 0.4 degC, 0.2 PSU, 0.8 volts, and 4 units of light,respectively.
Figure 4b. MarkIII CTD profiles at station 20 (cast 10-18) as retrieved by CTDPOST ascii output routine prior to post-processing. Note tick mark interval of the four variables are 0.4 degC, 0.2 PSU, 2.0 volts, and 4 units of light, respectively.
Figure 5. Results of bongo survey on eastern Georges Bank, 15-17 March 1995. Top panel shows location and bongo station numbers. Middle and bottom panels show the total number of cod and haddock larvae from both bongo nets (unstandardized). The 60-m isobath runs through the center of each figure with the 100-m and 200-m isobaths shown in the lower right corner.
Figure 6. Vertical distribution of gadid eggs (top panel) and cod and haddock larvae (bottom panel) from 1-m MOCNESS tow 14 at deep site 19, 21 March 1995, 1710-1756 EST, 86 m bottom depth.
Figure 7. Vertical distribution of gadid eggs (top panel) and cod and haddock larvae (bottom panel) from 1-m MOCNESS tow 26 at shoal site 20, 23 March 1995, 1712-1735 EST, 40-59 m bottom depth.
Figure 8. Digestion rates of Pleurobrachia Pileus (see text).
Figure 9. Digestion of calanus eggs by colonial hydroids (see text).