by
M.A. ROESSLER1, A.C. JONES2 and J.L. MUNRO3
Abstract
After descriptions of larval and postlarval stages of pink shrimp (Penaeus duorarum) were made from reared specimens and from specimens collected from the plankton in the Tortugas area it was possible to determine areas and seasons of spawning.
Spawning occurred in all seasons, but was greatest in spring and summer. Time of spawning was correlated with temperature and was greatest in the latter half of the lunar month. Spawning occurred mostly in the area where commercial fishing is conducted.
Approximately 87 × 1011 first protozoea were produced annually and there was a survival rate of approximately 80.4 percent per day. This produced about 85 × 108 postlarvae to enter the inshore nursery areas.
1 Institute of Marine Sciences, University of Miami, Miami, Florida 33149
2 Tropical Atlantic Biological Laboratory, Bureau of Commercial Fisheries, Miami, Florida 33149
3 University of the West Indies, Kingston, Jamaica
STADES LARVAIRES ET POSTLARVAIRES DE LA CREVETTE Penaeus duorarum DE LA FLORIDE MERIDIONALE
Résumé
Des descriptions de stades larvaires et postlarvaires de crevette Penaeus duorarum effectuées sur la base tant de spécimens d'élevage que d'individus provenant du plancton récolté dans la région de l'île de la Tortue, ont permis de déterminer les zones et les époques de reproduction.
La reproduction a lieu toute l'année, mais est plus intense au printemps et en été. Elle dépend de la température, et l'activité maximale est atteinte durant la seconde moitié du mois lunaire. Les frayères coïncident généralement avec la zone exploitée par la pêche commerciale.
La production annuelle de premières protozoés atteint le chiffre approximatif de 87 × 1011, avec un taux de survie d'environ 80,4 pour 100 par jour. En conséquence, environ 85 × 108 postlarves pénètrent dans les zones côtières où elles poursuivent leur développement.
FORMAS LARVALES Y POSTLARVALES DEL CAMARON ROSADO (Penaeus duorarum) EN EL SUR DE FLORIDA
Extracto
Después de las descripciones que se hicieron de las fases larvales y postlarvales del camarón rosado (P. duorarum) a partir de ejemplares criados y de ejemplares recogidos en el plancton de la zona de los cayos de las Tortugas, fue posible determinar las áreas y temporadas del desove.
El desove se verificó en todas las temporadas, pero fue máximo en primavera y verano. El período del desove se relacionó con la temperatura, siendo mayor en la segunda mitad del mes lunar. El desove tuvo lugar sobre todo en la zona en donde se realiza la pesca comercial.
Se produjeron anualmente 87 × 1011 primeros protozea, con una tasa de supervivencia de 80,4 por ciento por día, aproximadamente. Esto dió origen a que penetraran en las zonas de cría próximas a la costa de unas 85 × 108 postlarva.
In 1959, the Institute of Marine Sciences, University of Miami, began a study of the early life history of the pink shrimp Penaeus duorarum Burkenroad. This paper summarizes the work done to date, emphasizes the methods used, and suggests further research.
The purposes of this study were to delimit the spawning area and season of the pink shrimp that support the Tortugas fishery in southern Florida and to describe the distribution of the larval stages on the Tortugas shelf and in Florida Bay (Fig. 1). The study also attempted to estimate total annual production and mortality of larvae.
2.1 Identification and rearing of larvae
The larval stages of P. duorarum were described by Dobkin (1961) from specimens collected in plankton samples and from specimens reared from eggs spawned in the laboratory. Shrimp were reared in the laboratory from the first naupliar through the first protozoeal stage; older stages were collected in plankton samples. Specimens from plankton samples could be used because P. duorarum is the only member of the genus which has been found in the Tortugas area (Eldred, 1959). No evidence appeared during plankton sampling that larvae of other species of this genus were present. Ewald (1965) subsequently reared all stages in the laboratory and compared them with the published descriptions.
Dobkin (1961) failed to rear Penaeus larvae beyond the first protozoeal stage because the larvae either would not feed or they became entrapped in exuviae or algal food. Ewald (1965) used compartmented clear plastic boxes as described by Costlow and Bookhout (1959) and changed the water daily. Survival of the animals was better in sea water from the Gulf Stream than from Biscayne Bay. Food for protozoeal and early mysis stages was a mixture of Dunaliella tertiolecta and Tetraselmis sp. (flagellates), Phaedoctylum tricornatum (diatom), Chlorella sp. (green alga), and Rhodotorula affinis (yeast). Only young phytoplankton cultures (less than 11 days old) were used to minimize the proportion of less motile (older) stages. Terminal mysis stages and postlarvae were fed nauplii of brine shrimp (Artemia salina).
The optimum temperature for the larvae was 26°C (Ewald, 1965). At 31°C mortality was high, and at 21°C the growth rate was reduced. Protozoeal stages, when feeding began, were the most difficult to maintain. The number of mysis stages varied; more occurred at 21°C than at 25°C.
2.2 Plankton sampling
Plankton samples were collected from the waters of the Tortugas shelf, Florida, from 1959 to 1964. In 1959 to 1962 most samples were collected with a ¾ m modified ‘Discovery’ net. A 0.4 m (1 ft) (mouth diameter) ‘Turtox’ net of No. 6 mesh silk bolting cloth or Nitex No. 216 nylon cloth and a ½ m (mouth diameter) ‘Arts, Sciences Workshop’ net of either Nitex No. 75 or No. 175 nylon cloth also were used. A plankton sled (Frederick M. Beyer, University of Oslo, Oslo, Norway) was used to sample for eggs and nauplii near the bottom.
Discovery-type nets were submerged by using a lead weight attached to the end of the towing wire. The net was attached to the cable 1.5 m above the weight. Paired hauls were made at most stations, and sampling was carried out at three levels: 1.5 m above the bottom, in mid-depth, and 0.5 m below the surface. The levels were sampled either separately or with step-oblique hauls. The depth fished was determined by trigonometric calculations based on the towing angle and the length of cable.
Beginning in February 1960, Tsurumi Precision Instrument Company flow meters were attached in the mouth of the net to estimate the volume of water filtered. Salinity and temperature were recorded for each sample (Munro, Jones and Dimitriou, 1968).
Fig. 1 Spawning and nursery area of Tortugas population of pink shrimp (Penaeus duorarum). Arrows indicate current pattern.
Neither the Discovery-type net nor the Turtox net was completely satisfactory. Comparisons of catches of Discovery and Turtox nets showed that the Discovery nets caught relatively fewer nauplii (35 percent), more protozoea and mysis (133 percent), and more postlarvae (486 percent) than the Turtox nets. In August of 1962, a Gulf-V plankton net (Arnold, 1959) was introduced (Munro, Jones and Dimitriou, 1968). The 0.293 mm mesh opening of this net retained first protozoea (0.35 to 0.44 mm width) and older stages. Nauplii (0.17 to 0.22 mm width) were not retained. The net was equipped with TSK-type flow meters in the mouth. Thirty-minute tows were made in a step-oblique fashion at 10 equally spaced intervals between the bottom and surface.
2.3 Examination of samples
The naupliar, protozoeal, and mysis stages of P. duorarum were separated and counted in the laboratory. Postlarvae were sorted according to the number of rostral spines.
Discovery-net samples were brought to a volume of 400 ml and eight 24-ml aliquot portions sorted. The average percentage difference between total larvae as estimated by a subsample of eight aliquots and the total count was 14.7. Abundance was expressed as the number of shrimp per 700 m3 water filtered, because this volume was close to the average volume filtered.
The samples from the Gulf-V net were divided by the use of a two-compartment rotating plexiglass cylinder similar to that designed by T.R. Folsom (McEwan, Johnson, and Folsom, 1954). Abundance was calculated as the number of shrimp larvae per m3 of water filtered and the number of larvae under 10 m2 of surface.
An amorphous mucilaginous substance which appeared in many of the fresh samples, and which may have resulted from the presence of coelenterates in the plankton, made sorting difficult. This substance was removed from the samples by the following technique, developed by Dr. Eugene Corcoran, Institute of Marine Sciences. The sample of approximately 570 ml was treated with 10 ml of saturated copper ammonium solution and allowed to stand at least 12 h. The resulting precipitate was cleared by the addition of concentrated H2SO4 until a pH of 2 was reached. The sample was washed with water in a Buchner funnel until the solution returned to neutral. Suction was applied by a vacuum pump. The sample was then returned to 3 percent buffered formalin for storage.
The Discovery-net samples showed that the numbers of shrimp at the surface and at mid-depth generally decreased from midnight to midday and increased from midday to the following midnight. Nauplii were seldom taken because they probably inhabit the deepest part of the water column. Youngest protozoea were more common near the bottom, but second and third protozoea and mysis were found to undertake considerable vertical migrations. Postlarvae were more common in the surface tows than in deeper tows.
Results from monthly samples over the entire Tortugas shelf showed that spawning occurred all year and was greatest in the area of commercial fishing. Spawning occurred mostly in the south-central portion of the fishing grounds for most of the year, but shifted to the northwest edge of the area in winter. Shrimp generally did not spawn in water less than 6 fm (11 m) deep.
The numbers of larvae were lowest in winter. An increase in numbers, especially protozoeal and mysis stages, indicated an increase in spawning activity in spring. Several distinct periods of abundance were observed in summer. In fall, numbers of larvae were usually high on the fishing grounds, but numbers of postlarvae were low in the inshore areas.
Jones et al. (1969) found bottom temperature to be of major importance in determining time of spawning. Shrimp spawned, as indicated by the presence of first protozoea, over a temperature range from 19.6 to 30.8°C. Spawning was maximum in the temperature range from 27.0 to 30.8°C. In 1961, the temperature began its seasonal decline in July and August, and the abundance of larvae decreased markedly by early fall.
Salinity changes were small in the area, and the numbers of larvae were not related to salinity.
The methods of Winsor and Clarke (1940) and Silliman (1946) were used to determine the 95 percent confidence limits for the estimates of the number of protozoea present under 10 m2 of surface area from replicate Gulf-V net tows. These were found to be between 28.8 percent and 347.6 percent of the mean. Similarly, 90 percent confidence limits for Discovery net tows varied between cruises from 11 to 843 percent to 34 and 290 percent of the mean. These limits were wider than those reported by Silliman (1946) and Strasburg (1960), but probably reflected the replicate nature of the tows as compared to the true paired hauls used by Silliman and Strasburg. Data from several 24-h stations revealed no indication of significant changes between catches made in day-light or at night.
Estimates of total production of first protozoea indicated that 60.5 × 1011 were produced in 1963 (a poor year); average annual production is estimated at 87.0 × 1011 first protozoea (Munro, Jones and Dimitriou, 1968).
Munro, Jones and Dimitriou (1968) corroborated the seasonal cycles and the relation between spawning and bottom temperature noted by Jones et al. (1969) by an additional three years sampling. Spawning appeared to be more intense during the waning moon phases. The increased spawning in the latter half of the lunar month agrees with the work of Korringa (1957), who found that animals that display lunar rythms in spawning often have spawning peaks in the last lunar quarter.
By using data on the duration of each larval stage (Ewald, 1965) and our own catch data, we estimated mortality and survival rates from catch curves. Survival rates varied from 74 percent per day east of Rebecca Channel to 98 percent per day north of Northwest Channel. Survival appeared to be negatively correlated with distance from the keys or water depth. Munro, Jones and Dimitriou, (1968) obtained an average rate of 80.4 percent survival per day. Jones et al. (1969) calculated the survival rate of the larvae at 85 percent per day in 1961 and 76 percent per day in 1962.
As shown by Tabb, Dubrow and Jones (1962) and work in progress, postlarvae that are about 35 days old enter the Everglades nursery area in the 6 to 7 rostral-spine stage (Ewald, 1965). This information on survival and age implies that about 0.05 percent of the original population survive to enter the nursery grounds. On the basis of the number of first protozoea estimated for 1963 (60.5 × 1011), the survival amounts to 0.14 percent or 85 × 108 postlarvae entering the estuary; this would imply that about 6 percent of the postlarvae survived to produce the commercial catch of 5 × 108 individuals in 1963.
Both surface currents (Koczy, Rinkel and Niskin, 1960) and bottom currents (Rehrer, Jones and Roessler, 1967) showed a net drift to the southwest. Bottom drift was studied using the U.S. version of the Woodhead sea-bed drifter as described by Lee, Bumpus and Lauzier (1965).
A likely route of larval and postlarval migration is southwest across the grounds, through Rebecca Channel, along the western border of the Florida Current and back into Florida Bay through the channels between the Florida Keys (Munro, Jones and Dimitriou, 1968).
Work in Buttonwood Canal and in the Little Shark River was undertaken to estimate the number of postlarvae entering the Everglades nursery. Paired surface and bottom plankton hauls were made with a l-m plankton net with nylon mesh (471μ). Samples were taken from a vessel run at a constant engine speed. This procedure produced, as closely as possible, constant filtering behavior of the nets. Preliminary analysis showed catches of postlarvae were greatest in bottom samples made at night and during flood tides, in new and first quarter moon phases. Abundance increased in spring, reached a maximum in July and August, and fell off sharply in late September. Abundance was low during the winter.
The present studies show that more information is needed on the duration of the larval instars under natural conditions. Estimates of the production of nauplii and better estimates of the survival rates of the various larval stages are needed. The migration routes need thorough study. Survival and growth of postlarvae, length of time spent in the estuary, and abundance and behavior of postlarvae and juveniles should be studied. At the Institute of Marine Sciences, University of Miami, Dr. D. Hughes is studying some of the behavioral aspects; his results are published in these proceedings. Work by Dr. E. Iversen and B. Yokel at the Institute of Marine Sciences, University of Miami, considers the relation of the abundance of the juvenile stages to catches of adults; some of their results are also presented in these proceedings.
In summary:
We have learned to identify the various larval and postlarval stages.
We have successfully reared pink shrimp from eggs to postlarvae. (This has resulted in new handling techniques and a diet which should be useful in rearing crustacean larvae in general).
We have learned the vertical, areal and seasonal distribution of shrimp larvae and postlarvae.
We have developed a working hypothesis on the migration routes, but this point needs further evaluation.
We have found that temperatures above 27°C encourage highest production of larvae and that spawning occurs mostly during the waning moon phases.
Estimates show that approximately 87 × 1011 first protozoea are produced annually and that a survival rate of 80.4 percent per day for the larvae and early postlarvae will provide about 85 × 108 postlarvae for the inshore nursery area.
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