by
B.J. YOKEL, E.S. IVERSEN and C.P. IDYLL
Institute of Marine Sciences, University of Miami
Miami, Florida 33149
Abstract
Studies have been underway since 1962 on the juvenile stages of the pink shrimp (Penaeus duorarum Burkenroad) in the Everglades National Park estuary. The objectives of this study have been to increase knowledge of the biology and migration of the shrimp and to determine if a relationship exists between the relative abundance of emigrating juveniles and the catches of adults on the Tortugas commercial fishing grounds.
From January 1963 through June 1965 the relative monthly abundance of juveniles was estimated from catches in Buttonwood Canal at Flamingo in Everglades National Park using a large “channel net”. The entire canal was fished by this gear, which relied on tidal currents to collect samples. Subsequently experiments showed that “wing nets” could reliably subsample the shrimp moving in the canal, and all samples were thereafter taken with this gear.
Catches of juveniles made near the times of the new and full moon are used as indices of monthly abundance. These show a positive correlation with the commercial landings of the smallest size shrimp.
During periods of high abundance, when the average size of the emigrants is small (11 mm carapace length or smaller), there is a delay of 2 to 2-½ months from the time they leave the estuary until they appear in the commercial catches. In periods of high abundance, when the shrimp are larger, the delay between the estuary and the commercial catch is reduced to one month. Growth rates suggest that the differences in migration times are caused by a delay in the arrival of the shrimp at a size large enough for the trawls to capture. When an appropriate time lag is used the correlation is improved between the relative abundance of emigrating juveniles and the catch per unit of effort of small shrimp on the Tortugas grounds. Based on this relationship a forecast of increased abundance of small shrimp on the fishing grounds can be made before they are available to the fishery.
Using commercial landing data an apparent movement through the Tortugas fishery can be seen. This appears reliable enough that a forecast of up to 4 months can be made of increased abundance of 41 – 50 count shrimp.
PREDICTION DES RESULTATS DE LA PECHE COMMERCIALE DE LA CREVETTE SUR LES FONDS DES TORTUGAS, D'APRES LE COMPTAGE DES SUJETS EMIGRANT DE L'ESTUAIRE DU PARC NATIONAL DES EVERGLADES
Résumé
L'étude, menée depuis 1962, sur les stades juvéniles de la crevette Penaeus duorarum Burkenroad vivant dans l'estuaire du Paro national des Everglades, se propose d'accroître les connaissances sur la biologie et les déplacements de ce crustacé, et de déterminer s'il existe un rapport entre l'abondance relative des juvéniles émigrants et les captures d'adultes sur les fonds de pêche commerciale des Tortugas.
De janvier 1963 à juin 1965, on a estimé l'abondance relative des juvéniles à partir de pêches faites au moyen d'un grand “channel net” dans le Buttonwood Canal, à Flamingo, dans le Parc national des Everglades. Le filet en question, dans lequel les crevettes viennent se prendre sous l'effet des courants de marée, a été employé dans la totalité du chenal. Des essais ultérieurs ont révélé que des filets plus maniables dits “wing nets” permettaient d'obtenir des sous-échantillons des crevettes se déplaçant dans le chenal, et par la suite tous les spécimens ont été pêchés avec ces engins.
On a utilisé comme indices de l'abondance mensuelle les prises de juvéniles effectuées aux approches de la nouvelle lune et de la pleine lune, et constaté une corrélation positive avec les captures commerciales des crevettes de petite taille.
Durant les périodes de grande abondance, lorsque la taille moyenne des sujets émigrant est petite (longueur de carapace égale ou inférieure à 11 mm), de 2 mois à 2 mois et demi s'écoulent entre le départ de l'estuaire et l'apparition dans les prises commerciales; lorsque les crevettes sont plus grandes, ce laps de temps tombe à un mois. Sur la base des taux de croissance, il semblerait que cette différence dans la durée de la migration correspond au temps que met l'animal à parvenir à une taille “pêchable” par les chaluts. Lorsqu'on tient dûment compte du décalage dans le temps, on améliore la corrélation entre l'abondance relative des formes juvéniles émigrantes et les captures de petites crevettes par unité d'effort sur les fonds des Tortugas. Sur cette base, il est possible d'établir une prévision de l'abondance accrue des crevettes de petite taille dans la pêcherie avant qu'elles ne soient disponibles pour la capture.
Sur la base des données sur les apports commerciaux, on relève un mouvement apparent sur le fonds de pêche des Tortugas. Les conclusions paraissent suffisamment sûres pour que l'on puisse prévoir jusqu'à 4 mois à l'avance l'accroissement de l'abondance des crevettes de la catégorie “41–50 par lb”.
PRONOSTICO DEL EXITO DE LA PESCA COMERCIAL DE CAMARON EN LOS CALADEROS DE LAS TORTUGAS, BASADO EN LA ENUMERACION DE EMIGRANTES DESDE EL ESTUARIO DEL PARQUE NACIONAL DE EVERGLADES
Extracto
Desde 1962 se realizan estudios sobre las fases juveniles del camarón rosado (Penaeus duorarum Burkenroad) en el estuario del Parque Nacional de Everglades. Los objetivos de este estudio han sido aumentar los conocimientos sobre la biología y migración del camarón y determinar si existe relación entre la abundancia relativa de los juveniles emigrantes y las capturas de adultos en los caladeros de pesca comercial de las Tortugas.
Desde enero de 1963 hasta junio de 1965, se calculó la abundancia relativa mensual de juveniles basándose en las capturas efectuadas en el canal Buttonwood, en Flamingo, en el Parque Nacional de Everglades, empleando una gran “red de canal”. Se pescó a lo largo de todo el canal con este arte, que aprovechaba las corrientes producidas por las mareas, para recoger muestras. Los experimentos posteriores demostraron que con “redes de ala” podían recogerse satisfactoriamente submuestras del camarón del canal, por lo qué, más tarde, todas las muestras se recogieron con este arte.
Las capturas de formas juveniles, realizadas al aproximarse los períodos de luna nueva y luna llena, se emplean como índices de la abundancia mensual. Estos indican una correlación positiva con los desembarques comerciales del camarón de talla más pequeña.
En los períodos de gran abundancia, cuando la talla media de los emigrantes es pequeña (longitud del caparazón: 11 mm, 0 menos), existe un retraso de dos meses a dos meses y medio desde que abandonan el estuario hasta que aparecen en las capturas comerciales. En períodos de gran abundancia, cuando los camarones son mayores, el retraso entre la captura realizada en el estuario y la captura comercial se reduce a un mes. Los índices de crecimiento indican que las diferencias en las épocas de emigración se deben al retraso en la aparición de camarón de tamaño suficiente para su captura con las redes. Cuando se emplea un intervalo de tiempo apropiado, mejora la correlación entre la abundancia relativa de juveniles emigrantes y la captura de camarón pequeño por unidad de esfuerzo en los caladeros de las Tortugas. Basándose en esta correlación se puede pronosticar la mayor abundancia de camarón pequeño en los caladeros antes de que la pesquería pueda aprovecharlo.
Valiéndose de los datos de los desembarques comerciales, puede observarse un movimiento aparente a través de las Tortugas. Se puede hacer un pronóstico bastante seguro con una anticipación de hasta 4 meses acerca de la mayor abundancia de camarones de talla correspondiente a 41–50 ejemplares por libra.
Studies have been underway since late 1962 on the juvenile stages of the pink shrimp (Penaeus duorarum Burkenroad), in the Everglades National Park estuary. The principal objective has been to increase knowledge of the biology and migration of the shrimp, primarily in terms of changes in seasonal abundance and size distribution. A secondary objective has been to determine if a relationship exists between the relative abundance of emigrating juveniles and the catches of adults on the Tortugas commercial fishing grounds. It is this latter phase of the program which is discussed here. This work has been supported by the U.S. Department of the Interior, Bureau of Commercial Fisheries.
Fluctuations in availability of shrimp are of considerable importance to the industry. If they can be explained in relation to fluctuations in environmental conditions, or detected through changes in availability of younger stages, prediction of fishing success may be possible. More efficient operation of the commercial industry would result from this.
The pink shrimp is believed to be an annual crop. The success of a fishery which exploits annual crops is influenced to a large degree by short-term environmental changes. Furthermore, without the dampening effects of more than one year class, the fluctuations may be of greater magnitude. Hence, the problem of prediction of such species is difficult.
The Tortugas fishing grounds are south and west of the southern tip of Florida (Fig. 1.). Fishing takes place on adult populations in water about 15 to 20 fm (27.4 to 36.6 m) deep. Spawning takes place on the fishing grounds and the young hatch and move into shallower water, including the estuaries at the southern tip of the Florida peninsula. They enter the Whitewater-Coot Bay estuary as postlarvae in the 6–7 rostral spine stage (about 10 mm total length, i.e. tip of rostrum to tip of telson) at night on flooding tides (Tabb, Dubrow and Jones, 1962). After a period in the estuary ranging from two months or less to about six months (Costello and Allen, 1966), the juvenile shrimp move out of the estuary on ebbing tides to return to the offshore grounds to spawn.
From January 1963 through June 1965 the relative monthly abundance of juveniles was estimated from catches in Buttonwood Canal at Flamingo in EvergIades National Park using a large “channel net” (Yokel, Roessler and Iversen, 1967). The entire canal was fished by this net, which relied on tidal currents to collect samples. This gear took advantage of the behavior of the juvenile shrimp in moving out of the estuary with the ebbing tide. The net caught virtually all shrimp longer than about 8.0 mm carapace length (about 38 mm total length).
Subsequently, a series of experiments in early 1965 showed that “wing nets” could subsample reliably the shrimp moving in the canal, and the use of the cumbersome channel net was discontinued. The wing nets are made of 1.9 cm stretched mesh webbing, the same as that used in the channel net, and are 5.18 m long from mouth to codend. The webbing is attached to a rectangular frame made of 2.54 cm diameter galvanized pipe. The mouth of each net is 1.8 m by 1.07 m. The effective fishing area of the net is 1.8 m along the surface and 0.9 m deep, since the upper 15.2 cm of the net remains out of the water. The nets taper to codends about 0.46 m in diameter and are fished from both sides of a 5.5 m skiff.
For both the channel net and the wing nets, the standard sampling interval was 30 min. Records of water velocity, depth, temperature and salinity, air temperature, wind direction and velocity, and cloud cover were kept for each sample.
Fig. 1 Spawning and nursery area of the Tortugas population of pink shrimp (Penaeus duorarum).
To compare the catches of shrimp in Buttonwood Canal with commercial catches of shrimp on the Tortugas grounds, it is necessary to establish a standard measure of abundance of shrimp migrating out of Buttonwood Canal. A series of such standard measures of relative abundance can be compared with the catches of commercial pink shrimp. If a constant relationship can be shown to exist between the indices of abundance of migrating juveniles at Flamingo and catches on the commercial fishing grounds some time later, it is then possible to establish a system for the prediction of commercial catches from catches of juveniles.
To learn what factors are important in establishing a monthly index of abundance, monthly sampling times were selected randomly, and about equal numbers of flood and ebb tides were sampled. Some of the important factors affecting the numbers of shrimp were (1) light, (2) the direction of the tide, (3) the stage of the tide, and (4) moon phase.
Juvenile pink shrimp migrate almost exclusively at night, and virtually no shrimp were collected in daylight samples. A series of fourteen pairs of ebb-flood tides in which the two tides were fished on the same or adjacent nights during 1963 showed that relative abundance averaged over twenty times higher on ebb tides than on flood tides. Night samples collected at velocities in excess of 0.07 mi/h (0.31 m/sec) provided a more reliable index of abundance than samples taken at lower velocities.
Differences in relative abundance were also noted within months. The numbers of shrimp moving out of the estuary were higher in periods of new and full moons than during other times of the month. In 1963, 118 samples were collected on or near the time of new or full moons, and 130 samples about the time of the first or third quarter moons. The average 30 min catch during the new and full moon periods was 1,100 shrimp, compared to an average of 600 shrimp per 30 min on quarter moons. During seasons of low relative abundance the samples taken on quarter moons were about the same size as new and full moon catches, but in times of high abundance, new and full moon catches were larger than quarter-moon samples and showed a closer relationship to the commercial landings. With these data the monthly sampling schedule was adjusted to maximize the catch and provide the best prediction index.
A special study in 1964–65 showed that a consistently high proportion of the juvenile shrimp (average 78 percent on new moons; 91 percent on full moons) migrated in the upper 50 cm of water during the new and full moons (Beardsley, 1967). This suggests that on new and full moons the number of shrimp caught by both the channel net and the wing nets is strongly influenced by the volume of surface water (the upper meter) filtered and that the measure of relative abundance should be the number of shrimp per unit volume of surface water filtered. However, the volume filtered in the surface layer is essentially a function of velocity and it is possible to use velocity instead of water volume as an index.
Water velocity was measured at a depth of 50 cm with a Gurley current meter (Yokel, Roessler and Iversen, 1967) operated continuously during each 30 min sampling period. The number of revolutions of the meter during this period was used as the measure of water passing through the gear. The relative abundance estimate employed is a ratio of the number of shrimp caught divided by the total number of revolutions. Thus, the index of relative abundance for a particular month is the number of shrimp caught per revolution of the current meter on ebb tides at night during the periods of new and full moons.
An index of abundance of shrimp on the Tortugas grounds has also been established. This is the pounds per boat day of the smallest commercial size group (68 shrimp tails to the pound (150/kg), and smaller) caught in 15 fm (27.4 m) or less. These small shrimp are caught relatively close to the nursery areas and their numbers should show the most direct relationship with the numbers of emigrating juveniles. The data used to calculate this index are published by the United States Fish and Wildlife Service, Bureau of Commercial Fisheries in a monthly bulletin entitled “Gulf Coast Shrimp Data”.
For 1963 through 1965, the relative monthly abundance of shrimp emigrating from Buttonwood Canal is shown in Fig. 2. The mean size of shrimp, expressed in mm carapace length, for each mode of abundance appears in brackets inside the mode. The means have been calculated by weighting the average size for each month according to the abundance in that month.
After the shrimp leave the nursery area, they travel 50 to 80 mi (80 to 130 km) to the Tortugas fishing grounds. Thus, a time lag exists between the departure of the shrimp from Buttonwood Canal and their first appearance in the fishery. In our first attempt to correlate catches of emigrating juveniles with catches by commercial trawlers, this lag is disregarded, and the relationship is shown in Fig. 3. The solid line represents the number of pounds of “68-count-and-over” shrimp caught per boat day in 1963 through 1965. The broken line is the number of shrimp per revolution of the current meter in Buttonwood Canal. Breaks in the line in June and November 1963 represent months when samples were not taken during new or full moons.
In both 1963 and 1964, there were two seasonal peaks of commercial catches on the Tortugas grounds, whereas in 1965 there was a single peak. The data from Buttonwood Canal show similar peaks, but they occur slightly before those in the commercial catch. Estimates of growth rates and data from marked shrimp give evidence that these are the same groups of shrimp. On a yearly basis, emigrating shrimp range in size from 2 to 45 mm in carapace length, with a mean size of about 14 mm.
Kutkuhn (1966) gives equations for the conversion of carapace length measurements to total length for male and female shrimp. From these data a reasonably accurate size dependent conversion value (sexes combined) may be calculated. For shrimp with a carapace length of 30 mm or smaller total length may be estimated by multiplying carapace length by a constant of 4.6; for larger shrimp the constant is 4.4.
Direct information on the growth rates of juvenile shrimp within the estuary and during migration is not available and therefore it is necessary to use indirect methods to obtain an estimate of these rates. From data provided by Kutkuhn (1966) it is possible to estimate the mean rate of growth of shrimp from birth until they reach a size of about 24 mm carapace length and to make a more precise estimate of their growth during the recruitment phase on the eastern edge of the grounds. Kutkuhn estimates that a shrimp of 23 to 24 mm carapace length (70 headless count) is roughly 15 weeks old, which implies an average growth rate of 6.7 mm carapace length per month from birth through the juvenile stage. Present studies underway in Everglades National Park by the authors and others on the dynamics of postlarval and juvenile shrimp suggest this is a reasonable estimate of growth in the estuary. Accelerated growth inside the estuaries for this species has been noted by Williams (1955; 1965) and for the brown shrimp (Penaeus aztecus Ives) by St. Amant, Corkum and Broom, (1963), and Kutkuhn (1966).
During the recruitment phase on the Tortuges grounds Kutkuhn (1966) states that the shrimp increase from about 19.5 mm to 29.0 mm carapace length (123 to 37 headless count, 271 to 82/kg) in 12 weeks, which is a growth rate of 3.4 mm carapace length per month. These data suggest that the most rapid growth phase occurs in the larval, postlarval and juvenile stages, which are spent largely in the estuary, and that the growth rate slows when the shrimp arrive on the fishing grounds. For the purposes of this report the growth rate during migration is taken to be at 5.0 mm carapace length per month, a value intermediate between the average early growth rate and the growth rate on the grounds during recruitment.
Fig. 2 Monthly indices of abundance of juvenile shrimp emigrating from the Everglades National Park nursery grounds via Buttonwood Canal at Flamingo, Florida.
Fig. 3 Monthly indices of abundance of shrimp emigrating from the Everglades National Park nursery grounds and of commercial landings (in pounds) on the Tortugas grounds.
Shrimp in the smallest commercial category (68-count-and-over) have a carapace length of about 18 to 24 mm. Applying a growth rate of 5.0 mm carapace length per month, shrimp leaving the estuary at 14 mm (the average size on an annual basis) would be 19 mm long one month later, and would be available to the fishery.
Stained shrimp released by Costello and Allen (1966) at Shark River have been recovered on the Tortugas grounds in a minimum of 35 days. About 10 percent of the recovered shrimp were caught in 38 days or less (Costello and Allen, personal communication). Thus a “lag” of something over a month between the time shrimp caught in Buttonwood Canal and the time they are first caught on the Tortugas grounds seems reasonable.
The agreement between indices of abundance at Buttonwood Canal and on the Tortugas grounds is improved if the commercial catch is compared to the previous month's catch of juveniles in Buttonwood Canal (Fig. 4). The first three periods of abundance in Buttonwood Canal now match periods of abundance in the Tortugas landings and the mean sizes of the Buttonwood catch are such that in one month with a growth of 5.0 mm carapace length they are available to the fishery. However, the two larger peaks are still displaced by 1 to 1-½ months. Presumably this is caused by a delay in the arrival of the shrimp at a size large enough for the trawls to capture. This supposition is supported by the small average size of shrimp (about 11 mm carapace length) making up the catch at Buttonwood Canal during these two periods. From this small size a minimum of two months would be required for the shrimp to grow to the smallest exploitable size, and use of this increased lag time is justified.
Thus to predict an increase in the availability of small shrimp on the Tortugas grounds from the catches of juveniles leaving Buttonwood Canal, both the time of high relative abundance and the size of the individuals must be known, the latter being used to determine the extent of the lag period. Based on this relationship a forecast of availability of small shrimp on the fishing grounds can be made 1 to 2-½ months before they are available to the fishery.
The usefulness of these predictions of abundance is also enhanced, if there is more “lead time” (i.e., the period of time between prediction and harvest) and if the predictions include large shrimp as well as the small 68-and-over group. When plots are made of average monthly catches per boat day (reduced to a common base) by shrimp size category, a series of modes appear which represent periods when shrimp of a particular size were most available (Fig. 5). These modes first represent the smallest size group, and as the modes progress along the abscissa in successive size categories their displacement represents growth of the shrimp. Lines representing estimated growth rates have been fitted by eye through the values associated with these modes. The slopes of the lines are consistent and the growth rates implied are similar to those observed by Kutkuhn (1966) for this population.
This relationship can be used to extend the lead time of the forecasted abundance. The graph shows five periods of abundance in the 68-and-over size group. Four of these can be traced two months later to the 41 to 50 size category (90 to 110/kg). Thus if there is a two-month lag between the estuary and their appearance in the 68-and-over size group it is possible to anticipate increased abundance in the 41 to 50 size group up to four months in advance.
In addition to predicting that catches will increase in the commercial fishery at a particular time, it would be useful to predict the size of the group of shrimp entering the fishery. So far the variation around this relationship is too high to permit an accurate prediction of relative sizes of the groups.
Fig. 4 Monthly indices of juvenile shrimp abundance, advanced by one month, and of commercial landings (in pounds) on the Tortugas grounds.
Fig. 5 Average commercial catch per unit of effort of pink shrimp by size groups on the Tortugas grounds.
Standard measures of relative abundance for juvenile shrimp emigrating from estuarine areas in Everglades National Park and for shrimp in the smallest commercial category have been established which permit predictions of moderate accuracy of the appearance of groups of small shrimp on the Tortugas fishing grounds. Such influxes of small commercial shrimp may be detected in progressively larger size categories with time, thus increasing the lead time of the prediction and enabling it to be applied to larger size groups.
To improve these predictions we need more precise information on the factors which control the length of stay in the estuary and on the growth rates in the estuary and during migration. Also, the size distribution of the 68-and-over commercial size category is required to determine the size at recruitment into the fishery.
It is thus possible to make a qualitative prediction of shrimp abundance on the Tortugas grounds based on estimates of pre-recruit abundance. With refinements a quantitative prediction may be possible.
Beardsley, G.L., 1967 Distribution in the water column of migrating juvenile pink shrimp Penaeus duorarum Burkenroad, in Buttonwood Canal, Everglades National Park, Florida. University of Miami, Dissertation
Costello, T.J. and D.M. Allen, 1966 Migration and geographic distribution of pink shrimp, Penaeus duorarum, of the Tortugas and Sanibel grounds, Florida. Fishery Bull.Fish.Wildl.Serv.U.S., 65(2):449–59
Kutkuhn, J.H., 1966 Dynamics of a penaeid shrimp population and management implications. Fishery Bull.Fish Wildl.Serv.U.S., 65(2):313–38
St. Amant, L.S., K.C. Corkum and J.G. Broom, 1963 Studies on growth dynamics of the brown shrimp, Penaeus aztecus, in Louisiana waters. Proc.Gulf Caribb.Fish.Inst., 15:14–26
Tabb, D.C., D.L. Dubrow and A.E. Jones, 1962 Studies on the biology of the pink shrimp, Penaeus duorarum Burkenroad, in Everglades National Park, Florida. Tech. Ser.Fla.St.Bd Conserv., (37):32 p.
Williams, A.B., 1955 A contribution to the life histories of commercial shrimps (Penaeidae) in North Carolina. Bull.mar.Sci.Gulf Caribb., 5(2):116–46
Williams, A.B., 1965 Marine decapod crustaceans of the Carolinas. Fishery Bull.Fish Wildl. Serv.U.S., 65(1):298 p.
Yokel B.J., M.A. Roessler and E.S. Iversen, 1967 Fishes and juvenile stages of pink shrimp (Penaeus duorarum) collected in Buttonwood Canal, Florida, December 1962 to June 1965. Data Rep.U.S.Bur.comml Fish., (22)
