free-swimming copepod nauplii in Narragansett Bay. J. Fish. Res. Bd. Canada 23 (3): 415--422.

FISH, C. J. 1925. Seasonal distribution of the plankton of the Woods Hole region. Bull. U. S. Bur. Fish. 44: 91-179.

GIESBRECHT, W. 1891. Elenco dei copepodi pelagici raccolti dal tenente di vascello Gaetano Chierchia durante il viaggio della R. Corvefta "Vettor Pisani" negli anni 1882-1885, e dal tenente di vasceUo Fran- cesco Orsini nel Mar Rosso, nel 1884. Atti Rend. Accad. Lincei, Roma, ser. 4, 7 (sem. 1): 474--481.

GONZALEZ, J. G., AND T. E. BOWMAN. 1965. Planktonic copepods from Bahia Fosforescente, Puerto Rico, and adjacent waters. Proc. U. S. Nat. Mus. 117 (3513): 241-304.

GRICE, G. D. 1960a. Calanoid and cyclopoid copepods collected from the Florida Gulf coast and Florida Keys in 1954 and 1955. Bull. Mar. Sci. Gulf Caribbean 10 (2): 217-226.

- - - - - - . 1960b. Copepods of the genus Oithona from the Gulf of Mexico. Bull. Mar. Sci. Gulf Caribbean 10 (4): 485-490.

HEINLE, D. R. 1973. Copepods: microscopic animals, p. 16 17. In Lippson, Alice Jane, (ed.), The Chesapeake Bay in Maryland, an atlas of natural resources. The Johns Hopkins University Press, Baltimore and Lon- don, viii + 55 p.

Short Papers and Notes 1 3 7

HOPKINS, T. L. 1966. The plankton of the St. Andrew Bay System, Florida. Publ. Inst. Mar. Sci. Univ. Texas 11: 12-64.

JEEERIES, H. P. 1962. Copepod indicator species in estuaries. Ecology 43 (4): 730-733.

WASS, M. L. 1965. Check list of the marine invertebrates of Virgina. Virginia Inst. Mar. Sci. Spec. Sci. Rep. No. 24 (Third revision), 58 p.

WELLERSHAUS, S. 1969. On the taxonomy of planctonic Copepoda in the Cochin Backwater (a South Indian estuary). Ver~ffi Inst. Meeresforsch. Bremerhaven 11: 245 -286.

- - - - - - , 1970. On the taxonomy of some Copepoda in Cochin Backwater (a South Indian estuary). VerSffi Inst. Meeresforsch. Bremerhaven 12: 463-490.

WILSON, C. B. 1932a. The copepod crustaceans of Chesapeake Bay. Proc. U. S. Nat. Mus. 80 (2915): 1-54, pls. 1-5.

- - - - - - . 1932b. The copepods of the Woods Hole region, Massachusetts. Bull. U. S. Nat. Mus. 158, xix + 635 p., 4l pls.

THOMAS E. BOWMAN

Department o f Invertebrate Zoology Smithsonian Institution Washington, D.C. 20560

Modified Commercial Crab and Oyster Dredges as Sampling Devices for the Blue Crab Callinectes sapidus Rathbun z

ABSTRACT: Paired samples were collected using a lined versus unlined commercial crab dredge, a lined commercial crab dredge versus a lined commercial oyster dredge, and a lined commercial oyster dredge versus a weighted, lined oyster dredge. The densities of crabs of three size classes (1-59 ram, 60-119 mm, >120 mm) caught in each gear were calculated and analyzed by paired block design. The lining resulted in a larger catch of the smallest size class in the crab dredge, but did not inhibit the catch of the two larger size-classes. Catches in the crab dredge exceeded those for the oyster dredge in the two smaller size-classes, but not in crabs greater than 120 mm in carapace width. The addition of 18 kg of weight to the oyster dredge had no effect upon its ability to catch crabs.

Introduction

The behavior of the blue crab varies considerably depending upon ambient temperature. Although it is a strong swimmer, it becomes lethargic when the tempera- ture drops and enters a period of dormancy during the winter in the temperate part of its geographical range (Rathbun 1896; Van Engel 1958). In Chesapeake Bay, crabs tend to migrate to deeper waters in late autumn and burrow into the substrate during their dormant period

~Contribution No. 636, Center for Environmental and Estuarine Studies, University of Maryland.

Chesapeake Science Vot, 16, No, 1 June, 1975

(Churchill 1919). This behavior is the basis for an active winter dredge fishery in the Virginia portion of Chesa- peake Bay (Van Engel 1962). The dredge catch, consist- ing of up to 85% adult females, accounts for approxi- mately 20% of Virginia total landings. The most common dredge consists of a heavy metal rectangular frame bearing a toothed drag bar 1.8 meters wide (6 ft.) and a mesh bag with a capacity of 3 to 4 bushels of crabs (Van Engel 1962).

Scientific studies of blue crab populations in Chesa- peake Bay have been hampered by the differences in behavior of crabs from season to season. Although a dredge should be an effective winter sampling gear, catches are often highly variable, especially for small crabs (Sulkin 1973). In order to catch small crabs, a small mesh net liner must be placed inside the regular dredge bag, however, the effect of such modification upon the catch of larger-sized crabs is unknown.

It was the purpose of this study to compare the catch effectiveness of lined and unlined crab dredges for various sizes of blue crabs and also to compare the catch effectiveness of commercial crab and oyster dredges.

Methods and Results

LINED VERSUS UNLINED CRAB DREDGES

In February and March, 1972, eighty-five samples of blue crabs were collected with the modified crab dredge

1 3 8 Short Papers and Notes

shown in Fig. 1. The typical 1.8 meter wide commercial crab dredge was modified in the following manner. It was partitioned in the middle by welding a steel rod across the top and fastening a 7.6 cm (3 in.) mesh curtain between the rod and the dredge bag. A 1.3 cm (1/z in.) mesh liner was attached to the curtain and to the dredge bag on one of the two halves. The other half of the dredge bag consisted of unlined 7.6 cm mesh. This effectively divided the 1.8 m dredge into two 0.9 m halves, one lined and one unlined.

The areas sampled included stations in Tangier Sound, Md., in the Potomac River, and in the Patuxent River, Md. Depths ranged from 3 m to 9 m; bottom types were sandy, shelly, or muddy. The crabs caught in each half were sorted into three size categories (1-59 mm wide; 60-119 mm wide; and >120 mm wide) and counted. The distance that the dredge had been towed was determined by radar. Catch densities were calculated for each tow as number of crabs caught/sq, meter of bottom sampled. The data were analyzed by paired random block design. The results of this sampling and analysis are shown in Table 1. A larger catch was measured for each size class in the lined side of the dredge, although a statistically significant difference was found only in the smallest size class.

LINED CRAB DREDGE VERSUS LINED OYSTER DREDGE

In March, 1972, samples were collected using the commercial crab dredge and a commercial oyster dredge.

Fig. 2. Modified oyster dredge. The commercial oyster dredge is lined with 1.3 cm mesh netting. An additional 9 kg of weight could be added to each side by attaching the weighted cans to the side rails.

TABLE 2. Mean catch density of each of three size classes and results of paired random block statistical analysis for the crab and oyster dredges.

No/m ~ (• 10 -3)

lined lined crab oyster

dredge dredge Probability

1-59 mm 1.43 0 0.025 < p <0.05 wide

60-119 mm 7.89 3.85 0.025 < p<0.05 wide

> 120 mm 3.37 2.74 p > 0.05 wide

Fig. 1. Modified commercial crab dredge. The dredge is divided into two halves by a net curtain. The left half is lined with 1.3 cm mesh netting; the right side consists of standard 7.6 cm mesh netting.

TABLE 1. Mean classes and results analysis for the two s,,

/,

Lin~

1 59 mm 1.48 wide

60-119 mm 4.77 3., wide

> 120 mm 3.61 3.45 wide

ach of three size '~lock statistical

dredge.

',ility

The oyster dredge is shown in Fig. 2. It consists of a 106 cm (42 in.) wide steel frame with a toothed drag bar and a bag lined with 1.2 cm mesh netting. The effectiveness of this oyster dredge was compared to that of the lined crab dredge.

The relative effectiveness of the two dredges was determined by towing them simultaneously and compar- ing the catches. A total of eleven such hauls were made at two sites in the Patuxent River, Md. The stations were 5 m to 10 m deep, substrate was sandy or muddy. Three of the tows were made with the prevailing tide current, three against, and 5 either across the current or at slack tide. The distance of each haul was determined as described above and a catch density value was calculated for each size class.

The results of this sampling effort and of a paired random block analysis are shown in Table 2. The catches in the crab dredge were statistically higher for the small and medium size classes, but no statistical difference was found in the large size class.

The effect of adding weight to the standard oyster dredge described above was tested. The catch in the standard oyster dredge was compared to that of a similar dredge to which a 9 kg weight had been attached to each :de. As before, the two were towed simultaneously for a

~1 of eleven hauls. As before, the hauls were against "dal current (3), with it (3), and across it or at slack

TABLE 3. Mean catch density of each of three size classes and the results of paired random block statistical analysis for the two oyster dredges.

No/m = (• 3)

Standard Weighted Probability

1 59 mm 0.52 1.05 >0.05 wide

60-119 mm 3.55 2.65 >0.05 wide

> 120 mm 3.24 2.74 >0.05 wide

tide (5). Catch density values were calculated for each size class and subjected to paired random block statisti- cal analysis (Table 3). No statistical differences were found to exist between the two oyster dredges in any of the three size classes.

Discussion

The data supports the intuitive conclusion that small mesh lining increases the catch of very young crabs. Because the addition of small mesh may cause the dredge to fill with mud and debris, a question has been raised concerning its effect upon the catch of larger crabs. There is no indication of a reduction in catch for either medium or large crabs when a small mesh liner is added.

The 106 cm oyster dredge is much lighter than the commercial crab dredge and thus considerably easier to handle. The heavier crab dredge provides an increased catch of small and medium-sized crabs, but this increase is of only marginal statistical significance. The addition of 18 kg of weight to the oyster dredge does not improve its effectiveness.

The choice of gear in sampling for scientific purposes depends upon a number of considerations. Although catch effectiveness is of obvious significance, the size of boat required for the gear and the effort expended in using it must be considered. If the objective of the sampling is to capture as many crabs as possible per tow, the commercial crab dredge is clearly superior to the oyster dredge. On the other hand, if the objective is to

Short Papers and Notes 139

compile a relative catch density index, the logistical problems associated with the bulky, heavy crab dredge probably overshadows its marginally greater effective- ness in catching small crabs.

It is clear that in long-term studies changes in sampling gear should be undertaken with caution and quantitative comparisons between old and new gears should be made so that errors due to such changes may be taken into account.

ACKNOWLEDGMENTS

We wish to thank Captain William Keefe of the R/V Orion for his advice and help in selecting stations and conducting the sampling. Mr. John Cooper and Mr. Joseph Ustach also offered valuable assistance. The figures were prepared with the assistance of Mr. Michael Reber, biophotographer of the Chesapeake Biological Laboratory. The work was conducted under a contract funded jointly by the State of Maryland Fishery Admin- istration and the National Marine Fisheries Service (Project No. 3-108-R; Contract No. N-043-340-72 G).

LITERATURE CITED

CHURCHILL, 1~. P., JR. Life history of the blue crab. Fish. Bull. U.S. 36:91 128.

RATHBUN, M. J. 1896. The Genus Callinectes. Proc. U.S. Nat. Mus. 18(1070):349-375.

SULKIN, S. D. 1973. Completion Report--Blue Crab Study in Chesapeake Bay, Maryland. Natural Re- sources Institute Reference No. 73-94. Chesapeake Biological Laboratory, University of Maryland. 66 p.

VAN ENGEL, W. A. 1958. The blue crab and its fishery in Chesapeake Bay. Part 1. Reproduction, early develop- ment, growth, and migration. Commercial Fish. Rev. 20(6): 6-17.

- - . 1962. The blue crab and its fishery in Chesa- peake Bay. Part 2. Types of gear for hard crab fishing. Commercial Fish. Rev. 24(9): 1 - 10.

STEPHEN D. SULKIN and ROBERT E. MILLER

Chesapeake Biological Laboratory University of Maryland Solomons, Maryland 20688

Computer Simulation of Effects on Atlantic Menhaden Yield of Changes in Growth, Mortality, and Reproduction 1

ABSTRACT: A self-regenerating dynamic pool model fitted to the Atlantic menhaden population was applied to simulate yield as a function of the parameters for growth, reproduction, and mortality. A random variable was applied to simulate random environmental fluctuations. For each combination of parameters the yield for 50 replicates of 50 generations was calculated. Under normal conditions considerable fluctuation in yield from the menhaden population can be expected. Decrease in the

Gulf Breeze Laboratory Contribution No. 188.

Chesapeake Science Vol. 16, No. 1 June, 1975

growth parameter produced the largest decrease in yield and an increase in adult mortality produced the smallest decrease in yield. The combined effects of simultaneous changes in more than one parameter were approximately additive.

Introduction

In this study a self-regenerating dynamic pool model is applied to the Atlantic menhaden population to determine the effects of independent and combined changes in the parameters of growth, mortality, and