Observations on the Seasonal Occurrence, Abundance, 



and Distribution of Larval Lobsters 



(Homarus americanus) in Cape Cod Bay 



GEORGE C. MATTHIESSEN and MICHAEL D. SCHERER' 



ABSTRACT 



The seasonal occurrence, abundance, and dis(ribution of the larvae of the American lobster, Homarus 

 amerkanus, in Cape Cod Bay were studied over a 3-yr period (1974-76). Although larvae were observed during 

 the months of May-September, the great majority were found to occur during June, July, and August. In the 

 neuston net collections, mean larval densities for these 3 mo averaged 3.1, 3.2, and 1.6 larvae/1,000 m . respec- 

 tively. However, densities as high as 62.3 larvae/1,000 m J were observed in Tucker net collections near the east 

 end of Cape Cod Canal. 



The seasonal occurrence, pattern of distribution, and relative abundance of first-stage larvae near the 

 east end of Cape Cod Canal during June suggest the likelihood that the Canal may contribute significant numbers 

 of larvae to Cape Cod Bay. 



INTRODUCTION 



Despite the economic significance of the lobster (Homarus 

 americanus) fishery in New England, very little is known of 

 the origin and dispersal of larval lobsters and therefore of 

 the primary sources of recruitment to localized stocks. 



The increasing number of electric generating plants along 

 the New England coast prompted a series of investigations of 

 the seasonal abundance and distribution of larval lobsters 

 during the 1970's. The primary objective of many of these 

 investigations was to estimate the potential losses of larvae 

 resulting from their entrainment in the power plant's cooling 

 water, the relationship of the numbers of entrained larvae to 

 the population as a whole, and the resultant potential impact 

 upon the local or regional fishery. 



The Pilgrim Nuclear Power Station, located in Plymouth, 

 Mass., on the west side of Cape Cod Bay, draws its cooling 

 water from an area that supports a valuable and intensive 

 lobster fishery. The investigation described in this report was 

 initiated to compare the numbers of larvae occurring in the 

 vicinity of the station, and therefore potentially vulnerable 

 to entrainment, with larval abundance in other areas of Cape 

 Cod Bay, and to determine the seasonal occurrence and dura- 

 tion of the larval period in this area. 



METHODS 



Surface Sampling 



Larval lobster were sampled at Stations I-VIII (Fig. 1) bi- 

 weekly from 20 June through 19 August in 1974; weekly at 

 Stations I-VIII from 5 June through 8 October in 1975; and 

 weekly at Stations I-X from 4 May through 1 September 1976. 



Sampling gear consisted of a 1 mm mesh neuston net 

 measuring 1 m x 2 m at the mouth and 10 m in length. The 

 top edge of the net was held just above the surface by large 



floats. Tows were made at 4-5.5 km/h for approximately 

 30 min off the side of 12 m (1975 and 1976) and 20 m (1974) 

 vessels. Filtration volumes, estimated with a General Oceanics 

 2030 2 flowmeter mounted in the mouth of the net, averaged 

 about 3,000 mVtow. Surface temperature and salinity were 

 recorded at each station using a Beckman RS5-3 salinometer. 

 In 1974 duplicate tows were made irregularly at various 

 stations as time and weather permitted. Single tows were taken 

 at each station in 1975. In 1976 two vessels were used, towing 

 in parallel, to collect duplicate tows at each station. 



Vertical Sampling 



In 1976 samples were taken at 0, 3, 6, 9, and 12 m (near bot- 

 tom) at Station A located just off the easterly end of Cape Cod 

 Canal (Fig. 1). Samples were taken on easterly tides by holding 

 position into the current which averages about 6.5 or 7.4 km/h 

 on spring tides (Anraku 1964). One sample was taken at each 

 depth during daylight, and in most cases during darkness with- 

 in the same 12-h period, on a weekly basis 7 June through 8 Ju- 

 ly. 



Gear consisted of a messenger-operated 1 mm mesh Tucker 

 net (Tucker 195 1 ; Clarke 1969) with a mouth measuring 2 m x 

 2 m. Filtration volumes averaged 3,000-4,000 m 3 . 



All samples were preserved in 10% Formalin and returned 

 to the laboratory for analysis. Lobster larvae were enumerated 

 and staged following Herrick (1911). 



RESULTS 



Seasonal Occurrence and Distribution 



During the 1974-76 sampling seasons, lobster larvae were 

 found in Cape Cod Bay as early as 1 1 May (1976) and as late as 

 28 September (1975). Stage I larvae were found from 11 May 



Marine Research, Inc., 141 Falmouth Heights Road, Falmouth, MA 02540. 



Reference to trade names does not imply endorsement by the National 

 Marine Fisheries Service, NOAA. 



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