may be exaggerated, however, by the fact that the Bay was 

 sampled only once in June of 1974. 



Distribution by Larval Stage 



During the early part of the larval season, in both 1975 and 

 1976 the distribution of larval lobster in Cape Cod Bay ap- 

 peared to be related to stage of development. Not only the 

 highest densities of larvae but also the highest percentage of 

 stage I larvae were found in the southwest section of the Bay 

 (Fig. 2). A great percentage of larvae collected in the north- 

 west section (Station I) early in the season were also stage I lar- 

 vae; however, the numbers collected were low in comparison 

 with the southwest collections. 



No consistent distributional pattern could be detected 

 among total larvae densities in July and August of each year. 

 No pattern was apparent among stage II or stage IV larvae 

 during these months. In 1974, stage III larvae were most abun- 

 dant at Station IV on each sampling date of July and August 

 except the last (19 August) when this station ranked second. 

 In 1975 and 1976, however, no pattern among stage III larvae 

 was apparent. 



Paired Tows 



During 1976, when two vessels, towing in parallel, collected 

 simultaneous neuston samples at each station, a total of 164 

 paired samples were collected. The mean number of larvae per 

 individual tow was 3.11/1,000 m\ with a standard deviation 

 of 5.21. The mean of the variation between tows was 1.44/ 

 1,000 m 3 , with a standard deviation of 2.28. Comparing the 

 catch of the two vessels by means of a Wilcoxon paired sample 

 test indicated that no significant difference (P>0.05) 

 occurred. 



Vertical Distribution 



Results of vertical sampling at the easterly end of Cape Cod 

 Canal from 7 June to 8 July 1976 indicated that highest con- 

 centrations of larvae were generally found at a depth of 3 m 

 (Table 4). An analysis of these data using Friedman's test (Zar 

 1974) indicated a highly significant difference (P<0.01) be- 

 tween depths of collection. 



The data also indicate that the highest concentrations of lar- 

 vae tend to occur during periods of darkness (Table 4). How- 

 ever, when the data for dates which included both day and 

 night (1 h after sunset to 1 h before sunrise) sampling within a 

 24-h period were analyzed using the Mann-Whitney U test (Zar 

 1974), the results indicated no significant difference in larval 

 density occurred between day and night. 



Approximately 96% of the larvae captured in the Tucker net 

 were stage I. This is consistent with the June 1976 data for the 

 nearby neuston stations (Stations VIII, IX, and X) (Fig. 2). 



1951-61 was 12.2°C. J The occurrence of stage III and IV larvae 

 in the 20 June 1974 collections was therefore somewhat sur- 

 prising in relation to both the Massachusetts State Lobster 

 Hatchery data and observations by Sherman and Lewis (1967) 

 and Lund and Stewart (1970) relative to the onset of hatching 

 at 13.7 o -15.0°C and 14.0°C in Maine and Connecticut, 

 respectively. Surface water temperatures averaged 16.1 °C 

 on 20 June 1974. At this temperature, the time required to 

 reach the third and fourth larval stages is approximately 10 

 and 20 d, respectively (Templeman 1936). Bottom water 

 temperatures in Cape Cod Bay 14 d prior to 20 June averaged 

 only 7.7°C (based on 51 ichthyoplankton stations). 4 



In 1975 high concentrations of larvae were found in the col- 

 lections on the first sampling date (5 June), primarily in the 

 southwest sector of the Bay (Fig. 2). Although the great ma- 

 jority of these were stage I larvae (Fig. 2), bottom water tem- 

 peratures in Cape Cod Bay at this time, gathered at 18 ichthyo- 

 plankton stations on 3-4 June (MRP), averaged only 9°C. 

 Despite these low temperatures, it was evident from the large 

 numbers of stage I larvae, and moderate numbers of stage II, 

 in the collections that hatching must have been well underway 

 by 1 June. 



Cape Cod Canal water temperature records maintained by 

 the New England Gas and Electric Generating Station in 

 Sandwich, Mass., indicate that water temperatures may vary 

 by 5 °C or more in the Canal during a tidal cycle, depending 

 upon whether the water originates from Cape Cod Bay or 

 from the much warmer Buzzards Bay. Stations IX and X and 

 the Tucker net sampling station were added in 1976 for the 

 purpose of establishing whether the large number of larvae 

 found in the southwest area of the Bay in early June samples 

 might originate from Cape Cod Canal, or perhaps Buzzards 

 Bay, where temperatures at that time would be more condu- 

 cive to hatching. 



The June distribution of larvae in the Bay at Stations I-IX 

 (Fig. 2) and the abundance of larvae at vertical sampling sta- 

 tion A in 1976 (Table 4) raises the possibility that Cape Cod 

 Canal may contribute large numbers of larvae during June. 

 Data reported by the U.S. Army Corps of Engineers (1973) in- 

 dicated that water enters Cape Cod Bay via Cape Cod Canal at 

 an average rate of 2,060 m 3 /s during an easterly tide, and that 

 most of this water does not return to the Canal during the ebb- 

 ing (westerly) tide. It is estimated, therefore, that an average of 

 95.8 x 10 6 m 3 of water flows into Cape Cod Bay from the 

 Canal each day. 



For the period 7-30 June 1976 the mean density of larvae in 

 the water column at the mouth of the Canal was computed for 

 each day oi i ucKer net sampling. Densities for replicate or 

 day/night tows taken during the same 24-h period were aver- 

 aged. The total number entering the Bay each sampling day 

 was then estimated by multiplying mean larval density by daily 

 flow. By plotting the numbers of larvae introduced into Cape 

 Cod Bay on each sampling day against time, and computing 



DISCUSSION 



It was concluded on the basis of 10 yr of records maintained 

 at the Massachusetts State Lobster Hatchery that hatching of 

 Homarus americanus eggs usually begins when water tempera- 

 tures have risen to 15°C and is most intensive when tempera- 

 tures approximate 20°C (Hughes and Matthiessen 1962). The 

 lowest temperature at which hatching was recorded during 



'This information, in fact, was the basis for the decision not to initiate the 

 1974 sampling program until the latter part of June since the data collected 

 during previous ichthyoplankton surveys of the Bay (MRI 1974; see footnote 4) 

 indicated temperatures of Cape Cod Bay rarely exceeded 15°C before this 

 time. 



4 MRI (Marine Research, Inc.). 1974. Cape Cod Bay Study Quarterly 

 Progress Report, March-May 1974. 6 p. + appendix. 



'MRI (Marine Research, Inc.). 1975. Cape Cod Bay Study Quarterly 

 Progress Report, June-August 1975. 7 p. + appendix. 



43 



