length increases rapidly in early summer as gonad proliferates, then 

 decreases sharply with spawning. Weight declines in winter as food reserves 

 are used up. Shell length and shell weight showed no obvious seasonal 

 relationship, but a significant annual cycle in percentage organic content 

 of shell had distinct peaks at beginning and end of growing season. Three 

 peaks in calorific content were identified, in July, Sept, and Feb. Recruit- 

 ment was irregular. Successful recruitment at Hamble probably depends on 

 spawning of stocks in. the Test River, where conditions are more favorable for 

 larval development. Two mortality types were observed, in juveniles and 

 adults. These were related to predation. Weight-specific respiration (Q0 2 ) 

 decreased with increasing size of clam, and fell within the range reported 

 for other bivalves. Feces and pseudofeces made up a relatively consistent 

 proportion of food consumption (mean 59%) . Total energy consumption 

 (1292 Kcals/m 2 /yr) was deposited largely as feces (759 Kcals) or excreted 

 (160 Kcals). Other observations are mentioned without specific results. 

 - from thesis summary - J.L.I1. 



S27 



Hibbert, C. J. 1976. 



Biomass and production of a bivalve community on an intertidal mud-flat. 

 J. Exp. Mar. Biol. Ecol . 25(3): 249-261. 



The study was made in Southampton Water, the estuary of the Test and Itchen 

 rivers, on the mud flat of Hamble Spit at the mouth of the River Hamble in 

 the south of England. Sampling area was about 60 ha, with 4 transects about 

 200 m apart and with stations at 100 m intervals. Two surveys were made in 

 winter, in 1972 and 1973. Number of stations was 30. Stations were placed 

 in four groups, based on time of exposure on ebb tide, percent mud, and 

 total bivalve biomass. Of 12 bivalve species identified, Cerastoderma edule 

 and Mercenaria mercenaria dominated. Stations in Group I (above mean tide 

 level - 2 . 6 m or more above Port Datum) contained negligible numbers of 

 mollusks. Group II, with coarse sediments, was about 1.25 to 2.6 m above 

 Port Datum and had 503 g/m 2 live weight biomass of C. edule and 180 g/m 2 of 

 M. mercenaria, with total biomass all species of 772 g/m 2 ; Group III, about 

 0.8 to 1.8 m above Port Datum had 1969 g/m 2 C. edule and 1195 g/m 2 M. 

 mercenaria, and coarse sediments; Group IV, about 0.55 to 0.9 m above 

 Port Datum, was true mud flat, with 454 g/m 2 C. edule and 750 g/m 2 M. 

 mercenaria. Groups III and IV had total biomass of 3,315 and 1,205 g/m 

 respectively. Recruitment to the Mercenaria population was sporadic. 

 "Successful" years were correlated with summers of low water in the River 

 Test, which was assumed to favor retention of larvae. Recruitment to stocks 

 of other species, however, apparently did not vary greatly. Average shell 

 lengths of M. mercenaria measured between January and March 1972 and 1973 

 were: end of 1st yr 10.6 mm, 2nd yr 24.4 mm, 3rd 37.6 mm, 4th 49.0 mm, 5th 

 56.6 mm, 6th 61.3 mm, and 7th 67.6 mm. Ages were estimated from shell rings. 

 The relation between ash-free dry weight of meats and shell length was 

 expressed by the average equation: log weight = 3.328 log length -5.641; 

 between mean biomass and annual production in g ash-free wt/m2 was Group II 

 biomass 7.73, production 3.99, P/B 0.52; Group III biomass 50.04, production 

 14.00, P/B 0.28; and Group IV biomass 36.54, production 6.19, P/B 0.17. 

 Growth rates were similar over a wide range of sediment types, contrary to 

 findings of other authors. Percentage of mud in bottom samples ranged from 

 3 to 93. It was concluded that other factors have an important influence on 

 growth. Small clams, less than 25 mm long, probably are eaten by several 

 predators, including Carcinus maenas , smaller wading birds, and fishes. Most 

 gull and wading bird predation was on Groups II and III, in which 80% of 

 mortality was attributed to predation. Mortality was highest in Group II, 

 lowest in Group IV, related to period of exposure and depth of water. 

 Herring gulls took 95 g ash-free dry weight of Mercenaria per bird per yr, 

 and a total annual removal of 1.9 tonnes ash-free dry weight. Relation of 

 shell weight to shell length of M. mercenaria is expressed by the equation: 

 log shell wt in g =2.96 log length in mm -3.586. Shell contains 2.68% 

 organic matter. 



230 



